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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
VALVE TRIM
Use stainless steel valve stems and trim. Standard trim for
industrial valves is usually 316 stainless steel. Composition discs
leak and corrode at the seat and disc and are not used.
VALVE PACKING
Shredded Teflon, Teflon asbestos, graphite asbestos, and
spring loaded Teflon V-rings, are available in industrial valves
and acceptable to 230 C. A valve with a deep packing gland is
normally required.
VALVE FLOW CHARACTERISTICS
The flow characteristics of a valve is the relationship between
fluid flow expressed in percent of flow and valve stem travel
expressed in percent of travel. Always use equal percentage
characteristic valves for HTW control. Equal percentage valves
compensate for the nonlinear heat output of HW heat
exchangers so percent of heat output change is equal to percent
of valve travel. These valves provide the best control at low
flows. An industrial valve with an equal percentage plug has
the required rangeability of 50 to 1.
ACTUATORS
Valve actuators must close the valve on loss of power or
control air.
If the close-off rating is less than twice the expected pressure
difference across the closed valve, positioners on pneumatic
valves are used only, otherwise, sequencing of multiple valves
is required. Electro-pneumatic positioners or separate electro-
pneumatic transducers can be used to receive an electric signal
(4 to 20 mA) and operate the pneumatic valve.
Electric actuators are not satisfactory for HTW valves because
they are not available with a safe ambient temperature rating
and the operating speed is too slow.
VALVE SIZE
Accurate valve sizing is critical for satisfactory HTW control
since an oversized valve controls over only a small fraction of
its total stem travel and loses sensitivity. Safety factors should
be avoided but if used should be very small. See the Valve
Selection and Sizing section.
The pressure drop across a control valve should be between
50 and 80 percent of the drop across that part of the piping
system in which flow will vary as a result of valve action.
Always use a 35 kPa or greater pressure drop across the valve.
VALVE LOCATION
Always locate a HTW valve on the return side of the
converter, coil, steam generator, or other equipment because it
allows the valve to operate at lower pressures and temperatures.
HTW is less likely to flash when the valve is located on the
return side of the equipment.
INSTANTANEOUS CONVERTER CONTROL
An instantaneous converter is the standard type of converter
used in nearly all HTW heating installations because of its
extremely fast response. It is compact and available as standard
equipment.
The HTW flows through several passes of tubing within a
shell just large enough to accommodate the tube bundle.
Secondary water is supplied into the shell and surrounds the
tubes (Fig. 118). Only a small amount of HTW is needed to
heat a large volume of secondary water and the change is nearly
instantaneous. There is no room within the shell for a
temperature sensing element to quickly detect the change,
therefore, the temperature sensor must be located as close to
the converter as possible.
There must be a provision in the control loop to close the
HTW valve as soon as the secondary flow ceases. This requires
proving flow in case the flow stops for a reason other than the
pump is stopped by outdoor temperature, another signal, or
power failure. Even if the HTW valve closes as soon as
secondary water flow stops, flashing is likely to occur as there
is enough HTW in the tubes to overheat secondary water.
Flashing causes water hammer and possible equipment damage.
If shutdown is controlled, closing the HTW valve immediately
and delaying the secondary pump shutdown allows excess heat
in the converter to dissipate.
RELAY
C1
CONTROLLER
IMMERSION
SENSOR
N.C.
VALVE
INSTANTANEOUS
CONVERTER
HTWR
HTWS
Fig. 118. Control of HTW to Constant
Temperature Hot Water Converter.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
376
OUTDOOR
AIR SENSOR
OFF
DELAY
TIMER
FLOW
SWITCH
TO
ZONE(S)
PUMP
C2582
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