AHU - Air handling units, B.arch services

1. A.H.U
A.H.U

2. An air handler, or air handling unit (often abbreviated to AHU), is a device used to regulate and
circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is
usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers,
sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that
distributes the conditioned air through the building and returns it to the AHU. Sometimes AHUs
discharge (supply) and admit (return) air directly to and from the space served without ductwork.
Small air handlers, for local use, are called terminal units, and may only include an air filter, coil, and
blower; these simple terminal units are called blower coils or fan coil units. A larger air handler that
conditions 100% outside air, and no recirculated air, is known as a makeup air unit (MAU). An air
handler designed for outdoor use, typically on roofs, is known as a packaged unit (PU) or rooftop unit
(RTU).
AIR HANDLING UNIT

3. An air handling unit; air flow is from the right
to left in this case. Some AHU components
shown are
1 – Supply duct
2 – Fan compartment
3 – Vibration isolator ('flex joint’)
4 – Heating and/or cooling coil
5 – Filter compartment
6 – Mixed (recirculated + outside) air duct

5. Where to find A.H.U?
Air handling units 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 a building. AHU’s will serve a specified
area or zone within a building such as the east side, or floors 1 – 10 or perhaps a single purpose such as just the
buildings toilets. Therefore, it’s very common to find multiple AHU’s around a building.
Some buildings, particularly old high rise building, will have just one large AHU, 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 leaking out
of the building. This design isn’t so common anymore in new buildings because it’s very inefficient, now its most
common to have multiple smaller AHU’s supplying different zones. The buildings are also more air tight so we need
to have a return duct to regulate the pressure inside the building.

6. 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, clean it, heat
it or cool it, maybe humidify it and then force it through some ductwork around to the designed areas within a building.
Most units will have an additional duct run to then pull the used dirty air out of the rooms, back to the AHU, where a
fan will discharge it back to atmosphere. Some of this return air might be recirculated back into the fresh air supply to
save energy. Otherwise, where that isn’t possible, thermal energy can extracted and fed into the fresh air intake.
RETURN
SUPPLY

7. Mesh Damper
Filters
Heating coil
Cooling coil
Supply fan
Supply fan
Mesh Damper
RETURN
SUPPLY
BASIC A.H.U

8. • 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.
• At the inlet of the 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, they can open to fully allow air in or out, and it can also
vary their position somewhere in between to restrict the amount of air that can enter or exit.
• After the dampers we’ll have some filters. These are there to try and catch all the dirt and dust etc from entering the ahu and the building.
We have some panel filters or pre-filters to catch the largest dust particles. Then we have some bag filters to catch the smaller dust
particles.
• Across each bank of filters, 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 pickup dirt, the amount of air that can flow through is restricted and this causes a pressure drop across the
filters.
• The next thing we’ll find are the cooling and heating coils. The air temperature of the supply air is measured as it leaves the AHU and
enters the ductwork. This needs to be at a designed temperature to keep the people inside the building comfortable, this designed
temperature is called the set point temperature.
• If the air temperature is below this value the heating coil will add heat to increase the air temperature and bring it up to setpoint. If the
air is too hot then the cooling coil will remove heat to lower the air temperature and reach the setpoint.
• The coils are heat exchangers, inside the coil is a hot or cold fluid, usually something like heated or chilled water, refrigerant or steam.
• Next we’ll have a fan. This is going to pull the air in from outside and then through the dampers, filters and coils and then push this out into
the ductwork around the building. Centrifugal fans are very common in old and existing AHU’s but EC fans are now being installed and
also retro fitted for increased energy efficiency. Across the fan we’ll also have a pressure sensor, this will sense if the fan is running.
• Then we have the ductwork which will send the air around the building to the designed areas. We’ll also have some ductwork coming back
which is brining 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.
The return AHU in its simplest form has just a fan and damper inside. The fan is pulling the air in from around the building and then pushing it
out of the building. The damper is located at the exit of the ahu housing and will close when the AHU turns off.

