Air Handling Unit (AHU)

An air handler, or air handling unit (AHU), is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning  (HVAC) system. 

An air handler is usually a large metal box containing a  blower, heating or cooling elements  filter racks or chambers, sound attenuators, and  dampers. 

Air handlers usually connect to a  ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU.

AHU operation is fundamental to the Commercial Building HVAC Control. Increased comfort and operational efficiency may be achieved through advancing the control of an AHU- resulting in more sales, reduced energy costs and satisfied customers. 

An air handling unit (AHU) is used to calculate the energy consumption of heating and/or cooling (outside) air to a certain set point. This type is considered to be mainly used in multizone building for ventilation.

There are two main types of air handling unit — the ‘blow-through’ and the  ‘draw-through’.

The blow-through AHU has a fan which blows the air through the mixing box, cooling coil and filters before it goes to the ducting network. 

The draw-through AHU can be vertical or horizontal. It has a fan which pulls the air through the mixing box, cooling coil and filters before it goes to the ducting network.

In air handling unit if the air flow is from the right to left in this case AHU components are 

• Supply duct
• Fan compartment
• Vibration isolator (flex joint)
• Heating and/or cooling coil
• Filter compartment
• Mixed (recirculated + outside) air duct

AHU components

Filters

Air filtration is almost always present in order to provide clean dust-free air to the building occupants. Gas-phase and ultraviolet air treatments may be employed as well. Filtration is typically placed first in the AHU in order to keep all the downstream components clean. 

Heating and cooling elements

Air handlers may need to provide heating, cooling, or both to change the supply air temperature, and humidity level depending on the location and the application. Such conditioning is provided by heat exchanger coils within the air handling unit air stream; such coils may be direct or indirect in relation to the medium providing the heating or cooling effect.

Humidifier

Humidification is often necessary in colder climates where continuous heating will make the air drier, resulting in uncomfortable air quality and increased  static electricity. Various types of humidification may be used

Evaporative

Dry air blown over a reservoir will evaporate some of the water. The rate of evaporation can be increased by spraying the water onto baffles in the air stream. 

Vaporizer

Steam or vapor from a boiler is blown directly into the air stream. 

Spray mist: water is diffused either by a nozzle or other mechanical means into fine droplets and carried by the air. 

Ultrasonic

A tray of fresh water in the airstream is excited by an ultrasonic device forming a 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. As the air passes through the medium it entrains the water in fine droplets. This type of humidifier can quickly clog if the primary air filtration is not maintained in good order. 

Mixing chamber

In order to maintain indoor air quality, air handlers commonly have provisions to allow the introduction of outside air into, and the exhausting of air from the building. In temperate climates, mixing the right amount of cooler outside air with warmer return air can be used to approach the desired supply air temperature. A mixing chamber is therefore used which has  dampers controlling the ratio between the return, outside, and exhaust air.

Blower/fan

Air handlers typically employ a large  squirrel cage blower driven by an AC induction  electric  motor to move the air. The blower may operate at a single speed, offer a variety of set speeds, or be driven by a  Variable Frequency Drive to allow a wide range of  air flow rates. Flow rate may also be controlled by inlet vanes or outlet dampers on the fan. These are driven using high efficiency electronically commutated motors with built in speed control.

Balancing

Un-balanced fans wobble and vibrate. For home AC fans, this can be a major problem air circulation is greatly reduced at the vents, efficiency is compromised, and noise is increased. Another major problem in fans that are not balanced is longevity of the bearings (attached to the fan and shaft) is compromised. This can cause failure to occur long before the bearings life expectancy.

Heat recovery device

A heat recovery device  heat exchanger of many types may be fitted to the air handler between supply and extract airstreams for energy savings and increasing capacity. These types more commonly include for

Recuperator Plate Heat exchanger

A sandwich of plastic or metal plates with interlaced air paths. Heat is transferred between airstreams from one side of the plate to  the other. The plates are typically spaced at 4 to 6mm apart. Can also be used to recover coolth. Heat recovery efficiency up to 70%.

Thermal Wheel or Rotary heat exchanger

A slowly rotating matrix of finely corrugated metal, operating in both opposing airstreams. When the air handling unit is in heating mode, heat is absorbed as air passes through the matrix in the exhaust airstream, during one half rotations, 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 rotations, and absorbed during the second half rotation into the supply airstream. Heat recovery efficiency up to 85%. Wheels are also available with a  hydroscopic coating to provide latent heat transfer and also the drying or humidification of airstreams. 

Run around coil

Two airs to liquid heat exchanger coils, in opposing airstreams, piped together with a circulating pump and using water or a brine as the heat transfer medium. This device, although not very efficient, allows heat recovery between remote and sometimes multiple supply and exhaust airstreams. Heat recovery efficiency up to 50%. 

Heat Pipe

Operating in both opposing air paths, using a confined  refrigerant as a 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 up to 65%. 

Controls

Controls are necessary to regulate every aspect of an air handler, such as flow rate of air, supply air temperature, mixed air temperature, humidity, air quality. Common control components include temperature sensors, humidity sensors, sail switches,  actuators, motors, and controllers.

Vibration isolators

The blowers in an air handler can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the occupants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler and often also between the fan compartment and the rest of the AHU. 

Functions of AHU

• Heat the air flow 
• Cool the air flow 
• Dehumidify the air flow 
• Humidify the air flow 
• Clean the air flow 
• Distribute the conditioned air into the building space.