• SkillCat Team

Air Handling Unit: Complete Explanation - Part 1

Updated: Aug 25

Air Handling and Distribution: Chapter 1


Air Handling Unit


In this module, we will learn about air handling. We will also learn about equipment that handles the conditioned air in commercial buildings. Skip to quiz!


Introduction

Air handling deals with filtering the indoor air with an air filter. This clean filtered air is then cooled or heated to match the comfortable temperature. In this process, some amount of indoor air is also replaced with fresh outdoor air to improve air quality.


The equipment that handles the conditioned air in commercial buildings is called air handling unit. Recall that the air handling unit is a large rectangular metal box that is connected to the ductwork. We usually call it AHU for short.


Recall that an AHU contains air filters, blowers, and heating and cooling components. It is used to cool or heat and circulate a large amount of air throughout the room via ductwork.


AHUs come in different shapes and sizes with a range of configurations as required by the user. We usually place them on the roof or in the basement of the building. We can also place them on the floors of the mechanical rooms. Recall that a mechanical room contains HVAC equipment.


We can divide AHUs into two types based on where they are installed. The two types of AHUs are:

  • Floor mounted AHUs, and

  • Ceiling suspended AHUs.

Let us learn more about these AHUs.


Floor mounted AHUs are installed on the floor. Their advantages are:

  • low initial cost,

  • usually have a long life, and

  • easy to access during maintenance as they are placed on the floor.


As Floor mounted AHUs are placed on the floor, they occupy floor space in the room.This is not a good thing for offices in metropolitan cities as floor space is costly.


Ceiling suspended AHUs are hung from the ceiling. We can save floor space by choosing these AHUs. These comparatively cost more and have a low life span.Difficult to reach them for maintenance as they are hung from the ceiling.


Components


The main components of any AHU are:

  • Housing,

  • Blower,

  • Cooling coil,

  • Heating coil,

  • Air filter,

  • Dampers, and

  • Condensate drain.

Let us learn more about these components.


Housing is the insulated enclosure of the AHU. AHU’s components such as air filter, blower, and cooling and heating coils are placed inside it. It prevents heat transfer with outside air.

It also prevents air leakage in and out of the AHU.


Recall that blower is a type of fan that forces air to move through the ductwork into the rooms and office spaces. We generally use the centrifugal type of blower in AHUs.


Recall that the cooling coil in the AHU cools and dehumidify the air.The cooling coil in the AHU can be:

  • Cold water cooling coil, or

  • Refrigerant cooling coil.


Recall that chiller systems generate cold water. This cold water is circulated through the cooling coils in AHUs. In some applications, we connect an RTU or heat pump to the AHU.

Here refrigerant flows through the cooling coil.


The heating coil in the AHU heats the air. Similar to cooling coils, hot water or refrigerant circulates through the heating coil. We can install electrical heaters in AHUs in the place of the heating coils, but they consume more electricity.


All AHUs contain air filters to clean the air in the room. Recall that an air filter traps the dust particles and debris from the air.


Advanced air filters also remove pollen, bacteria, and allergens from the air. For example, AHUs installed in a hospital will require a high level of air filtration as the air in a hospital might contain contagious viruses and bacteria.


Recall that the damper is a plate that stops or regulates the flow of air.Dampers are placed at the supply and return ducts in the AHU. They regulate the amount of air entering and leaving the AHU.


Recall that the condensate drain consists of a drain pan and a drain line. The water that drips off the cooling coil is collected in the drain pan. Then, from the drain pan, the condensate drain line moves the water out of the house.


Some AHUs can have some other components depending on the complexity and requirements. The other components in AHU are:

  • Humidifiers,

  • Dehumidifiers,

  • Sound attenuators, and

  • Vibration isolators.

Let us learn more about these components.


During winters, the humidity level in the air can be less. Recall that low humidity can cause our skin to crack and feel itchy. So humidifiers are installed in an AHU to increase moisture content in the air.


Recall that in high humidity, our sweat cannot dry up as easily. This makes us feel warmer at room temperature. It can promote fungus growth that leads to harmful breathing problems. So some AHUs are built-in with desiccant dehumidifiers.


Recall that desiccant dehumidifiers are made up of silica gel. It absorbs moisture in the air.


The blower in the AHU produces a lot of noise. Ductwork transmits this noise to office space.

