Engineering Manual o.. - HVAC.Amickracing

PNEUMATIC CONTROL FUNDAMENTALSThe oil coalescing filter continues to coalesce and drain offaccumulated oil until solid particles plug the filter. An increasein pressure drop across the filter (to approximately 70 kPa)indicates that the filter element needs replacement. For verydirty air, a 0.005 millimeter prefilter filters out large particlesand increases the life of the final filter element.PRESSURE REDUCING VALVESA pressure reducing valve station can have a single-pressurereducing valve or a two-pressure reducing valve, depending onthe requirements of the system it is supplying.Single-Pressure Reducing ValveAfter it passes though the filter, air enters the PRV (Fig. 11).Inlet pressure ranges from 415 to 1035 kPa, depending on tankpressures maintained by the compressor. Outlet pressure isadjustable from 0 to 175 kPa, depending on the control airrequirements. The normal setting is 140 kPa.A safety relief valve is built into some PRV assemblies toprotect control system devices if the PRV malfunctions. Thevalve is typically set to relieve downstream pressures above165 kPa.Two-Pressure Reducing ValveA two-pressure reducing valve is typically set to pass 90 or124 kPa to the control system, as switched by a pilot pressure.The two-pressure reducing valve is the same as the singlepressurereducing valve with the addition of a switchoverdiaphragm and switchover inlet to accept the switchoverpressure signal. Switchover to the higher setting occurs whenthe inlet admits main air into the switchover chamber.Exhausting the switchover chamber returns the valve to thelower setting.The switchover signal is typically provided by an E/P relayor a two-position diverting switch. An automatic time clockcan operate an E/P relay to switch the main pressure for aday/night control system. A diverting switch is often used tomanually switch a heating/cooling system.In many applications requiring two-pressure reducingvalves, a single-pressure reducing valve is also required tosupply single-pressure controllers which do not perform wellat low pressures. Higher dual pressure systems operating at140 and 170 kPa are sometimes used to eliminate the needand expense of the second PRV.THERMOSTATSThermostats are of four basic types:— A low-capacity, single-temperature thermostat is the basicnozzle-flapper bleed-type control described earlier. It isa bleed, one-pipe, proportional thermostat that is eitherdirect or reverse acting.— A high-capacity, single-temperature thermostat is a lowcapacitythermostat with a capacity amplifier added. It isa pilot-bleed, two-pipe, proportioning thermostat that iseither direct or reverse acting.— A dual-temperature thermostat typically providesoccupied/unoccupied control. It is essentially twothermostats in one housing, each having its own bimetalsensing element and setpoint adjustment. A valve unitcontrolled by mainline pressure switches between theoccupied and unoccupied mode. A manual override leverallows an occupant to change the thermostat operationfrom unoccupied operation to occupied operation.— A dual-acting (heating/cooling) thermostat is anothertwo-pipe, proportioning thermostat that has two bimetalsensing elements. One element is direct acting for heatingcontrol, and the other, reverse acting for cooling control.Switchover is the same as for the dual-temperaturethermostat but without manual override.Other thermostats are available for specific uses. Energy conservationthermostats limit setpoint adjustments to reasonableminimums and maximums. Zero energy band thermostatsprovide an adjustable deadband between heating and coolingoperations.The thermostat provides a branchline air pressure that is afunction of the ambient temperature of the controlled spaceand the setpoint and throttling range settings. The throttlingrange setting and the setpoint determine the span and operatingrange of the thermostat. The nozzle-flapper-bimetal assemblyENGINEERING MANUAL OF AUTOMATIC CONTROL 69