Page 219 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

216 The HVAC World

These equations for energy consumed, energy applied, and system
efficiency are for the water system in total. Similar evaluations for
energy consumption must be made for all parts of a water system to
ensure that maximum system efficiency is achieved. In Chap. 6, wire-
to-water efficiency for pumping systems was addressed. Examples of
this efficiency will be found throughout this book.
The use of Eq. 8.3 to evaluate the efficiency of pumping for an
HVAC system may be cumbersome and the answer difficult to deter-
mine. Also, the use of small pipe and high friction losses may provide
a relatively high efficiency for a poorly designed system. The use of
Eq. 8.3 is more relative than absolute in comparing different piping
system designs with the same level of pipe friction.
Energy consumption of water distribution. There are other equations
for chilled water and hot water systems that are useful to determine
the effectiveness of pumping. They are easier to compute than Eq. 8.3
and provide absolute values. For example, for chilled water,

0.452 H
kW/100 tons (8.7)
P E T °F

where H system head
P pump efficiency

E motor efficiency or wire-to-shaft efficiency of a variable-

speed drive and motor for variable-speed pumps
T system temperature difference
For example, if the system head H is 100 ft, the pump efficiency 82
percent, the wire-to-shaft efficiency 89 percent, and the temperature
differential 12°F, then

0.452 100
kW/100 tons 5.16 kW/100 tons
0.82 0.89 12
If flow and watt transmitters are measuring system flow and kilo-
watts of the pumps for the chilled water, an alternate equation (Eq. 8.5)
utilizes values measured from the actual system. This equation
enables the operators of the water system to measure continuously
the energy consumed in distributing the chilled water.
2400 pump kW
kW/100 tons (8.8)
gal/min T °F

For example, if secondary pumps are pumping 1000 gal/min at a sys-
tem temperature difference of 12°F and are consuming 25.8 kW, then

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