Page 217 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

214 The HVAC World

systems. For example, an analysis can be made of a chilled water
system utilizing fan-coil and small air handlers versus all large air
handlers with extensive duct systems. A first cost evaluation against
energy consumption can be conducted to determinate the optimum
selection for a particular application.
The overall efficiency of a water system is like any other equation
for efficiency—the useful energy divided by the energy input. The use-
ful energy is the friction of the piping, fittings, and heating and cool-
ing coils; their sizing obviously affects the amount of useful energy.
The engineer must, as indicated in Chap. 3, balance friction against
first cost of the pipe to achieve an economical answer for the water
system under design.
The efficient energy for an HVAC water system K can be calculated
e
in kilowatts as follows:
useful frictions (ft) system flow (gal/min) 0.746
K
e 3960

useful frictions (ft) system flow (gal/min)
K (8.4)
e 5308
The energy consumed by such a system K can be the energy input
i
to the system if it is known, or it can be calculated (in kilowatts) as
follows:

total system head (ft) system flow (gal/min)
K (8.5)
i 5308 P E

The efficiency of a water system WS utilizing electric motor–driven

pumps can be computed by dividing the useful kilowatts by the kilo-
watt input to the pump variable-speed drives or motors K :
i
K
e
WS 100% (8.6)
K
i
Using as an example a chilled water system with variable-speed
pumping, if the friction of the piping, fittings, and cooling coils on this
system with a capacity of 1000 gal/min amounted to 75 ft of head, the
useful energy would be
1000 75
K 14.1 kW
e 5308
To calculate K , assume that the friction loss through control valves
i
and other appurtenances amounts to 20 ft; the total pump head would

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