Page 222 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

The Use of Water in HVAC Systems 219

Q H
v
v
hp (8.11)
v
3960 P

To convert to kilowatts,
hp 0.746
v
kW (8.12)
v
E

or
Q H
v
v
kW (8.13)
v
5308 P E

where P pump efficiency

E motor efficiency for constant-speed pumps and wire-to-

shaft efficiency for variable-speed pumps
This formula (8.11) computes the energy lost through part of a system
such as a balance valve by inserting the flow and head difference
recorded for each balance valve. The following discussion will be
based on balance valves.
A quick formula for estimating this energy loss without knowing
the pump and motor efficiencies is as follows: Assuming that the aver-
age pump efficiency is 75 percent and the average motor efficiency is
90 percent, Eq. 8.5 can be reduced to the following:
Q H
v
v
Lost kW (8.14)
v 3600
If the pressure drop H is indicated in psig, multiply it by 2.31.
v
The annual lost energy is
Annual kWh kW annual hours (8.15)
v
Although Eq. 8.11 is an approximation, it is accurate enough to deter-
mine generally how much energy is being lost in a building due to
balance valves. If the loss is appreciable, a further study should be
made using Eqs. 8.8 and 8.9 with actual pump and motor efficiencies.
Serious studies of the energy consumption of condenser, chilled, and
hot water systems are now under way. Out of this work has come evi-
dence of the great loss that can be incurred by balance valves on cool-
ing coils in particular. A specific study was conducted by a balancing
contractor and supervised by a consulting engineer that showed a
pronounced reduction in energy consumption by opening wide exist-
ing balance valves on a variable-volume, variable-speed chilled water
system in a high school. Table 8.1 is a summary of these data.

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