Page 299 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

296 The HVAC World

This table demonstrates that, in this case, the three-pump system
provides a higher wire-to-water efficiency throughout the load range
than does the two-pump system of 2500-gal/min pumps. The actual
pump selection has some bearing on these results, so this should not
imply that in every case it would be more efficient to use three
instead of two pumps. Such a wire-to-water efficiency program should
be run on every application where substantial motor horsepowers are
involved. A significant fact that can be secured from this particular
calculation is that two pumps should be operated from 1000 gal/min
upward even though one pump could hand the water system up to
1685 gal/min.

10.8.3 Wire-to-water efficiency for
constant-speed pumps
Wire-to-water efficiency can be computed for both constant- and
variable-speed pumping systems. One of the unique advantages of
this program is the ability to compute the wire-to-water efficiency
for a pumping system with constant- and variable-speed pumping.
Table 10.6 describes the wire-to-water efficiency of the two-pump
constant-speed system. The pump curve of Fig. 10.11b applies in this
case, and only one pump runs at a time.

TABLE 10.6 Wire-to-Water Efficiency Calculations for a Constant-Speed Pumping
System
System System Fitting Pump Pump Pump W/W
gal/min head, ft Water hp loss, ft head, ft rev/min efficiency Input kW efficiency
0500 23.6 3.0 0.4 145.0 1780 34.0 44.8 5.0
0625 25.4 4.0 0.6 144.0 1780 41.0 46.0 6.5
0750 27.5 5.2 0.9 142.9 1780 47.5 47.2 8.3
0875 30.0 6.6 1.2 141.5 1780 53.4 48.3 10.2
1000 32.9 8.3 1.6 139.9 1780 58.7 49.5 12.5
1125 36.0 10.2 2.0 138.1 1780 63.4 50.6 15.1
1250 39.4 12.4 2.5 136.1 1780 67.6 51.7 18.0
1375 43.2 15.0 3.0 133.8 1780 71.3 52.7 21.2
1500 47.2 17.9 3.6 131.2 1780 74.5 53.7 24.8
1625 51.5 21.2 4.2 128.3 1780 77.2 54.7 28.8
1750 56.2 24.8 4.9 125.1 1780 79.5 55.6 33.3
1875 61.1 28.9 5.6 121.6 1780 81.3 56.5 38.2
2000 66.3 33.5 6.4 117.8 1780 82.6 57.3 43.6
2125 71.8 38.5 7.0 113.8 1780 83.6 58.1 49.5
2250 77.6 44.1 8.1 109.5 1780 84.2 58.7 56.1
2375 83.7 50.2 9.0 104.9 1780 84.4 59.1 63.3
2500 90.0 56.8 10.0 100.1 1780 84.4 59.5 71.3
NOTE: Pump and wire-to-water efficiencies are in percent.

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