How Air Handling Units work, AHU working rule , HVAC ventilation
schematic diagram of hvac system
Hey guys, Today we're getting to be discussing
air handling units. We'll be watching some typical examples to know how they
work and where to seek out them.
what's the difference between an FAHU and an
AHU? Let me know your answers within the comment section below. If you do not
know, then provides it your best shot. No cheating, just test yourself and I'll
offer you the solution at the top .
So where can 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're usually located within the
basement, on the roof or on the floors of the building. and lots of large
buildings will likely have a mix of all of those . AHUs will serve a specified
area or zone within a building like the E side or office areas from floors one
to 10, or perhaps one purpose like just the building's toilets, therefore it's
extremely common to seek out multiple AHUs around a building. Some buildings,
particularly old high-rise buildings, will have only one large AHU which is
typically located on the roof. These will supply the whole building.
they could not have a return duct. Some older
designs believe the air just simply leaking out of the building. But this
design isn't so common anymore in new buildings because it's extremely
inefficient. Now it's commonest to possess multiple smaller AHUs supplying
different zones to offer better control and better quality space conditioning.
Buildings are now also far more airtight, so we'd like to possess a return duct
to manage the pressure inside the building. So what's the aim of an air handling
unit?
Air handling unit’s condition and distribute
air within a building. They take fresh ambient air from outside then clean
this, heat it or calm , maybe humidify or dehumidify it, then they'll force it
through some ductwork around to the designated areas within a building. Most
units will have a further duct run to then pull this dirty used air out from
the rooms, back to the AHU where a lover will discharge it back to the
atmosphere. a number of this return air could be re-circulated back to the
fresh air supply to save lots of energy. We'll have a glance at that later,
otherwise, where that may not possible, thermal energy are often extracted and
fed into the fresh air supply in fancy also save energy. Again, we'll check out
that in far more detail later.
Let's have a glance at an easy , typical AHU
design, then we'll check out some more advanced ones. during this very basic
model we've two AHU housings for flow and return air. At the very front on the
inlet and therefore the outlet of every housing we've a grill to stop objects
and wild life from getting into the mechanical components inside the AHU.
Here's a photograph of an AHU in take that would've sucked during a whole bunch
of trash if the grill wasn't there, so that's why it is vital to possess this
installed. At the inlet of a fresh air housing, and therefore the discharge of
the return air housing, we've some dampers.
The dampers are multiple sheets of metal which
may rotate. they will on the brink of prevent air from entering or exiting the
AHU. they will open fully to completely allow air in or out. and that they also
can vary their position somewhere in between to limit the quantity of air which
may enter or exit. I'll also show you some examples here of world dampers in
AHUs. The one on the left has the motorized controller visible which changes an
edge of the dampers. After the dampers, we'll have some filters. These are
there to undertake and catch all the dirt and therefore the dust et cetera from
entering the AHU, and also the building.
If we do not have these filters, the dust goes
to create up inside the ductwork and within the mechanical equipment. it is
also getting to enter the building and be breathed in by the occupants, also as
make the building dirty. So we would like to get rid of the maximum amount of
this as possible. Across each of the filter banks, we'll have a pressure
sensor. this may measure how dirty the filters are and warn the engineers when
it is time to exchange the filters. because the filters devour dirt, the
quantity of air which will 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.
subsequent thing we'll find are the cooling
and heating coils. These are there to chill or heat the air. The air
temperature of the availability air is measured because it leaves the AHU. This
must be at a designed temperature to stay the people inside the building
comfortable. This designed temperature is named the point . If the air
temperature is below this value, the heating coil will add heat to extend the
air temperature and convey it up to line point. If the air is just too hot,
then the cooling coil will remove heat to lower the air temperature and also
reach the point . The coils are heat exchangers. Inside the coil may be a hot
or cold fluid, usually something sort of a heated or chilled water, refrigerant
or perhaps steam.
Next we'll
have a lover . this is often getting to pull air in from outside then through
the dampers, the filters, the coils, then push this out through the ductwork
and round the building. Centrifugal fans are quite 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 even have a pressure sensor. this may
sense if the fan is running. If it's running, then it'll create a pressure
difference, and that we can use this to detect a failure within the equipment
and warn the engineers of a drag . We'll also likely have a duct pressure
sensor shortly after the fan. this may read the static pressure and in some
AHUs, the speed of the fan is controlled as a results of the pressure within
the duct.
this may also fairly often find a variable
speed drive connected to the fan for variable volume systems. We've covered VAV
systems separately. Then we've the duct work which sends the air round the
building to the designated areas. We'll even have some duct work returning ,
which is bringing all the used air from the building back to a separate a part
of the AHU. This return AHU is typically located near the availability , but it
doesn't need to be. It are often located elsewhere within the building.
