Handbook of air conditioning and refrigeration / Shan K

AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.21
The modulation of valve V1 in the cooling coil control loop of discharge air temperature control
can be performed in parallel with the control operations in 100 percent air economizer control. Although
mixed air control is part of air economizer control, it is also a part of the sequence control in
a discharge air temperature control. Mixed air control mixes the outdoor air with the recirculating
air by modulating the economizer damper and the recirculating damper as to provide a mixed air
temperature, such as 55°F (12.8°C) for the sake of free cooling in the interior zone. Mixed air control
functions only when T o � 55°F (12.8°C) and ends during minimum outdoor air heating mode.
For VAV reheat and fan-powered VAV systems, heating is often provided by a water heating coil or
an electirc heating coil in a terminal directly above the conditioned space. The modulation of the VAV
box and the control valve of the heating coil in sequence forms the zone cooling and heating control.
For a single-zone VAV system, a DDC unit actuates the economizer dampers, cooling coil valve,
economizer and recirculating dampers, and heating coil valves in sequence, based on the input
signal from a zone temperature sensor to maintain a predetermined zone temperature. The discharge
air temperature control is replaced by a zone temperature control.
Operation of Air Economizer and Outdoor Air Intake—Case Study
The outdoor air intake of a fixed dry-bulb air economizer in an AHU of a VAV system shown in
Fig. 23.7 is from Nabinger et al. (1994). In Fig. 23.7, the abscissa of the diagram is the indoor-outdoor
temperature difference of the conditioned space that the VAV system serves and the outdoor
FIGURE 23.7 The outdoor air intake of a temperature air economizer in a VAV system. (Modified from Nabinger
et al., ASHRAE Transactions, Part II, 1994. Reprinted by permission.)