The A-Z of HVAC&R

We are building an A-Z of HVAC&R to help define some of the terms in common (and not so common) use within the air conditioning and refrigeration industry.

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A-Z Index

Absolute Humidity – Air humidity refers to the water vapour content in the air.  As the temperature rises, the air’s capacity to absorb water vapour increases.  The amount of water vapour in grams contained in 1m³ of air is called the absolute humidity.

Absolute Pressure – Absolute pressure is a pressure that is relative to the zero pressure in the empty, air-free space of the universe.  This reference pressure is the ideal or absolute vacuum.  It is denoted with the subscript “a” i.e., Pa or Pabs

Absolute Zero Temperature – Absolute zero denotes the lower limit for the temperature i.e., the lowest possible temperature that can only theoretically be reached and cannot be fallen below.  This absolute zero defines the origin of the absolute temperature scale and is set as 0 Kelvin, which is equal to -273.15 degrees Celsius.

The existence and value of absolute zero can be plausibilised from various contexts. Gay-Lussac’s first law describes the relationship between the temperature and the volume of a gas – at absolute zero, this gas volume would be zero.  If one brings the thermal energy, which refers to the disordered movement of the particles in macroscopic matter, to the lowest possible value, where, graphically speaking, the movement of the particles can no longer be reduced, one has also reached absolute zero.

According to the third law of thermodynamics, absolute zero is not attainable; however, real temperatures can be realised indefinitely close to absolute zero.

Absorption Chiller – The thermodynamic cycle of an absorption chiller is driven by a heat source which is usually delivered to the chiller via steam, hot water, or combustion.  Compared to electrically powered chillers, an absorption chiller has very low electrical power requirements which just need to run two pumps: the solution pump and the refrigerant pump.  However, its heat input requirements are large, and its COP is only between 0.5 and 1.0.  For the same cooling capacity, an absorption chiller requires a much larger cooling tower than a vapor-compression chiller.  However, absorption chillers, from an energy-efficiency point of view, excel where cheap, low-grade heat or waste heat is readily available.  In extremely sunny climates, solar energy has been used to operate absorption chillers.

Adsorption Chillers – An adsorption refrigeration machine is a refrigeration machine in which, in contrast to the compression refrigeration machine, compression takes place through a temperature-influenced solution of the refrigerant.  This is also called a thermal compressor.  The refrigerant is absorbed in a solvent circuit at low temperature in a second substance and desorbed at higher temperatures.  The process makes use of the temperature dependence of the physical solubility of two substances.  The prerequisite for the process is that the two substances are soluble in each other in any ratio in the temperature interval used.

Alternating Current – Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction.  Alternating current is the form in which electric power is delivered to consumers, and it is the form of electrical energy that consumers typically use when they plug e.g. fans into a socket.  The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.  The frequency of the electrical system varies by country; most electric power is generated at either 50 or 60 Hertz.

AC Fans – Fans of various types driven by an AC (alternating current) motor providing air movement in heating, cooling and ventilation systems.

See also: EC Fans vs Axial Fans

Accumulator – The refrigeration compressor is designed to compress vapour only.  A suction line accumulator prevents compressor damage from a sudden surge of liquid refrigerant and oil that could enter the compressor from the suction line.  The suction line accumulator is a temporary reservoir for this mixture, designed to meter both the liquid refrigerant and oil back to the compressor at an acceptable rate.  This prevents damage to the reed valves, pistons, rods and crankshafts.

Adiabatic Cooling – Adiabatic cooling occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand, thus causing it to do work on its surroundings.  When the pressure applied on a parcel of gas is reduced, the gas in the parcel is allowed to expand; as the volume increases, the temperature falls as its internal energy decreases.

Adiabatic Process – In thermodynamics, an adiabatic process is a thermodynamic process which occurs without transferring heat or mass between the system and its surroundings.  Unlike an isothermal process, an adiabatic process transfers energy to the surroundings only as work.

Air Conditioning – Air conditioning is the process of removing heat and controlling the humidity as well as, in some cases, removing the dust and contaminants, of the air within a building to achieve a more comfortable interior environment.  This may be achieved using powered devices (‘air conditioners’), by passive cooling or by ventilative cooling.  An air conditioning system is a system that provides heating, cooling, filtering, ventilation and humidification/dehumidification (HVAC).  Air conditioners, which typically use vapour-compression refrigeration, range in size from small units used within vehicles to massive units that can cool an entire building.

Air Conditioning Unit– An air conditioning system is a ventilation and air conditioning system for producing and maintaining a pleasant or required indoor air quality (temperature, humidity, purity as well as CO2 content) independent of weather, waste heat and human and technical emissions. An air conditioning system has the task of bringing the air of a room into a certain state and maintaining it there (‘conditioning’)

Air Humidification – A distinction is made between three physical methods: evaporation, atomisation and vaporisation.  Evaporation is an isothermal process, whereas atomisation and evaporation are adiabatic processes.  In adiabatic air humidification, water is added to the air in liquid form and must therefore still reach the gaseous aggregate state.  This requires energy, which is extracted from the ambient air in the form of heat.  Since this results in a temperature reduction, this process is also called the adiabatic cooling effect.

Air Cooled Chiller – Each chiller needs to reject heat generated during the compression process of the phase-changing refrigerant.  Hot gas coming from the compressor(s) may reach temperatures well above 70°C.  To use this hot gas in the refrigeration circuits requires its condensation to a cooler fluid.  The heat can only be rejected by bringing it in indirect contact with a colder medium.  Air cooled chillers use air condensers as a component and the hot gas is cooled by colder air moved through heat exchangers of e.g. fin-and-tube or microchannel type.

The condenser coils maybe installed directly on the chiller or mounted somewhere in the vicinity of the chiller.  Such units are called split units or chillers with remote condensers.

Ammonia – Ammonia is a chemical compound of nitrogen and hydrogen with the molecular formula NH3.  It is a strong pungent smelling, colourless, water-soluble and poisonous gas that is tear-provoking and suffocating.  Under aqueous conditions it acts as a base.  Ammonia is one of the most produced chemicals and the basic material for the production of all other nitrogen compounds.  So far, it has been produced almost exclusively via the Haber-Bosch process from the elements hydrogen and nitrogen.

Although ammonia is produced synthetically for use in refrigeration processes, it is considered a “natural refrigerant”.  This is because NH3 occurs in the earth’s material cycles and is mainly produced in nature during the decomposition of organic, nitrogenous materials.  It has a long tradition as a refrigerant.  It has been used in cold stores, breweries and slaughterhouses for about 150 years, and for 120 years it has also been used in large refrigeration and industrial plants. In the middle of the last century, ammonia was often replaced by CFCs, which are banned today.  Nevertheless, it has always been able to maintain its important role in industrial refrigeration because of its very good thermodynamic properties.  Ammonia is used nowadays as a refrigerant in large refrigeration systems.

Atomisation (humidification) – In atomisation, water is delivered to the air to be humidified as small water droplets by mechanical atomisers or with the aid of nozzles.  Since the aggregate state of the water is also changed here and energy from the ambient air is required for this, adiabatic air humidification systems can also contribute to cooling in addition to humidification in environments with high heat generation.

Azeotropic Refrigerant Blends – An azeotrope or azeotropic mixture is a liquid consisting of two or more chemical compounds whose vapour phase has the same composition as the liquid phase. The mixture has a constant boiling point and cannot be separated by simple distillation; it therefore behaves like a pure substance. Azeotropy is the opposite of zeotropy.

Bernoulli’s Equation – Bernoulli’s equation is fundamental to the dynamics of incompressible fluids.  In many fluid flow situations, changes in elevation are insignificant and can be ignored.  With this simplification, Bernoulli’s equation for incompressible flows can be expressed as “static pressure + dynamic pressure = total pressure”.  Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own static pressure , dynamic pressure, and total pressure.  Static pressure and dynamic pressure are likely to vary significantly throughout the fluid but total pressure is constant along each streamline.

Bowen Ratio – The Bowen ratio (after Ira S. Bowen) is the ratio of sensible heat to latent heat.

BTU – BTU stands for British Thermal Unit.

See also: Dimensions of Energy

Bubble Point – The Normal Boiling Point (NBP) is the evaporation temperature of the refrigerant at atmospheric pressure (101.3 kPa).  This temperature basically determines the suitability of the refrigerant for the application.

Calorie – The old unit of measure of heat is kcal. It can be defined as the heat required to warm one kilogram of water by one degree Celsius.  Today the unit kW (kilowatt) is used.  Here, 860 kcal/h = 1 kW.

Capillary Tube – The capillary tube is the simplest expansion device and is mainly used in small systems.  It takes over the function of the injection line, the expansion and serves as a control valve.  The tube does not have a closing function, so that when the system is at a standstill there is a balancing between the high and low pressure sides.  This means that the refrigerant is at the coldest point of the system in the evaporator.  The start-up of the compressor is facilitated by the pressure compensation, but it must be ensured that no liquid refrigerant is sucked in during the start-up of the compressor (adjusted filling quantity).

See also: Expansion Valves

Carl von Linde – Carl Paul Gottfried Linde, Knight of Linde since 1897, (* 11 June 1842 in Berndorf near Thurnau; † 16 November 1934 in Munich) was a German engineer, inventor and founder of what is now an international corporation, Linde plc. With the help of his Linde process, the development of the first refrigerators with today’s cooling technology was possible.

Carnot Cycle – The Carnot cycle is a thought experiment that serves to realise a reversible heat-power machine for the conversion of heat into work.  The Carnot process was designed by Sadi Carnot in 1824, and he also laid the foundation for thermodynamics at the same time.  It comprises a reciprocating chamber, adjustable by means of a piston, which is exposed to heat and cold reservoirs and is otherwise thermally isolated.  Carnot intended this purely theoretical cycle not only as a description of mechanical processes, but used it to transfer the principle of causality to phenomena related to heat: Since the cycle is reversible, each stage can be represented as the sole effect of the others.

Carnot, Sardi – Nicolas Léonard Sardi Carnot (* 1 June 1796 in Paris; † 24 August 1832 ibid) was a French physicist and engineer who founded a new branch of science, thermodynamics, with his theoretical consideration of the steam engine (Carnot process).

Carrier, Willis – Willis Haviland Carrier (* 26 November 1876 in Angola (New York); † 7 October 1950) was an American engineer and inventor.  He is considered the father of modern air conditioning.  At the end of 1911, he presented his work on formulas of psychrometry.  He also developed the Carrier diagram.  See also: h-x Diagram.

Cascade Systems – In a cascade refrigeration system, two or more vapour-compression cycles with different refrigerants are used. The evaporation-condensation temperatures of each cycle are sequentially lower with some overlap to cover the total temperature drop desired, with refrigerants selected to work efficiently in the temperature range they cover.  The low temperature system removes heat from the space to be cooled using an evaporator, and transfers it to a heat exchanger that is cooled by the evaporation of the refrigerant of the high temperature system.  Alternatively, a liquid to liquid or similar heat exchanger may be used instead.  The high temperature system transfers heat to a conventional condenser that carries the entire heat output of the system and may be passively, fan, or water cooled.

