AHU types, components, and configurations
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 2016 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 430-2014 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.
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 1200 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 84-2013 — 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 90.1-2016 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.
AHU construction methods must be tight to avoid air leakage, minimize heat gains or losses from the air handler, and assist in reducing noise by providing sound attenuation. AHUs should have thermal breaks built into the structural components to alleviate any thermal pathways within the outer confines of the unit. An AHU that sweats and drips condensate on a mechanical room floor indicates poor construction.
One of the first AHU appurtenances to be considered from those mentioned above is the inlet air section for the AHU. Some AHUs have simple inlet only plenum sections (e.g. a 100% outside air unit), while others will have mixing plenums. A mixing plenum is where the different airstreams of outside air (OA) and RA mix prior to being conditioned for delivery of the supply air to the building. The inlet and/or mixing plenum will almost always contain other AHU components such as control dampers and filters, and may have other devices such as an air flow measuring stations. Photo 2 shows an AHU mixing plenum with the OA entering on the side and the RA entering on the top. In this picture an air flow measuring station is shown in the vertical section of the OA ductwork.
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 – 2014 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-2012, 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 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 – 2012 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 62.1-2013: 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 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 2016 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.
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 2016 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.