Handbook of air conditioning and refrigeration / Shan K

where Qrs � zone sensible cooling load, Btu/h (W), and Tr, Ts � zone and supply air temperature,
°F (°C). In Eq. (21.1), [m3 /(60 s)] means that in SI units, the unit should be � m3 1 V˙
s
�60 /s.
From Eq. (20.48b), the design cooling coil load Qcc, in Btu/h (W), can be calculated as
(21.2)
where h m � enthalpy of the mixture of outdoor and recirculating air, Btu/lb (J/kg), and h cc �
enthalpy of conditioned air leaving the cooling coil, Btu/lb (J/kg). From Eq. (20.85), the design
heating coil load Q ch, in Btu/h (W), which offsets the space heating load or sometimes maintains a
setback temperature during unoccupied periods to reduce warm-up load next morning and prevent
freezing, can be calculated as
where T hc, T en � air temperature leaving and entering heating coil, °F (°C).
Zone Temperature Control—Sequence of Operations
AIR SYSTEMS: VARIABLE-AIR-VOLUME SYSTEMS 21.17
Q cc � 60V˙ s� s(h m � h cc)
Q ch � 60V˙ s� sc pa(T hc � T en)
(21.3)
Consider a single-zone VAV system using an AHU with a water cooling coil, a water heating coil,
and a temperature economizer (air economizer), as shown in Fig. 21.1. The sequence of operations
of this VAV system to maintain a preset zone temperature T r is as follows:
1. When the AHU is in the off position, the outdoor and exhaust dampers are closed, the two-way
valves of the cooling and heating coils are also closed, and supply and return fan motors are off.
2. When the AHU is in the on position, zone temperature sensor T1 senses the zone temperature
T r, sends a signal to the DDC controller, and compares it with the set point in summer of 75°F
(23.9°F) in the controller. If T r > 75°F (23.9°C), the DDC controller calls for cooling—cooling
mode operation. At the same time, the outdoor temperature T o is measured by the outdoor temperature
sensor T2. If T o � 75°F (23.9°C) (a temperature economizer set point), the DDC controller
opens the outdoor and exhaust dampers to a minimum opening position, and fully opens the recirculating
damper. The DDC controller also starts supply and return fan motors with speed control
via variable speed drives.
3. The discharge air temperature T dis is sensed by temperature sensor T4. During cooling
mode operation, if the off-coil temperature T cc is 55°F (12.8°C), the set point of T dis � 55 � 2
(fan power) � 57°F (13.9°C). When T dis � 57°F (13.9°C) is sensed by T4, the DDC controller
opens and modulates the two-way valve of the water cooling coil so as to maintain T dis � 57°F
(13.9°C).
4. If T o � 75°F (23.9°C), using the free cooling of outdoor air is always beneficial in order to
save energy. The DDC controller fully opens the outdoor and exhaust dampers and, at the same
time, closes the recirculating damper. After the air economizer is energized, if T dis is still higher
than 57°F (13.9°C), the DDC unit controller will open and modulate the two-way valve of the water
cooling coil to maintain T dis � 57°F (13.9°C).
5. Zone temperature T r is sensed by a temperature sensor T1. This signal is compared with the
zone temperature set point T r,s, such as 75°F (23.9°C) in the DDC controller. If T r drops below or
rises above 75°F (23.9°C), an output from the DDC controller actuates and modulates the supply
fan speed, and thus the supply volume flow rate through an adjustable-frequency variable-speed
drive, to maintain T r � 75°F (23.9°C). For single-zone VAV system serving a large conditioned
space, several temperature sensors can be located in different areas. Either the mean value or the
highest or lowest temperature can be taken as sensed input to the DDC controller.
6. A pressure sensor P1 is used to sense the zone pressure �p r in the conditioned space. If �p r �
0.03 in. WC (7.5 Pa), the DDC controller starts the return fan and modulates its fan speed to maintain
a zone pressure �p r � 0.03 in. WC (7.5 Pa).