How do Air Handling Units work?
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| Air-handling-units-ws-696x385 jcool |
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So where do you find air handling units?
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| Location-of-AHUs-in-buildings jcool |
Air handlers, commonly known by the acronym A.H.U, are found in medium to large commercial and industrial buildings.
They are usually found in the basement, on the roof, or on the floors of a building. AHUs will serve a specific area or area in a building, such as the east side, or floors 1-10, or perhaps a single purpose, such as the building's bathroom. Therefore, it is very common to find multiple AHUs around a building.
Some buildings, especially older high-rise buildings, will only have one large CTA, usually located on the roof. These will power the entire building. They may not have a return duct, some older designs rely on exhaust air from the building. This layout is no longer as common in new buildings as it is very inefficient, it is now more common to have several smaller AHUs feeding different areas. Buildings are also more airtight, so we need a return line to regulate the pressure inside the building.
So what is an air handling unit used for?
Air handling units condition and distribute air in a building. They take fresh air from outside, clean it, heat or cool it, perhaps humidify it, and then force it through ductwork around designed areas in a building. Most units will have an additional duct to then extract used dirty air from the rooms, to the AHU, where a fan will return it to the atmosphere. Some of this return air can be recycled back into the fresh air supply to save energy – more on this later in the article. Alternatively, where this is not possible, the thermal energy can be extracted and introduced into the fresh air intake. Again, we'll look at this in more detail later.
Let's take a look at the simple and typical layouts and then look at the more advanced layouts.
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| AHU Intake Grille Air Handler |
In this very basic model we have the two AHU boxes for supply and return air. On the same front, entrance and exit of each enclosure, we have a grid to prevent the entry of objects and fauna into the mechanical components inside the AHU.
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| AHU Clogged Intake Grille |
You can see from this picture that the AHU entrance would have sucked in a lot of trash if the grate wasn't there, so that's why it's important.
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| AHU Records - Operation of air handling units |
We have dampers at the entrance of the outside air box and at the exit of the return air box. Shock absorbers are multiple sheets of metal that can rotate. They can be closed to keep air in or out, they can be opened to let air in or out completely, and they can also vary their position somewhere in between to limit the amount of air that can get in or out.
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| CTA filters |
After the dampers, we will have filters. These are there to try to catch all the dirt and dust etc. that penetrate the ahu and the building. If we do not have these filters, the dust will accumulate inside the ducts and in the mechanical equipment, it will also enter the building and be inhaled by the occupants and will dirty the building. So we want to remove as many as possible. Through each bank of filters we will have a pressure sensor. This will measure how dirty the filters are and notify engineers when it is time to replace the filters. As the filters collect dirt, the amount of air that can flow is limited, causing a pressure drop across the filters. We usually have panel filters or pre-filters to capture the larger dust particles. Then we have bag filters to trap the smallest dust particles. We have already covered ahu filters in detail. You can watch a video tutorial on this by clicking here.
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| AHU heating and cooling coil |
The next thing we will find are the cooling and heating coils. These are there to heat or cool the air. The supply air temperature is measured as it leaves the CTA and enters the ductwork. This must be at a temperature designed to guarantee the comfort of the people inside the building; this designed temperature is called the set temperature. If the air temperature is below this value, the heating coil will add heat to raise the air temperature to the set point. If the air is too hot, the cooling coil removes the heat to bring the air temperature down to the set point. Coils are heat exchangers, inside the coil is a hot or cold fluid, usually something like hot or cold water, coolant or steam. We have discussed this in detail before, you can watch a video tutorial on this by clicking here.
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| AHU power supply fan how it works |
Then we will have a fan. This will draw air in from outside, then through registers, filters, and coils, and then push it into ductwork around the building. Centrifugal fans are very common in old and existing air handling units, but EC fans are now being installed and retrofitted to increase energy efficiency. Through the fan we will also have a pressure sensor, this will detect if the fan is working. If it's working it will create a pressure difference, we can use this to troubleshoot equipment and notify engineers of the problem. We will probably also have a duct pressure sensor shortly after the fan, this will read the static pressure and in some ahu the fan speed is controlled according to the pressure in the duct so very often we will also find a variable speed drive. connected to the fan for variable volume systems. We have covered VAV systems separately, you can watch a video tutorial by clicking here.
Next we have the ducts that will send air around the building to the designed areas. We will also have ducts that will carry all the waste air from the building to a separate part of the AHU. This return AHU is usually located close to the supply, but is not required, it can be located elsewhere. If you would like to learn how to size and design ductwork, you can watch a video tutorial by clicking here.