10. Classifications of Air-Handling Units
Air-handling units may be classified according to their structure, location, and conditioning
characteristics.
Horizontal Unit.
In a horizontal unit, the supply fan, coils, and filters are all installed at the same level.
Horizontal units need more floor space for installation, and they are mainly used as large AHUs.
Most horizontal units are installed inside the fan room.
Occasionally, small horizontal units may be hung from the ceiling inside the ceiling plenum. In
such a circumstance, fan noise and vibration must be carefully controlled if the unit is adjacent to
the conditioned space.
Vertical Unit.
In a vertical unit, the supply fan is not installed at the same level as the coils and filters but is
often
at a higher level.
Vertical units require less floor space.
They are usually smaller, so that the height of the coil section plus the fan section, and the height
of the ductwork that crosses over the AHU under the ceiling, is less than the head room (the
height from the floor to
the ceiling or the beam of the fan room). The fan room is the room used to house AHUs and
other mechanical equipment.

11. Draw-Through Unit
The supply fan is located downstream from the cooling coil section, and the
air is drawn through the coil section. Conditioned air is evenly distributed
over the entire surface of the coil section.
Also the discharge air from the AHU can be easily connected to a supply
duct of similar higher velocity. Draw-through units are the most widely used
AHUs.
Blow-Through Unit
The supply fan is located upstream from the coil section, and the air blows
through the coil section.
Usually, a multizone air-handling unit adopts a blow-through unit.
In a multizone AHU, the coil section is divided into the hot deck and the cold
deck.
The heating coil is installed in the hot deck just above the cold deck, where
the cooling coil is located. The hot deck is connected to ductwork that
supplies warm air to the perimeter zone through the warm duct. The cold
deck is connected to a cold duct that supplies cold air to both
the perimeter and interior zones.
A blow-through unit also has the advantage of treating the supply fan heat
gain as part of the coil load and thus reduces the supply system heat gain.

12. Outdoor Air (or Makeup Air) AHU or Mixing AHU.
• Most mixing AHUs can be used to condition either outdoor air only or a mixture of
outdoor air and recirculating air, whereas an outdoor air AHU is used only to
condition 100 percent outdoor air.
• An outdoor air, or makeup air, AHU is a once-through unit; there is no return air
and mixing box. It may be a constant-volume system or a variable-air-volume
(VAV) system if the number of occupants varies.
• In an outdoor-air AHU, the cooling coil is usually a six- to eight-row depth coil
because of the greater enthalpy difference during cooling and dehumidification in
summer.
• Freeze protections for water coils are necessary in locations where the outdoor
temperature may be below 32°F (0°C) in winter.
• A heat recovery coil or a water economizer precooling coil is often installed in
makeup air AHUs for energy savings.
Single-Zone AHU or Multizone AHU.
• A single-zone AHU serves only a single zone.
• A multizone AHU serves two or more zones.
• A zone can be a large perimeter or an interior zone or one of the many control
zones which connect to a multizone AHU through ducts and terminals.
• A multizone AHU with a hot and cold deck is now often used for a dual-duct VAV
system.

13. Factory-Fabricated AHU or Field-Built AHU, Custom-Built or Standard
Fabrication.
• One important reason to use factory-fabricated AHUs or standard
fabrications is their lower cost and higher quality. Factory labor and
controlled manufacturing techniques provide more efficient and better
quality construction than field labor and assembly.
• Custom-built and field-built AHUs provide more flexibility in structure, system
component arrangements, dimensions, and specialized functions than
standard fabricated products. Custom built and field-built AHUs also need
more comprehensive, detailed specifications. Standard fabricating products
are usually less expensive and can be delivered in a shorter time.
Rooftop AHU or Indoor AHU.
• A rooftop AHU is an outdoor penthouse. It is usually curb-mounted on the
roof and should be completely weathertight. The outside casing is usually
made of heavy-gauge galvanized steel or aluminum sheets with corrosion-
resistant coating and sealant at the joints, both inside and outside. The fan
motor, water valves, damper actuator linkages, and controls are all installed
inside the casing. Access doors are necessary for service and maintenance of
fans, coils, and filters.
• An indoor AHU is usually located in the fan room. Small AHUs are sometimes
ceiling-hung.

14. SIZES of AHU

15. Benefits of Air
Handling Units
Air handling systems can do the jobs of many types of
ventilation equipment combined into just one slightly
larger system. They can improve the quality of the indoor
air, in some cases reducing allergy and asthma
symptoms. Additional benefits of AHUs include:
Improved climate control
Increased energy efficiency of the HVAC system
Fewer airborne allergens