What are the main parts of the AHU?

Air Handling and Distribution: Chapter 1

EZONG supply professional and honest service.

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...

For more ahu unit partsinformation, please contact us. We will provide professional answers.

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

Dampers are placed at the supply and return ducts in the AHU. They regulate the amount of air entering and leaving the AHU.

Question #4: Which of the following component absorbs the sound in an air handling unit?

1. Air filter

2. Dehumidifier

3. Sound attenuator

4. Vibration isolator

Scroll down for the answer...

Answer: Sound attenuator

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

Question #5: In a blow-through AHU, the blower is installed after the cooling or heating coils.

1. True

2. False

Scroll down for the answer...

Answer: False

In the blow-through AHU, the blower is installed before the cooling or heating coils.

In the draw-through AHU, the blower is installed after the cooling or heating coils.

Question #6: Which type of AHU is the best choice for where there is a space constraint?

1. Blow through AHU

2. Draw through AHU

Scroll down for the answer...

Answer: Draw through AHU

Draw through AHU is the best choice for where there is a space constraint. It occupies less space than blow through AHU.

Question #7: Which of the following sensor in the AHU detects the amount of carbon dioxide in the air?

1. Pressure sensor

2. Humidity sensor

3. CO2 sensor

4. Temperature sensor

Scroll down for the answer...

Answer: CO2 sensor

The CO2 sensor in the AHU detects the amount of carbon dioxide in the air.

Question #8: Building Management System (BMS) is also known as ________.

1. Building Air Conditioning System

2. Building Automation System

3. Building Operation System

4. Building Electrical System

Scroll down for the answer...

Answer: Building Automation System

Building Management System (BMS) is also known as the Building Automation System (BAS).

Question #9: Differential pressure switches are installed across the air filter and blower in the AHU.

1. True

2. False

Scroll down for the answer...

Answer: True

Differential pressure switches are installed across the air filter and blower in the AHU to monitor and control them through BMS.

Question #10: The makeup air unit is usually located on the top of the building.

1. True

2. False

Scroll down for the answer...

Answer: True

The makeup air unit is usually located on the top of the building, either in the mechanical room or out on the roof.

Question #11: Makeup air units recirculate indoor air.

1. True

2. False

Scroll down for the answer...

Answer: False

Makeup air units only circulate outdoor fresh. They do not recirculate indoor air.

Question #12: Which of the following is not the component of the makeup air unit?

1. Blower

2. Air filter

3. Cooling tower

4. Damper

Scroll down for the answer...

Answer: Cooling tower

The cooling tower is not the component of the makeup air unit.

Question #13: Which of the following component detects the temperature of the air in the makeup air unit?

1. Temperature sensor

2. Humidity sensor

3. Pressure sensor

4. C02 sensor

Scroll down for the answer...

Answer:Temperature sensor

The temperature sensor detects the temperature of the air in the makeup air unit.

The design of an air handling unit (AHU) within an HVAC system is dependent on the specific project requirements, as well as the designer's approach to design, experience level, knowledge of code-driven requirements and standard practices, and the ability to communicate the design intent clearly through the preparation of plans and specifications.

While the AHU is an important part of a building's HVAC system, the entire system should be considered during a complete design. The design, selection, and arrangement of an AHU for a project is based upon several factors, some of which are: application, performance, maintenance requirements, related size and building location, overall cost to purchase and install, and energy efficiency.

As HVAC designers, providing a system that can meet the building comfort requirements at a reasonable cost while minimizing maintenance costs and energy use, is the primary goal. Indoor air quality (IAQ), energy use, and occupant thermal comfort are a few of the concerns faced by building operations and maintenance (O&M) staff.

There are numerous articles and reviews regarding HVAC design. Many of these articles and other technical information can be found through ASHRAE, a global society for heating, refrigeration, and air conditioning engineers. Some information can be found in the ASHRAE Handbook ' HVAC Systems and Equipment, Chapter 4, Air Handling and Distribution. ASHRAE states 'The basic all-air system concept is to supply air to the room at conditions such that the sensible and latent heat gains in the space, when absorbed by supply air flowing through the space, will bring the air to the desired room conditions.'

The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is a trade association that represents global manufacturers of air conditioning, heating, commercial refrigeration, and water heating equipment. It works closely with ASHRAE in the development of standards for the HVAC industry.

For example, ANSI/AHRI Standard 430- defines a central station AHU (CSAHU) as 'a factory-made encased assembly consisting of a supply fan, or fans in parallel, which may also include other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air.'

