Handbook of air conditioning and refrigeration / Shan K

System Description
AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.19
systems. The following list relates parameters when �T s � 15 and 20°F except in an ice storage
system in which �T s may increase up to 30°F (16.7°C). All units are in °F (°C), except the space
relative humidity � r is in percent:
�T s T cc �T sf T dis �T du T s T r � r
15 (8.3) 55 (12.8) 2 (1.1) 57 (13.9) 3 (1.7) 60 (15.6) 75 (23.9) 50
20 (11.1) 50 (10.0) 2 (1.1) 52 (11.1) 3 (1.7) 55 (12.8) 75 (23.9) 45
Discharge air temperature control consists of the following control loops, as shown in Fig. 23.6a:
● Economizer control loop. For a temperature air economizer control, it consists of an outdoor temperature
sensor T2, a recirculating air temperature sensor T3, a DDC controller, outdoor damper
actuators D1 and D2, a recirculating damper actuator D3 and a relief damper actuator D 4.
● Cooling coil control loop. It consists of a discharge air temperature sensor T1, a DDC controller,
and a cooling coil valve actuator V1.
● Mixed air control loop. It consists of a mixed air temperature sensor T4, a DDC controller, outdoor
damper actuators D1 and D2, and a recirculating damper actuator D3.
● Heating coil control loop. It consists of a discharge air temperature sensor T1, a DDC controller,
and a heating coil valve actuator V2.
● Low-limit control loop (coil freeze protection). It consists of a temperature sensor T4, a DDC controller,
and outdoor air damper actuators D1 and D2.
For VAV systems, discharge air temperature control is a kind of cooling and heating capacity
control so that conditioned air is discharged from an AHU or PU with a predetermined temperature
and relative humidity, such as 57°F (13.9°C) and 95 percent relative humidity, and can perform the
following requirements:
1. Provide a kind of conditioned air for zone temperature control by varying the supply volume
flow rate.
2. Based on the conditions of air leaving the coil sensed by sensor T1, provide economizer control
and modulate the cooling or heating capacity of the AHU or PU to maintain a predetermined T dis.
3. In VAV systems, if an AHU or a PU serves both perimeter and interior zones, usually perimeter
heating is provided by the heating coil installed in the reheating VAV boxes or fan-powered VAV
boxes or by baseboard heating directly. In such a condition, T dis should meet the free-cooling
requirement in interior zones even in winter.
4. According to the outdoor temperature or system load, reset T dis to an optimum value for both
effective operation and energy saving.
For a cooling coil or heating coil in the AHU, the DDC controller modulates the coil valves continuously.
However, in a rooftop packaged unit, refrigeration capacity is often adjusted by cycling
the compressors on and off in the separate refrigerant circuits of the DX coils. The capacity control
of packaged units is discussed in Sec. 11.8.
Most VAV systems use the air economizer or water economizer as the first-stage cooling to save
energy. Cooling coil, mixed air, and heating coil control loops are operated in sequence so as to maintain
a predetermined discharge air temperature while preventing simultaneous cooling and heating.
For a typical AHU, the set point of the discharge air temperature at summer design conditions
may be 57°F (13.9°C). To avoid sudden repeated changes in operating mode among the cooling coil
and mixed air control loops, resulting in unstable operation, the set point for mixed air control is
better set at 55°F (12.8°C) as shown in Fig. 23.6b.