Air Handler: Blower Filter Dampers Ductwork Ventilation System
Air Handler: Blower Filter Dampers Ductwork Ventilation System
Air Handler: Blower Filter Dampers Ductwork Ventilation System
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
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).
Components
o
1 Filters
3 Humidifier
4 Mixing chamber
5 Blower/fan
6 Balancing
8 Controls
9 Vibration isolators
Construction
The air handler is normally constructed around a framing system with metal infill panels
as required to suit the configuration of the components. In its simplest form the frame
may be made from metal channels or sections, with single skin metal infill panels. The
metalwork is normally galvanized for long term protection. For outdoor units some form
of weatherproof lid and additional sealing around joints is provided.
Larger air handlers will be manufactured from a square section steel framing system
with double skinned and insulated infill panels. Such constructions reduce heat loss or
heat gain from the air handler, as well as providing acoustic attenuation. Larger air
handlers may be several meters long and are manufactured in a sectional manner and
therefore, for strength and rigidity, steel section base rails are provided under the unit.
Where supply and extract air is required in equal proportions for a balanced ventilation
system, it is common for the supply and extract air handlers to be joined together, either
in a side-by-side or a stacked configuration.
Components
The major types of components are described here in approximate order, from the
return duct (input to the AHU), through the unit, to the supply duct (AHU output). [1][2]
Filters
Air filtration is almost always present in order to provide clean dust-free air to the
building occupants. It may be via simple low-MERV pleated media, HEPA, electrostatic,
or a combination of techniques. Gas-phase and ultraviolet air treatments may be
employed as well.
Filtration is typically placed first in the AHU in order to keep all the downstream
components clean. Depending upon the grade of filtration required, typically filters will
be arranged in two (or more) successive banks with a coarse-grade panel filter provided
in front of a fine-grade bag filter, or other "final" filtration medium. The panel filter is
cheaper to replace and maintain, and thus protects the more expensive bag filters. [1]
The life of a filter may be assessed by monitoring the pressure drop through the filter
medium at design air volume flow rate. This may be done by means of a visual display
using a pressure gauge, or by a pressure switch linked to an alarm point on the building
control system. Failure to replace a filter may eventually lead to its collapse, as the
forces exerted upon it by the fan overcome its inherent strength, resulting in collapse
and thus contamination of the air handler and downstream ductwork.
Humidifier
Humidification is often necessary in colder climates where continuous heating will make
the air drier, resulting in uncomfortable air quality and increased static electricity.
Various types of humidification may be used:
Evaporative: dry air blown over a reservoir will evaporate some of the water. The
rate of evaporation can be increased by spraying the water onto baffles in the air
stream.
Vaporizer: steam or vapor from a boiler is blown directly into the air stream.
Spray mist: water is diffused either by a nozzle or other mechanical means into
fine droplets and carried by the air.
Wetted medium: A fine fibrous medium in the airstream is kept moist with fresh
water from a header pipe with a series of small outlets. As the air passes through
the medium it entrains the water in fine droplets. This type of humidifier can quickly
clog if the primary air filtration is not maintained in good order.
Mixing chamber
In order to maintain indoor air quality, air handlers commonly have provisions to allow
the introduction of outside air into, and the exhausting of air from the building. In
temperate climates, mixing the right amount of cooler outside air with warmer return air
can be used to approach the desired supply air temperature. A mixing chamber is
therefore used which has dampers controlling the ratio between the return, outside, and
exhaust air.
Blower/fan
Air handlers typically employ a large squirrel cage blower driven by an AC
induction electric motor to move the air. The blower may operate at a single speed, offer
a variety of set speeds, or be driven by a Variable Frequency Drive to allow a wide
range of air flow rates. Flow rate may also be controlled by inlet vanes or
outlet dampers on the fan. Some residential air handlers in USA (central "furnaces" or
"air conditioners") use a brushless DC electric motor that has variable speed
capabilities. Air handlers in Europe and Australia and New Zealand now commonly use
EC backward curve fans without scroll or "plug fans". These are driven using high
efficiency EC motors with built in speed control.
Multiple blowers may be present in large commercial air handling units, typically placed
at the end of the AHU and the beginning of the supply ductwork (therefore also called
"supply fans"). They are often augmented by fans in the return air duct ("return fans")
pushing the air into the AHU.
