How Air Handling Units work AHU working principle hvac ventilation

Hey there guys, Paul here from In this video, we're
going to be discussing air handling units. We'll be looking at some typical examples to understand how they work
and where to find them. Just before we jump in, I
want to say a quick thanks to Danfoss for sponsoring this video. Danfoss is committed to
spreading engineering knowledge around the world by both partnering with channels like this one, and also by offering free online classes through their online
portal Danfoss Learning. They have literally hundreds
of lessons available on a wide variety of topics including relevant classes for air handling units. You can also take exams
and earn certifications to give your career and
your confidence a boost. Just click the link in the
video description below to create your free
Danfoss Learning profile and gain access to a world of knowledge. Pop engineering quiz. What is the difference
between an FAHU and an AHU? Let me know your answers in
the comment section below. If you don't know, then
give it your best shot. No cheating, just test yourself and I'll give you the answer
at the very end of this video.

So where do we find air handling units? Air handling units, which
usually have the acronym of AHU, are found in medium to large commercial and industrial buildings. They are usually located in the basement, on the roof, or on the
floors of the building. And many large buildings will likely have a mixture of all of these. AHUs will serve a specified
area or zone within a building such as the E side or office areas from floors one to 10, or
perhaps a single purpose such as just the building's toilets, therefore it's very common to find multiple AHUs around a building. Some buildings, particularly
old high-rise buildings, will have just one large AHU which is usually located on the roof. These will supply the entire building. They might not have a return duct. Some older designs rely on the air just simply leaking out of the building. But this design is not so common anymore in new buildings because
it's very inefficient. Now it's most common to
have multiple smaller AHUs supplying different zones
to give better control and higher quality space conditioning. Buildings are now also much more airtight, so we need to have a return duct to regulate the pressure
inside the building.

So what is the purpose
of an air handling unit? Air handling units condition and distribute air within a building. They take fresh ambient air from outside, and then clean this, heat it or cool it, maybe humidify or dehumidify it, and then they'll force
it through some ductwork around to the designated
areas within a building. Most units will have
an additional duct run to then pull this dirty
used air out from the rooms, back to the AHU where
a fan will discharge it back into the atmosphere. Some of this return air
might be recirculated back into the fresh air
supply to save energy. We'll have a look at
that later in this video, otherwise, where that isn't possible, thermal energy can be extracted and fed into the fresh air supply
intake to also save energy.

Again, we'll look at
that in much more detail later in this video. Let's have a look at a
simple, typical AHU design, and then we'll look at
some more advanced ones. In this very basic model
we have two AHU housings for flow and return air. At the very front on the inlet and the outlet of each housing we have a grill to prevent
objects and wildlife from entering into the mechanical
components inside the AHU. Here's a photo of an AHU
intake that would've sucked in a whole bunch of trash if
the grill wasn't there, so that's why it's important
to have this installed. At the inlet of a fresh air housing, and the discharge of
the return air housing, we have some dampers.

The dampers are multiple sheets
of metal which can rotate. They can close to prevent air from entering or exiting the AHU. They can open fully to
fully allow air in or out. And they can also vary their
position somewhere in between to restrict the amount of
air which can enter or exit. I'll also show you some examples here of real world dampers in AHUs. The one on the left has the
motorized controller visible which changes a position of the dampers. After the dampers,
we'll have some filters. These are there to try and
catch all the dirt and the dust et cetera from entering the
AHU, and also the building. If we don't have these filters, the dust is going to
buildup inside the ductwork and within the mechanical equipment. It's also going to enter the building and be breathed in by the occupants, as well as make the building dirty. So we want to remove as
much of this as possible.

Across each of the filter banks, we'll have a pressure sensor. This will measure how
dirty the filters are and warn the engineers when it's time to replace the filters. As the filters pick up
dirt, the amount of air that can flow through them is restricted, and this causes a pressure drop. Typically, we'll have some panel filters, or pre-filters to catch
largest dust particles. Then we'll have some bag filters to catch the smaller dust particles. We've actually covered AHU filters in great detail previously. Links in the video description below, do check that video out. The next thing we'll find are
the cooling and heating coils. These are there to cool or heat the air.

The air temperature of the supply air is measured as it leaves the AHU. This needs to be at a designed temperature to keep the people inside
the building comfortable. This designed temperature
is called the set point. If the air temperature
is below this value, the heating coil will add heat to increase the air temperature and
bring it up to set point. If the air is too hot,
then the cooling coil will remove heat to
lower the air temperature and also reach the set point. The coils are heat exchangers. Inside the coil is a hot or cold fluid, usually something like a
heated or chilled water, refrigerant or perhaps steam. And we've discussed these
in great detail previously in other videos, do
check those videos out. Links are in the video description below. Next we'll have a fan. This is going to pull air in from outside and then through the dampers,
the filters, the coils, and then push this out
through the ductwork and around the building.

Centrifugal fans are very
common in old and existing AHUs, but EC fans are now being installed and also retrofitted for
increased energy efficiency. Across the fan, we'll also
have a pressure sensor. This will sense if the fan is running. If it is running, then it will
create a pressure difference, and we can use this to detect
a failure in the equipment and warn the engineers of a problem. We'll also likely have
a duct pressure sensor shortly after the fan. This will read the static
pressure and in some AHUs, the speed of the fan is controlled as a result of the pressure in the duct. This will also very often
find a variable speed drive connected to the fan for
variable volume systems. We've covered VAV systems separately, again, links down below for that. Then we have the ductwork
which sends the air around the building to
the designated areas.

