Engineered Systems - December 2008 - (Page 58)

60 50 System head — ft 40 Condensor water system with constant head 30 Minimum flow 20 Variable torque operation 10 Wire-to-water efficiency has been used for programming pumps, but it has been replaced by pump input kW that does not require the differential pressure transmitter for measuring pump head. Centrifugal pumps are usually considered variable torque machines where the torque required to turn the pump varies as the square of the speed. M1 = S(1)2 M2 S(2)2 where M is the torque and S is the pump speed (4). 0 0 1 2 3 4 5 6 7 8 Flow — 1,000’s gpm FIGURE 2. Condenser water system head curves. 100 90 80 70 60 50 40 30 20 Minimum speed 10 0 0 2 4 6 8 10 12 14 16 Pump speed — 100’s rpm FIGURE 3. Wire-to-shaft efficiency curve. This assumes that the pump performs in accordance with the affinity laws for centrifugal pumps that state that the head on a pump varies as the square of the pump speed. Such is not the case for most variable-speed chilled water or condenser pumps when a constant head exists in the water system. For chilled-water systems, a differential pressure is maintained on the system to provide pump head for the terminal units and their piping and control valves. The constant head in a condenser water system is the lift over the cooling tower plus the friction loss in the chiller condenser. Why is this important? It is important since this added head no longer allows the pump to operate as a normal variable-torque machine. Figure 1 describes the system head curve for a 600 ton, 1,200 gpm chilled water system operating with a fixed differential pressure of 15 ft and a total pump head of 75 ft; it also includes a curve for a 1,200 gpm system and 75 ft without the differential pressure and operating with normal variable torque pump performance. The curves begin at 360 gpm and 30% speed, minimums for most chilled water systems. Only one chiller is shown to simplify the calculations. Table 1 provides the actual data for these curves and demonstrates the difference in pump speed and brake hp for the two curves. In most cases, there is little energy savings in making condenser pumps variable speed. The pump energy saved is often less than the added energy consumed by the chiller with variable flow in its condenser. The only variable head is the system friction; in this instance, this is 35 ft. Only when there are a number of chillers and a sizeable amount of system piping friction due to remote cooling towers can variable speed be justified. Wire-to-shaft efficiency-% 58 En g i neer ed Sy stem s December 2008

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Engineered Systems - December 2008 Contents Editor's Note HVAC Challenge Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Applications Checklist Lakefront Library: Radiant Systems Meet Natural Ventilation Before (And After) The Flood Basics For Refrigerant Chillers Wire-To-Shaft Efficiency For HVAC Pumps Products Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - December 2008

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