All-Air System

An ALL-AIR SYSTEM provides complete sensible and latent cooling, preheating, and humidification capacity in the air supplied by the system. No additional cooling or humidification is required at the zone, except in the case of certain industrial systems. 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 provision 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 intermittently, 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 (multizone). Dual-duct systems generally vary the supply air temperature by mixing two airstreams of different temperatures, but can also vary the volume of supply air in some applications.

These categories may be further divided as follows:

Single duct
Constant volume
Single zone
Multiple-zone reheat
Bypass VAV
Variable air volume (VAV)
Variable diffusers
Dual duct
Dual duct
Constant volume
Variable air volume
Dual conduit
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 individual control of multiple zones, such as office buildings, schools and universities, laboratories, hospitals, stores, hotels, and even ships. All air systems are also used virtually exclusively in special applications 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 filtration equipment, vibration and noise control, and the selection of high quality and durable equipment.
• Keeping piping, electrical equipment, wiring, filters, and vibration and noise-producing equipment away from the conditioned area minimizes service needs and reduces potential harm to occupants, furnishings, and processes.
• These systems offer the greatest potential for use of outside air for economizer cooling instead of mechanical refrigeration for cooling.
• Seasonal changeover is simple and adapts readily to automatic control.
• A wide choice of zoning, flexibility, and humidity control under all operating conditions is possible, with the availability of simultaneous heating and cooling even during off-season periods.
• Air-to-air and other heat recovery may be readily incorporated.
• They permit good design flexibility for optimum air distribution, draft control, and adaptability to varying local requirements.
• The systems are well suited to applications requiring unusual exhaust or makeup air quantities (negative or positive pressurization, etc.).
• All-air systems adapt well to winter humidification.
• By increasing the air change rate and using high-quality controls, it is possible for these systems to maintain the closest operating condition of ±0.15 K dry bulb and ±0.5% rh. Today, some systems can maintain essentially constant space conditions.

All-air systems have the following disadvantages:
• They require additional duct clearance, which reduces usable floor space and increases the height of the building.
• Depending on layout, larger floor plans are necessary to allow enough space for the vertical shafts required for air distribution.
• Ensuring accessible terminal devices requires close cooperation between architectural, mechanical, and structural designers.
• Air balancing, particularly on large systems, can be more difficult.
• Perimeter heating is not always available to provide temporary heat during construction.

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