Cassette Units – For in-ceiling installation, cassette units are suitable  as they are hidden in suspended ceilings, invisible except for the ventilation outlets.  The air conditioner thus disappears discreetly into the ceiling.  Cassette units are commonly used in offices, sales areas, restaurants and hotels because of their suspended ceilings. Retrofitting is also very easy and decorative collars can be used where the ceiling depth is insufficient.

Centrifugal Compressors – The centrifugal compressor or radial compressor is a turbo compressor or compressor in which the gas to be compressed is set in rotation by an impeller running in an adapted housing and accelerated from the inside to the outside.  The kinetic energy is converted into pressure in the subsequent diffuser. Centrifugal compressors are designed for pressure ratios of 1.2 to 4, rarely also 6.

See also: Compressors

CFCs – Chlorofluorocarbons (CFCs, chemical nomenclature according to IUPAC: chlorofluorocarbons, CFCs, or also freons) are an extensive chemical group of low-molecular organic compounds that are used, among other things, as refrigerants.  CFCs are hydrocarbons in which hydrogen atoms have been replaced by the halogens chlorine and fluorine; they are a subgroup of the halogenated hydrocarbons.  It became apparent that the release of CFCs into the atmosphere is significantly responsible for the depletion of the ozone layer in the stratosphere (“ozone hole”), which is why the use of CFCs is now banned in many areas of application.

See also: Refrigerants

CFM – Cubic Feet per minute – Cubic feet per minute; parameter to describe a volume throughput, 1 cfm = 1.699 m³/h or 1 cfm = 0.471947 l/s.

Chiller – A chiller is a machine that removes heat from a circulating medium e.g. a phase-changing refrigerant via vapour-compression, adsorption refrigeration, or absorption refrigeration cycles.  A chiller  uses one or more compressors to transport heat energy from a colder place to be cooled to a warmer environment via this medium.  This medium can then be circulated through a heat exchanger to cool another process stream such as air or water.

Refrigeration machines such as chillers are based on a thermodynamic cycle.  Vapour compression systems use boiling cooling with the aid of refrigerants that have suitable evaporation temperatures for the desired temperature and pressure range. The refrigerant is constantly subject to a phase transition liquid/gaseous and vice versa in the circuit.

As a necessary by-product, refrigeration creates waste heat that must be exhausted to the ambient, or for greater efficiency, recovered for heating purposes.  Vapour compression chillers may use different types of compressors e.g. hermetic scroll, semi-hermetic screw, or centrifugal compressors.  The condensing side of the chiller can be either air or water cooled. Absorption and adsorption chillers require a heat source to function.

Close Control Systems – Air conditioning systems with precise close control for high reliability, better humidity and temperature control in, typically, IT cooling, in server rooms or data centers.

COP – The coefficient of performance or COP of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required.  The higher the COP value, the greater the efficiency.

Coil – Coil is a collective term for various types of heat exchangers used in refrigeration machines or air conditioning units.  A heat exchanger is a system used to transfer heat between two or more fluids.  Heat exchangers are used in both cooling and heating processes.  The fluids may be separated by a solid wall to prevent mixing, or they may be in direct contact.  They are commonly used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing and wastewater treatment.  There are three primary classifications of heat exchangers according to their flow arrangement.

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger.  The exchanger’s performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined.  In most simple systems this is the “log mean temperature difference” (LMTD).  Sometimes direct knowledge of the LMTD is not available and the  Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer (especially counter current exchangers).

See also: Condensers

Comfort Air Conditioning – A combination of reduced air temperature and reduced humidity helps people to remain cool.  The cooling of air alone, often described as ‘air conditioning’ is more correctly referred to as ‘comfort cooling’.  However, because it cools the air, comfort cooling may include some incidental dehumidification.

Compressor – A compressor is a fluid energy machine that supplies mechanical work to an enclosed gas; compressors are used to compress gases.  They increase the pressure and density of the gas.  Machines in which a small amount of compression is a side effect of transporting gases are called fans or blowers and are not counted as compressors.  Machines that increase the pressure of liquids are called pumps.  Compressors that create a negative pressure (vacuum) and work against the air pressure are called vacuum pumps.

See also: Compressors

Condensate – Condensate forms on the outside of air conditioning and ventilation ducts. This happens when warm air touches the cool pipe surfaces. In the worst case, the constant dripping can cause water damage.

Condenser – The condenser, whose purpose is to remove the heat from the hot gas after it leaves the compressor, is cooled by either air or water.  The air cooled condenser is a tube arrangement whose surface area is increased by shrunk-on copper or aluminium fins.  A fan conveys the cooling air through the set of fins.  In smaller cooling units, natural convection is used only, so that the fan can be omitted.  The condensing temperature, and thus also the coefficient of performance of the refrigeration system, depends on the cooling air temperature.

Evaporative condensers are technically more complex and require more maintenance, as very low condensing temperatures are possible here due to the partial evaporation of the water.  Due to the additional evaporation of the water, a very low condensing temperature can be achieved, which in the limiting case corresponds to the wet bulb temperature.  With dry air, the condensing temperature can be below the ambient temperature.  However, the evaporated cooling water must be replaced and treated.  In water-cooled condensers, the condensation heat is first transferred to the heat transfer medium water or water-brine mixture.  The cooling water is transported to the cooling tower via a centrifugal pump.  If an open cooling water circuit is used, evaporative cooling can also be used here by trickling the water.  In the cooling tower, the water is sprayed downwards through nozzle sticks.  Air is forced through the cooling tower in counterflow.  Heat is exchanged and part of the cooling water evaporates.  Droplet separators are arranged at the air outlet to drain water droplets back into the cooling tower cup.

See also: Condensers

Condensing Units – Condensing units are temperature-control devices in refrigerators, air conditioners, heat pumps and chillers.  They move energy in the form of heat by compressing a refrigerant, then pumping it through a system of coils and using the air around the coils to heat and cool spaces.

Constant Air Volume – Constant air volume (CAV) is a type of heating, ventilating, and air conditioning (HVAC) system.  In a simple CAV system, the supply air flow rate is constant, but the supply air temperature is varied to meet the thermal loads of a space.  Most CAV systems are small, and serve a single thermal zone.  However, variations such as CAV with reheat, CAV multizone, and CAV primary-secondary systems can serve multiple zones and larger buildings.

Convection – Convection (from Latin convehere ‘to bring’), or flow transport, is the transport of physical state variables in flowing gases or liquids.  Physical state variables are, for example, entrained heat, matter or momentum.  The convective transport of thermal energy is a mechanism of heat transport and is also called heat entrainment.  If static buoyancy occurs as the cause of the flow due to temperature differences, this is called thermal convection, natural convection, free convection or heat flow.  In addition, the flow can be caused by pumps or fans, for example, or by thermodynamic imbalances, this is called forced convection.  Convection cells in a vessel heated from below.  In the Anglo-Saxon language area, convection refers in a broader sense to any movement of molecules within a fluid and therefore includes not only pure advection through flow but also diffusion through movement at the atomic level (i.e., within fluids or solid matter).

Cooling Tower – A cooling tower (also known as a recooling plant) is a system that uses a heat exchanger to remove excess heat or heat that can no longer be used technically from a power plant or industrial processes.

Counter-flow Heat Exchanger – In counter-flow heat exchangers the fluids enter the exchanger from opposite ends.  The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium per unit mass due to the fact that the average temperature difference along any unit length is higher.

See also: Evaporators

CRAC Unit – A CRAC (Computer Room Air Conditioning) unit is a device that monitors and maintains the temperature, air distribution and humidity in a server room or data centre.

CRAH Unit – A computer room air handler (CRAH) is a device that is used in data centres to reduce the heat load caused by the operating equipment there.

Critical Point for Gases – The critical point (with the critical temperature and critical pressure) is a measure of the suitability of the refrigerant at higher temperatures of the heat sink.  In some substance systems, above the critical point, which is characterised by a critical temperature and pressure, the phase interfaces between the liquid and gaseous phases disappear.  Thus, under these conditions, liquid and gas are only one phase, which is called “supercritical”.  Thus, evaporation and condensation can no longer occur there.

Cross-flow Heat Exchanger –  In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

See also: Evaporators

Dalton, John – John Dalton (* 6 September 1766 in Eaglesfield, Cumberland; † 27 July 1844 in Manchester) was an English naturalist and teacher. He is considered one of the pioneers of chemistry because of his fundamental investigations into atomic theory.

Dampers – Dampers are typically located at the outside air entry and the discharge of the return air housing.  The dampers are multiple sheets of metal which can rotate.  They can close to prevent air from entering or exiting, they can open to fully allow air in or out, and it is possible to also vary their position somewhere in between to restrict the amount of air that can enter or exit.

Decibel – The Bel (unit symbol B) is an auxiliary unit of measurement named after Alexander Graham Bell to indicate the decadic logarithm of the ratio of two quantities of the same kind in levels and measures.  These are used in electrical engineering and acoustics, for example when specifying an attenuation measure or power level.  Examples of physical quantities for which logarithmic ratios are formed are electrical voltage, field strength and sound pressure.  As a rule, the decibel (unit symbol dB) is used instead of the bel i.e., the tenth part of a bel.

See also: The Perception of Sound

Density – The density ρ, also called mass density, is the quotient of the mass m of a body and its volume V.

Dew Point– The dew point, also the dew point temperature, is the temperature in air with a certain humidity that must be fallen below at constant pressure so that water vapour can separate as dew or mist. At the dew point, the relative humidity is 100 % and the air is (just) saturated with water vapour.  The more water vapour the air contains, the higher its dew point temperature.  The dew point can therefore be used to determine the absolute humidity.

Dimensions of Energy – Energy has a variety of different “units”.  A calorie is defined as the amount of heat required to raise the temperature of 1-gram of water by 1 degree Celsius or Kelvin from 14,5°C to 15,5°C.  Another definition forms the older physical quantity of a British Thermal Unit, abbreviated as BTU. This unit of energy, introduced in 1876, was originally defined as the quantity of heat needed to raise the temperature of 1 pound avoirdupois of air-free water by 1°F (Fahrenheit) under a constant pressure of 1 atmosphere, starting at the temperature at which water is most dense, 39.1°F.  One finds that 1 BTU converts into 252 cal.

Drop-in – Drop-in refers to the changeover of refrigerants in a chiller or air conditioning sytstem without changing the oil. The refrigerants required for this are often refrigerant mixtures, which are also called service or transition refrigerants and contain oil-soluble components.  This means that the chiller oil can remain in the system and the changeover of the refrigerant requires only minor changes to components of the refrigeration system.  Multi-component refrigerants have a temperature glide during evaporation or liquefaction (temperature change at constant pressure in the phase change).

See also: Refrigerants

Dry Bulb Temperature – Dry air contains very little moisture and therefore has a low dew point. The dry bulb temperature simply measures the air temperature without measuring the moisture it contains and is often referred to as the ambient air temperature or simply the ambient temperature.

