Handbook of air conditioning and refrigeration / Shan K

Conditioned spaces in buildings are primarily divided into two categories: (1) perimeter zone,
where there are external wall, roofs, and windows and space load varies depending on solar heat,
outdoor-indoor temperature difference and internal load; and (2) interior zone where space loads
are mainly internal loads. In an interior zone, the air system often operates at cooling mode in both
summer and winter except for morning warm-up. A perimeter zone can often be subdivided into
four subzones according to their orientations. Although all these subzones operate at cooling mode
in summer, in fall and winter, zones facing south in the northern hemisphere may need cooling,
whereas zones facing north may need heating. Under such circumstances, a VAV system that serves
a perimeter zone is preferable to having a modulating cooling device and a modulating heating
device to meet the variable cooling and heating requirements for each of its subzones facing different
orientations.
The system characteristics of different types of VAV systems vary mainly because of the zone
control of their different types of VAV boxes to serve perimeter zone, except a single-zone VAV system
which directly varies the fan speed or the inlet vanes and a dual-duct VAV system in which two
supply ducts (a cold air duct and a warm air duct) are used. A VAV cooling system with VAV boxes
usually serves an interior zone, and reheating VAV boxes or a VAV cooling system plus a perimeter
heating system (such as a hot water heating system) will serve a perimeter zone. A single-zone VAV
system can be used for either a perimeter zone or an interior zone. Excluding the terminals and
zone controls, the following system characteristics of different VAV systems are similar to each
other, and are discussed in later chapters.
● Fan combination
● System pressure analysis and space pressurization
● Smoke control
● Outdoor ventilation air
● Controls, including discharge temperature, duct static pressure, and safety controls
ASHRAE/IESNA Standard 90.1-1999, Energy Standard for Buildings Except Low-Rise Residential
Buildings, specifies that individual variable-air-volume (VAV) fans with 30-hp (22-kW) motors
and larger shall have other controls and devices (such as adjustable-frequency, variable-speed
drives) that will result in fan motor demand of no more than 30 percent of design wattage at 50 percent
of design volume flow rate when fan static pressure equals one-third of the total design static
pressure based on manufacturer’s certified data.
21.2 SINGLE-ZONE VAV SYSTEMS
System Description
AIR SYSTEMS: VARIABLE-AIR-VOLUME SYSTEMS 21.3
A single-zone VAV system is a VAV system that varies the air volume flow rates supplied to and
returned from a single-zone conditioned space to maintain a predetermined space parameter at
reduced load and to conserve fan power. Single-zone systems are widely used in indoor stadiums,
assembly halls, shopping malls, and factories.
Figure 21.1 shows the schematic diagram and the air conditioning cycles for a typical single-zone
VAV system. During the summer cooling mode operation, the mixture of outdoor air and recirculating
air at point m is filtered at the filter and cooled and dehumidified at the cooling coil. The conditioned
air leaves the coil at point cc. It then flows through the supply fan of the air-handling unit
(AHU) or packaged unit (PU) and the supply duct before it is discharged to the single-zone conditioned
space at point s. After absorbing the space cooling load, the supply air becomes the space air
r and returns to the AHU or PU through the return air system.