Handbook of air conditioning and refrigeration / Shan K

21.2 CHAPTER TWENTY-ONE
21.1 SYSTEM CHARACTERISTICS OF VARIABLE-AIR-VOLUME
SYSTEMS
Variable-Air-Volume Systems
A variable-air-volume (VAV) system is an air system that varies its supply air volume flow rate to
match the reduction of space load during part-load operation to maintain a predetermined space
parameter, usually air temperature, and to conserve fan power at reduced volume flow. A constantvolume
system varies its supply air temperature to match the reduction of space load during partload
operation to maintain a predetermined space air temperature.
Compared with a constant-volume system, a VAV system has mainly the following advantages:
● Reduced fan energy use during part-load operation when the supply volume flow rate is reduced
● A slightly lower or nearly the same zone relative humidity when the supply volume flow rate is
reduced during summer cooling mode part-load operation
● More individual control zones
● Reduction of the construction cost because of taking into consideration of the supply air volume
flow diversity factor instead of the sum of zone peak loads
● Capability of self-balancing of zone supply volume flow rates
● Convenience during the relocation of the terminals and space diffusion devices during future
expansion or retrofit
Compared with a constant-volume system, the primary disadvantages of a VAV system are:
● Inadequate outdoor ventilation air when the supply volume flow rate is reduced
● A more complicated system structure and controls, which need more demanding design, installation,
operation, and maintenance
VAV systems are applicable to air systems whose space load varies significantly so that there are
fan energy savings. VAV systems became popular after the energy crisis in 1973. They are widely
used in large commercial buildings in the United States.
Types of Variable-Air-Volume Systems
Most medium-size and large buildings need multizone air systems. However, many indoor stadiums,
convention centers, factories, residential buildings, and small retail stores employ single-zone
air systems. Currently used variable-air-volume systems can be mainly classified into the following
types:
● Single-zone VAV systems
● VAV cooling systems
● VAV reheat systems
● Dual-duct VAV systems
● Fan-powered VAV systems
Recently, another VAV system has been developed called the variable diffuser VAV system. In
variable diffuser systems, the aperture of each diffuser can be varied so that the discharge velocity
is relatively constant while the supply volume flow is reduced, and the throw from the variable
diffuser may also be maintained above a certain limit. Variable diffuser VAV systems provide more
individual control zones as well as a more complicated diffuser construction and controls. More
field perfomance data and experience are needed to make an appropriate selection.