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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
RA
DAMPER
ACTUATOR
N.O.
OA
N.C.
DAMPER
ACTUATOR
TO
MIXED AIR
CONTROLS
RELAY CLOSES
OA DAMPER
WHEN FAN STOPS
Fig. 121. HTW for Mixed Air Reheat.
HTW SPACE HEATING UNITS
Unit heaters, convectors, radiation, and radiant panels can
be supplied directly with HTW. This allows use of smaller units
and smaller piping and eliminates intermediate heat exchangers.
Most of the additional control cost is in the valves suitable for
HTW as the small unitary control valves normally used on low
and medium temperature heating systems are not able to stand
the pressures and temperatures of HTW. Actuator design, body
ratings, disc materials, packing gland design, and lack of low
flow ports are all likely limitations for this type of valve. Flanged
connections are required as union connections do not seal well
enough for HTW applications.
These units are most often used in storage areas, shops,
hangers, or other areas where people do not work in close
proximity to the units. Where people do work near HTW
supplied unitary equipment, some type of limiting control is
required to reduce surface and/or discharge temperatures.
STORAGE CONVERTERS
A storage converter is most often used for heating domestic
water (Fig. 122). It can also be used for building heating.
Separate converters are required for domestic water and building
heating (Fig. 123). Heating system water is treated and
recirculated while domestic water is used and replenished.
RELAY CLOSES FACE
DAMPER AND VALVE
OPTIONAL RESET
WHEN FAN STOPS
LINE FROM
OUTDOOR
CONTROLLER
DAMPER
ACTUATOR
DISCHARGE
AIR TEMP
CONTROLLER
N.O.
DISCHARGE
AIR
N.C.
LOW-TEMP
LIMIT
HTWS
VALVE
HTWR
RELAY OPENS VALVE ON
LOW TEMPERATURE
C2584
TEMPERATURE CONTROL
HTWS
HTWR
Fig. 122. Direct HTW to Domestic Hot Water.
SPACE HEATING CONVERTER
CONTROLS NOT SHOWN
HTWR
HTWS
TEMPERATURE
CONTROL
PUMP
Fig. 123. Indirect HTW to Domestic Hot Water.
In a storage converter, the tube bundle is immersed in the
bottom of a storage tank (shell) with capacity to provide for
large intermittent demands. There is little lag in transferring
heat from the tube bundle but because of the large capacity of
secondary water in the shell, the system provides more stable
control. Rate of change might be 11 C to 28 C per hour. Large
installations frequently employ a system circulating pump to
assure quick availability of hot water at remote usage points
and help eliminate stratification in the tank.
MULTIZONE SPACE-HEATING HTW
WATER CONVERTERS
Figure 124 shows a storage converter application. It is
controlled as a conventional converter with only the HTW valve
and pump interlock having special requirements because of the
heating medium.
Because the response rate of a storage converter is slow, the
storage capacity must be sufficient to supply any sudden
demands. Warm-up is the demand period where the converter
recovery rate becomes the determining factor for length of time
required for warm-up. Normally all other changes in a building
heating system are gradual.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
378
RELIEF VALVE
TO
BUILDING
50 C
FROM
BUILDING
STORAGE
CONVERTER
MAKE UP
WATER
C4625
SPACE
HEATING
LOAD
RELIEF VALVE
DOMESTIC
HOT WATER
TO BUILDING
50 C
STORAGE
MAKE UP
CONVERTER
WATER
C4624
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