Engineering Manual o.. - HVAC.Amickracing

PNEUMATIC CONTROL FUNDAMENTALSMROOMTHERMOSTATM BDA WINTERRA SUMMERRESTRICTORSWITCHINGRELAYC O CAPMPXDA OUTDOOR AIRTHERMOSTATMM BPXMC OSNAP ACTINGRELAYDA ROOMTHERMOSTATMBVAV TERMINAL UNITDAMPER ACTUATORVALVECOILFig. 35. Typical Switching Relay for Application.During the cooling cycle, the 90 kPa mainline pressure atthe pilot port (P) is below the minimum switching pressure ofthe preset limits. The common port (C) connects to the normallyconnected port (O), which is capped. The normally disconnectedport (X) is closed and removes the low-limit controller fromthe system.In a dpdt model, the common, normally connected, andnormally disconnected ports are duplicated in the secondswitch section.SNAP ACTING RELAYLIMITCONTROLLERDISCHARGEAIRC2379The snap acting relay is a spdt switch that provides twopositionswitching action from a modulating signal and has anadjustable switching point. The switching differential is lessthan 7.0 kPa. The switching pressure is manually adjustablefor 20 to 100 kPa operation.Figure 36 shows a snap acting relay application. Operationis similar to the switching relay. When the branchline pressurefrom the outdoor air thermostat equals or exceeds the presetswitchover pressure, the relay connects the normallydisconnected port (X) and blocks the normally connected port(O) to deliver main air to the normally open heating valveand provide positive close off. When the outdoor air thermostatpressure drops below the relay setpoint, the normallydisconnected port (X) is blocked and the normally connectedport (O) connects to the common port (C) to connect the valveactuator to the room thermostat.MMN.O. HEATINGVALVEMIXED AIRTEMPERATURECONTROLLERM BEXHRETURN AIRTHERMOSTATM BSNAP ACTINGRELAYX COPBLEEDLOCKOUTRELAYP 1BP 2C2360Fig. 36. Typical Application for Snap Acting Relay.LOCKOUT RELAYThe lockout relay is a three-port relay that closes off onepressure signal when a second signal is higher. Figure 37 showsa typical application in which mixed air control becomesdisabled when outdoor air temperature is higher than return airtemperature. To prevent air from being trapped in the linebetween the lockout relay and the snap acting relay, a smallbleed must be present either in the pilot chamber of the snapacting relay or in the line.MEXH P 1P 2BRESTRICTORLOCKOUTRELAYOUTPUTTHERMOSTATOUTDOOR AIRDAMPER ACTUATOROUTDOOR AIRTHERMOSTATC2355M BRESTRICTORTOACTUATORMMC2362Fig. 37. Lockout Relay in Economizer Cycle.Figure 38 shows the lockout relay used as a repeater. Thisapplication provides circuit isolation by repeating the pilotsignal with a second air source.Fig. 38. Lockout Relay as Repeater.78ENGINEERING MANUAL OF AUTOMATIC CONTROL