Handbook of air conditioning and refrigeration / Shan K

29.12 CHAPTER TWENTY-NINE
System Characteristics
maximum total pressure loss in any of the branch takeoffs connected to the supply main duct. And
0.1 in. WG (25 Pa) should be added as a safety factor.
For a supply duct system with similar configuration in branch take-offs, the static pressure sensor
of a duct static pressure control should be located near the remote end of the main duct as well
as at a location where steady static pressure can be properly measured, as shown in Fig. 29.1.
If there are two or three supply main ducts, two or three static pressure sensors should be installed.
Each should be located near the remote end of each main duct. A comparator is used so that
the DDC controller can pick the lowest static pressure as the feedback value to modulate the variable-speed
drive to provide a duct static pressure higher than the preset value in all main ducts.
System characteristics of a VAV cooling packaged system are listed in Table 29.1.
29.6 VAV REHEAT PACKAGED SYSTEMS
System Description
A VAV reheat packaged system is a multizone system that uses a packaged unit with DX coil and
filters to condition the air and that supplies the conditioned air to various control zones in the
perimeter zone of a building through reheating VAV boxes, ducts, and diffusers, and to various control
zones in the interior zones through VAV boxes, ducts, and diffusers. The zone supply volume
flow rate of the cold supply air is modulated to match the variation of the zone loads during partload
operation.
Heating provided by the reheating coil is used to offset the zone heating load in winter in the
perimeter zone as well as to prevent the zone temperature drops below a preset temperature when
the zone supply volume flow rate has been reduced to the minimum setting during cooling mode
part-load operation. A reheating coil is not required in the VAV boxes in the interior zone.
As discussed in Sec. 21.3, for energy saving, the zone supply volume flow rate must be reduced
to a minimum setting, such as 30 percent of the peak supply volume flow rate during cooling mode
part-load operation before the reheating coil is energized. Figure 29.2 shows a VAV reheat packaged
system with a rooftop packaged unit. The air conditioning cycle for cooling mode operation is
the same as that shown in Fig. 21.5b. Usually, the same rooftop packaged unit is used to supply
conditioned air to both perimeter and interior zones. However, in winter, if two packaged units are
used to serve the perimeter and interior zone separately, the mixing air temperature for the perimeter
zone can be raised to a higher value, to save energy.
VAV reheat packaged systems are simple and effective. However, simultaneous cooling and
heating processes should be minimized. VAV reheat packaged systems are used in many commercial
buildings.
Supply Volume Flow Rate and Coil Load
In a VAV reheat packaged system, the zone peak supply volume flow rate , in cfm [m3 V˙
sn
/(60 s)],
can be calculated from Eq. (21.4) as
V˙ sn �
Q rsn
60� sc pa(T r � T sn)
(29.12)