The return AHU in its simplest form has just a
lover and a damper inside. The fan is pulling air in from round the building,
then pushing it all the answer of the building into the atmosphere. The damper
is found at the exit of the AHU housing and can close when the AHU turns off.
That's a really simple and typical AHU, so what else might we find? If you're
during a cold a part of the planet where air temperatures reach melting point
or on the brink of it, then we'll find a pre-heater within the inlet of the fresh
air intake. this is often usually an electrical heater.
When the surface air gets around six degrees
Celsius, or 42.8 degrees Fahrenheit, the heater will activate and warmth up the
air to guard the components inside from frost. Otherwise this might freeze the
heating and cooling coils inside and burst them.
What about humidity control? Some buildings
got to control the humidity of the air they provide into the building. We'll
find a humidity sensor at the outlet of the availability AHU to live the moisture
within the air supply. this may even have a group point for a way much moisture
should be within the air intentionally . If the air's moisture content is below
this value, then we'd like to introduce moisture into the air employing a
humidifier. this is often usually one among the last things within the AHU.
This device will usually either add steam or a sprig 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 lots of
energy.
Although they're still crucial for places like
document stores and computer rooms. If the air is just too humid, then this
will be reduced through the cooling coil. because the air hits the cooling
coil, the cold surface will cause the moisture within the air to condense and
flow off . you will find a drain pan under the cooling coil to catch the water
and drain this away. The cooling coil are often wont to further reduce the
moisture content by removing more heat, but in fact this may decrease the air
temperature below the availability point . If this happens , then the heating
coil are often turned on to bring the temperature copy .
this may work, although it's very energy
intensive. Energy recovery. If the availability and extract AHUs are located in
several areas, then a standard thanks to recover a number of the thermal energy
is to use a frolic coil. This uses a coil in both AHUs, and a pump circulates
water between the 2 . this may devour waste heat from the extract AHU and add
this to the availability AHU. this may reduce the heating demand on the heating
coil when the surface air temperature is below the availability point
temperature and therefore the return air temperature is above the point . the
warmth would rather be wasted because it is just rejected to atmosphere.
because the pump will consume electricity, it's only cost effective to show on
if the energy saved is quite the pump will consume.
Another quite common version we'll encounter
is to possess a duct sit between the exhaust and therefore the fresh air
intake. this enables a number of the exhaust air to be re-circulated back to
the fresh air intake to offset the heating and cooling demand. a further damper
sits within the connecting duct to regulate what proportion air are often
re-circulated. this is often safe and healthy to try to to so, but you'll got
to make sure that the exhaust air features a low CO2 count, so we'd like some
CO2 sensors to watch that. If the CO2 level is just too high, then the air
cannot be reused. the blending damper will close and every one the return air
are going to be rejected from the building.
When in recirculation mode, the most inlet and
outlet dampers won't fully draw in this setup because we'll still need a
minimum amount of fresh air to enter the building. we will use this within the
winter if the return air is warmer than the surface air. and that we also can
use this within the summer if the return air is cooler than the surface air,
respective to the availability point temperature. We'll also need some
temperature sensors at the intake return and just after the blending region.
Some buildings require 100% fresh air, so this strategy cannot be used everywhere.
The local laws and regulations will dictate
this. Another variation we'd encounter is that the heat wheel. this is often
quite common in newer compact AHUs. This uses an outsized rotating wheel. half
it sits within the exhaust air stream, and half it sits within the fresh air
intake. The wheel will rotate, driven by alittle induction motor. because 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, therefore
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 often very effective, but some air
will leak from the exhaust into the fresh air stream, so this can't be utilized
in all buildings.
Another version we'd encounter is that the air
plate device . This uses thin sheets of metal to separate the 2 streams of air
in order that they are doing not inherit direct contact or mix in the least .
The temperature difference between the 2 air streams will cause the warmth to
transfer over from the recent exhaust stream, through the metal walls of the
warmth exchanger, and into the cold intake stream. the 2 air streams got to
crossover for this to occur. So it are often a touch confusing to seem at. Just
remember the airstreams aren't mixing.
what's the difference between an AHU and an
FAHU is just that FAHU stands for Fresh Air Handling Unit, meaning it's an Air
Handling Unit or an AHU, except it can only handle 100% fresh outside air. It
doesn't re-circulate any return air back to the availability stream. An AHU on
the opposite hand, can re-circulate a number of its return air into the
availability stream. The building application and native regulations will
dictate when and if this strategy are often utilized in a building.
Okay guys, that's it , many thanks
considerably . I hope you've enjoyed this and it's helped you. If so, please do
not forget to love , subscribe and share. And also, leave your questions within
the comment section below.
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