Handbook of air conditioning and refrigeration / Shan K

29.3 SINGLE-ZONE VAV PACKAGED SYSTEMS
System Description
System Calculations
Controls
AC SYSTEMS: PACKAGED AND DESICCANT-BASED SYSTEMS 29.7
In a single-zone VAV packaged system, zone temperature is maintained by a DDC controller
through variation of the opening of the inlet guide vanes at the supply fan inlet or the supply fan
speed through a variable-speed drive. Therefore, the supply volume flow rate is varied to match the
zone load variation. A typical single-zone VAV packaged system is similar to that shown in Fig.
21.1 except for the following:
● The AHU is replaced by a packaged unit.
● The water-cooling coil is replaced by a DX coil.
● The heating coil is replaced by a gas furnace with primary and secondary heat exchangers.
An air economizer or a water economizer is often used to save energy. The cooling mode operation
at design conditions and part-load operation and the heating mode operation at design and partload
operation are similar to that discussed in Sec. 21.2.
The supply volume flow rate to offset the zone sensible load at cooling mode design conditions can
be calculated by Eq. (21.1) as
V˙ s �
The DX coil load required at cooling mode design conditions can be calculated by Eq. (21.2) as
Q cc � 60V˙ s� s(h rn � h cc)
(29.5)
(29.6)
The heating capacity of the gas furnace required to offset the single-zone heating load during heating
mode design conditions can be calculated by Eq. (21.3) as
Q ch � 60V˙ s� sc pa(T hc � T en)
(29.7)
Zone temperature control and sequence of operations are similar to those discussed in Sec. 21.2 except
for the following:
● Cooling capacity is varied by cycling multiple compressors, or cylinder unloaders, or hot gas pass
at low part-load operation. Cooling capacity control is discussed in later sections.
● Heating capacity is varied by controlling the gas valve in on/off or two-stage control.
● Discharge air temperature control is still required for a better control quality and energy saving.
However, for a small packaged unit with only on/off cooling capacity control, zone temperature
control in cooling mode operation is actually the same whether the single-zone VAV packaged
system is employed with or without a discharge air temperature control.
Minimum ventilation control, head pressure control, and safety controls are similar to those discussed
in Chap. 23. Shirey (1995) recommended an improved fan cycling strategy based on the field
operating experience in the Salvador Dali Museum, St. Petersburg, Florida. The museum areas (storage
Q rs
60� sc pa(T r � T s)