Dry Expansion Evaporator – Liquid refrigerant is fed into the dry expansion evaporator by an expansion device which meters the liquid into the evaporator at a rate such that all the liquid is vaporised by the time it reaches the end of the evaporator coil.

See also: Evaporators

EC Fan – The EC fan is driven by a brushless DC motor and powered by an electronic circuit board containing the permanent magnet in the rotor and a copper coil in the stator. The EC fan is characterised by high efficiency and the advantage of speed control.

See also: EC Fans vs Axial Fans

EcoDesign Directive – The Ecodesign Directive provides regulations for specific products.  These so-called lots apply to specific product groups and thus also describe which air conditioning units are affected by the Ecodesign Directive.  While Lots 1 ENER or 6 ENER are relevant for air-conditioning units, Lots 1 ENER and 21 ENER define the framework for chillers.  In this context, specific regulations within the Ecodesign Directive define the requirements for the individual product groups.  The fundamental aspects include, among other things, specifications for energy efficiency, an evaluation of the environmental impact as well as an analysis of the product life cycle.  In addition, the Ecodesign Directive also defines, for example, the maximum permissible energy consumption and the recycling quantities.

Economiser – Economisers are integrated into compression refrigeration systems to increase the coefficient of performance.  The refrigerant is compressed by a compressor and then condensed in the condenser.  A side stream is taken from the main stream, which is expanded at an intermediate pressure.  The main and side streams are fed to a heat exchanger.  The refrigerant in the main flow is subcooled by the evaporating bypass flow.  The bypass flow is pressure-regulated and returned to the suction side of the compressor.  The increase in the coefficient of performance is very dependent on the refrigerant used and the pressure levels of the refrigeration system.  An alternative to the eco-subcooler is the installation of a medium pressure cylinder in a two-stage compression refrigeration system.  The liquid refrigerant from the condenser is fed into the medium-pressure cylinder via an expansion fitting.  The evaporated refrigerant is sucked off by the compressor, which is regulated to the intermediate pressure.  The economiser is a type of subcooler where part of the refrigerant, usually 10 to 20 %, evaporates at a higher temperature than in the main evaporator, while the remaining refrigerant flow is strongly subcooled.

EER – The EER Energy Efficiency Ratio describes the ratio between power consumption and delivered cooling capacity.  The greater the value of EER, the better the efficiency.

See also: Annual Energy Efficiency of Chillers – a comparison

Electronic Expansion Valve – The electronic expansion valve has the same function as a thermostatic expansion valve, but regulate the liquid quantity by means of resistance thermometers.  The resistance thermometers are installed at the inlet and outlet and the control unit evaluates the temperature difference and gives a corresponding control signal to the actuator.

See also: Expansion Valves

Enthalpy – The enthalpy, formerly also heat content, of a thermodynamic system is the sum of the internal energy U of the system and the product of pressure ρ and volume V of the system: H=U+ρV.  It has the dimension of energy and is measured in the unit joule.  Enthalpy is an extensive quantity: the enthalpy of a total system is the sum of the enthalpies of the subsystems.

Entropy – Entropy is a fundamental thermodynamic state variable with the SI unit Joule per Kelvin J/K.  One also explains the term entropy with the “measure of disorder” in the system in question.  All processes that occur spontaneously within a system cause an increase in its entropy, as does the addition of heat or matter.  Such processes are, for example, mixing, heat conduction, chemical reaction or conversion of mechanical energy into thermal energy through friction.  The entropy of a system can only decrease through the release of heat or matter.  Therefore, in a closed system i.e., a system in which there is no exchange of energy or matter with the environment, the entropy cannot decrease, but only increase over time (see also : Second law of thermodynamics).  Processes in which entropy increases in a closed system cannot proceed in the reverse temporal direction without external intervention; they are called irreversible.  To return a system to its initial state after an irreversible process, it must be coupled with its environment, which absorbs the increase in entropy and thereby also changes its own state.

ESEER – The ESEER (European Seasonal Energy Efficiency Ratio) was introduced to describe the energy efficiency of a chiller over the entire year.  This means that the calculation is not carried out exclusively in full-load operation, as with the EER, but also includes the partial load conditions.  The following weighting is applied: 100% load ≡ weighted by 3%, 75% ≡ 33%, 50% ≡ 41%, and 25% ≡ 23%.

See also: Annual Energy Efficiency of Chillers – a comparison

Eurovent – Eurovent is Europe’s Industry Association for Indoor Climate (HVAC), Process Cooling, and Food Cold Chain Technologies.  Its members from throughout Europe represent more than 1.000 companies, the majority small and medium-sized manufacturers.  The organisation’s activities are based on highly valued democratic decision-making principles, ensuring a level playing field for the entire industry independent from organisation sizes or membership fees.

Evaporating Temperature – The evaporation point or boiling point of a pure substance is a pair of values in its phase diagram and consists of two variables: the saturation temperature (especially also boiling temperature) and the saturation vapour pressure (especially also boiling pressure) at the phase boundary line between gas and liquid.  It is therefore made up of the two state variables pressure and temperature during the transition of a substance from the liquid to the gaseous state of aggregation.  For an open liquid, the boiling point is therefore the point on the temperature scale at which the vapour pressure is equal to the atmospheric pressure.  Below and above the boiling point, heating the liquid or gas only leads to an increase in temperature.  The energy supplied is converted into kinetic energy of the particles.  During the phase transition of the liquid to the gas, however, the temperature remains constant, provided that the pressure also remains constant.  All thermal energy supplied is invested in the change of state.  Once the boiling point is reached, the chemical-physical interactions between the particles are dissolved when further energy is added – the particles enter the gas phase.  The temperature of the liquid stagnates, since the thermal energy supplied is completely used to dissolve the intermolecular bonds.  The energy required for this for one mole of the substance is also called the enthalpy of vapourisation and its counterpart, which is not related to the quantity of substance, is called the heat of vapourisation.  Only when all particles are in the gas phase does the temperature of the system rise again.

Evaporation (humidification) – Water is guided over evaporation mats while the air to be humidified simultaneously passes over these mats and is thereby enriched with moisture.  The advantage of humidification by evaporation is that not only the operating costs but also the investment costs for a humidification system of this type are favourable.  Moreover, evaporators can be used not only for humidifying but also for cooling buildings.  Indirect evaporative cooling makes cooling particularly energy-efficient.  Conventional cooling units can be dimensioned much smaller and operating costs can be significantly reduced.

Evaporative Cooling– If a liquid is left to stand in an open vessel for a long time, it gradually changes into a gaseous state, it evaporates.  As with evaporation, energy is also required for evaporation.  It is usually extracted from the liquid itself.  This process is called evaporative cooling.  Indirect evaporative cooling is a process that uses the evaporative cooling produced in the phase change for cooling, e.g. for cooling the air in a ventilation and air-conditioning system.  The water evaporation takes place on the extract air side of an air handling unit, whereby warm outside air is cooled via a subsequent heat recovery.  The achievable cooling of the outside air thus depends on the evaporated water quantity on the extract air side and the design as well as the efficiency of the heat recovery system used.  The extract air can be humidified to near saturation without an increase in humidity in the supply air.

Evaporator – In refrigeration, flooded evaporators or heat exchangers with refrigerant injection (dry expansion) are used.  Evaporators with dry expansion are equipped with a throttling element at the inlet, which is designed so that the refrigerant is completely evaporated and superheated at the outlet. The refrigerant is injected into the evaporator in droplet form.  In other chillers with dry expansion, thermostatic expansion valves with internal or external pressure equalisation are often used.  Mechanically operating expansion valves work without auxiliary energy.  A sensor is connected to the pipe at the outlet of the evaporator.  The stroke of the injection valve and thus the injection mass flow are controlled by the pressure of the enclosed sensor charge.  The superheat of the escaping refrigerant is used as the controlled variable.  Electronic expansion valves have been increasingly used, which allow finer tuning of the control behaviour and improve the coefficient of performance of the refrigeration system.

Systems with flooded operation have a refrigerant tank on the low-pressure side, the so-called separator, in which liquid refrigerant with a gas overlay is kept under saturated steam conditions.  The level of the separator is either controlled by a mechanical float regulator (high or low pressure float) or a level control system with a level probe and a control valve is installed in the liquid supply line.  The pressure in the separator is kept at a certain value, which is determined by the required temperatures at the refrigeration points.  The refrigerant compressors extract the gas from the upper part of the separator, which maintains the saturated vapour equilibrium at the required temperature in the separator.

See also: Evaporators

Expansion Valve – An expansion valve (also called a throttle valve) is a device that reduces the pressure of the fluid flowing through it by locally narrowing the flow cross-section, thus causing an increase in volume or expansion.  The pressure reduction takes place without dissipation of work and, ideally, also without dissipation of heat, i.e. thermodynamically isenthalp.  Expansion valves can be controlled or uncontrolled.   Thus, a constriction of the pipeline represents an unregulated expansion valve.  Such a constriction, also called a throttle, is realised in the domestic refrigerator with a piece of pipe of very small diameter, also called a capillary.  In systems with a larger capacity, controlled expansion valves are used.  Here, the cross-section and thus the pressure drop is controlled according to a suitable reference variable.  In the case of a refrigerating machine, such a reference variable can be, for example, the pressure in the adjoining evaporator, which then results in an approximately constant temperature of the cooled medium.

See also: Expansion Valves

Fan Coil Unit – A fan coil unit is an air handling unit consisting of a heat exchanger and a fan. It is part of a decentralised air conditioning system and is used in residential, office and industrial buildings as well as on ships to change the indoor climate. The fan coil unit influences the air temperature and relative humidity and is either controlled manually or regulated by a thermostat.

F Gas – Fluorinated gases (F gases for short) are greenhouse gases. They include the substance groups of partially fluorinated hydrocarbons (HFC), fully fluorinated or perfluorinated hydrocarbons (PFC) and sulphur hexafluoride (SF6).

See also: Refrigerants

F Gas Regulation – The F-Gas Regulation (fully: Regulation (EC) No. 842/2006 of the European Parliament and of the Council of 17 May 2006 on certain fluorinated greenhouse gases) is an EC regulation on the control of installations containing certain hydrofluorocarbons (HFCs) that contribute to the greenhouse effect.  It was in force from 4 July 2006 and 4 July 2007 respectively.  On 1 January 2015, it was replaced by Regulation (EU) No 517/2014 on fluorinated greenhouse gases. The regulation stipulated that installations, for example air-conditioning and refrigeration units as well as fire extinguishing systems, with certain gases had to be tested for leakage at regular intervals and this had to be recorded.  A ban on hydrofluorocarbons was not imposed.  The requirements depend on the filling quantity of the individual system.