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| AHU damper and return fan |
The rear AHU in its simplest form just has a fan and damper inside. The fan draws air around the building and then pushes it out of the building. The gate is located at the exit of the AHU box and will close when the AHU is turned off.
It is a very simple and typical UTA. So what else could we find?
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| Frosted Coil Air Handler |
If you are in a cold part of the world where the air temperature is at or near freezing. Then we will probably find a preheater at the entrance of the fresh air intake. This is usually an electric heater. When the outside air reaches approximately 6*C (42.8 F), the heater will turn on and heat the air to protect the components inside from freezing. Doing so could freeze the heating and cooling coils inside and cause them to burst.
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| Humidity control Air handling unit |
What about humidity control? Some buildings need to control the humidity of the air they supply to the building. We will find a humidity sensor at the outlet of the supply AHU to measure the humidity in the supply air, this will also have a set point of how much humidity there should be in the air by design.
If the moisture content of the air is less than this value, we must introduce moisture into the air through a humidifier, this is usually one of the last things in the CTA. This device will usually add steam or spray a mist of water into the air. Many standard office buildings in Northern Europe and North America have turned off their humidity units or have them uninstalled to save energy. Although they are still crucial for places like document stores and computer rooms.
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| Dehumidifying with a Cooling Coil: How Air Handling Units Work |
If the air is too humid, this can be reduced with the cooling coil. When air hits the cooling coil, the cold surface causes moisture in the air to condense and escape. You will find a drain pan under the cooling coil to catch the water and drain it. The cooling coil can be used to further reduce the moisture content by removing more heat, but of course this will reduce the air temperature below the supply set point; if this happens, the cooling coil heater can also be turned on to lower the temperature. it will work even if it consumes a lot of power.
Energy Recovery
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| Circular coil - Air handling unit |
If the supply and exhaust air handling units are located in different areas, a common way to recover some of the heat energy is to use a circular coil. This uses a coil on the supply and return AHUs that are connected through pipes. A pump circulates the water between the two. This will capture waste heat from the exhaust AHU and add it to the supply AHU. This will reduce the heating demand from the heating coil when the outside air temperature is lower than the supply temperature set point and the return air temperature is higher than the set point; otherwise, the heat would be rejected back into the atmosphere. Therefore, we will need an air temperature probe in the return air handler at the intake and will likely have air temperature probes after the return coil and before the fresh air intake. These will be used to control the pump as well as measure efficiency. As the pump will consume electricity, it is only profitable to run it if the energy saved is greater than the consumption of the pump.
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| AHU air saver air saver |
Another very common version that we will find is to place a duct between the exhaust and the fresh air intake. This allows some of the exhaust air to recirculate to the fresh air intake, to meet the demand for heating or cooling. It's safe and healthy to do so, but you'll need to make sure the exhaust air is low in CO2, so we need CO2 sensors to monitor that. If the Co2 level is too high, the air cannot be reused, the mixing damper will close and all return air will be exhausted from the building. In recirculation mode, the main intake and exhaust dampers will not close completely in this configuration because we still need a minimal amount of fresh air to enter the building. We can use it in winter if the return air is warmer than outside air and we can use it in summer if the return air is cooler than outside air, depending on the supply air set point temperature, so We will also need temperature sensors at the entrance, return and just after the mixing area. Some buildings require 100% fresh air, so this strategy cannot be used everywhere, local laws and regulations will dictate.
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| AHU heat wheel, heat wheel |
Another variant that we can find is the thermal wheel. This is very common on new compact AHUs. This uses a large rotating wheel, half of which is on the exhaust airflow and half on the fresh air intake. The wheel will spin, powered by a small induction motor, as it spins it picks up unwanted heat from the exhaust stream and absorbs it into the material of the wheels. The wheel then rotates into the fresh air intake stream, this air is at a lower temperature than the exhaust stream, so heat will be transferred from the wheel to the fresh air stream which obviously heats this air stream. incoming air and thus reduces the demand on the heating coil. This is very efficient, but some of the air will leak from the exhaust into the fresh air stream, so it cannot be used in all buildings.
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| AHU Plate Heat Exchanger - Operation of Air Handling Units |
Another version that we can find is the air plate heat exchanger. This uses thin sheets of metal to separate the two air streams so they do not come into contact or mix at all, the temperature difference between the two air streams will cause heat to be transferred from the hot exhaust streams through the metal walls of the heat exchanger and into the cold intake stream.
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