We'll also have some ductwork coming back, which is bringing all the
used air from the building back to a separate part of the AHU. This return AHU is usually
located near the supply, but it doesn't have to be. It can be located
elsewhere in the building. The return AHU in its simplest form has just a fan and a damper inside. The fan is pulling air in
from around the building, and then pushing it all
the way out of the building into the atmosphere. The damper is located at
the exit of the AHU housing and will close when the AHU turns off. That's a very simple and typical AHU, so what else might we find? If you're in a cold part of the world where air temperatures reach
freezing point or close to it, then we'll find a pre-heater in the inlet of the fresh air intake. This is usually an electrical heater.

When the outside air gets
around six degrees Celsius, or 42.8 degrees Fahrenheit,
the heater will turn on and heat up the air to protect the components inside from frost. Otherwise this could freeze the heating and cooling coils inside and burst them. What about humidity control? Some buildings need to
control the humidity of the air they supply into the building. We'll find a humidity sensor
at the outlet of the supply AHU to measure the moisture in the air supply. This will also have a set
point for how much moisture should be in the air by design.

pexels photo 3964704

If the air's moisture
content is below this value, then we need to introduce moisture into the air using a humidifier. This is usually one of the
last things in the AHU. This device will usually either add steam or a spray of water mist into the air. Many standard office-type
buildings in Northern Europe and Northern America have
turned off their humidity units or uninstall them to save energy. Although they are still crucial for places like document stores and computer rooms.

If the air is too humid,
then this can be reduced through the cooling coil. As the air hits the cooling coil, the cold surface will cause
the moisture within the air to condense and flow away. You'll find a drain pan
under the cooling coil to catch the water and drain this away. The cooling coil can be
used to further reduce the moisture content
by removing more heat, but of course this will
decrease the air temperature below the supply set point. If this occurs, then the
heating coil can be turned on to bring the temperature back up. This will work, although it
is very energy intensive. Energy recovery. If the supply and extract AHUs are located in different areas, then a common way to recover
some of the thermal energy is to use a run around coil.

This uses a coil in both AHUs, and a pump circulates
water between the two. This will pick up waste
heat from the extract AHU and add this to the supply AHU. This will reduce the heating
demand on the heating coil when the outside air temperature is below the supply set point temperature and the return air temperature
is higher than the set point. The heat would otherwise be wasted as it is simply rejected to atmosphere. As the pump will consume electricity, it is only cost effective to turn on if the energy saved is more
than the pump will consume. Another very common
version we'll come across is to have a duct sit between the exhaust and the fresh air intake. This allows some of the
exhaust air to be recirculated back into the fresh air intake to offset the heating and cooling demand.

An additional damper sits
within the connecting duct to control how much air
can be recirculated. This is safe and healthy to do so, but you will need to ensure that the exhaust air has a low CO2 count, so we need some CO2
sensors to monitor that. If the CO2 level is too high,
then the air can't be reused. The mixing damper will close and all the return air will
be rejected from the building. When in recirculation mode, the main inlet and outlet
dampers will not fully close in this setup because we will
still need a minimum amount of fresh air to enter the building. We can use this in the
winter if the return air is warmer than the outside air. And we can also use this in the summer if the return air is cooler
than the outside air, respective to the supply
set point temperature.

We'll also need some temperature sensors at the intake return and
just after the mixing region. Some buildings require 100% fresh air, so this strategy can't be used everywhere. The local laws and
regulations will dictate this. Another variation we might
come across is the heat wheel. This is very common in newer compact AHUs. This uses a large rotating wheel. Half of it sits within
the exhaust air stream, and half of it sits within
the fresh air intake. The wheel will rotate, driven
by a small induction motor. As it rotates, it picks up unwanted heat from the exhaust stream and absorbs this into
the wheel's material. The wheel then rotates into
the fresh air intake stream.

This air is at a lower temperature
than the exhaust stream, so the heat will transfer from the wheel and into the fresh air stream which obviously heats the
incoming air stream up, and thus reduces the
demand on the heating coil. This is very effective,
but some air will leak from the exhaust into
the fresh air stream, so this cannot be used in all buildings. Another version we might come across is the air plate heat exchanger. This uses thin sheets of metal to separate the two streams of air so that they do not come into direct contact or mix at all. The temperature difference
between the two air streams will cause the heat to transfer over from the hot exhaust stream,
through the metal walls of the heat exchanger, and
into the cold intake stream. The two air streams need to
crossover for this to occur. So it can be a little
confusing to look at. Just remember the air
streams are not mixing. Just before we wrap up,
I just want to remind you to sign up for your free
Danfoss Learning profile.

Doing so gets you access to
hundreds of engineering-focused e-lessons including several
about heat exchangers. It also enables you to
earn certifications. So what are you waiting for? Go give it a try now. Links are in the video description below. The answer to the question
I asked at the beginning of the video for what is the difference between an AHU and an FAHU, is simply that FAHU stands for Fresh Air Handling Unit, meaning it is an air
handling unit or an AHU, except it can only handle
100% fresh outside air. It does not recirculate any return air back into the supply stream. An AHU on the other hand, can recirculate some of its return air into the supply stream.

The building application
and local regulations will dictate when and if this strategy can be used in a building. Okay guys, that's it for this video. Thank you very much for watching. I hope you've enjoyed this
and it has helped you. If so, please don't forget
to like, subscribe and share. And also, leave your questions
in the comment section below. Don't forget to follow us on
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Once again, thanks for watching..

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