TABLE OF CONTENTS The primary purposes of an air-conditioning system are to supply adequate ventilation air to, and to control the dry-bulb temperature in, each of the zones served by the system. A secondary purpose is to control relative humidity. These goals are typically accomplished by controlling the following parameters:
volume of outside air supplied by the central system,
total volume of air supplied by the central system,
temperature of air supplied by the central system,
volume of air supplied to each zone, and
temperature of air supplied to each zone.
In a typical VAV system, a zone volume control terminal modulates the volume of conditioned air supplied to the zone, in order to satisfy the zone temperature sensor. A variable-speed supply fan, discharge damper, or fan inlet vane modulates the total volume of air supplied by the system to maintain a duct static pressure setpoint. A cooling coil valve modulates the volume of fluid supplied to the coil to maintain the system supply air temperature at its setpoint. The fluid temperature is determined by a variety of chiller and central plant control sequences. An outside air damper, controlled by pressure relationships, a fan-tracking schedule, or air volume measurements, modulates to maintain the required volume of outside ventilation air. The various temperature and pressure setpoints may be modified according to appropriate reset schemes to optimize system operation. There are many variations and modifications to the standard VAV control systems that are beyond the scope of this guide. Additional coverage of the...
