How do air handling units work?
 |
| Air treatment units |
Air treatment units. In this article, we will
learn how Air Handling Units or AHUs work. We'll look at several typical AHU
examples along with animations for components like dampers, heating and cooling
coils, heat wheels, humidifiers, coils, heat exchangers, and more to help you
learn HVAC engineering.
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So where do you find air handling units?
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| Location of CTA in buildings |
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
bathroom of the building. 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 design is no longer as common in new buildings as
it is very inefficient, it is now more common to have several smaller AHU's
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 the 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 pull the used dirty
air out of the rooms, to the AHU, where a fan will blow it back into 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, 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 housing for flow
and return air. At the very front on the inlet and outlet of each housing we have
a grille to prevent objects and wild life entering into the mechanical
components inside the AHU.
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| Blocked intake grille AHU |
You can see in this picture that the AHU entrance would have
sucked in a lot of trash if the grate wasn't there, so it's important.
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| AHU Records - Operation of Air Handling Units |
At the inlet of the outside air box and at the outlet of the
return air box we have dampers. Shock absorbers are multiple sheets of metal
that can rotate. They can close to keep air in or out, they can open 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 shock absorbers, 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 don't have these filters, dust will accumulate inside the ducts
and mechanical equipment, it will also enter the building and be inhaled by the
occupants and make the building dirty. 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 catch the smallest dust particles. We have already covered ahu filters
in detail. You can see a video tutorial on this by clicking here.  |
| AHU heating and cooling coil |
Then we will have a fan. This will draw air from outside, then
through registers, filters, and coils, and then push it into ductwork around
the building. Centrifugal fans are very common in older 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 is working it will create a pressure
difference, we can use this to detect equipment failure 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 on 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 view a video tutorial. clicking here.  |
| AHU power supply fan how it works auta fan |
Then we will have a fan. This will draw air from outside, then
through registers, filters, and coils, and then push it into ductwork around
the building. Centrifugal fans are very common in older 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 is working it will create a pressure
difference, we can use this to detect equipment failure 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 on 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 view 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 ductwork that will take all of the waste
air from the building to a separate part of the AHU. This return AHU is usually
located near the supply but is not required, it can be located elsewhere. If
you want 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 only has a fan and damper
inside. The fan draws in air around the building and then pushes it out of the
building. The damper is located at the outlet of the AHU box and will close
when the AHU shuts down.
It is a very simple and typical UTA. So what else could we find?
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| Iced 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 interior components from freezing. Otherwise, it 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 supply AHU outlet to measure humidity in the supply air, this will also
have a set point of how much humidity should be in the air by design.
If the moisture content of the air is less than this value, we
need to introduce moisture into the air using 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 uninstalled them to
save energy. Although they are still crucial for places like document
warehouses and computer rooms.
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| Dehumidify 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
collect the water and drain it off. 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 may also come 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 treatment unit |
If the supply and extract air handling units are located in
different areas, a common way to recover some of the thermal 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 of the heating coil when the outside air temperature
is less than the supply temperature setpoint and the return air temperature is
greater than the setpoint; otherwise, the heat would be rejected back into the
atmosphere. So we will need an air temperature sensor on the return air handler
at the inlet and we will probably have air temperature sensors after the return
coil and before the fresh air inlet. These will be used to control the pump as
well as to measure efficiency. Since 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 inlet. This allows some of the exhaust air
to recirculate back into the fresh air intake, to offset heating or cooling
demand. It's safe and healthy to do, 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 the
return air will be exhausted from the building. In recirculation mode, the main
inlet and outlet 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 the winter if the return air is warmer than the outside air and
we can use it in the summer if the return air is cooler than the outside air,
depending on the supply air setpoint temperature, so We will also need
temperature sensors at the inlet, return and just after the mixing zone. 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 spinning wheel, half of
which is in the exhaust airflow and the other half in the fresh air intake. The
wheel will spin, driven 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. Then the wheel rotates into the fresh air inlet stream, this air is at
a lower temperature than the exhaust stream, so the heat will be transferred
from the wheel to the fresh air stream which obviously heats this stream of
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 can't be used in all buildings.
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| AHU Plate Heat Exchanger - Air Handling Units Operation |
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 don't come into contact or mix at all, the temperature difference between
the two air streams will cause heat to transfer from the hot exhaust streams
through the metal walls of the heat exchanger and into the cold intake stream.
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(JCOOL) products
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