Engineering Manual o.. - HVAC.Amickracing

PNEUMATIC CONTROL FUNDAMENTALSHIGH-PRESSURE SELECTOR RELAYThe high-pressure selector relay is a three-port relay thattransmits the higher of two input signals to the output branch.The high sensitivity of the relay allows it to be used in sensorlines with an accuracy of 1 to 1.5 degrees C.The application shown in Figure 39 uses pressures from twozones and a high-pressure selector relay to determine control.A separate thermostat controls each zone damper. Thethermostat that calls for the most cooling (highest branchlinepressure) controls the cooling valve through the high-pressureselector relay.DA ZONETHERMOSTATMM BMDA ZONETHERMOSTATM BLOAD ANALYZER RELAYThe load analyzer relay is a bleed-type, diaphragm-logicpressure selector. The relay selects the highest and lowest branchpressure from multiple inputs to operate final control elements(Fig. 41). The relay contains multiple diaphragms and controlnozzles. Each input pressure connects to two diaphragms.MM BMDA ZONE THERMOSTATSM BMM BMM BTOZONEDAMPERSN.C. ZONEDAMPER15-50 kPaDAMPERACTUATORP1P2BHIGH-PRESSURESELECTOR RELAYN.C. ZONEDAMPER15-50 kPaDAMPERACTUATOR55-85 kPaN.C. COOLINGVALVEMHIGHEST 1 2 3 NMLOWESTLOAD ANALYZER RELAY55-85 kPaCOOLING COILVALVEM1514715-50 kPaN.O. HEATINGVALVEM15149Fig. 39. Typical Application forHigh-Pressure Selector Relay.Fig. 41. Load Analyzer Relay inMultizone Air Unit Application.LOW-PRESSURE SELECTOR RELAYThe low-pressure selector relay is a three-port relay thatselects the lower of two input pressure signals and acts as arepeater for the lower of the two inputs. The relay requires anexternal restrictor on the input to the branch port. Figure 40shows a low-pressure selector relay controlling the heating coilvalve from the thermostat that calls for the most heat.N.O. ZONEDAMPERM55-85 kPa DAMPERACTUATORRESTRICTORDA ZONETHERMOSTATM BEXHMP 1P 2DA ZONETHERMOSTATM BN.O. ZONEDAMPERIn Figure 41, the load analyzer relay selects the lowestpressure signal from the thermostat in the coldest zone andtransmits that signal to a normally open heating valve. The relaytransmits the highest pressure signal from the thermostat in thewarmest zone to a normally closed cooling valve.CAPACITY RELAYThe capacity relay is a direct-acting relay that isolates aninput and repeats the input pressure with a higher capacityoutput. Figure 42 shows a capacity relay enabling a single bleedtypethermostat to operate multiple damper actuators quicklyby increasing the output capacity of the thermostat.BLOW-PRESSURESELECTOR RELAY55-85 kPaDAMPERACTUATOR15-50 kPa N.O.HEATING COILVALVEM15148Fig. 40. Typical Application forLow-Pressure Selector Relay.ENGINEERING MANUAL OF AUTOMATIC CONTROL 79