Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSFEATURES1. Preheat coil conditions large quantities of low temperatureOA before it enters the system.2. Bypass damper controls temperature of air leaving theface-and-bypass section without endangering the preheatcoil which operates in full valve open mode duringfreezing conditions.3. Upon fan shutdown, valve controls at 100°F leaving airtemperature in freezing weather and positions closed uponfan shutdown in non-freezing weather.CAUTION: If steam (or very hot water) valvesposition full open during off periods, airtemperatures may melt fire damperlinkages.The heating coil valve shall position from 0 to 100% open asthe OA temperature varies from 50°F to 35°F. The face-andbypassdampers shall modulate to maintain an average faceand-bypassleaving air temperature of 50°F.PSYCHROMETRIC ASPECTSIn the following chart it is assumed that:1. The coil has a 50 Fahrenheit degree temperature rise withfull air flow 0°F air and a 70 Fahrenheit degreetemperature rise with reduced air flow as in the example.2. The coil water flow modulates from closed to open asOA temperature drops from 50°F to 35°F.3. OA temperature is 30°F DB.CONDITIONS FOR SUCCESSFUL OPERATION1. Preheat coil sized to raise a maximum quantity of OAfrom outdoor design temperature to 50°F.2. Averaging element sensor positioned in face-and bypassair to sense average air temperature.3. Bypass air well mixed with face (and return) air prior toentering down-stream water coils (additional lowtemperature controls may be required for down-streamwater coils).4. On large 100% OA systems the OA damper opens priorto fan start up to protect ducting from collapse.5. The setpoint of air leaving the preheat section is usuallydictated by the downstream AHU temperature controls.SPECIFICATIONSSee FAN SYSTEM START-STOP CONTROL.Anytime the supply fan runs, heating control shall be enabled,and the OA dampers shall open for ventilation requirements.OA AND AIRLEAVING BYPASSSECTION 30°F DB2AIR LEAVINGPREHEATSECTIONThe following results are obtained:ItemNo.1ExplanationAIR LEAVINGHEATING COIL100°F DBC2552-21 Heating of OA occurs along a line of constantmoisture content from 30°F to 100°F.2 The PID control loop maintains thetemperature of the preheat section leaving air.The heating coil valve shall open upon loss of actuator motiveforce, shall close upon fan shutdown if the OA temperature isabove 35°F, and shall control to maintain 100°F leaving airtemperature upon fan shutdown if the OA temperature is below35°F.ENGINEERING MANUAL OF AUTOMATIC CONTROL212