Balancing
Un-balanced fans wobble and vibrate. For home AC fans, this can be a major problem:
air circulation is greatly reduced at the vents (as wobble is lost energy), efficiency is
compromised, and noise is increased. Another major problem in fans that are not
balanced is longevity of the bearings (attached to the fan and shaft) is compromised.
This can cause failure to occur long before the bearings life expectancy.
Weights can be strategically placed to correct for a smooth spin (for a ceiling fan, trial
and error placement typically resolves the problem). But for a home / central AC fan or
big fan are typically taken to shops, which have special balancers for more complicated
balancing (trial and error can cause damage before the correct points are found). The
fan motor itself does not typically vibrate.
Run around coil: Two air to liquid heat exchanger coils, in opposing airstreams,
piped together with a circulating pump and using water or a brine as the heat
transfer medium. This device, although not very efficient, allows heat recovery
between remote and sometimes multiple supply and exhaust airstreams. Heat
recovery efficiency up to 50%.
Heat Pipe: Operating in both opposing air paths, using a confined refrigerant as a
heat transfer medium. The heat pipe uses multiple sealed pipes mounted in a coil
configuration with fins to increase heat transfer. Heat is absorbed on one side of the
pipe, by evaporation of the refrigerant, and released at the other side, by
condensation of the refrigerant. Condensed refrigerant flows by gravity to the first
side of the pipe to repeat the process. Heat recovery efficiency up to 65%.
Controls
Controls are necessary to regulate every aspect of an air handler, such as: flow rate of
air, supply air temperature, mixed air temperature, humidity, air quality. They may be as
simple as an off/on thermostat or as complex as a building automation system
using BACnet or LonWorks, for example.
Vibration isolators
The blowers in an air handler can create substantial vibration and the large area of the
duct system would transmit this noise and vibration to the occupants of the building. To
avoid this, vibration isolators (flexible sections) are normally inserted into the duct
immediately before and after the air handler and often also between the fan
compartment and the rest of the AHU. The rubberized canvas-like material of these
sections allows the air handler components to vibrate without transmitting this motion to
the attached ducts.
The fan compartment can be further isolated by placing it on a spring suspension, which
will mitigate the transfer of vibration through the floor.
An exposed fan coil unit may be wall-mounted, freestanding or ceiling mounted, and will
typically include an appropriate enclosure to protect and conceal the fan coil unit itself,
with return air grille and supply air diffuser set into that enclosure to distribute the air.
A concealed fan coil unit will typically be installed within an accessible ceiling void or
services zone. The return air grille and supply air diffuser, typically set flush into the
ceiling, will be ducted to and from the fan coil unit and thus allows a great degree of
flexibility for locating the grilles to suit the ceiling layout and/or the partition layout within
a space. It is quite common for the return air not to be ducted and to use the ceiling void
as a return air plenum.
The coil receives hot or cold water from a central plant, and removes heat from or adds
heat to the air through heat transfer. Traditionally fan coil units can contain their own
internal thermostat, or can be wired to operate with a remote thermostat. However, and
as is common in most modern buildings with a Building Energy Management
System(BEMS), the control of the fan coil unit will be by a local digital controller or
outstation (along with associated room temperature sensor and control valve actuators)
linked to the BEMS via a communication network, and therefore adjustable and
controllable from a central point, such as a supervisors head end computer.
Fan coil units circulate hot or cold water through a coil in order to condition a space. The
unit gets its hot or cold water from a central plant, or mechanical room containing
equipment for removing heat from the central building's closed-loop. The equipment
used can consist of machines used to remove heat such as a chiller or a cooling
towerand equipment for adding heat to the building's water such as a boiler or a
commercial water heater.
Fan coil units are divided into two types: Two-pipe fan coil units or four-pipe fan coil
units. Two-pipe fan coil units have one (1) supply and one (1) return pipe. The supply
pipe supplies either cold or hot water to the unit depending on the time of year. Fourpipe fan coil units have two (2) supply pipes and two (2) return pipes. This allows either
hot or cold water to enter the unit at any given time. Since it is often necessary to heat
and cool different areas of a building at the same time, due to differences in internal
heat loss or heat gains, the four-pipe fan coil unit is most commonly used.
Depending upon the selected chilled water temperatures and the relative humidity of the
space, it is likely that the cooling coil will dehumidify the entering air stream, and as a by
product of this process, it will at times produce a condensate which will need to be
carried to drain. The fan coil unit will contain a purpose designed drip tray with drain
connection for this purpose. The simplest means to drain the condensate from multiple
fan coil units will be by a network of pipe work laid to falls to a suitable point.
