Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSSUPPLYV8CHILLER1CHILLER2DP1DPHEAT/COOLCOIL1HEAT/COOLCOIL2HEAT/COOLCOIL3HEAT/COOLCOIL4HEAT/COOLCOIL5HEAT/COOLCOIL6V7V1V2V3V4V5V62M15065B1B2B3B4B5B6RETURNFig. 78. High AHU Valve Differential Pressure ControlIn these examples, the design differential pressure acrossLoad 1 is 84 kPa (Fig. 74), and pressure bypassing occurs at103.5 kPa. The maximum setpoint for DP-2 (Fig 78) shouldbe about 87 or 90 kPa. This initial setpoint is then slowlylowered based upon the percent-open values of load controlvalves V1 through V6, to a minimum value of 30 to 36 kPa.Specification:Anytime either chiller pump starts, DP-2 shall be enabledto control pressure reduction valve V8 at an initial setpoint of60 kPa. Anytime any load control valve is greater than95 percent open, the DP-2 setpoint shall be incremented at therate of 1.5 kPa every 2.0 minutes up to a maximum of 90 kPa.Anytime all control valves are less than 80 percent open theDP-2 setpoint shall be decremented at the same rate to aminimum value of 36 kPa. All values shall be user adjustable.Specification Discussion:The value of 2.0 minutes in the specification assumes thatvalves V1 through V6 are controlled from discharge airtemperature and should recover in less than 2 minutes fromV8-caused changes. If V1 through V6 were controlled fromspace temperature directly, the time rate for V8 adjustmentsmay need to be extended to 12 to 15 minutes to allowincremental space temperature control recovery from the flowreductions brought about by V8. Valve V8 is normally openand line size to minimize its pressure drop during full loadoperation. This valve is applicable for direct or reverse returnpiping configurations where significant piping friction lossesmigrate out to the control valves upon low flow conditions.In reverse return systems, locate Valve V8 in the main linepiping just before the AHU 1 takeoff and the DP-2 sensor acrossAHU 1 set for approximately 57 kPa, this should be adequatefor any variation in uneven loading. Allow 57 kPa for close-offand good control. Resetting the DP-2 setpoint lower isunnecessary in this constant-speed pumping reverse returnexample.VARIABLE SPEED PUMP CONTROLDecoupled Variable Speed PumpControl, Direct ReturnSimilar to Figure 74, Figure 79 decouples the loads from thesource pumps and heat exchangers and uses of a variable speedpump to provide significant energy savings at reduced loads andsimpler control. Variable speed pump control matches pump speedto system flow demands. If source devices perform well withvariable flow, the primary pumps may be replaced with variablespeed pumps and controlled similarly to the decoupled example.NOTE: The variable speed pump pressure is only 108 kPasince the primary pumps account for the 36 kPa chillerpressure drop.ENGINEERING MANUAL OF AUTOMATIC CONTROL353