Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSThe examples in this section on Flow And Pressure ControlSolutions use a distribution system that has six equal loads(coils) as shown in Figure 73. These control solutions are:1. Single constant speed pump, single chiller system, twowayAHU control valves, and pressure bypass valve tocontrol chiller flow to a minimum of 90 percent full flow.a. direct return.b. reverse return.2. Dual constant speed pumps, dual chiller systems, andpressure bypass valve to control chiller flow to a minimumof 90 percent full flow.a. direct return.b. reverse return.3. High control valve differential pressure control.4. Decoupled variable speed secondary pumping systemwith two-way AHU control valves.a. direct return.b. reverse return.1'DROP1'DROPSUPPLY2'DROP1'DROP1'DROPEACH COIL AND ASSOCIATED PIPING TAKES AN 8' DROPthrough the chiller (1080 gpm), the chiller and equipment roompiping drops are 81 percent of design (90 percent squared).The pump curve (not shown) indicates a pump head of 50 ft at1080 gpm.VALVE LOCATION AND SIZINGSince the system piping between Loads 1 and 2 is designedfor only 1000 gpm and the low load bypass flow could exceedthat, the bypass valve is located remotely before Load 1. Ifnecessary to locate the bypass valve after Load 1, redesign thepiping to carry the 90 percent flow.If the differential pressure sensor is located across the mainlines at Load 1 as shown in Figure 75 (see DIFFERENTIALPRESSURE SENSOR LOCATION), the best place for thebypass valve is the same location. Because the differentialpressure is lower than in the chiller room, valve wear is less.The valve is sized for approximately 1000 gpm with a 34foot drop. A double seated valve is appropriate to reduce actuatorclose off requirements and the inherent leakage will not be asignificant factor.DIFFERENTIAL PRESSURE SENSOR LOCATIONHEAT/COOLCOIL1HEAT/COOLCOIL2HEAT/COOLCOIL3200 GPMPERAHU COILHEAT/COOLCOIL4HEAT/COOLCOIL5EACH CONTROL VALVE (NOT SHOWN) TAKES AN 8' DROPHEAT/COOLCOIL6As previously stated the chiller design flow is 1200 gpm at12 ft of head and requires a minimum flow of 1080 gpm. At1080 gpm the pump curve shows a head of 50 ft.From the formula:⎛gpm 2 ⎞2h 2⎜gpm ⎟ =⎝ 1 h⎠ 1B11'DROPB2BALANCINGVALVE1'DROPB32'DROPRETURNB41'DROPFig. 73 Typical Example Loads.B5 B61'DROPM15060Calculate the drop across the chiller as 9.6 ft.1080( 1200)2 h 2 =1212 (0.90) 2 = h 2h 2 = 12 x 0.81 = 9.6 ft.Single Pump, Pressure Bypass, Direct ReturnFigure 74 analyzes Figure 70 pumping system at full flowand at half flow. The flow reduction at half flow is taken evenlyacross each coil. At half flow with no pressure bypass the controlvalve pressure drops increase from 8 ft to 44 ft as system frictiondrops reduce to one-forth of the design values and the pumphead rises from 48 to 54 ft.Figure 75 shows a pressure controlled bypass valve set tomaintain 90 percent minimum flow through the chiller to satisfythe chillers minimum flow requirement. At 90 percent flowSimilarly calculate the reduced drop in the supply and return toLoad 1 as 3.2 ft.With the differential pressure controller located across Load 1,the setting is:50 ft – 9.6 ft – 3.2 ft – 3.2 ft = 34 ftThis location provides a lower head across the load controlvalves at light loads than if located across the pump and chiller.To ensure the best sensing, be sure that the system strainer islocated up stream from the differential pressure controller returnpickup, so that a dirty strainer is not sensed as an increasingpressure drop (decreasing flow).ENGINEERING MANUAL OF AUTOMATIC CONTROL326