Filter Drier – The filter drier has the task of cleaning and drying the mixture of refrigerant and oil in the refrigeration circuit of a chiller.  Both the refrigerant and the lubricating oil of the compressor are highly hygroscopic, i.e. they absorb water from their environment.  The moisture in the refrigerant interferes with the proper functioning of the system and must be removed.  Even if virtually no moisture can penetrate the completely closed cooling circuit during operation, the operating fluids already absorb water during filling of the system or contain small amounts of moisture from production.  Therefore, constant drying is necessary.  The filter drier is a small cartridge with substances that bind the water even more strongly than the mixture of refrigerant and oil.  The cartridge also has a mechanical filtering effect and catches small foreign bodies or chips that may have accumulated in the air-conditioning compressor.  The filter drier is located at the outlet of the condenser in the system’s liquid receiver.  The refrigerant flows through the filter drier in its liquid phase.  The filter driers keep the refrigeration circuit free from contamination by water, acids and solids.  In case of contamination, corrosion and ice formation as well as malfunction of the compressor may occur.

First Law of Thermodynamics – The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body’s state, called Internal energy.  The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.  Based on this statement, energy balances can be formed for closed and open systems. The energy of a closed system remains unchanged. Different forms of energy can therefore transform into each other, but energy can neither be created out of nothing nor can it be destroyed. Therefore, a perpetual motion machine of the first kind is impossible ie. no system performs work without supplying another form of energy and/or without reducing its internal energy.

For a thermodynamic process without transfer of matter, the first law is often formulated as ΔU = Q – W, where ΔU denotes the change in the internal energy of a closed system, Q denotes the quantity of energy supplied to the system as heat, and W denotes the amount of thermodynamic work done by the system on its surroundings.

Fluids – In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress, or external force.  Fluids are a phase of matter and include liquids, gases and plasmas.  Although the term “fluid” includes both the liquid and gas phases, in common usage, “fluid” is often used synonymously with “liquid”.  Liquids form a free surface ie. a surface not created by the container while gases do not.

Fluids and Pressure – The concept of pressure is central to the study of fluids.  A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion.  The concepts of total pressure and dynamic pressure arise from Bernoulli’s equation and are significant in the study of all fluid flows.  To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use the term static pressure to distinguish it from total pressure and dynamic pressure; the term static pressure is identical to the term pressure, and can be identified for every point in a fluid flow field.

Free Cooling – Free cooling in air cooled and water cooled chillers is a significant factor in reducing the operating costs of a refrigeration system.  When outside temperatures are cold, compressors are switched off completely and only energy is needed to operate the fans.  During partial free cooling, the water is cooled partly in the free cooling coil and partly in the evaporator using mechanical cooling.  During the operating mode “100 percent free cooling”, the full cooling capacity is provided by the free cooling coil.

See also: Free-Cooling by ‘Thermosyphon’ method

Free Energy – The thermodynamic free energy is a concept useful in the thermodynamics of chemical or thermal processes.  The change in the free energy is the maximum amount of work that a thermodynamic system can perform in a process at constant temperature, and its sign indicates whether a process is thermodynamically favorable or forbidden.  Since free energy usually contains potential energy, it is not absolute but depends on the choice of a zero point.  Therefore, only relative free energy values, or changes in free energy, are physically meaningful.

Free Surfaces – Both solids and liquids have free surfaces, which cost some amount of free energy to form.  In the case of solids, the amount of free energy to form a given unit of surface area is called surface energy, whereas for liquids the same quantity is called surface tension.  The ability of liquids to flow results in different behaviour in response to surface tension than in solids, although in equilibrium both will try to minimise their surface energy: liquids tend to form rounded droplets, whereas pure solids tend to form crystals.  Gases do not have free surfaces, and freely diffuse.

Freezing Point – The freezing point or solidification point are those points in a phase diagram that lie at the boundary between the states of aggregation “solid” and “liquid”.  The most commonly known freezing point is that of water.  The freezing point represents the conditions that exist during the phase transition of a substance from the liquid to the solid phase, which is called freezing.  The freezing point depends on the substance and is indicated by a freezing temperature at a certain pressure.

Gauge Pressure – The gauge pressure is defined as the difference between an absolute pressure Pabs and the prevailing atmospheric pressure Pamb.  It is denoted with the subscript “e”: Pe and is calculated as follows: Pe = Pabs – Pamb.

Glycol System – Both glycol and water chillers can be used to satisfactorily dissipate the heat being generated by e.g. a production process.  The fundamental difference between both chiller types lies in variations in their freezing points as well as their heat conduction capacities.  Pure glycol or a mix of glycol and water has a much lower freezing point than that of pure water.  By implication, glycol-based chillers are better suited to low-temp environments.  Conversely, water has a better ability to retain and conduct heat from an associated process than a glycol mixture can.  As a result, the efficiency of heat transfer for a water chiller will be higher than that of a glycol chiller.

See also: Water Glycol Mixtures

GWP – The GWP (Global Warming Potential, CO2 = 1), is a measure of the contribution of a refrigerant to the greenhouse effect.

h, x Diagram – The states and changes of state of humid air can be represented in the h-x diagram (enthalpy vs. vapour loading of the air).  In Europe, the Mollier diagram is mainly used, while in the USA the Carrier diagram is used.  In the Mollier diagram, the area under consideration of the air unsaturated with water vapour was enlarged by rotating the abscissa of the coordinate system.  The rotation was made so that the isotherm for zero degrees Celsius becomes straight horizontal.  Accordingly, the enthalpy lines (isenthalps) do not run horizontally, but run with a gradient of the slope corresponding to the heat of evaporation of water at zero degrees Celsius.  In addition to the isotherms, the relative humidity and the density of the air mixture are also entered as parameters in the diagram.

HC (Hydrocarbon) Refrigerants – Hydrocarbons are a substance group of chemical compounds that consist only of carbon and hydrogen.  This substance group is diverse, as hydrocarbons can contain carbon chains, rings or combinations thereof.  There are several subgroups such as alkanes, alkenes, alkynes and aromatics (arenes).  The hydrocarbons have gained great technical importance mainly due to their use as fossil fuels and in organic synthesis.  Chemically speaking, a hydrocarbon [HC] is an elementary compound of hydrogen and carbon which occurs naturally and is found in large concentrations in crude oil.  Used as a modern refrigerant, non-toxic hydrocarbons are an alternative to the CFC/HCFC/HFC fluorocarbons linked to ozone damage.  Though a range of HCs have refrigerant applications, isobutane (R600a) is the HC most frequently found in domestic fridges and freezers, whilst propane (R290) is common in commercial heat pump, air conditioning, refrigeration, and freezer applications.

See also: Refrigerants

HCFC – HCFCs are “partially halogenated” chlorofluorocarbons, their hydrogen atoms are only partially replaced by chlorine and fluorine atoms: they have a much lower ozone depletion potential than CFCs, their “global warming potential” is also far below that of CFCs.  Moreover, HCFCs are already degraded in the troposphere and only partially reach the stratosphere.

See also: The Ozone Depletion Process

Heat Capacity – The specific heat capacity, also known as specific heat or heat capacity for short, is a material property of thermodynamics.  It measures the ability of a substance to store thermal energy.

Heat Exchanger – A heat exchanger is a device that transfers thermal energy from one material flow to another.  For good efficiency, the component that separates the media must have good thermal conductivity and large surface area.  Turbulent flow is favourable for good heat transfer.  This occurs mainly at high Reynolds numbers.  Therefore, the flow velocity should be high and the viscosity of the media should be low.  However, high velocity and a large wetted surface also require a high energy input to pump the media through the heat exchanger.  In heat exchangers where one medium is a liquid and the other medium is a gas (usually air), the heat capacities per volume of the media differ greatly.  Much more gas than liquid must flow through, and the surface area for heat transfer must be increased on the gas side.  This is often done by fins applied to pipes.

Heat Pump – A heat pump is a machine that uses technical work to absorb thermal energy from a reservoir at a lower temperature (usually the environment) and – together with the drive energy – transfers it as useful heat to a system to be heated at a higher temperature (space heating).  In principle, the process used is the reverse of a heat-power process, in which heat energy at a high temperature is absorbed and partially converted into useful mechanical work, and the residual energy at a lower temperature is dissipated as waste heat, usually to the environment.  The principle of the heat pump is also used for cooling, while the term “heat pump” is only used for the heating unit.  In the cooling process, the useful energy is the heat absorbed from the room to be cooled, which together with the drive energy is dissipated to the environment as waste heat.

Heat Reclaim Chillers – During cooling only operation, the chiller produces a controlled source of chilled water leaving the evaporator while dissipating heat through the condenser and ultimately to the environment.  When there is a simultaneous need for chilled water and hot water, these chillers have the capability to operate in heat recovery mode.  The recovered heat can be redirected for various heating applications, which saves energy while maintaining conditions.

Heat Transfer – Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems.  Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes.

Hermetic Compressors – Fully hermetic compressors – motor and compressor are in an encapsulated welded housing and in direct contact with the refrigerant, the housing cannot be opened for repairs.

See also: Compressors

HFC – Hydrofluorocarbon (HFC), any of several organic compounds composed of hydrogen, fluorine, and carbon.  HFCs are produced synthetically and are used primarily as refrigerants.  They became widely used for this purpose beginning in the late 1980s, with the introduction of the Montreal Protocol, which phased out the use of chemicals such as halons and chlorofluorocarbons (CFCs) that contribute to the depletion of Earth’s ozone layer.  However, while HFCs have an ozone depletion potential of zero, they are potent greenhouse gases, and thus their manufacture and use became increasingly regulated in the 21st century.  In general, HFCs are relatively nonflammable and chemically stable.

See also: The Ozone Depletion Process

HFO – Hydrofluoroolefins (HFO) are unsaturated organic compounds of hydrogen, fluorine and carbon.  These organofluorine compounds are of interest as refrigerants.  Unlike traditional saturated hydrofluorocarbons (HFCs) and chlorofluorocarbons (CFCs), HFOs are alkenes (olefins).  HFO refrigerants have a low global warming potential and are therefore more climate-friendly than previous generations of synthetic refrigerants.  Systems with hydro-fluoro-olefins (HFO) achieve very high energy efficiency.  They can be used to replace systems with the refrigerant R-134a, for example.

Hot Aisle – Hot aisle/cold aisle is a layout design for server racks and other computing equipment in a data centres.  The goal of a hot aisle/cold aisle configuration is to conserve energy and lower cooling costs by managing air flow.  In its simplest form, hot aisle/cold aisle data center design involves lining up server racks in alternating rows with cold air intakes facing one way and hot air exhausts facing the other.  The rows composed of rack fronts are called cold aisles. Cold aisles face air conditioner outlet ducts.  The rows the heated exhausts pour into are called hot aisles.  Hot aisles face air conditioner return ducts.  A containment system can be used to isolate hot aisles and cold aisles from each other and prevent hot and cold air from mixing.  Containment systems started out as physical barriers that simply separated the hot and cold aisles with vinyl plastic sheeting or Plexiglas covers.  Plenums that combine containment with variable fan drives (VFDs) to prevent cold air and hot air from mixing are a potential solution.

