Handbook of air conditioning and refrigeration / Shan K

of a VAV system using inlet vane modulation. A fan total pressure of less than 3 in. WC (750 Pa)
was available. After the inlet vanes had been dismantled, the fan total pressure increased to more
than 4 in. WC (1000 Pa).
For a BC fan with a 22.25-in.- (565-mm-) diameter impeller, the ratio p t,i/p t,o � 0.86; and for an
FC fan of 15-in.- (381-mm-) diameter impeller, the ratio p t,i/p t,o � 0.94.
During a cost comparison between inlet vanes and adjustable-frequency variable-speed drives,
the total pressure loss due to the inlet vanes when they are fully opened must be taken into consideration.
Therefore, for centrifugal fans in either AHUs or PUs, installation of inlet vanes is not
recommended in backward-curved centrifugal fans with impeller diameters smaller than 25 in.
(635 mm).
30.4 TYPES OF VAV CENTRAL SYSTEMS
Similar to packaged systems, VAV central systems can be subdivided into the following air conditioning
systems according to their configuration and operating characteristics:
● Single-zone VAV central system (SZVAVCS)
● VAV cooling central system (VAVCCS)
● VAV reheat central system (VAVRCS)
● Perimeter-heating VAV central system (PHVAVCS)
● Dual-duct VAV central system (DDVAVCS)
● Fan-powered VAV central system (FPVAVCS)
The dual-duct VAV central system is the only system with a dual-duct VAV air system. Regarding
the operating characteristics of the air system, the difference between a VAV central system and a
VAV packaged system is mainly due to the fact that water heating and cooling coils in an AHU are
used to heat and to cool the supply air in the central system instead of gas or electric heater and DX
coils in a PU to heat and cool the supply air in packaged systems. Their differences are discussed in
detail in Sec. 29.1.
30.5 SINGLE-ZONE VAV CENTRAL SYSTEMS
System Description
AC SYSTEMS: CENTRAL SYSTEMS AND CLEAN-ROOM SYSTEMS 30.7
A single-zone variable-air-volume central system is an air conditioning system that has a central plant
and water systems to supply hot and chilled water to the water heating and cooling coils in AHUs to
heat and cool the supply air, and distribution to the conditioned space through ducts and diffusers, as
shown in Fig. 21.1. A relief or return fan is employed to extract the recirculating air to the AHU, or it is
exhausted outdoors during the air economizer cycle. The zone air is maintained at a preset temperature
or relative humidity by modulating the water flow rate to the coils by means of a sensor and a DDC
controller. Hot water is heated in the boiler, and the chilled water is cooled in the chiller in a central
plant and transported to the AHU through hot and chilled water systems. Single-zone VAV systems are
widely used in arenas, indoor stadiums, airport terminals, and many industrial applications.
Supply Volume Flow Rate, Coil Load, and Zone Temperature Controls
The supply volume flow rate V˙
s,
in cfm [m/(60s)], of a single-zone VAV central system based on
the zone sensible cooling load at summer design conditions can be calculated from Eq. (29.1); and