Understanding Air Handling Units
We find HVAC systems in buildings and industrial processes frequently these days. Such systems move air around a particular space and the technical term for this is air handling unit, often abbreviated to AHU.
What is meant by ventilation and air conditioning? Ventilation and air conditioning systems keep the condition of room air, or air to be supplied to processes, constant within certain limits of purity, temperature, and humidity.
AHUs consist of a series of sections through which air is moved. In each of these sections the air is treated and leaves it in a different state than when it entered.
Depending on the section, air is moved, heated or cooled, mixed, humidified or dehumidified, filtered, soundproofed or diverted. There are also sections that work with energy recovery equipment.
The sections are each self-contained in function, but are connected to preceding or succeeding sections in the air flow. Each of the sections usually has side openings for cleaning and maintenance. Sections usually consist of metal frame structures and thermally insulated panels, which ideally have no thermal bridges between their inside and outside panels.
A distinction is made between those AHUs that exclusively condition outside air, which also transfer the used air to the outside again, and those that mix part of the air present in rooms with a defined proportion of outside fresh air, for example, and also those that merely condition the air present in rooms and operate without outside air.
A further distinction is made between units that are installed outdoors, so-called Packaged Units or Roof Top Units, and those that are operated indoors, described as Terminal Units.
The vast majority of the uses of AHUs relate to the following applications:
- AHUs for commercial and residential buildings such as office buildings, hotels, shopping malls, and houses and may include fan coils. These units need to comply with standard requirements.
- AHUs for service areas with high relative humidity, specific requirements apply.
- AHUs for use in medical fields e.g. hospitals, laboratories, universities and in industrial areas. These hygienic units present specific requirements.
- AHUs for data centre applications, special control requirements may apply for ensuring safe operation in an area of sensitive equipment.
- AHUs for use in hazardous environments e.g. for service areas of the oil and gas industry where special certificates such as IECex or ATEX are necessary.
Basic Structure of an AHU
The mostly cuboid sections with square sections are arranged in a frame system which also gives the AHU its static structure. The frame structure is made of a penta-post construction and is reinforced with panels side by side or perpendicularly. Doors or other inspection openings are provided in suitable places to allow easy maintenance.
The panels are made of double-skinned sheet metal, the inside and outside of which are thermally separated by insertion of thermal insulation made of PU foam, or rock or glass wool suitable for this purpose. The metal is galvanized and electrostatically powder coated.
AHUs may contain many sections and may be several meters long. To support the sectional composition, and for strength and rigidity, welded base rails are provided to form an integrated base on the full perimeter under the unit.
In the case that supply and return air are required in equal proportions for a balanced HVAC system, the two parts of the AHU are joined together, either side-by-side or in a stacked manner. AHUs may be constructed in nearly any shape and dimensions.
Air quality is an important feature of conditioned air. Air conditioning filters are used to remove solid contaminants such as smoke, pollen, dust, grease and pollen to ensure better air quality.
The MERV ratings (Minimum Efficiency Reporting Value) indicate the effectiveness of the filters’ ability to remove airborne particulates. Air filters utilise porous membranes that maintain airflow while trapping particles and contaminants.
When choosing a filter for an HVAC system, it is very important not to exceed the recommended MERV rating that the system was designed to handle. High MERV filters, while very effective at air filtration have a negative impact on system airflow due to their density. This may cause air leakage within the system in areas where there is less resistance. High performance filtration can lead to increased energy use, more wear and tear on the system and the chances of a coil freezing (where installed).
Filters are placed first in an AHU to keep the downstream components clean.
- Flat-Panelled Fiberglass Filters: These filters are on the low end of the MERV ratings. The disposable filters contain layered fiberglass and a metal reinforcing grate to prevent collapsing surrounded by a frame
- Pleated Media Filter: These filters are also disposable and carry a MERV rating of between 5 and 13 with high-efficiency versions rating between 14 and 16. The pleats are designed to increase the surface area of the filter, which in turn, raises its filtration efficiency
- HEPA Filter: HEPA filters provide the highest level of protection against air borne particles. The filters are capable of trapping particles as small as 0.3 microns or larger and catch 99.97% of air borne particles. HEPA air filters carry a MERV rating of between 17 and 20
- Reusable Air Filters: Another HVAC filter option but with low MERV ratings is a washable reusable air filter. While this filter type is initially more expensive than its disposable counterpart, it can be re-used after cleaning
Bag Filter | Pleated Filter | HEPA Filter
Heating and Cooling Elements
AHUs may need to provide heating or cooling or both by means of heat exchanger coils in the air stream to change the physical state of the supply air, i.e. temperature and/or humidity.
Direct heat exchangers include gas-fired heaters, electrical heaters, heat pumps or refrigeration evaporators or evaporative coolers placed in the air stream.
Indirect heat exchangers use water, hot or chilled, from other sources in buildings or a process. Such coils are made of copper tubes and copper or aluminium fins. Cooling coils need to remove or drain condensate by eliminator plates. Temperature sensors are used to monitor and control “off coil” temperatures.
For dehumidification, the cooling coil needs to be employed to over-cool so that the dew point is reached, and condensation occurs. A re-heater coil after the cooling one reheats the air up to the desired supply temperature by reduction of the relative humidity. Also, pre-heat coils are employed in case of low ambient temperatures to protect downstream filters and cooling coils.
