Handbook of air conditioning and refrigeration / Shan K

29.14 CHAPTER TWENTY-NINE
From Eq. (21.8), the DX coil load Q cc, in Btu/h (W), can be calculated as
(29.15)
The reheating coil load depends primarily on the winter zone heating load. From Eq. (21.10), the
reheating coil load for a control zone in the perimeter zone Q chxn, in Btu/h (W), can be calculated as
Night Setback and Morning Warm-up
(29.16)
In multizone VAV reheat rooftop packaged systems, the heating of the zone air during the occupied
period in the perimeter zone in winter is often provided by zone electric reheating coils, or sometimes
by water heating coils. The gas furnace, heat pump, or electric heater in the rooftop packaged
unit is only used to maintain a night setback temperature and for morning warm-up purposes.
The reasons to supply warm air to maintain an indoor night setback temperature (such as 55°F,
or 12.8°C) are
● To prevent freezing of water pipes and water surfaces in areas where the outdoor temperature at
night is below 32°F (0°C)
● To provide an acceptable indoor temperature for emergency access
● To reduce the time required to warm up to a required temperature, say, 68 or 70°F (20.0 or
21.1°C), prior to an occupied period the next morning
During the night setback and morning warm-up periods, the following hold:
1. Outdoor dampers and exhaust dampers should be completely closed.
2. Recirculating dampers, inlet vanes, and inlet cones of the supply fan should be fully open; or the
variable-speed drive should be running at full speed.
3. All VAV boxes or reheating boxes should be fully open.
4. Both the refrigeration compressor and the relief fan should be turned off.
5. When the zone temperature exceeds a certain limit, both the supply fan and the furnace, heat
pump, or heater in the packaged unit can be turned off. The heating device in the packaged unit
will be energized again when the zone temperature drops below a certain limit.
6. The termination of the night setback period is the beginning of the warm-up period. The warm
air supply temperature from the rooftop packaged unit during the warm-up period is generally
between 100 and 120°F (37.8 and 48.9°C).
Evenly Distributed Airflow at DX Coils
Q cc � 60V˙ s� s(h m � h cc)
Q chxn � Q rhxn � Q venxn
� Q rhxn � 60V˙ sxn� sc pa(T rxn � T enn)
In a VAV reheat packaged system, if the air flowing through the DX coil is not evenly distributed
over the entire coil surface, then liquid slugging of the reciprocating compressor, hunting of the
thermostatic expansion valve, and a decrease in the DX coil capacity may all occur at the same
time.
Refrigerant enters the various refrigerant circuits of the DX coil and the evaporator, typically as
a mixture of 75 percent liquid and 25 percent vapor, after passing through the thermostatic expansion
valve and the distributor tubes. If some of the refrigerant circuits have heavy refrigeration
loads and others have only very light loads, the refrigerant in circuits with heavy loads expands
rapidly to vapor, resulting in a greater vapor velocity and greater pressure loss. The refrigerant in
circuits with very low loads remains in a liquid state and flows to the compressor in the form of