Humid Air – All conditions of humid air can be represented as a gas-vapour mixture of dry air (referred to as “gas”) and water vapour (referred to as “steam”).  The gas-vapour mixture thus consists of condensing and non-condensing components and, like its two components, behaves similarly to ideal gases under the boundary conditions typical in meteorology and air-conditioning technology (room ventilation) with regard to temperature and pressure.

HVAC – Heating, Ventilation and Air Conditioning, sometimes completed by R for Refrigeration: HVAC&R

Ideal Gas – In physics, an ideal gas is a certain idealised model of a real gas.  It assumes a large number of particles in disordered motion and considers only hard, elastic collisions between the particles and with the walls as interactions between the particles.  Although this model is a great simplification, it can be used to understand and mathematically describe many thermodynamic processes of gases.

Internal Energy – The internal energy is the total energy available for thermodynamic transformation processes of a physical system that is at rest and in thermodynamic equilibrium.  The internal energy is composed of a multitude of other forms of energy; according to the first law of thermodynamics, it is constant in a closed system.  The internal energy is an extensive state variable and a thermodynamic potential of the system.  From the caloric equation of state of the system it follows how the internal energy is to be calculated from other state variables (e.g. pressure, temperature, number of particles, entropy, volume).

Inverter – A frequency converter is a power converter that generates a different AC voltage from the AC voltage it feeds.  In most cases, the output frequency and output amplitude can be changed.  In contrast to “simple” converters, the devices are usually used to supply three-phase asynchronous motors, as they use sensor technology to adjust the frequency and amplitude of the output AC voltage according to the applications of the motor and its current load.  Depending on their design, frequency converters can be fed with single-phase AC voltage, three-phase AC voltage or DC voltage and generate a three-phase AC voltage from this to supply three-phase motors.

IPCC – The Intergovernmental Panel on Climate Change IPCC was established in November 1988 by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) as an intergovernmental institution to summarise the state of scientific research on climate change for policy makers with the aim of providing a basis for science-based decision-making without recommending action.

Joule – The SI unit of energy is Joule [J], named after James Prescott Joule of Manchester, a British brewer and autodidact.

Joule-Thomson Effect – The Joule-Thomson effect refers to the temperature change of a gas in the case of an isenthalpic pressure reduction, i.e. without a change in enthalpy.  The direction and strength of the effect is determined by the strength of the attractive and repulsive forces between the gas molecules.  Under normal conditions, the temperature of most gases and gas mixtures decreases with expansion.  On the other hand, it rises, e.g. for hydrogen, helium, neon.  In an ideal gas there are no molecular forces, therefore it does not show a Joule-Thomson effect.  The Joule-Thomson effect plays an important role in the thermodynamics of gases.

Kigali Amendment – The global reduction of hydrofluorocarbons (HFCs) has been agreed: The Parties to the Montreal Protocol, which include all 197 United Nations countries, have agreed in Kigali, Rwanda to gradually reduce the production and consumption of HFCs.  The Montreal Protocol for the protection of the ozone layer thus becomes an effective climate agreement.  The new agreements – the so-called “Kigali Amendment”, include different timetables for gradual quantity restrictions of HFCs in industrialised and developing countries.

See also: F-Gas & ODS Regs – Post Brexit

Kyoto Protocol – The Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC) is an additional protocol to the United Nations Framework Convention on Climate Change (UNFCCC) adopted on 11 December 1997 with the aim of protecting the climate.  The agreement, which entered into force on 16 February 2005, set binding targets for greenhouse gas emissions in industrialised countries under international law for the first time.  Participating industrialised countries committed to reduce their annual greenhouse gas emissions by an average of 5.2 per cent compared to 1990 levels within the so-called first commitment period (2008-2012).  These emission reductions have been achieved.After five years of negotiations, the Parties agreed on a second commitment period (“Kyoto II”) from 2013 to 2020.  The main points of contention were the scope and distribution of future greenhouse gas reductions, the inclusion of newly industrialising and developing countries in the reduction commitments and the amount of financial transfers.  For the period after 2020, the Parties to the Framework Convention on Climate Change agreed on the Paris Agreement.

Latent Cooling Capacity – Any proportion of the cooling capacity inadvertently used to dehumidify the air is called latent cooling capacity.  In an ideal situation with no unwanted dehumidification, sensible cooling capacity is the same as the total cooling capacity.

Latent Heat – Latent heat (also called latent energy or heat of transformation) is energy released or absorbed by a body or thermodynamic system during a constant temperature process, usually a first order phase transition, in the unit joule.  The technical term used in thermodynamics for this is transformation enthalpy, since the phase transitions are isothermal and take place at a constant ambient pressure. Strictly speaking the term latent heat is incorrect from a thermodynamic point of view, since heat is an energy instead of an enthalpy, i.e. it would be defined for processes at constant volume of the substance.

Latent heat can be understood as energy in hidden form that is supplied or extracted to change the state of a substance without changing its temperature.  Examples are the latent heat of fusion and the latent heat of vaporisation in phase transformations, i.e., the condensation or vaporisation of a substance at a certain temperature and pressure.  The term was introduced around 1762 by the British chemist Joseph Black.

Liquefaction – In physics and chemistry, the phase transitions from solid and gas to liquid (melting and condensation, respectively) may be referred to as liquefaction.  The melting point (sometimes called liquefaction point) is the temperature and pressure at which a solid becomes a liquid.  In commercial and industrial situations, the process of condensing a gas to liquid is sometimes referred to as liquefaction of gases.

Loading / Unloading of Chillers – Chiller loading and unloading are terms associated with capacity modulation of a chiller operating with one or more compressors, either by modulating the capacity of a single compressor or a number of compressors with fixed capacity – depending on the load the chiller is operating with.

Load refers to the chiller’s cooling demand. Full load means that the chiller is operating at its maximum cooling capacity, this is typically only about 2% of the year.  Partial load means that the chiller is operating at less than its maximum capacity, which is normal for most of the year in typical comfort applications.  Let’s assume the following conditions for the chiller water: the outgoing water has a temperature of 7°C, the incoming warmer water has 12°C.  The setpoint for the process is 7°C and the chiller operates based on the difference between this setpoint and the actual temperature of the leaving chilled water.  As soon as the chiller stops operating, e.g. during the night, no more chilled water is produced and the water slowly gains heat from the environment.

When the chiller is switched on again and the chilled water pumps are also running, the chiller starts with water in the pipes that has reached a higher temperature.  So the chiller must first pull down this load before chilled water can be supplied to the airside equipment such as AHUs or fan coils.  If your chiller is a scroll chiller with multiple fixed capacity compressors, one of the compressors will start first and attempt to operate until the water temperature has dropped to the set temperature.  If the chilled water temperature does not drop within a set period of time, the microprocessor control commands another compressor to start and operate at full load until the chilled water set point and the chilled water discharge temperature are equal.  This process of loading one compressor at a time to reach the setpoint temperature is called loading. The load is maintained until the actual chilled water outlet temperature falls slightly below the set temperature. When the load is met, the low-pressure refrigerant in the evaporator cannot evaporate as it used to when the water was at a higher temperature. This reduces the volume of boiled-off gas and causes a lower suction pressure.

If the compressors continue to operate as they did at full load, the suction pressure they generate will be high enough to draw liquid refrigerant into the compressor, which is very detrimental.  When lower load conditions are detected, the chiller control will command operation with fewer fixed capacity compressors or the capacity modulation of a modulating compressor will begin to unload the chiller.  This can be done either by VFD-controlled compressors or slide valves (screw compressors) or guide vanes (centrifugal compressors).

log p-h diagram – Most important instrument for the saturated, wet vapour, and superheated area.  The refrigeration circuit can be graphically visualized.  A disadvantage is the fact that the values required for the calculations can only be read inaccurately.  Instead, vapor tables are used in such cases.  The log p – h diagram of a refrigeration circuit is the basis for dimensioning the system components such as compressor(s), condenser, throttle device, evaporator, piping.  The diagram identification of crucial parameters like the amount of energy needed to evaporate eg. 1 kg of the refrigerant at a certain temperature or pressure, the amount of energy needed to compress the gaseous refrigerant, to determine the final temperature of the refrigerant after compression, the amount of energy required to dissipated via the condenser as well as the proportion of throttling vapour after the expansion.

Low Pressure Stage – The simple refrigerant circuit consists of the four components compressor, condenser, throttling device and evaporator.  In the single-stage refrigeration system, a distinction is made between the high-pressure and low-pressure sides.  The high-pressure side (abbreviated: HP) extends from the pressure side of the compressor to the throttling device.  The low-pressure side (abbreviated: LP) includes the refrigeration circuit downstream of the throttling element up to the compressor inlet.

Microchannel Coil – Microchannel is a coil type used in evaporator and condenser coils.  It can easily be identified by its flat tubes, and its fins appear as waves between the tubes.  The technology was developed for use in the automotive industry first and is now widely used for radiators and condenser coils.  These coils are made of all aluminum, and they are used because of their superior heat transfer due to increased surface contact between the refrigerant and the metal.  They also have lighter weight and smaller refrigerant charge.  These coils have come under a lot of criticism by technicians due to an undisputed high failure/leak rate of the condenser coils in some systems.  Some have felt these failures occur due to inherent issues with the design, while others have stated that the leaks were due to specific manufacturing issues on a few coils and that these issues are largely in the past.

See also: Condensers

Mollier Diagram – The Mollier h, x diagram (formerly i-x diagram), enthalpy-water loading diagram, makes it possible to describe changes of state of humid air by heating, humidifying, dehumidifying, cooling and mixing different amounts of air.  It is valid for a certain air pressure p (usually for atmospheric air pressure, e.g. 100 kPa), i.e. for isobaric changes of state.  The variables temperature, humidity, enthalpy and density can be read directly.  Changes of state can be determined graphically.  The diagram was proposed by Richard Mollier in 1923 (see also psychrometry).  In the Mollier diagram, mixing processes of two air streams, heating of an air stream, cooling of air with and without dehumidification as well as humidification by water evaporation or steam can be shown.

Mollier, Richard – Richard Mollier (* 30 November 1863 in Trieste; † 13 March 1935 in Dresden) was a professor of applied physics and mechanical engineering in Göttingen and Dresden and a pioneer in the study of physical data for thermodynamics, especially for water, steam and moist air.

Montreal Protocol – The Montreal Protocol on Substances that deplete the ozone layer is a multilateral environmental agreement and thus a binding treaty under international environmental law.  It was adopted on 16 September 1987 by the Parties to the Vienna Convention for the Protection of the Ozone Layer and is a concretisation of this Convention.  It entered into force on 1 January 1989.  In the Montreal Protocol, the states commit themselves to take appropriate measures to protect human health and the environment from harmful effects caused, likely to be caused, or likely to be caused by human activities that modify the ozone layer.

