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Figure 115B shows the valve in the half-open position. The
output of the coil is cut approximately in half (16 kW). This is
in contrast to the oversized valve application (Fig. 100B) where
the heat was cut only 5 percent.
Figure 115C shows the output cut to 8 kW with the valve in
the quarter-open position. In contrast, the oversized valve in
the quarter-open position produced 15 kW.
The conclusions reached from Figure 115 are:
1. A valve with a large pressure drop will be effective in
controlling heat output over its entire stroke.
2. The valve, not the trap, takes up most of the pressure
drop between supply and return mains.
SUPPLY AND RETURN MAIN PRESSURES
The supply main pressure should be constant and sufficient
to allow an 80 percent drop through the control valve and still
leave enough steam pressure downstream from the valve to
produce the desired heat output. If boiler pressure is not
constant, install a pressure reducing valve ahead of all steam
supplied devices where output temperatures may vary rapidly
with steam pressure fluctuations.
Even though the control valves do not change position,
variations in return main pressure causes fluctuations in steam
flow through control valves. From a control standpoint, an
atmospheric return with a condensate pump is more effective
than a vacuum return with a vacuum pump that can cycle over
a range of several inches of vacuum.
As an example of the effect of fluctuating supply and return
main pressures, assume a system where the boiler cycles so that it
shuts off at 145 kPa and cuts in at 115 kPa. On the same system
assume that a vacuum pump is used which cuts in at 90 kPa and
shuts off at 80 kPa. The pressure difference between supply and
return mains can vary from a minimum of 25 kPa to a maximum
of 65 kPa as the boiler and vacuum pump cycle. This means a
60 percent variation in capacity of the control valves in the building
as the pressure fluctuates. Control valves correctly sized for
28 kPa are 60 percent too large during periods when a 65 kPa
difference exists across the supply and return mains.
SYSTEM DESIGN CONSIDERATIONS FOR
STEAM COILS
Figure 116 shows the optimum design conditions and piping
arrangement for a steam supplied heating coil. Considerations
for effective control are:
1. Steam mains held close to design pressures. Refer to
SUPPLY AND RETURN MAIN PRESSURES.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
373
2. Returns at atmospheric pressure, unless lifts (condensate
pumps) are required in the returns.
3. Traps sized to pass design condensate flow at 7 kPa drop.
4. An equalizer line to prevent formation of a vacuum
within coil.
5. A control valve pressure drop of 80 percent of the
difference between supply and return main pressures.
EQUALIZER
LINE
AUTOMATIC
CONTROL
VALVE
STEAM
MAIN
STEAM
COIL
F & T
TRAP
RETURN
MAIN
Fig. 116. Steam Supplied Air Heating Coil.
High vacuum systems are an exception to these considerations
since they lower the steam temperature and pressure as the
heating load decreases. Vacuum systems are adaptable to
automatic control valves, since usual practice is to maintain a
controlled difference between supply and return main pressures
while varying supply main pressure with heating load.
LOW TEMPERATURE PROTECTION FOR
STEAM COILS
Any steam coil exposed to outdoor air is in danger of freeze
up in cold climates. A coil begins to freeze in the –1 C to 0 C
temperature range. Steam coils designed for heating cold air
contain internal distributing tubes to ensure that steam reaches
all parts of the coil as the control valve modulates.
Another approach to freeze-up control is to design coils with
dampers so that the control valve does not modulate but remains
open when air entering the coil is below freezing. If too much
heat is being delivered, face and bypass dampers control the airflow
across the coil. Above freezing, the valve can be modulated.
In all cases, a low limit temperature controller, which
responds to the coldest portion of the capillary sensing element,
should be part of the design. For addition examples of control
with freezing air conditions entering a coil see Air Handling
Systems Control Applications section.
CHECK
VALVE
C2927
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