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ALL-AIR SYSTEMS AN ALL-AIR SYSTEM provides complete sensible and latent cooling, preheating, and humidification capacity in the air sup-plied by the system. No additional cooling or humidification is required at the zone, except in the case of certain industrial systems. 23755
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Heating may be accomplished by the same airstream, either in the central system or at a particular zone. In some applications, heating
is accomplished by a separate heater. The term zone implies the
pro- vision of, or the need for, separate thermostatic control, while the
term room implies a partitioned area that may or may not require
separate control. The basic all-air system concept is to supply air to the room at
such conditions that the sensible heat gain and latent heat gain in the
space, when absorbed by the supply air flowing through the space,
will bring the air to the desired room conditions. Since the heat
gains in the space will vary with time, a mechanism to vary the energy
removed from the space by the supply air is necessary. There are
two such basic mechanisms: vary the amount of supply air
delivered to the space, either by varying the flow rate or supplying air inter-
mittently, or vary the temperature of the air being delivered to the
space, either by modulating the temperature or conditioning the air
intermittently. All-air systems are classified in two categories:
• Single-duct systems, which contain the main heating and cooling
coils in a series flow air path; a common duct distribution
system at a common air temperature feeds all terminal apparatus. Either
capacity varying mechanism (varying temperature or varying
volume) can be used with single-duct systems.
• Dual-duct systems, which contain the main heating and cooling
coils in parallel flow or series-parallel flow air paths with either
(1) a separate cold and warm air duct distribution system that
blends the air at the terminal apparatus (dual-duct systems), or
(2) a separate supply air duct to each zone with the supply air blended
to the required temperature at the main unit mixing dampers
(mul- tizone). Dual-duct systems generally vary the supply air temper-
ature by mixing two airstreams of different temperatures, but
can also vary the volume of supply air in some applications.
These categories may be further pided as follows:
Single duct Constant volume Single zone Multiple-zone reheat Bypass VAV Variable air volume (VAV)
Throttling Fan-powered The preparation of this ch
Reheat
Induction
Variable diffusers
Dual duct
Dual duct
Constant volume
Variable air volume
Dual conduit
Multizone
Constant volume
Variable air volume
Three-deck or Texas multizone
All-air systems may be adapted to many applications for comfort
or process work. They are used in buildings that require inpidual
control of multiple zones, such as office buildings, schools and uni-
versities, laboratories, hospitals, stores, hotels, and even ships. All-
air systems are also used virtually exclusively in special applica-
tions for close control of temperature and humidity, including clean
rooms, computer rooms, hospital operating rooms, research and
development facilities, as well as many industrial/manufacturing
facilities.
All-air systems have the following advantages:
• The location of the central mechanical room for major equipment
allows operation and maintenance to be performed in unoccupied
areas. In addition, it allows the maximum range of choices of fil-
tration equipment, vibration and noise control, and the selection
of high quality and durable equipment.
• Keeping piping, electrical equipment, wiring, filters, and vibra-
tion and noise-producing equipment away from the conditioned