See alsoAbout Ozone Depletion

Microchannel Coil – Microchannel is a coil type used in evaporator and condenser coils.  It can easily be identified by its flat tubes, and its fins appear as waves between the tubes.  The technology was developed for use in the automotive industry first and is now widely used for radiators and condenser coils.  These coils are made of all aluminum, and they are used because of their superior heat transfer due to increased surface contact between the refrigerant and the metal.  They also have lighter weight and smaller refrigerant charge.  These coils have come under a lot of criticism by technicians due to an undisputed high failure/leak rate of the condenser coils in some systems.  Some have felt these failures occur due to inherent issues with the design, while others have stated that the leaks were due to specific manufacturing issues on a few coils and that these issues are largely in the past.

Mollier Diagram – The Mollier h, x diagram (formerly i-x diagram), enthalpy-water loading diagram, makes it possible to describe changes of state of humid air by heating, humidifying, dehumidifying, cooling and mixing different amounts of air.  It is valid for a certain air pressure p (usually for atmospheric air pressure, e.g. 100 kPa), i.e. for isobaric changes of state.  The variables temperature, humidity, enthalpy and density can be read directly.  Changes of state can be determined graphically.  The diagram was proposed by Richard Mollier in 1923 (see also psychrometry).  In the Mollier diagram, mixing processes of two air streams, heating of an air stream, cooling of air with and without dehumidification as well as humidification by water evaporation or steam can be shown.

Mollier, Richard – Richard Mollier (* 30 November 1863 in Trieste; † 13 March 1935 in Dresden) was a professor of applied physics and mechanical engineering in Göttingen and Dresden and a pioneer in the study of physical data for thermodynamics, especially for water, steam and moist air.

Montreal Protocol – The Montreal Protocol on Substances that deplete the ozone layer is a multilateral environmental agreement and thus a binding treaty under international environmental law.  It was adopted on 16 September 1987 by the Parties to the Vienna Convention for the Protection of the Ozone Layer and is a concretisation of this Convention.  It entered into force on 1 January 1989.  In the Montreal Protocol, the states commit themselves to take appropriate measures to protect human health and the environment from harmful effects caused, likely to be caused, or likely to be caused by human activities that modify the ozone layer.

Natural Refrigerant – Natural refrigerants do not contribute to the depletion of the ozone layer and have only a comparatively small direct influence on the greenhouse effect.  Ammonia, carbon dioxide and water, but also hydrocarbons and air, in contrast to halogenated hydrocarbons, are also referred to as natural refrigerants.  Halogenated hydrocarbons are substances that also occur in nature.  They are released in not inconsiderable quantities by microorganisms and plants as well as as a result of volcanic activity.

See also: Refrigerants

Navier-Stokes Equation – The behavior of fluids can be described by the Navier–Stokes equations—a set of partial differential equations which are based on four laws of conservation: the continuity equation describing the conservation of mass, and the conservations laws for linear momentum, angular momentum and energy.

Near Azeotrope – Near azeotropic mixtures are mixtures with small temperature variation during phase change and a small difference in composition in liquid and vapour phases at equilibrium.  The use of near azeotropic refrigerant mixtures is to extend the range of refrigerant alternatives beyond pure fluids and azeotropic mixtures.

Non-Condensable Gas – Non-condensables are gases that will not condense into a liquid within the operating temperatures of the refrigeration system.  Air and nitrogen are the most likely non-condensables.  Air can enter a chiller or an air conditioning system when improper service and evacuation procedures are not followed or when there is eak on the low pressure side.  Nitrogen can enter the system in other ways.  Sometimes chillers are shipped from the factory with a holding charge of dry nitrogen, not refrigerant.  Once the refrigeration lines are connected, the service valves need to be opened and all of this nitrogen holding charge must be removed.  The condensing unit should then be evacuated along with the piping and the evaporator.

ODP – The ozone depletion potential (ODP) of a chemical compound is a measure of the relative effect of ozone layer depletion (ozone hole) that can be triggered by the substance, related to the substance trichlorofluoromethane R11 specified in the Montreal Protocol with the ODP value 1.

See also: The Ozone Depletion Process

Oil Separator – Most compressors in a chiller must be lubricated, an exception is centrifugal compressors for oil-free operation.  The oil used for this purpose flows partly with the refrigerant into the discharge line and thus into the circuit.  If it is not guaranteed that the oil flows back to the compressor, the oil must be separated out after the compressor and returned.  In most cases it is unavoidable that compressor always conveys a portion of oil from the crankcase into the discharge line along with the hot refrigerant gas.  Nowadays, the oil content is so low (< 3 %) that correct suction line dimensioning (sufficient speed) ensures safe oil return from the evaporator.  However, problems could possibly occur in partial load operation of compressors.  In these cases, care should be taken by installing oil separators.

Oil-Free Compressors – see: Turbocompressors

Open Compressors – In an Open Compressor, the compressor is driven via belt, gear or gears and the housing can be completely disassembled and repaired via screwed covers.

Ozone Depletion Hypothesis – Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of ozone in Earth’s atmosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth’s polar regions.  The latter phenomenon is referred to as the ozone hole.  The main cause of ozone depletion and the ozone hole is manufactured chemicals, especially manufactured halocarbon refrigerants, solvents, propellants, and foam- blowing agents (CFCs, HCFCs, halons), referred to as ozone-depleting substances (ODS).  These compounds are transported into the stratosphere by turbulent mixing after being emitted from the surface.  Once in the stratosphere, they release atoms from the halogen group through photodissociation, which catalyze the breakdown of ozone O3 into oxygen O2.  Both types of ozone depletion were observed to increase as emissions of halocarbons increased.

Packaged Unit – Compressor(s), heating and/or cooling coils, air handler features are all housed in a single-boxed housing.

Parallel Flow Heat Exchanger – In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side.

See also: Evaporators

Paris Agreement – The Paris Agreement is an agreement of 195 Parties to the United Nations Framework Convention on Climate Change (UNFCCC) with the aim of protecting the climate as a successor to the Kyoto Protocol.  The agreement was adopted on 12 December 2015 at the UN Climate Change Conference in Paris by all Parties to the UNFCCC, at that time 195 states and the European Union, and provides for limiting man-made global warming to well below 2 °C above pre-industrial levels.  The EU formally ratified the Convention on 5 October 2016.  This allowed it to enter into force on 4 November 2016.  For it to enter into force, at least 55 countries responsible for at least 55% of global emissions had to deposit their instruments of ratification.

Partial Pressure – Partial pressure refers to the pressure of an individual component or fraction in an (ideal) gas mixture.  The total pressure is composed additively of several parts, namely the partial pressures of the individual gas components.  The sum of all partial pressures is equal to the total pressure.  The partial pressure corresponds to the pressure that the individual gas component would exert if it were present alone in the volume concerned.

Phase Changes – Phase transitions can occur between solid, liquid and gaseous phases, among others.  There are special designations for phase transitions between certain states of aggregation.

Potential Energy – In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.  Types of potential energy include the gravitational potential energy of an object that depends on its mass and its distance from the center of mass of another object, the elastic potential energy of an extended spring, and the electric potential energy of an electric charge in an electric field. The unit for energy in the International System of Units (SI) is Joule [J].

Pound – Various definitions for a pound have been used. The most common today is the international avoirdupois pound, which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces.  The international standard symbol for the avoirdupois pound is lb.  The term “avoirdupois” is an Anglo-French term and translates literally into “goods of weight”.

Precision Air Conditioning – The precision air conditioners, also known as a CCU (close control units) or CRAC (computer room air conditioner) are refrigerating equipment specifically designed to provide precise control of temperature and humidity in all applications in which is required a very high degree of precision.

Psychrometric Chart – See: Psychrometry.

Psychrometry – Psychrometry (from Greek “psychros” – cold, and “metron” – measure) is the determination of humidity by comparative temperature measurement with two thermometers.  One of the two thermometers is in the environment of the humidity to be determined (room humidity), the other in the same place at steam saturation.  The humidity results from the temperature difference as a function of the room temperature, taking into account an instrument constant.  Such a measuring device is called a psychrometer.  Sometimes the entire investigation of a system consisting of a vapour and a gas is called psychrometry and the properties of the (usually air-water) system are called psychrometric properties.

PUE – Power Usage Effectiveness (PUE) is a technical indicator that shows the energy efficiency of a data centre.  The PUE value puts the total energy consumed in a data centre in relation to the energy consumption of the IT infrastructure.

Rack – Rack, also IT rack, data distributor, IT cabinet, server cabinet or data cabinet, is the name given to a rack with a mounting grid in which servers, switches, routers, bridges, storage modules, patch panels and other IT or network components are mounted.

Radiation – In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.  This includes: (1) electromagnetic radiation, such as radio waves, microwaves, infrared, visible light, ultraviolet, acoustic radiation, such as ultrasound, sound, and seismic waves (dependent on a physical transmission medium) x-rays, and gamma radiation (γ), (2) particle radiation, such as alpha radiation (α), beta radiation (β), proton radiation and neutron radiation (particles of non-zero rest energy), (3) acoustic radiation, such as ultrasound, sound, and seismic waves (dependent on a physical transmission medium), and (4) gravitational radiation, radiation that takes the form of gravitational waves, or ripples in the curvature of spacetime.

Rankine Cycle – The Rankine cycle or Rankine Vapour Cycle is the process widely used by power plants such as coal-fired power plants or nuclear reactors.  In this mechanism, a fuel is used to produce heat within a boiler, converting water into steam which then expands through a turbine producing useful work.  This process was developed in 1859 by Scottish engineer William J.M. Rankine.  This is a thermodynamic cycle which converts heat into mechanical energy—which usually gets transformed into electricity by electrical generation.

Receiver  – A common accessory used on many refrigeration systems is the liquid receiver.  It is basically a storage vessel designed to hold excess refrigerant not in circulation.  Refrigeration systems exposed to varying heat loads, or systems utilizing a condenser flooding valve to maintain a minimum head pressure during low ambient temperatures, will need a receiver to store excess refrigerant. Liquid receivers are installed in the liquid line as close as possible to the outlet of the condenser.  The piping between the condenser and the receiver should be arranged to allow free drainage.  The piping should also not cause excessive friction pressure loss or gas binding and must have adequately sized valves and connection fittings.

Refrigerant – A fluid used for heat transfer in a refrigeration system, which absorbs heat at low temperature and pressure and releases heat at higher temperature and pressure, usually involving changes of state of the fluid.  Refrigerants transport enthalpy (i.e. heat energy) from the consumer to be cooled to the environment.  The difference to the cooling fluid is that a refrigerant in a refrigeration circuit can do this along a temperature gradient, so that, with the expenditure of supplied energy (mostly volume change work in the form of compression), the ambient temperature may even be higher than the temperature of the object to be cooled, whereas a cooling fluid is only capable of transporting enthalpy in a refrigeration circuit against the temperature gradient to a point of lower temperature.

See also:  Refrigerants

Refrigerant Blend – A mixture of two or more refrigerants blended in a specific ratio which can be separated by distillation.  Regular blends may have up to 10°C or more temperature glide.

Refrigerant Charge – The amount of refrigerant charged to a chiller to operate the chiller within permitted limits i.e., to safeguard the design parameters like, for example, COP and cooling capacity.