In addition, AHU appurtenances are equipment or components that may be added to an AHU for purposes, including but not limited to, control, isolation, safety, static pressure regain, and wear. Common appurtenances include, but are not limited to, coils, filters, energy recovery devices, dampers, air-mixers, spray assemblies, eliminators, discharge plenums, and inlet plenums. Figure 3 is an example of a preliminary AHU elevation selection that illustrates the various physical internal components for a dual duct AHU with multiple plenum fans.

Common Types of Air Handlers

By definition, AHUs are used to condition and/or circulate air as part of an HVAC system. The control and operation of AHUs is beyond the scope of this article, but these items become increasingly more complicated with the HVAC application and the number of components within the AHU. AHUs can be small, used in local environmental spaces, and include minimal components such as: a fan or blower, a single heat transfer coil, and filter(s).

These small, terminal-type AHUs are referred to as fan coil units (FCUs) or blower coil units (BCUs), based on size and capacity. They generally have simple controls and serve a single zone within a building such as a loading dock or maybe a stairwell in a larger building. Although not universal, typical load capabilities for FCUs are 200 to cfm while BCUs might range from 400 to 3,000 cfm.

Slightly larger AHUs selected for outdoor use, located on grade outside or on the roof, are usually referred to as packaged units or rooftop units (RTUs), respectively. In addition to the components noted above, these units will typically have control dampers and serve larger areas or multiple zones within a building. The HVAC load capabilities of these units generally range from a few thousand cfm to tens of thousands. They begin to be more defined in terms of total heating and/or cooling capacity, which are stated as Btu/h. Cooling capacity may also be stated in tons (where 1 ton equals 12,000 Btu/h), with typical applications ranging from 3 to 10 tons or more.

The next level of AHUs could be considered semi-custom, highly flexible, but cataloged, AHUs that can be selected to meet most any commercial, institutional, or even industrial applications. Most manufacturers have a line of these type of AHUs, which can be modified to meet a designer's specific job requirements for new or existing building projects. These units typically use a building-block or modular construction methodology to allow for a wide range of standard and custom engineered modules.

The modules, sometimes called splits, are engineered in a manner that allows for ease of shipping and then assembled in the field. These modules can be stacked or arranged in a variety of configurations to address a project's constraints (i.e. access and space requirements). Available standardized components'including a wide assortment of fans, coils, filters and controls packages'allow for optimal AHU performance. Many optional components can also be added and may include: air-to-air, fixed-plate heat exchangers, sensible and total energy wheels, and face-and-bypass dampers. The energy related components follow rating and testing requirements by AHRI 106 ' Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation Equipment, and ASHRAE Standard 84- ' Method of Testing Air-to-Air Heat/Energy Exchangers (ANSI Approved).

Finally, designers may have a need to specify a custom AHU. A custom AHU may be used if there are special application or capacity requirements for the project beyond standard manufactured equipment, there are physical size constraints, or abnormal inlet and outlet connections. Custom AHUs are often used in applications such as laboratories, large industrial and manufacturing facilities, or in renovations where a semi-custom unit cannot fit. These units are engineered and designed such that their size, material type, and thickness of construction, insulation, and internal components can be altered to meet specified performance. These units, however, can be very expensive.

For example, in a semiconductor manufacturing plant, custom AHUs might be sized at 200,000 cfm with a 150 hp motor on the fan, and have a preheat coil, a primary and secondary chilled water coil, a glycol coil, a reheat coil, and a humidifier section to pre-condition and treat outside airstreams.

An HVAC system's AHU may connect to a corresponding ductwork system that distributes the conditioned discharge or supply air to a section or HVAC zone of the building. Typically, the HVAC ductwork system returns some, if not all, of the return air (RA) back to the AHU; however, AHUs can simply supply and return air directly to and from the space they serve with very little or no ductwork. Fan and blower coils above are typical examples.

By most definitions, an HVAC zone has very similar occupancy and similar HVAC thermal characteristics but does not necessarily have a defined area or size. An exterior row of three or four offices in a commercial office building, for example, may be considered a small HVAC zone because the occupant density and usage patterns are similar. Likewise, an interior area comprised of multiple work spaces or cubicles within the building also could be considered a larger HVAC zone. Physical size does not matter, as a room or area is only partitioned from others and may not need to be controlled separately. In fact, ASHRAE Standard 90.1- defines an HVAC zone as 'a space or group of spaces within a building with heating and cooling requirements that are sufficiently similar so that desired conditions (e.g., temperature) can be maintained throughout using a single sensor (e.g., thermostat or temperature sensor).'