A high level of noise can irritate the people in the office and lower their productivity. So sound attenuator is installed in some AHUs to reduce this noise.


The sound attenuator is an assembly that is installed in the air handling unit to absorb sound.

It contains sound-absorbing materials.We can also install sound attenuators in the supply and return ducts of AHUs.


Vibration isolators are flexible devices that absorb vibrations.These are mounted between the blower and the AHU casing. These absorb the vibration created by the blower.



AHU Types


The main types of air handling units are:

  • Blow through AHU, and

  • Draw through AHU.

Let us see a video to learn more about these air handling units.


In a blow through AHU, the blower is installed before the cooling or heating coils.The blower pushes air upon the coils.


The disorder of airflow across the coil is more in the blow through AHU. We have to increase the length between the blower and coil to provide a better airflow pattern. So, blow through AHU’s length is more.


In a draw through AHU, the blower is installed after the cooling or heating coils. The blower pulls air from the coils.


Airflow is evenly distributed in the draw through AHU because air is properly pulled through the coil. There is no need to provide a gap between the blower and coil. So the AHU’s length is shorter.Draw through AHU is the best choice for where there are space constraints.


Air handling deals with filtering the indoor air with an air filter. Air distribution deals with the transfer of cool or hot air to rooms inside the building.


Keep in mind that AHUs do not produce hot or cold water.They take refrigerant or water from another large HVAC equipment and act as a middle man to give hot or cold air to each floor or office. The main types of AHUs are:

  • Blow through AHU, and

  • Draw through AHU.



Air Handling Unit Controls


In this module, we will learn about controls of the air handling unit. We will also learn about BMS control and monitoring of the air handling unit. Skip to quiz!


Introduction


Recall that HVAC control systems are used to control the operations of HVAC equipment.

Control systems use a small amount of control voltage to control larger voltage-connected equipment. Recall that control voltages are usually 24 volts DC.


Controls used in the Air Handling Units:

  • To maintain the temperature and humidity of the air,

  • To maintain optimum air quality,

  • To reduce energy consumption,

  • To reduce manpower,

  • To identify maintenance problems, and

  • To prevent the hazard from occurring.


Some of the sensors that we find in the AHU are:

  • Temperature sensor,

  • Humidity sensor,

  • Pressure sensor, and

  • CO2 sensor.

We will learn about these sensors in detail.


Most AHUs have an electronic control board that receives the signals from these sensors and controls different components. A temperature sensor is installed in the AHU to detect the temperature of the air.


The temperature sensor sends the control signals to the system to control the flow of water or refrigerant in the cooling coil. It can also control the speed of the blower in the AHU.


We install the humidity sensor in the AHU to detect the moisture content of the air. It sends signals to the AHU to control the humidity. It controls the operation of humidifiers or dehumidifiers if the AHU has them.


We install the pressure sensor in the AHU to detect the pressure of the air. Blower speed increases or decreases depending on the input given by the pressure sensor. It tells us if the air filters are dirty or clean. Recall that dirty air filters will not allow air to pass through them easily.


We install a CO2 sensor in the AHU to detect the amount of carbon dioxide in the air. It sends signals to the air handling unit to remove stale air and to add fresh air. It is important to maintain CO2 levels below a certain level to improve the quality of air.


BMS


BMS stands for ‘Building Management System.It is also known as Building Automation System (BAS). It is a computer-based control system that monitors and controls the connected HVAC equipment. Nowadays, it is common in most commercial buildings.


The objective of the BMS is to centralize and simplify the monitoring, operation, and management of the HVAC systems. The user can control the HVAC system by changing temp, humidity, and many other functions from one BMS control room.


BMS shows the alarm on the computer if there are any faults in the HVAC equipment and alerts the users. Let us see a video to understand Building Management Systems.


AHU control systems can also be controlled and monitored by BMS. Some BMS controls attached to AHU are:

  • Damper actuators,

  • Control valve actuator, and

  • Differential pressure switches.

Let see a video to understand how BMS works to monitor and control these controls.


Some of the sensors that we find in the air handling unit are:

  • Temperature sensor,

  • Humidity sensor,

  • Pressure sensor, and

  • CO2 sensor.

AHU control systems can also be controlled and monitored by BMS.




Makeup Air Unit


In this module, we will learn about the makeup air unit. We will also learn about its working principle. Skip to quiz!


