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 2 to 3 degrees F.The application shown in Figure 38 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 BN.O. ZONEDAMPERM8-12 PSI DAMPERACTUATORRESTRICTORDA ZONETHERMOSTATM BEXH2-7 PSI N.O.HEATING COILVALVEP 1P 2Fig. 39. Typical Application forLow-Pressure Selector Relay.BMDA ZONETHERMOSTATM BLOW-PRESSURESELECTOR RELAYN.O. ZONEDAMPER8-12 PSIDAMPERACTUATORC2364N.C. ZONEDAMPER2-7 PSIDAMPERACTUATOR8-12 PSI N.C.COOLING COILVALVEP1P2BHIGH-PRESSURESELECTOR RELAYN.C. ZONEDAMPER2-7 PSIDAMPERACTUATORC2363LOAD ANALYZER RELAYThe load analyzer relay is a bleed-type, diaphragm-logicpressure selector. The relay selects the highest and lowestbranch pressure from multiple inputs to operate final controlelements (Fig. 40). The relay contains multiple diaphragmsand control nozzles. Each input pressure connects to twodiaphragms.Fig. 38. Typical Application forHigh-Pressure Selector Relay.DA ZONE THERMOSTATSLOW-PRESSURE SELECTOR RELAYMM BMM BMM BMM BThe 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 39shows a low-pressure selector relay controlling the heating coilvalve from the thermostat that calls for the most heat.MHIGHEST 1 2 3 NMLOWESTTOZONEDAMPERS8-12 PSIN.C. COOLINGVALVELOAD ANALYZER RELAY2-7 PSIN.O. HEATINGVALVEC2369Fig. 40. Load Analyzer Relay inMultizone Air Unit Application.In Figure 40, 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.ENGINEERING MANUAL OF AUTOMATIC CONTROL 77