An AHU is just a metal box of various sizes, dependent upon the necessary internal components (e.g. appurtenances of various sorts). It typically is constructed around a framing system with insulated roofing, flooring, and side panels, sometimes built in modules, sections, or splits as required for the overall configuration of the components. Depending upon the manufacturer and size of the AHU, the framing system can be constructed from metal c-channels or square steel framing, with internal wall posts and sectionalized steel base rails under the unit. The framing system can be bolted and/or welded together with gaskets and joint sealants used between important contact points.

The floor is typically insulated and covered with a thick metal plate, sometimes in a diamond pattern to assist in providing a walkable surface. The sides or wall panels can be single or double skin insulated metal panels. Fiberglass insulation can be laid into the panel voids prior to closure, or with recent construction methods, sprayed in as a foam product that then dries and adheres to the metal. The roof can be like the sides of the AHU unless it's in an outdoor application where additional weatherproofing and joint sealing may be required. Much of the metal for the AHUs is galvanized, or of aluminum or similar construction as necessary, for long-term protection and strength and typically can be painted as required. Photo 1 indicates a variety of AHUs of different size arranged on the rooftop of a building.

Photo 2: A multi-zone (2 zones) cooling-only air handling unit (AHU) that serves fan power boxes (FPB) variable air volume (VAV) boxes with hot water reheat coils. Courtesy: Stanley Consultants Inc.[/caption]

Dampers

Automatic control damper assemblies within the mixing plenum should be corrosion resistant and, because mixing and controlling the amounts of OA and RA airstreams is sometimes critical, the selection, size, and orientation and location of the dampers is important. Both parallel- or opposed-blade dampers can be used for controlling the overall proportions of the airstreams, but pressure relationships and sizing for wide-open and/or modulating pressure drops needs to be considered.

Opposed-blade dampers typically have lower pressure drops when modulating. ASHRAE Guideline 16 ' Selecting Outdoor, Return, and Relief Dampers for Air-Side Economizer Systems can be referenced for input on these dampers as well as for relief dampers during economizer mode of operation. Other references a designer should review are AMCA 511 ' Certified Ratings Program Product Rating Manual for Air Control Devices and ANSI/AMCA STANDARD 500-D-, Laboratory Methods of Testing Dampers for Rating.

The OA damper can be one size and equal to the OA intake louver or ductwork for use in the 100% economizer operation; or, as a better option, can be split into a smaller minimum OA damper and another larger one to accommodate the additional OA needed for 100% intake. Because most designers oversize the dampers, the advantage is better control. Due to the installed flow characteristics of a damper, they are somewhat linear between 10% and 80%.

There is no need to fully open or fully close a damper (except in emergencies or when off-line), and splitting the OA damper will help limit the dampers to their respective portions and can be used for controlling the required airflows more efficiently. The RA damper is then selected on the difference between the total design supply air and the minimum OA flow rate, and considered the maximum RA flow rate. During economizer mode, this RA damper would be closed. The dampers should be specified to meet the leakage requirements of ASHRAE Standard 90.1 and of the International Energy Conservation Code by leaking less than 3 cfm/sq ft at 1 in. of static pressure, and shall be AMCA (Air Movement and Control Association International Inc.) licensed as a Class 1A damper.

To ensure the proper amount of ventilation or OA is provided to an AHU, many designers today add airflow measuring stations either in the OA ductwork (see Photo 2) or at the entrance of the mixing plenum with a combination damper and air flow measuring device. Again, AMCA is involved with ratings standards: AMCA 610 ' Laboratory Methods of Testing Airflow Measurement Stations for Performance Rating, and AMCA 611 ' Product Rating Manual for Airflow Measurement Stations.

Filters

Filters are typically placed in the section of an AHU ahead of other components, such as fans, coils, etc. as their primary function is to filter out dirt and other contaminants and protect the AHU's other components. Based upon the application, filters may be arranged in one or more layers, or sets. The filters are placed within a filter holding frame assembly or racking system that could be configured as either flat, or of an angular bank arrangement. The filter frame is typically constructed from a heavy gauge galvanized steel with vertical stiffeners and appropriate frame-to-frame sealant and/or gaskets to provide a rigid leak tight assembly. The filter frame is generally built to accommodate standard size filters (e.g. 24×24-in. or 12×24-in.) with an appropriate type fastener to meet or exceed the face area specified by the AHU schedule.

In applications with more than one set of filters, a preliminary or rough grade filter would be provided first in the direction of airflow. Intermediate and/or final filters, where provided, will be of varying grades of filtration or efficiency to assist in removing smaller and smaller contaminants. The function of each set of filters is to help extend the life and improve the efficiency of the next set of filters. The first set of filters are typically the cheapest to replace and thus maintain, while succeeding sets are more expensive to replace.