Introduction


The makeup air unit is a type of air handling unit which only circulates fresh outdoor air.

There is no recirculating of indoor air. We usually call it MAU for short.


Recall that exhaust fans are used in rooms like kitchens and bathrooms to suck the air from inside the building. Because the building is tightly sealed, fresh air coming into the building through the doors and windows is not equal to the amount of removed air. So we need to install the MAU to replace the exhaust air.


MAUs are becoming more popular these days because:

  • buildings are getting significantly tighter, and

  • exhaust fans are getting more powerful.


MAU supplies 100% fresh air. It is usually located on the top of the building, either in the

  • mechanical room, or

  • out on the roof.

MAUs are installed to replace exhaust air by bringing in fresh outdoor air. The fresh outdoor air is cooled or heated and then supplied to the room. It eliminates the negative pressure caused by the exhaust system.



Components


MAU contains the same main components as an AHU. Some of the main components of MAU are:

  • Housing,

  • Blower,

  • Cooling coil,

  • Heating coil,

  • Air filter,

  • Dampers, and

  • Condensate drain.

Let us recall these components.


Housing is the insulated enclosure of the air handling unit. MAU’s components such as air filter, blower, cooling, and heating coils are placed inside it. It prevents heat transfer with outside air. It also prevents air leakage from the MAU.


Recall that blower is a type of fan that forces air to move through the ductwork into the rooms and office spaces. We generally use the centrifugal type of blower in MAUs.


The cooling coil in the MAU cools and dehumidify the air.


Similar to the cooling coils in an AHU, cooling coils in the MAU can be:

  • Cold water cooling coil, or

  • Refrigerant cooling coil.

Recall that chiller systems generate cold water. This cold water is circulated through the cooling coil. In some applications, we connect an RTU or heat pump to the MAU.

The refrigerant flows through the cooling coils of the MAU in these cases.


The heating coil in the MAU is used to heat the air.Similar to cooling coils, hot water or refrigerant circulates through the heating coil. We can install electrical heaters in the place of the heating coil, but electricity is expensive in many places. So we only use them as backup heaters.


All MAUs contain air filters to clean the air in the room. Recall that an air filter traps the dust particles and debris from the air.


All MAUs contain air filters to clean the air in the room. Recall that an air filter traps the dust particles and debris from the air. Advanced air filters also remove pollen, bacteria, and allergens from the air.


Recall that the damper is a plate that stops or regulates the flow of air. Dampers are placed at the supply and return ducts in the MAU. They regulate the amount of air entering and leaving the MAU.


Recall that the condensate drain consists of a drain pan and a drain line. The water that drips off the cooling coil is collected in the drain pan. Then, from the drain pan, the condensate drain line moves the water out of the house.


Controls


Some of the sensors that we find in the makeup air unit are:

  • Temperature sensor,

  • Humidity sensor, and

  • Pressure sensor.

Recall that MAU supplies 100% fresh air to replace exhaust air in the room. So, it does not need a CO2 sensor like the AHU. Let us recall the above controls.


Recall that the temperature sensor controls the flow of water or refrigerant in the cooling coil. Recall that the humidity sensor detects the moisture content of the air. Recall that the pressure sensor gives input to increase or decrease the blower speed.


The makeup air unit is a type of air handling unit which only circulates fresh outdoor air.

MAUs are becoming more popular these days because

  • buildings are getting significantly tighter, and

  • exhaust fans are getting more powerful.


MAU supplies 100% fresh air. It is usually located on the top of the building, either in the

  • mechanical room, or

  • out on the roof.

MAU contains the same main components as AHU.




Question #1: Where do we find air handling units in a building?

  1. Roof

  2. Basement

  3. Mechanical rooms

  4. All the above

Scroll down for the answer...














Answer: All the above

Air handling units are usually placed on the roof or in the basement of the building.

They might also be placed in the mechanical rooms on the floor.


Question #2: Which of the following component prevents air leakage from the AHU?

  1. Air filter

  2. Ductwork

  3. Damper

  4. Housing

Scroll down for the answer...







Answer: Housing

Housing is the insulated enclosure of the air handling unit. It prevents heat transfer with outside air. It also prevents air leakage from the AHU.


Question #3: Dampers regulate the amount of air entering and leaving the AHU.

  1. True

  2. False

Scroll down for the answer...







Answer: True