Page 220 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

The Use of Water in HVAC Systems 217

2400 25.8
kW/100 tons 5.16 kW/100 tons
1000 12
for hot water,

23.48 H
kW/1000 mbh (8.9)
P E T °F

where is the specific weight of the hot water at operating tempera-
ture. See Tables 2.3 and 2.4.
Like chilled water, if watt transmitters are available to measure the
energy input to the hot water pumps, the following equation can be
used:
124,700 pump kW
kW/1000 mbh (8.10)
gal/min T °F
For example, if the secondary hot water pumps are pumping 500
gal/min of 180°F water at a system temperature difference of 40°F
and are consuming 10.2 kW, then

124,700 10.2
kW/1000 mbh 1.05 kW/1000 mbh
500 40 60.57
where 60.57 is the specific weight of water at 180°F.
Examples of these energy rates in kilowatts per 100 tons or kilo-
watts per 1000 mbh will be included in various chapters of this book.

8.1.3 Energy lost to mechanical
flow-control devices
In this day of concern over energy conservation, as we begin the
design of an HVAC water system, it is imperative that we reevaluate
our standard practices to see where we are wasting energy. With
computer-aided design, it is much easier to develop part-load informa-
tion and a closer evaluation of diversity on hot and chilled water sys-
tems. This provides the basis for more efficient piping designs that do
not need mechanical devices to circulate the water throughout the
system. For example, on a recent evaluation of a Midwestern univer-
sity, a chilled water pumping system was in operation with balance
2
valves on the pump discharges. There was a 58-lb/in pressure drop
across these balance valves, and each year around 900,000 kWh was
wasted by them.
As discussed in Chap. 2, the thermal equivalent of a brake horse-
power is 2545 Btu/h, and that for a kilowatthour is 3412 Btu/h. This
energy must be accounted for in the calculation of heating and cooling

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