Engineered Systems - December 2008 - (Page 60)

Wire-To-Shaft Efficiency For HVAC Pumps Chilled water system with constant head Pump Head - ft 20.40 24.60 30.00 36.60 44.40 53.40 63.60 75.00 Pump rpm 867 964 1,077 1,201 1,637 1,695 1,739 1,770 Pump bhp 2.5 3.8 5.5 7.9 11.1 15.1 20.2 26.4 Pump Head - ft 6.75 12.00 18.75 27.00 36.75 48.00 60.75 75.00 Variable torque operation Pump rpm 531 708 885 1,062 1,239 1,416 1,593 1,770 Pump bhp 0.7 1.7 3.3 5.7 9.0 13.5 19.2 26.4 System flow-gpm 360 480 600 720 840 960 1,080 1,200 TABLE 1. Comparison of brake hp and pump speeds. Chilled water system with constant head Pump Head - ft 26.4 30.6 37.6 47.4 60 Pump rpm 1,422 1,468 1,554 1,634 1,770 Pump bhp 16.9 18.8 21.5 25.2 31.7 Pump Head - ft 2.4 9.6 21.6 38.4 60 Variable torque operation Pump rpm 71 283 443 1133 1770 Pump bhp 0.3 2 6.8 16.2 31.7 System flow-gpm 1,800 3,600 5,400 7,200 8,400 For chilled-water h d t 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. develop data for such hybrid pump operation so that HVAC engineers can compute energy estimates for variable speed pumps with reasonable accuracy. The brake hp and pump speed data in Tables 1 and 2 for the chilled water system and for the condenser water system should be adequate for the computation of these wireto-shaft efficiency curves. Since almost all new chilled water pumps are now variable speed, it is imperative that the procedure for computing part-load energy consumptions be readily available with accurate data on the wire-to-shaft efficiency for the known motor size and type, pump speeds, and brake hp. ES Rishel is a professional engineer in Cincinnati, who has devoted much of his life to the improvement of HVAC pumping and chilled water distribution. He is director of mechanical operations for TekWorx in Cincinnati. TABLE 2. Comparison of brake hp and pump speeds. Pump speed-rpm 400 600 800 1,000 Wire-to shaft efficiency 39 55 67 77 Pump speed-rpm 1,200 1,400 1,600 1,770 Wire-to shaft efficiency 83 86 88 89 TABLE 3. Wire-to-shaft efficiencies. As shown, the constant head is the lift over the cooling towers plus the friction loss in the chiller condenser. Figure 2 illustrates the system head curve for the condenser water system as well as a similar pump curve for the pump operating as a variable torque machine. Table 2 describes an even greater disparity between the two system head curves for condenser water than that in Table 1 for chilled water. There are only five flows in Table 2, since there is constant flow in the chiller condensers, and five chillers staged on and off with the load on the chilled water system. It is obvious from the data in Tables 1 and 2 that the wire-to-shaft efficiency curves for many HVAC water systems are different than those for variable torque operation. Unfortunately, there are no known published data for a wire-to-shaft efficiency curve for a pump operating at higher heads than those for true variable torque operation. The wire-to shaft efficiency for these higher heads should be greater than that for variable torque operation since the motor is outputting higher horsepowers at reduced speeds. Table 3 provides wire-to-shaft efficiencies at various speeds, and Figure 3 is the resulting wire-to-shaft efficiency curve for a 30 hp, high-efficiency, open frame, drip-proof motor that would be required for the above chilled water system. This data is from the author’s experience with variable-speed pumps and VFDs. It should be replaced by a curve based upon the actual brake hp and speeds of the pump under evaluation. One reason for publishing this article is to encourage people in the VSD industry to REFERENCE 1. Rishel, Durkin, and Kincaid, HVAC Pump Handbook, McGraw Hill Companies, New York City, NY, 2006: 299. 60 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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