Refrigerant Dilemma – The correct selection of the refrigerant essentially determines the application limits and areas of application as well as the efficiency of a refrigeration system.  In addition to thermodynamic suitability, other selection criteria such as environmental relevance, safety, economic criteria and also availability play a decisive role in the selection of refrigerants.  In addition to its thermodynamic suitability, the ideal refrigerant would be neither toxic nor flammable, available everywhere in the world and also cheap and highly efficient.  Typically nowadays refrigerants consist of hydrocarbons, chlorine and fluor in various combinations.  With increasing chlorine content, the ozone depletion potential (ODP) rises; with higher fluorine content, however, the contribution of the refrigerant to the greenhouse effect (GWP) is higher.  With an increasing proportion of hydrogen, the GWP decreases but the flammability tends to increase.

Refrigeration – The process of removing heat from an enclosed space or from a substance for the purpose of lowering its temperature.

Refrigeration Cycle – The chiller is a refrigeration machine that uses the physical effect of the heat of evaporation when the phase changes from liquid to gaseous.  A refrigerant that is moved in a closed circuit undergoes various changes of state in succession.  The gaseous refrigerant is first compressed by a compressor. In the following condenser it condenses (liquefies) the gas while releasing heat.  Then the liquid refrigerant is expanded due to the change in pressure via a throttle, for example an expansion valve or a capillary tube.  In the downstream evaporator, the refrigerant evaporates while absorbing heat at low temperature (boil cooling).  The cycle can now start again from the beginning.  The process must be kept running from the outside by supplying mechanical work in form of drive power via the compressor.

Relative Humidity – Air humidity – or air moisture – refers to the proportion of water vapour in the gas mixture of the air.  Liquid water (e.g. raindrops, fog droplets) or ice (e.g. snow crystals) are therefore not counted as humidity.  Depending on temperature and pressure, a given volume of air can only contain a certain maximum amount of water vapour.  Relative humidity, which is the most common measure of humidity, is then 100 %.  Relative humidity, expressed as a percentage (%), is defined as the weight ratio of the current water vapour content to the maximum water vapour content possible for the current temperature and pressure.  The addition of water vapour reduces the density of the air because, with the total pressure remaining the same, an added number of water molecules displaces the same number of heavier nitrogen or oxygen molecules.

Retrofit – In our industry retrofitting is understood as the active adaptation of an existing refrigeration system to a new refrigerant as a replacement for a refrigerant that is no longer approved.  This is often accompanied by the replacement of the lubricating oil as well as an extensive overhaul of the complete system.  A retrofit makes sense if longer residual running times are planned, the availability of the system is important and costs for conversion and operation play a role.  From the cost point of view, a new chiller is always associated with a significant investment.  Often, for spatial or process-related reasons, a new chiller is not possible because of longer plant downtimes or building restrictions.  Retrofit should not be confused with drop-in, i.e., the mere replacement of the refrigerant without having to carry out further conversion measures on the refrigeration circuit.

Reverse Rankine Cycle – see “Vapour Compression Cycle”.

R Number – The general designation of refrigerants is given by the letter R followed by three (special cases: two or four) digits, i.e., in the form R-zzz, possibly also with appended letter b in the form of an abbreviation R-zzzbb. The development of the nomenclature of refrigerants is documented in the book “Advances in Fluorine Chemistry” and was first proposed by Henne, Midgley and McNary.  The date of introduction of the nomenclature by these three scientists, all of whom were DuPont employees, is not known.  The reason for its introduction was the desire to simplify studies with halocarbons.  The nomenclature was also used by DuPont itself, which ultimately led to the well-known brand names with the same numbering for the refrigeration industry.

Screw Compressor – The screw compressor belongs to the rotating, twin-shaft displacement compressors with internal compression.  It has a simple design, small dimensions, a low mass, uniform, pulsation-free delivery and smooth running because it lacks oscillating masses and control elements.  It reaches overpressures of up to 30 bar.

See also: Compressors

Scroll Compressor – A scroll compressor is a compressor consisting of two intermeshing spirals whose counter-rotating motion compresses the gas.  The scroll compressor was invented by Léon Creux in 1905 and is mainly used as a compressor in refrigeration machines and heat pumps because it is relatively cheap to mass-produce and runs quite quietly compared e.g. to reciprocating compressors.

See also: Compressors

Second Law of Thermodynamics – The second law of thermodynamics makes statements about the direction of physical processes and the principle of irreversibility.  The definition of the thermodynamic temperature and the state variable entropy can be derived from the second law.  Many different, partly equivalent formulations have been established for the Second Law of Thermodynamics e.g. (1) heat cannot transfer by itself from a body of low temperature to a body of higher temperature or, (2) the efficiency of the Carnot process cannot be surpassed or, (3) equalisation and mixing processes are irreversible.

SEERSEER (Seasonal Energy Efficiency Ratio) and SCOP (Seasonal Coefficient of Performance) stand for energy efficiency in year-round operation.  Compared to the previously used COP and EER ratios, these ratios are much closer to the actual consumption.

See also: Annual Energy Efficiency of Chillers – a comparison

Semi-hermetic Compressors – Semi-hermetic compressors – motor and compressor are in the same housing which can be completely disassembled and repaired via screwed covers.

See also: Compressors

Sensible Heat – Sensible heat is thermal energy that is directly expressed in an increase or decrease in temperature when supplied or removed.

SEPR – For air cooled and water cooled process chillers with high and medium operating temperatures, the Seasonal Energy Performance Ratio (SEPR) must be specified.  The different calculation formula results from the fact that the part load ratio of process and comfort chillers differs significantly.  In the case of process cooling, there is a high cooling demand all year round, even at lower outdoor temperatures.

See also: Annual Energy Efficiency of Chillers – a comparison

Shear Stress – In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of strain rate.  A consequence of this behavior is Pascal’s law which describes the role of pressure in characterizing a fluid’s state.  Depending on the relationship between shear stress, and the rate of strain and its derivatives, fluids can be characterized as one of the following: Newtonian fluids where stress is directly proportional to rate of strain and Non-Newtonian fluids where stress is not proportional to rate of strain, its higher powers and derivatives.

Sound Attenuator – A sound attenuator, also called a duct silencer or silencer trap, is an acoustic soundproofing measure for heating, ventilation and air conditioning ducts that reduces the transmission of noise through the ducts, either from equipment to occupied spaces in a building or between occupied spaces.  In its simplest form, a sound attenuator consists of a baffle within the duct system.  These baffles often contain sound-absorbing materials.

The dimensions and configuration of the silencers are chosen to attenuate a specific frequency range.  Unlike conventional internally lined ducts, which only attenuate mid and high frequencies, sound attenuators can achieve a wider band attenuation at relatively short lengths.  Certain types of silencers are essentially a Helmholtz resonator used as a passive sound attenuation device.

Sound Pressure Level – The sound pressure level is the decadic logarithm of the squared ratio between the effective value of the measured sound pressure and its reference value of 20 µPa, which is commonly used in acoustics.  The sound pressure level is a measure of the sound power, which is proportional to the square of the sound pressure. I t belongs to the sound field quantities.  Although physically ambiguous, the sound pressure level is often simply called the sound level.

See also: The Perception of Sound

Split Chiller Systems – Split chiller systems are used in applications where the compressor(s), the evaporator and a cold water pump and/or tank are installed indoors near the process on its own dedicated skid.  The air cooled condensing unit includes the condenser and fans and is installed outside the building e.g. on the roof or beside the building.

SSCEE – With the entry into force of Directive 2016/2281 for comfort chillers, the annual space cooling energy efficiency ηs,c (SSCEE = seasonal space cooling energy efficiency) and the Seasonal Energy Efficiency Ratio (SEER value) become mandatory information in cooling operation.

See also: Annual Energy Efficiency of Chillers – a comparison

State Functions – Besides the directly measurable ordinary physical variables that identify a thermodynamic state of a system, a system is characterized by further quantities called state functions, which are also called state variables, thermodynamic variables, state quantities, or functions of state.  They are uniquely determined by the thermodynamic state as it has been identified by the original state variables.  There are many such state functions.  Examples are internal energy, enthalpy, Helmholtz free energy, Gibbs free energy, thermodynamic temperature, and entropy.  For a given body, of a given chemical constitution, when its thermodynamic state has been fully defined by its pressure and volume, then its temperature is uniquely determined.  Thermodynamic temperature is a specifically thermodynamic concept, while the original directly measureable state variables are defined by ordinary physical measurements, without reference to thermodynamic concepts; for this reason, it is helpful to regard thermodynamic temperature as a state function.

State Variables – A thermodynamic system can be identified or described in various ways.  Most directly, it can be identified by a suitable set of state variables. Less directly, it can be described by a suitable set of quantities that includes state variables and state functions.  The identification of the thermodynamic state of a body of matter is by directly measurable ordinary physical quantities.  For some simple purposes, for a given body of given chemical constitution, a sufficient set of such quantities is ‘volume and pressure’.

Static Pressure – The concept of pressure is central to the study of fluids.  In fluid dynamics, we use the term static pressure in preference to just pressure to avoid ambiguity.  The word ‘static’ may be dropped and in that usage pressure is the same as static pressure at a nominated point in a fluid.

Subcooling – In refrigeration technology, subcooling a fluid means lowering the temperature below the condensing temperature at the associated vapour pressure.  Targeted subcooling increases the operational reliability of the chiller and also has an economic benefit. Sufficient subcooling ensures bubble-free refrigerant upstream of the expansion valve.  This avoids cavitation and ensures that the valve capacity is achieved. In this way, the usable evaporation enthalpy is also increased.

Temperature Glide – The temperature glide is the difference between the temperature at the beginning and at the end of the evaporation process at constant pressure for zeotropic mixtures. (For azeotropic mixtures and pure substances, the temperature glide is zero).  With single-substance refrigerants, the same temperature prevails at every point in the heat exchangers at a given pressure during the phase change (evaporation/condensation).  In refrigerant mixtures, the refrigerant components have different boiling points, which leads to different phase change temperatures.  At a given pressure, the refrigerant with the lower boiling point evaporates more than the refrigerant with the higher boiling point at the beginning of evaporation.  This leads to the so-called “temperature glide”.  For simplification, superheating in the heat exchangers is neglected here.  This temperature difference must be taken into account in the design and construction of heat exchangers and in the control of refrigeration systems.

Thermal Advection – Advection is the transport mechanism of a fluid from one location to another, and is dependent on motion and momentum of that fluid.

Thermal Conduction – The transfer of energy between objects that are in physical contact.  Thermal conductivity is the property of a material to conduct heat and evaluated primarily in terms of Fourier’s Law for heat conduction.

Thermal Convection – The transfer of energy between an object and its environment, due to fluid motion.  The average temperature is a reference for evaluating properties related to convective heat transfer.

Thermal Energy – Thermal energy usually has two components: the kinetic energy of random motions of particles and the potential energy of their configuration.  The more formal definition is that potential energy is the energy difference between the energy of an object in a given position and its energy at a reference position.

