How to Size a Chiller

Correctly sizing a chiller is of critical importance to ensure that it can meet the anticipated cooling load.  One should also be careful not to oversize a chiller as this impacts not only the capital cost, but also incurs unnecessary whole-life costs.  In this article we look at what information is required in order to properly select the right chiller for the job.

A chiller is a machine in which a working medium goes through a cyclic process, similar to the ideal Carnot process. The working medium is called refrigerant. It is a chemical substance that undergoes phase transitions as it passes through the cycle. Phase transitions occur between liquid and gaseous phases.

This so-called vapor compression cycle starts in a compressor where refrigerant in its gaseous state is compressed to a smaller volume. This results in the refrigerant being at a higher temperature and higher pressure.

Excess heat is taken first from this hot gas in a condenser where the gas is liquefied at or below its condensation temperature. Both sensible and latent heat are dissipated through a heat exchanger to either ambient air (air-cooled) or water at lower temperature than the gas (water-cooled). The previously hot gas now cools and becomes liquid at the same pressure as when the hot gas left the compressor.

The now cooler liquid is expanded in a special valve device to lower pressure and lower temperature.  This liquid then enters an evaporator where it is evaporated in the heat exchanger, generating cold water for use in air-conditioning or industrial processes. At the end of the evaporation process the refrigerant is in its gaseous phase again.

Once the capacity is known, a decent spare capacity should be added.  Conservative estimates allow for up to an additional 20% to make sure that the nominal capacity is reached even in the case of any shortcomings of other components.

The next important step is the selection of compressor types and refrigerants. Note that different types of compressor run with different types of refrigerant or gas.
The currently utilized compressor types include scrolls, screws, centrifugal types, which can be with or without oil, and in some cases, piston type compressors. Depending on the prevailing requirements of a user, the chiller design needs to reflect e.g. the demand for high efficiency, robustness or low budgets. It is almost inevitable that satisfying all three criteria is impossible to reach at the same time.

High efficiency, i.e. the best possible ratio of energy input in relation to the cooling capacity achieved, is achieved if chillers are designed in such a way that a high proportion of partial load occurs during operation. For physical reasons, chillers have almost identical efficiency at full load when comparing with other chillers with similar compressor types. Differentiation between manufacturers is usually achieved by using higher quality heat exchangers or advanced control algorithms.
Depending on the compressor type there are various accessories or options which may be used. For oil-free compressors a proper non-return valve needs to be installed after the discharge port to avoid hot gas returning to the compressor. For those compressors with oil circuits, dedicated accessories may be selected.

The choice of the proper refrigerant is also crucial for a sound chiller sizing and selection. Among many other technical boundary conditions, the pressure in the refrigeration circuit plays an important role in the selection of a refrigerant. Depending on the operating pressure, only (selected) certain refrigerants can be considered for the different compressor types. Due to the F-Gas Regulation in the EU, the trend is clearly towards gases with a low GWP or HFOs or natural refrigerants. A more detailed discussion of this topic is beyond the scope of this article.

No specific discussion is made here regarding the controls of a chiller system which is a topic on its own.

What contributes to the efficiency of a chiller?
Sub-cooling of liquid refrigerant in chillers is a MUST for reasons of operational safety, to ensure bubble-free refrigerant before the expansion device and to increase the useful evaporation enthalpy. In physical terms it is cooling the refrigerant in liquid state, at uniform pressure, to a temperature that is less than the saturation temperature corresponding to condenser pressure.

Superheat is the difference between the measured temperature at the evaporator outlet with a thermometer,and the temperature corresponding to the suction pressure measured with a manometer. Superheating is the sensible heating of refrigerant vapor at invariable pressure in the evaporator to a temperature more than the temperature of saturation corresponding to the evaporator pressure and ensures total evaporation of the liquid refrigerant before it goes into the compressor.

How to size a Chiller
Chillers are used to reject heat from e.g. buildings or industrial processes by means of cooling water. Proper sizing of a chiller is crucial to ensuring that the correct chiller is selected.

The most important factor is to determine the amount of heat rejection into the cooling loop of a chiller. Information about the required volume flow in m³/s of the water to be cooled, water entry Tin and leaving temperature Tout, density ρ and specific heat cp at Tavg = (Tin + Tout)/2 can be used to calculate the required capacity by utilizing

Q=V˙ρ cp (TinTout)

The unit of the capacity is Kilowatt [kW]

Chillers have two sides, the process water side and the refrigeration side. Air-cooled and water-cooled chillers have the same process water side but different refrigeration sides. Air cooled chillers reject heat through fans, water-cooled chillers reject heat to another water source (building water, cooling tower).
The following sketch is an example of how chillers may be built. Note that not all chillers may have all the components shown below.

