Handbook of air conditioning and refrigeration / Shan K

21.40 CHAPTER TWENTY-ONE
● Modulates the discharge volume damper of the mixing VAV box so that the total supply volume
flow rate is maintained at mixing mode setting 30 percent of the peak cold air supply V˙ scx1,d during
the mixing mode operation
The cold deck discharge air temperature T c,dis is still 52°F (11.1°C). If the cold air supply in the
perimeter zone drops nearly to its minimum setting (30 percent of the peak supply volume flow
rate), the duct and fan power heat gain may have a temperature rise of 10°F (5.6°C) which raises
the cold supply air temperature in the perimeter zone to about 60°F (15.6°C).
If the hot-deck discharge air temperature T h,dis cannot be maintained at a value 5°F (2.8°C)
higher than the zone temperature T rxn, the DDC system controller opens the two-way valve of the
hot water coil and maintains a T h,dis � T rxn � 5. And T h,dis increases as T o falls.
The relationship between the outdoor air temperature T o and T h,o as well as T o and T c,o depends
on the characteristics of the zone load and the construction of the building shell.
6. When the outdoor air temperature T o � T c,o, the mixing VAV box is in heating mode operation.
In heating mode operation, the DDC system controller positions the outdoor and recirculating
dampers to provide required minimum ventilation as well as to maintain a mixture temperature of
50°F (10°C) if possible. The cold deck discharge air temperature remains at 52°F (11.1°C). Also the
two-way valve of the water cooling coil remains closed.
If the duct heat loss is about 2°F (1.1°C), the hot deck discharge air temperature is equal to
T h,dis � 72 � 15 � 2 � 89°F (31.7°C). The DDC system controller modulates the two-way valve
of the hot water coil in the AHU to maintain T h,dis � 89°F (31.7°C).
In the mixing box X1 in the perimeter zone, the DDC terminal controller modulates the inlet
mixing dampers and the discharge volume damper according to the sensed zone temperature and
airflow signal, to maintain a preset zone temperature of 72°F (22.2°C). As the zone heating load
increases, the warm air supply volume flow rate increases accordingly to a maximum value at winter
design conditions with a warm air supply temperature differential not exceeding 15°F (8.3°C).
Discharge Air Temperature Control
In general, the discharge air temperature from the cold deck must meet the year-round cooling requirements
for the interior zone. An air economizer cycle using free cooling should be applied to
the cold deck whenever possible, to meet the outdoor air requirements and to minimize refrigeration
in order to save energy. In the hot deck, internal heat gains carried by the recirculating air
should be fully utilized.
Cold deck discharge air temperature T c,dis is often reset according to zone demands at part load
or outdoor air temperature by means of a DDC unit controller. Hot deck discharge air temperature
T h,dis is often reset by outdoor temperature during winter heating mode.
Figure 21.11 shows the cold and warm deck discharge air temperature from the AHU of a
typical dual-fan dual-duct VAV system at various outdoor temperatures. The cold deck discharge
air temperature T c,dis is maintained at 52°F at all outdoor air temperatures, in order to
meet high zone cooling loads in the interior zone. However, when the cold air supply volume
flow rate in the perimeter zone reduces to about 30 percent of its peak volume flow during
part-load operation, the temperature rise because of the fan power and duct heat gain may increase
to 10°F (5.6°C), which results in a supply air temperature of cold air up to 60°F
(15.6°C).
When the outdoor air temperature T o drops from 70 to 60°F (21.1 to 15.6°C), the mixing of minimum
outdoor air and recirculating air lowers the hot deck discharge air temperature T h,dis from 80
to 76°F (26.7 to 24.4°C). If T o � 50°F (10.0°C), heating may be required in the perimeter zone, so
T h,dis increases as T o falls. If the duct heat loss is 2°F (1.1°C), the warm air supply temperature differential
T h,dis � 2 � T rxn should not exceed 15°F (8.3°C) even at winter design conditions, in order
to avoid stratification.