Air handlers or Air Handling Units or AHUs are explained along with it’s basic details, definition, working principle, components, description of all components, applications, etc.
In this article we will learn how Air Handling units, or AHU’s, work. We’ll look at different componenst of typical AHU’s along with diagrams to clear our basics.
Let’s explore air handler!
Let’s learn the basics of the air handlers. We have already learned the HVAC system and understood that air needs to be conditioned based on our requirements.
Now, how do we condition air?
There are many equipment to do that and air handler or air handling unit is one of them. As the name suggest, it simply air handles as well as conditioning the air.
An air handler, or air handling unit (often abbreviated to AHU), is a device used for cleaning, conditioning (heating or cooling or humidify or dehumidify), regulate and circulate air as part of a heating, ventilating, and air-conditioning or HVAC system.
The main purpose of Air Handling Unit (AHU) is to handle the air in terms of cooling, heating, filtration, humidifications etc. as part of a heating, ventilating and air-conditioning system.
The purpose of air handlers shall be as follows,
Air handling units, as the name suggests, it handles air. It helps to cool or heat, cleans the air for conditioning. It is abbreviated as AHU, and we get the filtered air from it.
There are many components in air handling unit, are as follows,
Casing is the outer most part of air handling unit. Air handler unit casing is factory assembled, modular, double skinned and insulated.
Fuction of AHU Casing
The function of AHU casing, are as follows:
Normally sheet metal is used for AHU casing, howver, for hazardous area application it can be considered as stainless stell.
Typical technical parameters are as follows,
Intake louver is the first part of the AHU through which air is entered into the unit.
The main function of the intake louver are,
Normally it is made of sheet steel.
Filters means the thing which cleans the air. Filters along with filter frame is one the main part of AHU and arranged in banks in sufficient numbers. The numbers of filters are calculated based based on air quantity, air quality.
Filters has the below characteristics,
The following filters are used for air handling units, like
Prefilters are the first basic filters in air handling unit. It filtrates comparatively course particles.
Medium Efficiency Filters
Fine Filters / Bag Filters
The chemical filters are not used in normal areas, however, in case if it is corrosive atmosphere, chemical filters are used for at fresh air intake.
HEPA filters are normally used for precision application, like medical applications.
Cooling coil is the main part for cooling of air or water. It cools the air or water whatever comes in contact. Normally air comes in contact with cooling coil but for a bigger system like chilled water system, cooling coil cools water circuits.
Cooling coil can be classified into two types,
It is basically acts as a heat exchanger. Copper is used for the material of cooling coil and it is properly insulated to restrict heat loss.
Drain pan is provided below the cooling coil to collect the condensate. It is normally made of stainless steel (SS316L) is common option.
The U Trap shall be designed for a water level according to the relevant air pressure differential and in consideration of additional 50 mm W.G. for reserve.
In case of high humid area, moisture content is becoming more and the same needs to be removed to make it comfortable or acceptable.
Heating coil is used for this purpose. In many places, if humidity is too high, duct heaters shall be provided in the individual rooms instead of a common heating coils in air handling unit.
Air handling fan or air handler fan is one of the main component for supplying air into the required areas. Normally double inlet double with type centrifugal fans are widely used for air handling units. Now a day, EC fans are also using in many AHUs, to optimize the power consumption. The fan can be driven by motor or variable speed drive (VFD) based on requiremments.
In many big AHU units, multiple fans are also considered. Normally, double inlet double width type centrifugal fans are used with forward or backward curved impellers.
Fans can be mounted as below based on project requirements,
Fuction of AHU Fan
The function of AHU fan, are as follows:
Normall the material of fans shall be based on application or type of projects, however, the following materials are widely used,
The fan is driven by a motor.
Heaters are integral parts of air handlers, based on the requirements. If moisture content is too high in the atmosphere, it is required to reduce to make the conditioned air comfortrable. Here, the requirement of heater comes. Whenever required it will be switched on.
AHU shall be provided with a mixing box and the name suggests, it mixes return air and fresh air.
In case air handlers are kept outside, a factory installed weather hood is used to protect the unit from the sun.
AHU dampers are provided at the inlet of the fresh air and at the discharge of the return air. It controls the air.
To control the air handling unit, a dedicated control system is inbuilt in it. Control system used to control the followings:
It can be operated through thermostat, and connected to direct digital controls or even PLC system.
There are many items are associted with control system,
AHU fan creates vibration on the base of AHU. Hence, to reduce this vibration, vibration isolator is provided at he base of fan.
AHU fan is connected to a ducting system and the vibration of the fan will be transmitted to the ducting system. It can reduce the rigidity of the duct supports. Hence, a flexible connection is provided between the fan and ducting system to limit the transmission of vibration.
It is the main working fluid in the HVAC system and it flows through within the cooling coil for the DX-type system.
Nowadays, the following environment-friendly refrigerants are used in various industries,
Air handlers are classified as follows:
Based on type of fan’s operation
Based on cooling coils
Based on Nos. of AHU fans and motors
Let’s see all types of AHUs to have a basic idea,
In the Draw-Through type air handlers, the fan is used to pull the air through filters, cooling coil, mixing box before discharge.
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.
In the Blow-Through type air handlers, the fan is used to blow the air through filters, cooling coil, mixing box before discharge. A positive pressure will be created after the fan.
DX type AHU
DX is the abbreviation of direct expansion. Direct expansion means when the air is directly comes in contact with cooling coils. Cooling coil is with refrigerant and heat transfer happens between refrigerant and the air. This cooling coil is called as DX coil.
Chilled Water type AHU
Chilled water type AHU means the AHU which is having chilled water cooling coil. As this cooling coil is having chilled water instead of refrigerant, it is called as chilled water cooling coils.
Single Fan AHU
Normally AHU comes with single fan with one motor. Single fan AHU means only one no. of AHU fan.
Duel Fans AHU
Duel fans AHU are not generally used, however, it can be used in the below case,
Duel Motors AHU
Duel motors AHU is not generally used, however, it can be used in the below case,
Location of Air handling units, or AHUs depend on the type of project, space availabilty, building sizes etc. AHUs can be placed,
Within a Room, called AHU Room.
Not in a Room
Before going to the working principle, let’s have look of the diagram of air handlers.
Let’s learn the working principle of air handling unit. Its very simple and we will disucss in step by step:
Fresh air enters through the intake louver of the AHU. Louver helps to avert any animals or bird entry into the AHU.
It has two options,
After filters, air gets cleaned and pass through the cooling coil.
Cool air then pass thorugh moisture eliminator and remove all moisture from the air.
After moisture eliminator, air is pass through heaters to control the moisture content. In many cases, heaters are not provided in AHU, instead, it is provided in the duct based on the project requirements.
The air then sucked by the fan and transferred to the required areas to be conditioned. The fan is connected to a ducting system. This ducting system helps to distribute the air properly. For small capacity or small room, ducting system is not used.
Conditioned air is taking heat in the areas and comes back to the AHU through return air opening/damper and mixed with the fresh air. This process continues and AHU works.
There are many instruments in the AHU to understand the filter health, AHU health, chilled water temperatures, pressures, air flow rate etc, which helps to meets the demands as well as maintained the good health of AHU.
The basic parameters for AHU specification, shall be as follows:
There are so many companies manufacture Air handlers, few of them are listed,
So, finally we have got a basic idea about Air handler or Air Handling Units along with it’s basic details. If you have any question, please let us know.
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