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Air-handling
systems consist of the fans, filters, dampers, and ducts that
deliver cooled or heated air throughout a building. The fans
required are included in packaged equipment, except when separate
powered exhaust is required to balance the pressure contribution
of the incoming ventilation air. Fans are separately specified
for built-up central systems. Alternative systems can be compared
along several dimensions: their fan energy requirement; the
degree to which they can respond to heating and cooling needs
of different building zones; and their ability to take advantage
of cooler outside air or water through economizers or to supply
reheat energy for temperature control to zones in the building.
In general, more flexible systems cost more to install but
are often more efficient on a system basis, and provide greater
comfort for building occupants.
Economizers
In most
commercial applications, at least some interior spaces need
cooling during times when the outdoor air temperature and
humidity are sufficiently low to economically provide cooling
without using the mechanical refrigeration cycle. Economizers
use controls and supply and return air dampers to control
outside air quantities. Economizers are an option for virtually
all packaged air-conditioning equipment. However, for very
small units and in some climates, economizers may not be economical.
When bringing in large quantities of outdoor air, equivalent
quantities of indoor air must be exhausted at the same time.
This exhaust is called "relief air."
At some
upper outside temperature limit, it is no longer economical
to bring in 100 percent outdoor air because the energy to
cool it will be greater than cooling the building return air
mixed with the minimum quantity of outdoor air. This point
is called the economizer changeover point and, depending on
the climate, there are several ways to determine and control
the changeover.
- Fixed
dry bulb controls shift from economizer to refrigeration
cycle at a specific outdoor temperature, such as 75°F
maximum outside temperature.
- Differential
dry bulb systems compare the dry bulb temperature of the
outdoor air to the dry bulb temperature of the return air
and make the changeover when the outdoor temperature is
near the return temperature. Dry bulb systems are appropriate
in dry climates, but may cause problems where high humidity
and moderate to high outdoor temperatures occur together.
- For
these situations (such as the climate belt from Houston
through the Southeast and into the Mid-Atlantic states),
"enthalpy" controls are better, since they consider
the work required to dehumidify the outdoor air. (Enthalpy
is a measure of the total heat in the air, made by measuring
both the dry bulb temperature and the relative humidity.)
- Fixed
enthalpy controls work like fixed dry bulb controls,
except they consider the enthalpy of the outdoor air
(in Btus per pound of air) rather than the dry bulb
temperature.
- Differential
enthalpy controls compare the enthalpy of the outdoor
air with the enthalpy of the return air and change from
economizer/outside air to refrigeration whenever the
outdoor air enthalpy is greater that the return air
enthalpy. In humid climates, the added cost and complication
of the enthalpy controller will generally be justified
by increased comfort and energy savings.
Ventilation Air and Energy Efficiency
As a
bonus, during economizer operation abundant outside air is
brought into the space, diluting and exhausting contaminants
generated there. However, unlike residential applications,
most commercial applications require the HVAC system to bring
in, condition, and distribute specified amounts of outdoor
air for ventilation at all times that the space is occupied.
This means that some form of outdoor air intake must be integrated
into the design of the system. It is important to know what
the outdoor air requirements are for the space being conditioned.
For example, restaurant kitchens need to exhaust large quantities
of air through cooking hoods. Some applications require so
much ventilation air that the need cannot be met with conventional
packaged equipment, or economically met by built-up systems.
In these cases, energy recovery ventilation, heat recovery
ventilation, or air-to-air heat exchange is needed. These
devices use desiccants, heat pipes, or other technologies
to recover energy from the exhaust air and transfer it to
the outdoor supply air. In winter, they use this energy to
preheat the incoming air by cooling the exhaust air, and the
opposite in summer. These devices can save enormous amounts
of energy (U.S. Department
of Energy. 1999.). However, they also can waste energy
if not carefully engineered. For example, devices that have
high peak load efficiency may have so much pressure drop that
at part load they require more electricity for their fans
than they save in cooling. Such situations required engineered
solutions that may raise first costs.
Dehumidification
If the
indoor temperature rises above the temperature set point,
the thermostat energizes mechanical cooling; if temperature
drops, cooling is de-energized. If the unit is properly sized,
the cooling will be running most of the time during extremely
hot weather conditions. Since most days are not extreme, under
normal conditions the mechanical cooling cycles on and off
to maintain the desired temperature. As the unit only dehumidifies
when the mechanical cooling is energized and has run long
enough to cool the evaporator coil, it is not uncommon for
spaces to be uncomfortably humid for many hours each year.
A simple
way to help the unit do more dehumidification is to reduce
the supply fan speed (being careful not to violate ventilation
requirements.) This can be accomplished with fan speed switches,
changing drive pulleys, or variable speed fans. This both
cools the coil (increasing condensation) and lets the unit
operate longer, providing additional dehumidification. Another
option is to install humidity controls that override the temperature
controls. If the relative humidity exceeds control levels,
the humidistat energizes the cooling mode, enabling dehumidification.