Alternatively a condensate pump may be employed where space for such gravity pipe
work is limited.
Speed control of the fan motors within a fan coil unit is effectively used to control the
heating and cooling output desired from the unit. Some manufacturers accomplish
speed control by adjusting the taps on an AC transformer supplying the power to the fan
motor. Typically this would require adjustment at the commissioning stage of the
building construction process and is therefore set for life. Other manufacturers provide
custom-wound Permanent Split Capacitor (PSC) motors with speed taps in the
windings, set to the desired speed levels for the fan coil unit design. A simple speed
selector switch (Off-High-Medium-Low) is provided for the local room occupant to
control the fan speed. Typically this speed selector switch is integral to the room
thermostat, and is set manually or is controlled automatically by the digital room
thermostat. Building Energy Management Systems can be used for automatic fan speed
and temperature control. Fan motors are typically AC Shaded Pole or Permanent Split
Capacitor. More recent developments include brushless DC designs with electronic
commutation. While these motors do offer significant energy savings, initial cost and
return on investment should be carefully considered.
Areas of use
Fan coil units are typically used in spaces where economic installations are preferred
such as unoccupied storage rooms, corridors, loading docks.
In high-rise buildings, fan coils may be stacked, located one above the other from floor
to floor and all interconnected by the same piping loop.
Fan coil units are an excellent delivery mechanism for hydronic chiller boiler systems in
large residential and light commercial applications. In these applications the fan coil
units are mounted in bathroom ceilings and can be used to provide unlimited comfort
zones - with the ability to turn off unused areas of the structure to save energy.
Installation
In high-rise residential construction, typically each fan coil unit requires a rectangular
through-penetration in the concrete slab on top of which it sits. Usually, there are either
2 or 4 pipes made of ABS, steel or copper that go through the floor. The pipes are
usually insulated with refrigeration insulation, such as acrylonitrile butadiene/polyvinyl
chloride (AB/PVC) flexible foam (Rubatex or Armaflex brands) on all pipes or at least
the cool lines.
Unit ventilator
A unit ventilator is a fan coil unit that is used mainly in classrooms, hotels, apartments
and condominium applications. A unit ventilator can be a wall mounted or ceiling hung
cabinet, and is designed to use a fan to blow outside air across a coil, thus conditioning
and ventilating the space which it is serving.
depending on the actual space temperature and the temperature setpoint, while the
compressor modulates the refrigerant flow to maintain a constant supply air
temperature. The result is more precise space temperature control.
Another advantage is energy savings and reduced wear. VAV fan control, especially
with modern electronic variable-speed drives, reduces the energy consumed by fans,
which can be a substantial part of the total cooling energy requirements of a building.
Modulating control of the compressor also reduces wear and delivers further energy
savings.
A final advantage is increased dehumidification. Because VAV air flow is reduced under
part-load conditions, air is exposed to cooling coils for a longer time. More moisture
condenses on the coils, dehumidifying the air. Thus, although a constant-volume and a
single-zone VAV unit maintain the same room temperature, the VAV unit provides more
passive dehumidification and more comfortable space conditions.
Fan control
Control of the system's fan capacity is critical in VAV systems. Without proper and rapid
flow rate control, the system's ductwork, or its sealing, can easily be damaged by over
pressurization. In the cooling mode of operation, as the temperature in the space is
satisfied, a VAV box closes to limit the flow of cool air into the space. As the temperature
increases in the space, the box opens to bring the temperature back down. The fan
maintains a constant static pressure in the discharge duct regardless of the position of
the VAV box. Therefore, as the boxes close, the fan slows down or restricts the amount
of air going into the supply duct. As the boxes open, the fan speeds up and allows more
air flow into the duct, maintaining a constant static pressure.
One of the challenges for VAV systems is providing adequate temperature control for
multiple zones with different environmental conditions, such as an office on the glass
perimeter of a building vs. an interior office down the hall. Dual duct systems provide
cool air in one duct and warm air in a second duct to provide an appropriate
temperature of mixed supply air for any zone. An extra duct, however, is cumbersome
and expensive. Reheating the air from a single duct, using electric or hot water heating,
is often a more cost-effective solution.