Filters can be a variety of types and sizes, from throwaway 2-in.-thick type through reusable 36 in. deep type, based upon the application for which they are needed in the AHU. Filters are rated by ASHRAE Standard 52.2 ' test methods and classified by minimum efficiency reporting value (MERV). Filter MERV ratings range from 1 to 20. Nearly all AHUs will have a MERV 7 or 8 filter assembly that have dust spot efficiencies of 25% to 30% or 30% to 35%, respectively. In addition to filter assemblies of standard AHU applications, a hospital inpatient care application would also use a MERV 15 with greater than 95% efficiency while a cleanroom would use MERV 20 at greater than or equal to 99.999% on 0.10 to 0.20 micron particles. There are other types of filters such as chemical impregnated gas-phase as well as electrostatic and ultraviolet for air treatment requirements.

Filters should also be Underwriters Laboratories Inc. (UL) classified. Classification for HVAC air filters confirms that the filters will meet local and state requirements for most applications, and particularly in accordance with the standards of the NFPA. A UL 900 classified filter is 'an air filter which, when clean, will burn moderately when attacked by flame, or emit moderate amounts of smoke, or both' as tested within the standard. UL 900 covers both washable and throwaway filters, used for the removal of dust and other airborne particles from mechanically circulated air in equipment and systems.

This mixing plenum and/or filter section of the AHU should have service access door(s) as well as differential pressure devices across filter assemblies for indication when the filters are dirty. By monitoring the pressure drop through the filters, as related to the airflow through the AHU, the filter life can be assessed and decisions can be made regarding the appropriate time to change them. Dependent upon the existence of a BAS, this monitoring can also be done by using a visual display from a simple magnehelic differential pressure gauge, or by a pressure switch linked to an input (I/O) alarm point.

Refer to Figure 4, a typical AHU schematic which illustrates multiple options for component positions within an AHU. The OA portion of the schematic is at (A) as it connects with the RA portion of the schematic at (B). This section is the mixing plenum with the associated OA and RA dampers (OAD and RAD, respectively), and with the filters represented a little further to the right. The amount of OA required to enter the mixing box is dictated by ASHRAE Standard 62.1-: Ventilation for Acceptable Indoor Air Quality, while optimizing the energy use as dictated by ASHRAE Standard 90.1. Figure 4 shows an airflow measuring station (AMS) that is sometimes provided to ensure the amount of OA entering the AHU meets the ventilation requirements. Measuring the OA will allow a BAS to modulate the OA damper and, in turn, the RA damper as needed to maintain the appropriate amount of ventilation air at varying operating conditions.

An HVAC system's AHU may contain other components necessary to perform a combination of the four basic psychrometric processes of cooling, heating, dehumidification, and humidification. Psychrometrics is the study of the thermodynamics of air and its moisture content (or air/vapor mixtures), and is used to analyze conditions and processes which require control of moisture content and temperature. For any good project design, HVAC designers must have a working knowledge of psychrometrics.

The result of performing a variety of these HVAC processes is the delivery of properly conditioned supply air to the spaces it serves. For heating, these components could be steam or heating-hot water coils, direct or indirect gas fired heat exchangers, or even electric strip heat coils. For cooling, the components could be chilled water or direct expansion (DX) cooling coils, or direct and indirect evaporative cooling devices. There also may be several other components used such as energy recovery devices that could be employed to assist in the processes.

Referring again to Figure 4, there is both a heating and cooling coil in the center of the horizontal section. Dependent upon the application, if this heating coil were located to the OA inlet ductwork near (A) it could be referred to as a pre-heat coil; or, if moved downstream of the cooling coil closer to (C) referred to as a reheat coil.

Fans

Fans are very important components of an AHU as well. ASHRAE and AMCA are also involved in testing and rating fans through both ANSI/AHSRAE Standard 51 and ANSI/AMCA 210 ' Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating; and, AMCA 300 ' Reverberant Room Method for Sound Testing of Fans. All AHUs will have a supply air fan (SAF, or SF for short). Dependent upon the size and application, this supply fan component can be composed of a single or double inlet centrifugal fan, one or more plenum or plug fans, or a vane or mixed-flow, axial fan, etc. The AHU can also have a RA fan (RAF or RF) in some instances where needed. The ASHRAE Handbook ' HVAC Systems and Equipment, Chapter 21, Fans, provides information on fans and helps in understanding selection criteria.

The HVAC system's AHU fans (SF, RF, exhaust fan or EF) considered should be based upon the desired design operation point for the airflow required and at the static pressure of the system. Fan efficiencies, sound levels, and redundancies are also considerations in fan selections.

