The definition of air handling unit from ANSI/AHRI Standard 430-2009 states that it is "A factory-made encased assembly consisting of a fan or fans and other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air...."
The AHU is used to control the following parameters of the space.
Draw-Through or Blow-Through
There are basically two types of air handling unit that are in used and they are the "Draw-Through" or "Blow-Through".
In the Draw-Through type, the fan pulls the air through the mixing box, filters and cooling coil before discharging it from the fan outlet to the space to be conditioned or to the ducting network. The design can be vertical or horizontal. In this case, the section before the fan has negative pressure.
In the Blow-Through type, the fan blows the air through the mixing box, filters and cooling coil before discharging them to the space being conditioned or the ducting system. In this case, the section after the fan has positive pressure.
An air handling unit where (1) is the supply air, (2) fan section, (3) vibration isolator, (4) cooling coil, (5) filter and (6) mixed air duct.
Components of Air Handling Unit
Here are some of the air handling unit components that may be contained in the equipment.
The housing that contains all the other components of an AHU is usually make of metal, some are painted to prevent corrosion.
In sections where the fans and the coil are located, 1-2 inches of polyurethane foam or PU is used to insulate them to prevent the condensation on the panel. Drain pan is also used as a precaution in the event of condensation of water.
Centrifugal fan is used to circulate the air to the various parts of the sections in the building. The typical types of fan available are Backward Inclined, Backward Curved, Forward Curved and Airfoil.
The selection of the fan will depend on the air volume and the static pressure required of the system. Usually, the designer of the system will use a specialized software to do this selection.
In order to reduce the effect of vibration on the panel, the motor and the fan are usually installed on the vibration isolator except when the drive assembly is external to the fan casing.
In recent years, the use of variable air volume (VAV) system is becoming more popular as the volume of the air being discharged can be varied depending on the load condition. If the load is high, the fan speed will be higher and if the load is lower, the speed of the fan will be lower.
The speed of the fan is varied by using frequency inverter instead of conventional motor such as PSC motor. Frequency inverter provides better control of the fan speed as a whole range of fan speed from super low to super high can now be utilized based on the load conditions required.
This technology has enabled better use of energy and is in tandem with the move to go for greener energy.
Cooling Coil is used to cool and dehumidify the air. Both DX (direct expansion) cooling and CW (chilled water) cooling coils are available for use depending on the system design.
These coils are arranged in rows with different fin spacing. Aluminium fins and copper tubes are used in the design of the coils. The corrosion resistance hydrophilic fins are also used due to its lower cost and lower resistance to the air velocity.
Filters are to remove particles and contaminants of various sizes from the air. The type of air filter being used will very much depend on the application of the system.
Panel Filter is a flat and rectangular in shape and provides a minimum low efficiency filtration which is acceptable to the air conditioning industry. The high velocity filter is arranged vertically whereas the low velocity filter is arranged in V shape. Typical air velocity that moves through the filters is in the range of 2-3 m/s.
HEPA Filter is very efficient and is able to achieve efficiencies up to 99.97%, removing minute particles and airborne bacteria from the air. It is usually used in clean room applications such as semiconductor production floor, operating theaters and critical processes.
Electrostatic Filter is used to remove particles from the air by using highly charged electrodes that ionized the air. Bag Filter is able to remove dust particles and is thrown away after use. Roll Filter is used for high velocity filtration where the used part is rolled up automatically/manually.
During winter, the humidity level of the air can be low hence causing discomfort to the occupants. The humidity of the air is increased by using the humidifiers. Here are the commonly used humidifiers:
Spray Type has a header and spray nozzles that spray water with a pressure of 15 psi or more.
Steam Pan Type has a pan and a heating coil to heat up the water of the pan. The evaporation of water caused by the heating will increase the humidity level of the surrounding air.
Steam Grid Type has tiny holes on the pipe to distribute the steam that flows through it. In this case, the water that is heated up to produce the steam to be supplied to the grid is conditioned to prevent odor being discharged to the room.
This box has air inlets that is attached to the dampers. This is the place where the outside air and the return air are mixed to provide the correct proportion of air to be distributed to the space that is to be conditioned.