 

Process Water Side

Refrigeration Side

 

For a proper chiller sizing and design, some other factors are of relevance.  Choosing the right low ambient controls needs to be considered in colder climates.  There are different options to choose from e.g. fan speed control or liquid receivers.

Pump Selection

Pump selection is a very important topic as it is vital to make sure that the pumps can overcome the pressure drops of the volume flow of the chiller and the piping.  Some basic remarks on pump selection are below:

  • The chilled water pump is a part of a chilled water system.  Such systems may include a chiller, piping, valves/fittings, expansion tank, and air side units (fan coils, AHUs)
  • The chilled water pump is used to circulate chilled water in closed systems and circulates return chilled water from the air side system back to the chiller.  The chiller then cools the return water and the chilled water is then supplied back to the air side units again

The chilled water is referred to as fluid in what follows:

  • Information about such fluids includes fluid type, temperature, density and kinematic/dynamic viscosity
  • Fluid types include water, propylene, ethylene glycol mixtures or others
  • Density and viscosities are used to determine the Reynolds number.
  • To allow proper selection of a chilled water pump detailed information about the piping is required e.g. the flow rate, pipe materials, pipe types, pipe sizes and lengths of pipes

The following information needs to be calculated from chiller and system information:

  • Fluid velocity = function (flow rate, inner area of pipes)
  • Reynolds number = function (density, velocity, inner diameter, viscosity)
  • Friction factor = function (Reynolds number, relative roughness)
  • Pressure drops need to be determined for four categories: (1) piping, (2) valves and fittings, (3) equipment and (4) expansions and reducers. Only for the equipment (3) manufacturer data e.g. of the chiller can be used, all other values need to be calculated by utilising equations.
  • Necessary design information comprises pump type, flow rate, total dynamic head, location, pump speed, efficiency and motor data
  • Flow rate max flow of the chiller or max flow required by building or process
  • Total dynamic head pressure drop in the most remote run of the pump to the chilled water return of the pump
  • Location in a mechanical room of near/on the chiller with sufficient net positive suction head

Types

  • Mostly centrifugal pumps are used for chillers (in-line or end-suction types, horizontal or vertical set up, number of stages, long or close coupled with motor)
    Efficiency typically 60 – 80%
  • Speed select lowest pump speed, either fixed or VFD
  • Motor pump power = function (flow rate, pressure, pump efficiency)
  • Motor power = function (pump power, motor efficiency)

Of major significance is the correct dimensioning of the system volume to ensure that the cooling power can made available for the process side. System volume refers to the total available volume in the chiller and the pipework.  So, this can be either in piping or a separate tank which can be considered as nothing other than a big piece of piping. As a rule of thumb, 26 – 30 litres for each 10 kW of cooling capacity is needed.  In the case that the tank is set up as a recirculation tank, enough volume for both the process loop and the evaporator loop must be safeguarded.

Other important components

  • A filter is used to remove particles and contaminants and prevent them from entering the refrigeration cycle
  • Filter driers are devices used in chillers and are a combination of a filter and a dryer
  • The dryer function is used to remove moisture from the refrigerant. Moisture may freeze inside the tube causing the refrigerant flow to be restricted and may cause the formation of acids and sludge when in contact with oil in the system
  • There are two types of filter dryers:
    • The Liquid Line Type
    • The Suction Line Type
  • The Liquid Line Type is placed after the condenser and before the expansion valve. Some devices also have a sight glass that enables technicians to look into it to see the level of refrigerant. Such glasses may contain chemicals indicating the amount of moisture in the system
  • Suction Line Types are placed after the evaporator and before the compressor thus preventing particles of foreign objects from being sucked into the compressor
  • Suction accumulators may prevent compressor damage from a sudden surge of liquid refrigerant and oil that could enter from the suction line. The accumulator is a temporary reservoir for this mixture and is designed to meter both the liquid refrigerant and oil back to the compressor
  • A flow switch is a device that monitors the flow of a gas or a liquid. It sends a trip signal to a different device in the system, such as a pump. The flow switch can indicate to the pump to shut off or to turn on. The switch needs to be installed after the evaporator to detect low water flow and shut off the chiller
  • A discharge bypass valve opens on a decrease in suction pressure and can be set to automatically maintain a desired minimum evaporating pressure regardless of the decrease in evaporator load

In a nutshell the following parameters need to be evaluated to make a proper chiller sizing:

  • Entering and leaving water temperature
  • Water flow rate
  • Expected pressure drops
  • Percentage of glycol
  • Lowest and highest ambient temperatures in case of air-cooled chillers
  • Condenser water temperatures in case of water-cooled chillers
  • System volume to set up a tank or not
  • Proper pump selection
  • Addition of other components to the refrigeration cycle