Engineering Manual o.. - HVAC.Amickracing

PNEUMATIC CONTROL FUNDAMENTALSBRANCHLINEINLETFLOWF3F1F2OUTLETFLOWNORMALLY OPEN VALVEC2612Two- and three-way butterfly valves can be operated by longstroke pneumatic actuators and appropriate linkage (Fig. 31).One or two low pressure actuators powered directly bybranchline pressure can operate butterfly valves up to about300 millimeters, depending on the differential close-off ratingof the valve. For other applications high pressure pneumaticcylinders can be used to provide the force required by the valve.A pneumatic positioner provides an appropriate high pressuresignal to the cylinder based on a 20 to 100 kPa inputsignal.Fig. 29. Double-Seated Valve.Figure 30 shows three-way globe valve assemblies. Themixing valve has two inlets and a common outlet. The divertingvalve has a common inlet and two outlets.BRANCHLINEF1F2INLETFLOWOUTLETFLOWINLET FLOWMIXING VALVE, NORMALLY CLOSED TOSTRAIGHT-THROUGH FLOWFig. 31. Butterfly Valve Assembly.M10403BRANCHLINEF1F2For a more detailed discussion of valves, see the ValveSelection And Sizing section.DAMPERSINLETFLOWOUTLETFLOWDampers control the flow of air in air-handling systems. Themost common type of damper, a multiblade louver damper,can have parallel or opposed blades (Fig. 32).OUTLET FLOWDIVERTING VALVE, NORMALLY OPEN TOSTRAIGHT-THROUGH FLOWC2615Fig. 30. Three-Way Valve Assemblies.Three-way valves may be piped to be normally open ornormally closed to the heating or cooling load. If a three-wayvalve has linear characteristics and the pressure differentialsare equal, constant total flow is maintained through the commoninlet or outlet port.PARALLELBLADESOPPOSEDBLADESC2604Fig. 32. Parallel- and Opposed-Blade Dampers.76ENGINEERING MANUAL OF AUTOMATIC CONTROL