Where necessary, there are several options for warming the
dehumidified air for increased comfort. Some are inefficient
(e.g., resistance reheat or hot water coils), and others have
lower energy penalties (e.g., hot gas reheat or downstream
air-to-air heat exchangers)
Alternatively,
desiccant dehumidifiers are viable in some applications. Desiccants
naturally attract moisture, efficiently removing latent (humidity-related)
load from the air. Conventional air conditioners are then
typically used to reduce the temperature (called the sensible
load) of the dried air to desired occupant comfort levels.
Latent and sensible loads are handled more efficiently because
each component is optimized to independently remove these
loads. When heated, the saturated desiccant is regenerated
to be used again (Slayzak, Pesaran, and Hancock. 1996. Experimental
Evaluation of Commercial Desiccant Dehumidifier Wheels.
Golden, Colo.: National Renewable Energy Laboratory).
Commercial
desiccant dehumidification systems have been applied primarily
in supermarkets and hotels and motels. In supermarkets they
displace antisweat heaters and defrosters that consume considerable
energy to control moisture levels in freezer display cases.
The systems also have been used to effectively control humidity
levels in hotels and motels, where mold and mildew can damage
wallpaper, paint, carpet, and other materials. In addition,
desiccant systems can improve indoor air quality, improve
ventilation rates, and remove air pollutants and odors (National
Renewable Energy Laboratory. 2000. Desiccant
Cooling: A Non-CFC, Energy-Efficient Space-Conditioning Technology.
Golden, Colo.: National Renewable Energy Laboratory).
Constant
Volume Systems
Single
zone: The most commonly applied air-handling system is a constant
volume, single-zone system-just like a house with one thermostat
in an "average" location controlling the air delivered
to every room. Packaged air-conditioning units of all sizes
are available as constant volume, single-zone units, as are
heated fan coils and water-loop heat pumps. As the name implies,
this equipment is characterized by a single space thermostat
that controls temperature and a fan that delivers a constant
volume of heated or cooled air (not a mixture of the two)
to every space. Some constant volume, single-zone systems
(e.g., packaged rooftop units) can easily integrate air-side
economizers to take advantage of "free" cooling
but usually there is no reheat in these systems for temperature
control. Of the three systems presented here, fan energy for
constant volume, single-zone systems is generally the highest.
Fan energy could be reduced if the fan were allowed to cycle
with calls for heating and cooling; however, this practice
is regulated by most standards and building codes, in order
to maintain ventilation by outside air. These systems can
cause discomfort under rather common conditions. Consider
a system that is designed to meet 95ºF outdoor conditions.
When it is 80ºF outside, the refrigeration system will
cycle off and on. But the circulation fan runs full-time to
bring in outdoor air and mix it with the air returned from
the system. In the summer, the system will distribute much
warmer (and often much more humid) air when the fan is on
but the compressor is off.
Multizone:
Constant volume, multizone systems were designed to provide
simultaneous heating and cooling to multiple temperature control
zones from a single packaged unit. Individual temperature
control zones receive air from two ducts, one equipped with
a heating coil (or another heating method, such as a small
gas furnace) and the other with a cooling coil. The space
thermostat energizes either the heating coil or the cooling
coil as needed to maintain the space temperature. These systems,
which were widely applied 20 to 30 years ago, are very inefficient
because they mix heated and cooled air streams for space temperature
control. They can be very expensive because of the need for
two duct systems, each sized to meet virtually the entire
load.
Variable Air Volume Systems
Variable
air volume (VAV) systems can be very efficient if well designed
and carefully operated. These systems vary the amount of air
supplied to a space, rather than leaving the supply volume
constant and varying the temperature of the air, as with constant
volume systems. Typically, these systems are more efficient
since: (1) less energy is used for cooling because the volume
of air cooled is reduced; (2) less energy is required for
heating because less air needs to be reheated; and (3) fan
energy is reduced because the amount of air the fan needs
to move is reduced. These systems generally employ economizers.
Variable
air volume, variable temperature systems have been applied
to packaged equipment to overcome the constant volume, single-zone
system's lack of zone control. One thermostat is acceptable
as long as the zone served has uniform heating or cooling
demands. But this is seldom the case. The boss' office is
much too cold or the conference room is much too warm. Fortunately,
in variable air volume, variable temperature systems, a given
area is served by a single unit but the area is divided into
temperature control zones and each zone gets a thermostat.
Each thermostat operates an automatic control damper located
in the branch duct that serves the zone. In the cooling mode,
if the zone is too cold, the damper is closed. If the zone
is too warm, the damper is opened. Just the opposite occurs
in the heating mode. All the thermostats "talk"
to a central panel that decides which zone demands the most
heating or cooling. Based on this decision, the central panel
operates the unit. By not allowing the dampers to close fully,
ventilation requirements can be met. However, with these systems
it is critical that all zones have comparable loads, so some
are not calling for heating while others require cooling.
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