Download 2. There is a danger that warm return water will

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 circulators 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 maintenance 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 pressure 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 differential 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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Configuring an HVAC Water System
Configuring an HVAC Water System
271
T
From pumps
To
building
To pump
speed control
∆P
To pumps
Building
pump
Locate differential pressure
transmitter for system
pumps if this is a critical
building or at the point
of highest pressure loss
in system.
Figure 9.13 Heat exchanger for high risk building. (From Patterson Pump
Company HVAC Pumps and Systems Manual, 2004.)
One of the concerns for the control valve of Fig. 9.13 is the great loss
that may be incurred in it at light loads on the building. Through
adaptive control, the set point for the differential pressure transmitter
can be reset on light loads through a valve position indicator/transmitter
on the control valve as per Fig. 9.14. The differential pressure across
Valve position indicatortransmitter
To pump speed
control
T
To
building
From pumps
To pump
speed control
∆P
To pumps
Heat exchanger
Building
pump
Set point is adjusted by
valve position to hold
valve to 80 to 85 % open
Figure 9.14
Valve position control on heat exchanger.
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272
The HVAC World
Valve position indicatortransmitter
To pump speed
control
From pumps
To pump
speed control
T
∆P
To pumps
Set point is adjusted by
valve position to hold
valve to 80 to 85% open
Figure 9.15
Air-handling
unit coil
Valve position control on air-handling unit coil.
the heat exchange is adjusted so that the control valve is always 80 to
85 percent open. For example, if the set point is 20 ft at design load, it
may be only 5 ft at minimum load. Not only is friction loss reduced
through the control valve, but the constant pressure established for
the system is reduced by the lower set point. Similar control can be
utilized on large air-handling units, Fig. 9.15, where high set points
such as 35 ft are required for them, and only 15 ft is required for the
other coils. The set point for the large air handler may be down near
the 15 ft for other coils much of the time when part-load conditions
exist on the air-handling unit.
9.11 Bibliography
James B. Rishel, Variable Water Volume Is Hydro-Electronics, SYSTECON, Inc., West
Chester, Ohio, 1982.
James B. Rishel, The Water Management Manual, SYSTECON, Inc., West Chester,
Ohio, 1992.
James B. Rishel, “Simplifying Contemporary HVAC PIPING,” ASHRAE JOURNAL,
Vol. 47, No.2, pp.16–25, FEBRUARY 2005.
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