VRFs come in two system formats, two pipe and three pipe systems. In a heat pump 2 pipe
system all of the zones must either be all in cooling or all in heating. Heat Recovery (HR)
systems have the ability to simultaneously heat certain zones while cooling others this is
usually done through a 3 pipe design, with the exception of Mitsubishi which is able to do
this with 2 pipes. In this case the heat extracted from zones requiring cooling is put to use in
the zones requiring heating. This is made possible because the heating unit is functioning
as a condenser, providing sub-cooled liquid back into the line that is being used for cooling.
While the heat recovery system has a greater initial cost, it allows for better zoned thermal
control of a building and overall greater efficiencies.
Ventilation
An air handling unit is used for the heating and cooling of air in a central location
Ventilating (the V in HVAC) is the process of "processing" or replacing air in any space
to provide high indoor air quality (i.e. to control temperature, replenish oxygen, or
remove moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide).
Ventilation is used to remove unpleasant smells and excessive moisture, introduce
outside air, to keep interior building air circulating, and to prevent stagnation of the
interior air.
Ventilation includes both the exchange of a air to the outside as well as circulation of air
within the building. It is one of the most important factors for maintaining
acceptable indoor air quality in buildings. Methods for ventilating a building may be
divided into mechanical/forced and natural types.
"Mechanical" or "forced" ventilation is used to control indoor air quality. Excess humidity,
odors, and contaminants can often be controlled via dilution or replacement with outside
air. However, in humid climates much energy is required to remove excess moisture
from ventilation air.
Ventilation increases the energy needed for heating or cooling, however heat recovery
ventilation can be used to mitigate the energy consumption. This involves heat
exchange between incoming and outgoing air. Energy recovery ventilation additionally
includes exchange of humidity.
Kitchens and bathrooms typically have mechanical exhaust to control odors and
sometimes humidity. Kitchens have additional problems to deal with such as smoke and
grease. Factors in the design of such systems include the flow rate (which is a function
of the fan speed and exhaust vent size) and noise level. If ducting for the fans traverse
unheated space (e.g., an attic), the ducting should be insulated as well to prevent
condensation on the ducting. Direct drive fans are available for many applications, and
can reduce maintenance needs.
Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing
the perceived temperature because of evaporation of perspiration on the skin of the
occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in
the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans
do not provide ventilation as defined as the introduction of outside air.
Natural ventilation is the ventilation of a building with outside air without the use of a
fan or other mechanical system. It can be achieved with openable windows or trickle
vents when the spaces to ventilate are small and the architecture permits.
In more complex systems warm air in the building can be allowed to rise and flow out
upper openings to the outside (stack effect) thus forcing cool outside air to be drawn
into the building naturally through openings in the lower areas. These systems use very
little energy but care must be taken to ensure the occupants' comfort. In warm or humid
months, in many climates, maintaining thermal comfort solely via natural ventilation may
not be possible so conventional air conditioning systems are used as backups. Airside economizers perform the same function as natural ventilation, but use mechanical
systems' fans, ducts, dampers, and control systems to introduce and distribute cool
outdoor air when appropriate.
Definition
Ventilation is the intentional movement of air from outside a building to the inside.
Ventilation air, as defined by the American Society of Heating, Refrigerating and Air-
Necessity
When people or animals are present in buildings, ventilation air is necessary to dilute
odors and limit the concentration of carbon dioxide and airborne pollutants such as dust,
smoke and volatile organic compounds (VOCs). Ventilation air is often delivered to
spaces by mechanical systems which may also heat, cool, humidify and dehumidify the
space. Air movement into buildings can occur due to uncontrolled infiltration of outside
air through the building fabric (see stack effect) or the use of deliberate natural
ventilation strategies. Advanced air filtration and treatment processes such
as scrubbing, can provide ventilation air by cleaning and recirculating a proportion of the
air inside a building.
Types
Mechanical or forced ventilation: through an air handling unit or direct injection to a
space by a fan. A local exhaust fan can enhance infiltration or natural ventilation, thus
increasing the ventilation air flow rate.
Natural ventilation occurs when the air in a space is changed with outdoor air
without the use of mechanical systems, such as a fan. Most often natural ventilation
is assured through operable windows but it can also be achieved through
temperature and pressure differences between spaces. Open windows or vents are
not a good choice for ventilating a basement or other below ground structure.
Allowing outside air into a cooler below ground space will cause problems with
humidity and condensation.
control measure to regulate the natural ventilation process, for example, to restrict
the air change rate during periods of high wind speeds.