Page 236 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

The Use of Water in HVAC Systems 233

Figure 8.7 Heating or cooling coil pump for freeze
protection or laminar flow. (From The Water
Management Manual, Systecon, Inc., West Chester,
Ohio, 1992, Fig. 4.3.)

8.5 The Use of Balance Valves on
Variable-Volume Systems
Most HVAC chilled and hot water systems are variable volume, not
constant volume. Figure 8.8 describes why balance valves cannot be
used on these systems.
Figure 8.8 describes the problem with trying to balance a variable-
volume system with manual-balance valves. Two cooling coils demon-
strate the problem with mechanically balancing direct return systems
through the use of balance valves. Coil No. 1 has no load on and yet
has 96 ft of head on its control valve while Coil No. 2 is fully loaded
with 88.9 ft of head on its coil and control valve. This demonstrates
the great amount of head that can be imposed upon the coil valves
where the coils are located near the central plant. If the coil, its pip-
ing, and control valve have a total loss of 20 ft, the balance valve
would be set to eliminate around 76 ft of head. If you try to manually
balance out this high head at full flow in the system, you will starve
the coils when they operate at full flow but with reduced total load on
the system. Therefore, a variable-volume system cannot be balanced
with manual or automatic balance valves. This demonstrates that the

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