Page 273 - HVAC Pump Handbook

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Configuring an HVAC Water System

270 The HVAC World

2. There is a danger that warm return water will be bypassed through
the crossover bridge back to the supply side of a coil that is fully
loaded and needs design supply temperature at that moment. The
result is that the coil will not supply the load required of it. For
example, in Fig. 9.12b, the fully loaded coil no. 5 needs 44°F water
to produce 10 tons of cooling; since it is receiving 50°F water, it can
produce only around 5 tons of cooling.
3. If certain coils are exhibiting laminar flow, circulating pumps can be
installed on the coils to avert this problem, as described in Fig. 8.4.
This is an excellent method for controlling laminar flow in a heating
coil. It was stated in Chap. 8 that, in most cases, pumps or circula-
tors should not be piped in series with the coil but in the bypass, as
shown in Fig. 8.7.
4. Return temperature-control valves obliterate the true return water
temperature from the system and prevent the operating and main-
tenance people from understanding which coils or control valves
are creating problems. Return temperature is the operator’s guide to
efficient use of water in HVAC systems.
5. Return water temperature control lulls maintenance people into
believing that there is no problem with any of their coils when
some of those coils may be very dirty on the air or water side.
In conclusion, one of the designer’s most important tasks is the
selection of a sound temperature differential that will provide
maximum possible system efficiency. The second step in this
process is to ensure that differential is maintained after the system
is commissioned.

9.10 Heat Exchangers for Hot
and Chilled Water

High-rise buildings, as shown in Fig. 9.12b, can create static pressure
problems on water systems where the remainder of the system is low
rise. The answer to this problem, often, is the use of a heat exchanger,
as shown in Fig. 9.13. The heat exchanger eliminates the static pres-
sure of the tall building from the water system pressure. The supply
temperature to the building is controlled by the supply control valve. If
this is a critical load or at the highest pressure loss of the system, a dif-
ferential pressure transmitter should be connected across the supply
side of the heat exchanger to ensure that adequate water is available to
the building at all loads on it.

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