Various types of humidification techniques inside a dedicated section may be applied:
- Evaporative: spraying water onto baffles or diffuse water by a nozzle into fine droplets and charge to the air stream.
- Vapourising: blowing steam or vapour from a boiler directly into the air stream.
- Ultrasonic: a tray of fresh water in the airstream is agitated by an ultrasonic device forming fog or water mist.
- Wetted Medium: A fine fibrous medium in the airstream is kept moist with fresh water from a header pipe with a series of small outlets.
AHUs typically employ large fans driven by induction motors to move air through the unit. Such fans may run at single speed, staged speeds or driven by VFDs or high efficiency EC motors with built in speed control to allow a wide range of air flow rates.
Two major categories of fans are used in AHUs, axial fans or centrifugal (radial) ones. A centrifugal fan operating at a constant speed moves a relatively constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though the mass flow rate through the fan is not. Centrifugal fans displace air radially, changing the direction of the air flow typically by 90°.
Axial fans cause air to flow through it in an axial direction parallel to the fan shaft. The flow is axial at entry and exit and fans are designed to produce a pressure difference causing a flow through the fan.
Flow rates are controlled by inlet vanes or outlet dampers of the fans.
Several fans may be present in AHUs, typically placed at the end and the beginning of the ductwork. Such supply fans may be augmented by fans in the return air stream pushing air into the AHU.
In ventilation systems, noise comes from a variety of components such as fans or dampers. To comply with specified noise levels, sound attenuators may be added to suitable locations in the AHU. Sound attenuators must have adequate silencing capabilities and low air resistance to attenuate noise before it enters the ductwork by means of a duct accessory consisting of an inner perforated baffle with sound-absorptive insulation.
Beyond that, the casing may be equipped with panels with double thickness and proper insulation inside the panels.
Energy recovery devices may be fitted as heat exchangers to an AHU between supply and return air streams for energy savings, increased capacity and a better efficiency.
- Plate Heat Exchanger: A sandwich of metal plates with interlaced air paths. Heat is transferred between air streams from one side of the plate to the other. Heat recovery efficiency is up to 70%
- Rotary Heat Exchanger: A slowly rotating matrix of finely corrugated metal, operating in both opposing airstreams. When the AHU is in heating mode, heat is absorbed as air passes through the matrix in the exhaust airstream, during one half rotation, and released during the second half rotation into the supply airstream in a continuous process. When the air handling unit is in cooling mode, heat is released as air passes through the matrix in the exhaust airstream, during one half rotation, and absorbed during the second half rotation into the supply airstream. Heat recovery efficiency is up to 85%.
Enthalpy wheels are also available with a hydroscopic coating to provide latent heat transfer and also the drying or humidification of air streams.
- Run around coil: Two air to water heat exchanger coils, in opposing air streams, piped together with a circulating pump and using water as heat transfer medium. This device, although not very efficient, allows heat recovery between remote and supply and exhaust airstreams. Heat recovery efficiency is up to 50%
- Heat Pipe: Operating in both opposing air paths, using a confined refrigerant as heat transfer medium. The heat pipe uses multiple sealed pipes mounted in a coil configuration with fins to increase heat transfer. Heat is absorbed on one side of the pipe, by evaporation of the refrigerant, and released at the other side, by condensation of the refrigerant. Condensed refrigerant flows by gravity to the first side of the pipe to repeat the process. Heat recovery efficiency is up to 65%
To maintain high indoor air quality, AHUs have provisions for mixing room air with the right amount of cooler outside air, with warmer return air to achieve the desired supply air temperature. Mixing sections have dampers controlling the ratio between return, outside and exhaust air.
Sophisticated controls are necessary to operate an AHU efficiently e.g. air flow rate, supply air temperature, mixed air temperature, humidity, air quality. The range of available controls comprises simple thermostats or building automation system. Control components include temperature sensors, humidity sensors, various actuators, motors, and controllers.
A huge variety of accessories are available and vibration isolators are in most cases mandatory. AHUs create substantial vibrations and the ductwork may transmit vibrations and related noise into the building. Vibrations isolators are inserted into the duct before and after the AHU and also between fan and adjacent sections. Fans may be further isolated by putting (it) them on spring suspensions mitigating the transfer of vibrations and noise through AHU and ductwork.
A number of protective devices are capable (to contribute) of contributing to the longevity of outdoor AHUs.
- Rain Louvres prevent rain (to intrude) entering the AHUs
- Roof Covers helps to dispose of rain water and may be added either as single or double sloped canopies
- Bird Screens are wire meshes placed at the fresh air entrance as a component to prevent insects and birds from entering the AHU
- Sand Trap Louvres are also available for dedicated applications.
Click here to see our full range of options.
Since AHUs are used for a wide range of applications to provide conditioned air, there are also cases that have to meet special requirements in addition to those listed above.
Two special versions, which are frequently asked for, should be mentioned here as examples.
Firstly, AHUs in hygienic design conditions, which must meet all requirements for specially conditioned air e.g. in hospitals or laboratories. Secondly, there are AHUs which are designed for operation in hazardous areas.
The new field of Data Centre cooling also requires special designs of AHUs for cooling the sensitive CPUs in accordance with stringent requirements.
Petra AHU – PAHHC 150