Page 213 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

210 The HVAC World

higher water temperatures to avoid condensing. Noncondensing boil-
ers cannot operate with water at temperatures below 140°F. The
development of the condensing boiler has enabled the designer to
select great differential temperatures between the supply and return
temperatures. This has reduced appreciably the water flow and,
therefore, the pump flow and horsepower.
Likewise, on chilled water systems, it is recognized that chillers
operate at lower kilowatt per ton rates with higher chilled water tem-
peratures. In recognition of these facts, HVAC water systems should
be designed for lower hot water temperatures and higher chilled
water temperatures. Reset of system water temperature at boilers or
chillers should be used rather than zone reset in most cases. Zone
reset should use efficient pumping such as distributed pumping,
which will be described in Chaps. 15 and 20 on chilled and hot water
distribution systems.
Condenser water systems are realizing greater efficiencies through
the use of variable-speed pumps. The types of controls that are avail-
able today for removing heat from the chillers have allowed the use of
these variable-speed condenser pumps. The consumption of energy for
boiler feedwater is relatively less than that for the other major HVAC
water systems.

8.1.1 Determination of useful energy
The advent of digital electronics, as discussed in Chap. 1, provides the
means to determine rapidly the efficient use of pumping energy for all
HVAC water systems. It remains for us to develop what is efficient
use and what is not. Following are some guidelines that will help us
in this determination of the value of various practices that consume
pumping energy.
Useful consumption of pumping energy
1. Pipe friction required to transport water
2. Friction in fittings to connect the pipe
3. Friction in heating and cooling coils
4. Properly sized pumps and motors
5. Properly selected variable-speed drives
6. Friction in properly evaluated chillers and boilers

Inefficient use of energy
1. Temperature-control valves
2. Balance valves, manual or automatic

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