Referring again to Figure 4 at point (C), the schematic illustrates a draw-through system configuration with the supply fan where the airstream is drawn through both a heating and cooling coil, respectively. Many designers prefer this arrangement for a better air distribution over the coils. If the supply fan was positioned back prior to the coils and closer to the filters (shown dashed), this would be considered a blow-through system configuration. This figure also indicates how a return fan might be configured into the HVAC system. A return fan may be needed to assist in overcoming static pressures to get the RA back to the AHU; or, typically because of air-side economizer applications and/or building pressurization controls may require a return fan and/or an exhaust fan application.

Coils

Most coils found in AHUs are used to provide sensible heating or sensible and latent cooling, and/or in conjunction with humidification and dehumidification. Coils are primarily constructed of tubes, typically of copper, with copper or aluminum fins pressed or extruded on the external surface of the tubes for several heat transfer processes. The tubes may be staggered or installed in line with respect to the airflow, and can come in various styles to enhance performance. They are typically interconnected by return tube bends to form several different serpentine arrangements which create multi-pass circuiting options for the tube circuits. Chilled water and refrigerants for cooling, and hot water, steam and even refrigerants (e.g, hot gas reheat, variable refrigerant flow systems) for heating, are typically used for various psychrometric applications, respectively. The ASHRAE Handbook ' HVAC Systems and Equipment, Chapters 23 and 27 provide more information on coils.

Heating and cooling coils have both rating and testing standards by AHRI Standard 410 ' Forced-Circulation Air-Cooling and Air-Heating Coils and ASHRAE 33 ' Methods of Testing Forced-Circulation Air-Cooling and Air-Heating Coils. Coils are typically selected using coil programs, with the performance dictated by the system designer and provided by a manufacturer's representative that is providing the various options included in the AHU.

There are several basic things to know about coils. For cooling coils, they are built to be piped in a counter-flow arrangement; or, the air flows in the opposite direction as the chilled water (CHW), and/or the refrigerant in DX coils. For a 4-row coil, the air would pass through rows 1-4 at the same time the CHW or refrigerant is flowing through rows 4-1. The coil programs default to this configuration. A designer can select a coil that is not counter-flow, but the coil's performance will be reduced, based on its size, by anywhere from 8% to 12%. In particular, the dehumidification capabilities for the coil are significantly reduced.

Secondly, all water coils should be fed from a bottom connection so once the header piping is full, and the air is bled out of the system, every tube in the coil will be fed evenly with water. Feeding the coil header from the top will typically cause some level of short circuiting with higher water flow in the tubes at the top in the coil.

In heating coils, circuiting the coil is not as critical except to ensure the piping connections are on the side you want. However, steam coils need to be piped at the top connection so that all the condensate will be able to leave from the bottom connection of the coil, which should be below the lowest tube. In addition, steam coils must be pitched to the return end of the coil. If condensate blocks part of a steam coil, one part of the coil will be warm and another will be cool. In colder climates, another problem when condensate builds up in the coil and steam is unable to move through the top portion of the tubes, the coil could freeze and break.

Designers should pay close attention to the location of the fan in relation to the AHU coils. Fan heat is always added to the airstream. If the fan is located after the air has crossed the cooling coil, this fan heat must be considered when calculating the desired leaving supply air temperature. Because any AHU fan will convert its input energy to move the air through the unit and into an HVAC system, the air temperature will rise slightly. Designers need to understand that this is an additional load for the building load calculations (cooling as well as heating) for which it must be accounted.

The ASHRAE Handbook-Fundamentals provides a general estimates of fan heat as approximately 0.5°F per inch of total fan pressure. However, using the basic formulas for fan horsepower (hp), and the psychrometric sensible heat (Qs) equation, it is recommended that the designer calculate the temperature rise across the fan to be more accurate.

As an example, assume an AHU supplies 100,000 cfm using a 125 hp SF, the fan heat is calculated as:

125 hp = (0.746 kW/hp) x 3,413 Btu/h/kW = 318,262 Btu/h.

Using the sensible heat equation below, the airstream temperature rise can be determined as:

Qs (Btu/h) = 1.085 x cfm x (T1 ' T2)

(T1 ' T2) = 318,262 Btu/h / (1.085 x 100,000)

(T1 ' T2) = 2.9°F for a temperature rise

Note that with a draw-through fan arrangement, this example shows that the fan leaving air temperature will be higher than the cooling coil leaving air temperature by almost 3°F.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.

For more information, please visit glass swing doors.