Working of AHU (Air Handling Unit)

 Working of AHU (Air Handling Unit)

Introduction

Air conditioning is a process used to maintain the temperature, humidity, and human comfort for the occupants in the room, it works on air refrigeration cycle. Air Handling Unit (AHU) is the equipment which is used to achieve the above parameters and is used when a large space is to be maintained at an isolated comfort zone from the outer atmospheric temperature.

The cooler air from the AHU is then transferred to the desired location with the help of ducts.

 

Parts of AHU

AHU mainly has 3 section.

1.       Filter section

2.       Coil section

3.       Blower section

Components of AHU

·         Damper

·         Pre filter

·         Bag filter

·         Cooling coil

·         Heating coil

·         Blower fan

·         Damper

all the above-mentioned components are placed in order.

 

Working of AHU

The fresh air from outside and 20% return air from the room get mix in the mixing chamber. Then it passes through a pre filter where dust particles, hairs from the air get filtered. Air than passes through bag filter where minute particles left from the pre filter gets trapped. Then it passes through the cooling coil where chilled water is circulated from the chiller. Which reduces the temperature of the air to the desired point. It is than passed on to the occupied zone with the help of ducts. These types of AHU are widely used also called AHU with Recirculating Air.

Image Credit - ciqa.net

FAHU (Fresh Air Handling Unit)

FAHU is the one in which no return air is used. 100 % fresh air is sucked from the atmosphere, conditioned and passed on to the serving area.

Types of AHU

AHUs are of 2 types

i) blow through type

ii) draw through type

 

i) blow through type

Here the blower fan is placed before the cooling coil as represented in the image below. 

In this the air from the mixing chamber is sucked with the help of a blower and pushed on to the cooling coil. Which makes the air cooler and sends it to the desired space.

 

Advantages

1. The motor which runs the blower heats up due to continuous use. The fan attached to the motor used to cool it removes the heat formed in the motor to the out of the motor, but not out from the AHU since it is a closed box. This hot air too passes from the cooling coil and used in conditioning.

2. A water trap is used to remove the dew formed from the air coming on to the coil. When the AHU is used for cooling purpose it's fine, but when using it for heating purpose there will be no dew formation on the coil and the trap pipe is empty. Since the air is blown onto the coil the chance of bad outside air entering the AHU is zero.

 

Disadvantages

1. Since the air is blown at a high pressure on to the coil some of the dews formed get carried away by the air itself and can damage the duct. Which increase the maintenance cost.

2. To avoid the above point the distance between the blower and the coil can be increased so as to make a specific gap between both. Which increase the manufacturing cost of AHU. And sometimes cannot be placed in the room with small size.

 

 

i) draw through type

Here the blower is placed after the cooling coil as represented in the image below.

The air from the mixing chamber is sucked by the help of blower but here it first passes through the cooling coil and then it is pressurized by blower to the occupant space.

 

 

Advantages

1. This type of AHU controls the temperature along with humidity. As the coil is placed before the blower the chance of dews to be carried with air to the spaces gets neglected.

2. This types of AHU can be made smaller in size as there is no effect of distance between the coil and the blower.

 

Disadvantages

1.  Due to high blower pressure the bad air from the trap below the coil can enter the AHU and pass on to the occupied space. Which creates a discomfort.

To avoid this

1. the trap design should be good.

2. The drain line of pipe not. To Be entered into sewage, instead place the pipe at an outer position where there is atmospheric air so even if the air is sucked from the drain line it mixes with the air inside the ahu and eliminated the bad smell.

Due to above mentioned reasons the draw through type AHU is widely used and recommended.

controls of AHU

Many sensors and actuator are used in AHU at various points as shown in figure below.

T = Temperature sensor

RH = Relative humidity sensor

DP = differential pressure switch

CO2 = carbon dioxide sensor it checks the quality of air. 

actuators are used to control the opening of the damper. They are of 2 types one which only opens and close the damper. The other one modulating actuator controls the flow from 0 to 100% opening.

The DP switch checks the air pressure difference before and after the damper. If the difference is less, it's ok, if it is high the filters are need to be washed or replaced (the filters are periodically washed depending upon the working of that AHU it is either once in a month or once in 3 months).

The CO2 sensor at the end checks the quality of the air.

The RH sensor checks the relative humidity of the air at every point after passing through each component.