Page 250 - HVAC Pump Handbook

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Configuring an HVAC Water System

Configuring an HVAC Water System 247

water systems, whereas the energy economies of condenser water sys-
tems are included in Chaps. 11 and 16.
Figure 9.1 describes the basic pump head losses in a chilled water
distribution system. A condenser water or cooling tower circuit would
be similar by changing the chiller evaporator to the chiller condenser
and replacing the cooling coil with the cooling tower. Likewise, hot
water systems would be similar by exchanging the chiller evaporator
with the loss through the boiler. The losses through both of these sys-
tems are usually much less than those for the chilled water system.
Figure 9.2 describes the hydraulic gradient and the losses for Fig. 9.1.
Note that unnecessary losses such as balance valves or multiple-duty
valves are not shown.

9.2 Selection of Temperature Differential
The first decision a designer of a chilled or hot water system must
make is the selection of the temperature differential. Temperature dif-
ferential is the difference between the supply water and the return
water temperatures. A number of conditions must be recognized before
making the final selection of temperature differential:

1. An increase in temperature differential decreases water flow and
therefore saves pumping energy.
2. An increase in temperature differential may increase the cost of
coils that must operate with a higher mean temperature difference.
3. Higher temperature differentials increase the possibilities of loss
of temperature difference in coils due to dirt on the air side and
chemical deposits on the water side of them.
4. Laminar flow on the water side due to lower velocities at low loads
on a coil is always a concern of the water system designer. The possi-
bility of laminar flow is greater with higher temperature differences.

The quick promise of energy savings with higher temperature dif-
ferences can be offset later by problems in system operation due to
failure to have enough pumping capacity after the system has aged.
Only experienced designers should entertain water temperature dif-
ferences in excess of 12°F on chilled water and 40°F on hot water sys-
tems. A careful balance between energy savings and first cost should
be made by the designer. There is no one temperature difference for the
chilled or hot water systems.
Following are two formulas for calculating the water flow, in gallons
per minute, for hot and chilled water systems:

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