Page 307 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

304 The HVAC World

TABLE 10.10 Total kW Input Calculations for a 100 Percent Constant-Speed Pump

System System Water Fitting Pump Pump Pump Motor Input
gal/min head, ft hp loss, ft head, ft rpm effy effy kW
300 30.9 2.3 0.1 138 1780 20 96.0 40.6
600 33.2 5.0 0.3 137 1780 37 96.0 43.6
900 36.7 8.3 0.7 136 1780 50 96.2 47.9
1200 41.4 12.5 1.3 133 1780 60 96.2 52.1
1500 47.2 17.9 2.0 129 1780 68 96.2 55.8
1800 54.1 24.6 2.9 126 1780 74 96.1 60.1
2100 62.0 32.9 3.9 122 1780 80 96.1 62.8
2400 71.0 43.0 5.1 117 1780 84 96.1 65.5
2700 81.0 55.2 6.5 110 1780 88 96.1 66.1
3000 92.0 69.7 8.0 100 1780 90 96.1 65.4

NOTE: pump efficiencies are in percent.

3. The constant-speed, 100 percent capacity pump is very inefficient
excepting at full load. This is due to the elimination of the ineffi-
ciency of the variable speed drive 100 percent speed.
4. A combination of two 50 percent pumps and one 100 percent pump
would offer the optimum efficiency.
Conclusions. The installed pump horsepower is 200 hp for the two 100
percent pumps, 150 hp for the three 50 percent pumps, and 200 hp for a
system consisting of two 50 percent pumps and one 100 percent pump.
Before a final selection of pump capacity is made, a thorough evalua-
tion must be made of the actual load curve for the water system. If most
of the loads are 1500 gal/min or less with peak loads occurring momen-
tarily, there probably is no justification for the use of a 100 percent
capacity pump. If the daily load is above 2000 gal/min with light loads
occurring during the night hours, the combination of two 50 percent
pumps and one 100 percent pump may be the best selection.
Other factors that must apply are, of course, first cost, cost of power,
and all other pertinent information that must be reviewed in each
case of pump application. This study reveals that the proper selection
of pumps, both in size and number requires a sizeable amount of
information and calculations to determine the most feasible pumping
equipment for a particular installation.
It should be noted that the above tables are based upon standard
water at 60°F. If kW inputs are computed for liquids of other densities
or viscosities, the head inserted in the tables should be corrected for
the actual density and viscosity.
This evaluation was made on a specific installation without any
preconceived results. The important factor about this analysis is the
method used, not the results that are specific for this water system.

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