ABSTRACT
Air is heated and cooled by passing it over a heat transfer device called a “coil,” which is a tube-and-plate heat exchanger with air on the outside and the primary heating/cooling medium on the inside. Two aspects of the selection of these coils are important: provision must be made to de-energize the coils if airflow is interrupted to prevent their overheating, and capacity variation can be done only in “steps,” unless expensive silicon-controlled rectifier controllers are utilized. When the outdoor air temperature is above freezing, the control valve can be modulated to maintain discharge air temperature set point. An alternative design for hot water preheat control is to incorporate a modulating two-way control valve and integral face and bypass dampers. Virtually all heating and cooling systems for commercial buildings rely on the distribution and circulation of air as the medium for heat transfer to, within, or from the occupied space.
