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Air Handling Unit

Air Handling Units are used in engineering management of environment engineering and are often called AHU. The air-handling unit is box-like equipment with a fan and a cooling coil inside. Some units also contain air filters. The whole fan and motor assembly, comprising shaft, bearings, pulley, belting is usually put inside the AHU.

The basic function of the AHU is to suck air from the rooms, let it pass through chilled water cooling coils and then discharging the cooled air back to the rooms. Normally, letting it pass through panel or bag filters also filters the air. A certain amount of fresh air may be introduced at the suction duct so that air in the rooms may be gradually replaced. AHU's come in many sizes and shapes. Usually, the air conditioning designer will choose a particular AHU based on the air flow requirements and the cooling capacity. If humidity of the air has to be controlled, steam coils, or other heating coils may be installed. If the air has to be very cleaned, special HEPA filters have to be installed at the ducting outlets or at the AHU filter box. Moisture in the air is condensed out when it comes into contact with the chilled water coils. At the bottom of the AHU, a pipe is installed so that water that is collected can be drained out.

The fan and motor assembly is usually mounted on vibration dampers that absorb any vibrations generated. Removable panels are installed so that personnel can enter into the AHU for maintenance. Maintenance is mostly changing or washing of air filters, greasing of bearings, changing of belts, and general inspection and cleaning work.

Temperature Control

Controlling the flow of chilled water through the cooling coils alters the temperature of the discharged air into the rooms. Control valves are used to throttle chilled water through the chilled water coils. A simple temperature control system uses thermostats to control on-off solenoid valves. A better control system uses engineering software  with temperature sensors, controllers, and motorized control valve. More complicating systems may have motor speed control for the fan.

Humidity Control

Some critical processes may require that the humidity of the air-conditioned space be controlled. During the normal cooling process, as the air becomes cooler, the relative humidity of the air tends to increase. If the relative humidity have to be brought down, the air have to be heated by steam coils or other means. Steam coils, if installed will have their own controls. A typical control system has a temperature sensor, controller, and control valve. Usually, humans monitor the relative humidity, and the steam controller settings are adjusted accordingly. In advanced systems engineering, engineering software are used for controls.


Persons who want to control temperature and humidity of the air in the air conditioning system should have an understanding of psychrometry. They make use of psychrometric charts to determine various parameters of the air at any condition. For example, by knowing two parameters, you can look up the chart to find the other values. The values found in a typical psychrometric chart are:


  • Dry Bulb Temperature
  • Wet Bulb Temperature
  • Dewpoint Temperature
  • Relative Humidity
  • Humidity Ratio

Other parameters that can be found from the psychrometric chart are:


  • Enthalpy
  • Sensible Heat Ratio
  • Vapor Pressure
  • Specific Volume

However, nowadays, engineering software is used for engineering management. More and more engineering schools offer engineering degrees that includes systems engineering and computer engineering.

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