Thermal Radiation – The transfer of energy by the emission of electromagnetic radiation.

Thermodynamic Process – Classical thermodynamics considers three main kinds of thermodynamic process: (1) changes in a system, (2) cycles in a system, and (3) flow processes.

(1) A change in a system is defined by a passage from an initial to a final state of thermodynamic equilibrium. (2) A cyclic process carries the system through a cycle of stages, starting and being completed in some particular state. (3) Defined by flows through a system, a flow process is a steady state of flows into and out of a vessel with definite wall properties.

Thermodynamics – Thermodynamics, or heat theory, is an engineering discipline and natural science.  It originated in the study of steam engines and pursued the question of how to convert heat into mechanical work. To this end, it describes systems of sufficiently many particles and their state transitions on the basis of macroscopic state variables that represent statistical functions of the detailed many-particle states.

As an engineering science, it has significance for the various possibilities of energy conversion, and in process engineering it describes the properties and behaviour of substances involved in processes.  The French physicist Sadi Carnot, who wrote his groundbreaking work in 1824, is considered the founder.  As a branch of physics it deals with heat, work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is governed by the four laws of thermodynamics which convey a quantitative description using measurable macroscopic physical quantities, but may be explained in terms of microscopic constituents by statistical mechanics.

Thermostatic Expansion Valve – This type of throttle element regulates the injected refrigerant quantity depending on the temperature.  The control takes place depending on the superheating of the refrigerant vapour at the evaporator outlet.  Only enough refrigerant is injected into the evaporator so that the entire injected quantity can evaporate completely.  A poorly functioning or non-functioning thermostatic expansion valve can be recognised by too high or no superheat.  Excessive superheat is reached at over 10 K.  If the superheat is 0 K, the suction line to the compressor will become frosted and there is a risk of liquid being sucked in.

See also: Expansion Valves

Thermosyphon – A variant of the flooded system is the thermosyphon chiller.  It can be used if a refrigerant circuit such as a chilled water or brine circuit is used.  A plate heat exchanger is installed below the separator.  The lower inlet of the plate heat exchanger is connected to the separator and liquid refrigerant is charged.  The refrigerant evaporates due to the heat transfer from the brine and the refrigerant vapour enters the upper chamber of the separator.  Due to the plate heat exchanger used and a low feed volume in the separator, these refrigeration systems have low filling quantities.

See also: Free Cooling by the Thermosyphon Method

Third Law of Thermodynamics – A variant of the flooded system is the thermosiphon chiller.  It can be used if a refrigerant circuit such as a chilled water or brine circuit is used.  A plate heat exchanger is installed below the separator.  The lower inlet of the plate heat exchanger is connected to the separator and liquid refrigerant is charged.  The refrigerant evaporates due to the heat transfer from the brine and the refrigerant vapour enters the upper chamber of the separator.  Due to the plate heat exchanger used and a low feed volume in the separator, these refrigeration systems have low filling quantities.

Thomson, William – William Thomson, 1st Baron Kelvin or Lord Kelvin for short, OM, GCVO, PC, FRS, FRSE, (born 26 June 1824 in Belfast, province of Ulster, United Kingdom of Great Britain and Ireland; † 17 December 1907 in Netherhall near Largs, Scotland) was a British physicist in the fields of electricity and thermodynamics.  The unit kelvin was named after William Thomson, who introduced the thermodynamic temperature scale at the age of 24.  Thomson is known for both theoretical work and the development of measuring instruments.  The international World Refrigeration Day is held on the 26th June every year in recognition of Lord Kelvin.

Trane, James – James Alex Trane (April 29, 1857 – January 24, 1936) was a Norwegian-American inventor and industrialist.  He was the co-founder of Trane, a manufacturer of heating, ventilating and air conditioning (HVAC) systems and building management systems and controls.  The company was a subsidiary of Ingersoll Rand Inc. which was the successor company to the American Standard Companies. Trane Technologies is now an independent company listed on NASDAQ (TT).  It makes products under the Trane brand name and has its international headquarters in Swords, Ireland.

Triple Point – In some substance systems there may be a triple point at which both a solid and a liquid and a gaseous phase are in equilibrium with each other and, accordingly, all forms of phase transitions occur simultaneously.

Turbo Compressors – The turbo compressor is a compressor and belongs to the turbo machines as a thermal turbo working machine.  It works in the reverse of the principle of the turbine.  Turbo compressors are divided into the main types of radial and axial compressor.  In the axial compressor, the gas to be compressed flows through the compressor in a direction parallel to the shaft.  In the radial compressor, the gas flows axially into the impeller of the compressor stage and is then deflected outwards i.e., radially.

Turbocor Compressor – A recent innovation in compressor technology is the Turbocor compressor based on magnetic bearings allowing frictionless operation without any lubrication required.  In its field of application, the turbocompressor achieves high performance figures, especially under part load conditions and/or with very low condensing temperatures that occur more frequently on an annual average.  Due to the contactless triple magnetic bearing, the turbocompressor operates completely free of lubricating oil.  As standard, the impeller for compressing the refrigerant is driven by a motor cooled with liquid refrigerant with frequency converter and speeds up to 48,000 rpm.  By combining it with a swirl throttle in the intake at low load operation, the oil-free turbo compressor adapts optimally to the different load conditions at every operating point.  All functions and operating states of the compression process are monitored and controlled via the electronics integrated in the Turbocor compressor.

UPS – An uninterruptible power supply (UPS) ensures the supply of critical electrical loads in the event of faults in the power grid.  This is to be distinguished from the mains backup system (also referred to as “backup power supply”), as this has a short interruption of the power supply when switching over.

Vacuum Test – Vacuum is the term for an absolutely gas-free space.  The vacuum test is carried out to check whether the device to be tested is tight.  Gas is extracted up to a certain value with a vacuum pump.

Vapour Compression Chiller – A vapour compression chiller typically uses one of four types of compressor: reciprocating compression, scroll compression, screw-driven compression, and centrifugal compression are all mechanical machines that can be powered by electric motors, steam, or gas turbines.  Using electric motors in a semi-hermetic or hermetic configuration is the most common method of driving the compressors since electric motors can be effectively and easily cooled by the refrigerant, without requiring fuel supply or exhaust ventilation and no shaft seals are required, reducing maintenance, leaks, operating costs and downtime.  They produce their cooling effect via the reverse-Rankine cycle, also known as vapour-compression.  With evaporative cooling heat rejection, their coefficients of performance (COPs) are very high ie. > 4.0.  Utilising oil-free centrifugal compression technology part load COP >> 4.0 can be achieved.

Vapour Compression Cycle – Vapour compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from a space or a substance to be cooled and subsequently rejects that heat elsewhere.  Such systems usually have four components: a compressor, a condenser, a thermal expansion valve, and an evaporator.  Circulating refrigerant enters the compressor as saturated vapor and is compressed to a higher pressure, resulting in a higher temperature of the gas as well.  The hot, compressed gaseous refrigerant becomes superheated vapor at a temperature and pressure at which it can be condensed with either cooling water or cooling air.  The superheated vapor then passes through the condenser where the circulating refrigerant rejects heat from the gas as it cools and condenses completely.  The rejected heat is carried away by either water or air.  The condensed liquid refrigerant, now in the state of a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure.  That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant.  The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the space or substance to be cooled.  The cold mixture (flash gas) is then routed through the evaporator where the circulating refrigerant absorbs and removes heat which is subsequently rejected transferred else by the water or air used in the evaporator.  The fully evaporated refrigerant is now again a saturated vapour and is routed back into the compressor.

Vapour Pressure – The vapour pressure is the pressure that occurs when a vapour is in thermodynamic equilibrium with the associated liquid phase in a closed system.  The vapour pressure increases with increasing temperature and depends on the substance or mixture present.  If the vapour pressure of a liquid in an open system is equal to the ambient pressure, the liquid begins to boil.  In a liquid, at any temperature above absolute zero, i.e. greater than zero Kelvin, the particles strive to leave the liquid bond.  This is counteracted by the surface tension and the external pressure acting on the liquid.  The surface tension decreases as the temperature rises, causing the vapour pressure to increase.  At the critical point, the surface tension is zero and the vapour pressure is equal to the external pressure.

Vapour Pressure Curve – The vapour pressure curve describes the relationship between the pressure and the respective evaporation / or condensation temperature.  Pressure ratios, pressure differences and the basic pressure partings have an influence on the efficiency of the system (essentially the compressor), but also on other variables such as the final compression temperature.  The pressure partings is also relevant for the classification in various directives and standards (or the Pressure Equipment Directive).

Water Vapour – Water vapour or aqueous vapour is the gaseous phase of water.  It is one state of water within the hydrosphere.  Water vapour can be produced from the evaporation or boiling of liquid water or from the sublimation of ice.  Water vapour is transparent, like most constituents of the atmosphere.  Under typical atmospheric conditions, water vapour is continuously generated by evaporation and removed by condensation.  It is less dense than most of the other constituents of air and triggers convection currents that can lead to clouds.  In more general terms vapour means the gaseous phase of a substance e.g., of a refrigerant.

Water Cooled Chiller – A water cooled chiller is a machine that removes heat from a liquid via e.g., vapour compression cycle.  This liquid can then be circulated through a heat exchanger (evaporator) to cool equipment or other process streams like air or process water.  An unavoidable by-product of the refrigeration process is waste heat stemming from the condensing of the refrigerant, which must be removed to the environment.  The condensing side of the chiller can be either air cooled or water cooled.

Watt – The watt is the SI unit of power (energy turnover per period of time).  It was named after the Scottish scientist and engineer James Watt.  The capital letter “W” is used as the unit symbol.

Wet Bulb Temperature – The theoretical limit of cooling, measured as wet bulb temperature, is the lowest temperature that can be achieved by direct evaporative cooling.  Here, the water release of a moist surface is in equilibrium with the water absorption capacity of the surrounding air.  Due to the evaporative cooling, the cooling limit temperature is below the air temperature, depending on the relative humidity.  The drier the surrounding air, the greater the temperature drop; conversely, the humidity can be determined from the temperature difference.

Working Medium – In refrigeration technology, a working medium is a chemical substance (refrigerant) that periodically undergoes phase transitions between liquid and gaseous in the cycle process for refrigeration.  In the process, the working medium is exposed to changing temperatures and pressures.

Zeotrope – A mixture of chemical substances is called zeotropic if the composition of liquid and vapour is always different in vapour-liquid equilibrium.  This means that the dew curve and boiling curve do not touch at any point.  Mixtures whose dew and boiling curves touch in at least one point and thus the composition in vapour and liquid is the same are called azeotropic mixtures.  The vapour pressure and boiling temperature of a zeotropic mixture are characterised by the fact that they always lie between the vapour pressures and boiling temperatures of the pure substances, whereas in azeotropic mixtures a pressure maximum/temperature minimum or pressure minimum/temperature maximum occurs which lies outside the range limited by the pure substance values.