Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSPREHEAT CONTROL PROCESSESThe preheat process heats the air in preparation for subsequentconditioning. Preheat is sometimes necessary when highpercentages of low temperature OA must be handled by thesystem. Considerations before a preheat component is installedin an air conditioning system are:1. Preheat coils are often exposed to subfreezingtemperatures. Provision to prevent freezing the coils mustbe made.2. Accurate sizing of preheat coils is important. They shouldbe sized so it is possible to allow OA for cooling yet notoverheat the space.In most of the following illustrations, preheat is shown in theOA preceding the mixing of OA and RA. In complete HVACsystems, the final preheat coil leaving air temperature setpointshould be dictated by demands for heating or cooling.PREHEAT CONTROL WITH FACE AND BYPASS DAMPERSFunctional Description11CONTROLPROGRAM8950RA3OPEN44PERCENTOPENTO COIL1050OAN.C.N.O.N.C.MASA44066N.O.5NORMAL21ON7VALVE POSITIONSCHEDULEOUTDOORAIRTEMPERATURE50VALVEPERCENTOPEN06CONTROLPROGRAM35100M10462ItemNo.Function1-2 Control system energizes when fan is turnedon (See FAN SYSTEM START-STOPCONTROL).3 OA damper opens on fan startup, closes uponfan shutdown (in some cases the OA dampermay be part of a mixed air system).4 OA temperature determines valve position.5 Valve controls heating medium flow to coil.6 Control program coordinates fan interlock andvalve control.7 Valve position determined by OAtemperature.8-10 Face-and-bypass dampers controlled tomaintain face-and-bypass leaving airtemperature setpoint.11 Control program coordinates fan interlock andface-and-bypass damper control.ENGINEERING MANUAL OF AUTOMATIC CONTROL211