ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
338
Fig. 51. Typical Water Distribution System.
CONTROL REQUIREMENTS FOR WATER
DISTRIBUTION SYSTEMS
The requirements of a properly applied distribution system are:
1. Maintain controllable pressure drop across the control
valves.
2. Maintain required flow through the heating (or cooling)
source.
3. Maintain desired water temperature to the terminal units.
4. Maintain minimum flow through the pump(s) that are
running.
5. Maintain NPSH.
6. Stage the pumps of a multipump system to satisfy
conditions 1 through 5.
CENTRIFUGAL PUMPS USED IN HOT AND
CHILLED WATER SYSTEMS
The pump is a key component of a water distribution system.
It is essential to understand pump characteristics in order to
understand and design distribution systems and to understand
pumping system control solutions. Centrifugal pumps are
commonly used to distribute hot and chilled water through
commercial buildings. Many varieties of centrifugal pumps are
available, as shown in Table 2. Figure 52 shows a typical base-
mounted pump.
M15053
HEAT/
COOL
COIL
3
RETURN
HEAT/
COOL
COIL
2
HEAT/
COOL
COIL
1
AIR
SEPARATION
HEAT/
COOL
SOURCE
MAKE-UP
WATER
EXPANSION
TANK
PUMP
SUPPLY
PRESSURE
REGULATING
VALVE
The expansion tank is charged with compressed air to place
the system under the minimum pressure required at the inlet to
the pump to prevent pump cavitation and the resultant impeller
erosion. The minimum inlet pressure required by a pump is
referred to as the net positive suction head (NPSH). Figure 54
indicates the NPSH for a particular pump. The air volume in
the tank is sized, based upon the volume of water in the system
and the expected water temperature variations, to allow the water
to expand and contract as water temperatures vary throughout
the year. The expansion tank static pressure does not effect the
closed system control valve differential close-off pressure, but
must be considered, in addition to the pump head, for valve
body and other piping system component pressure rating
selection.
The air separator and expansion tank are omitted from the
examples in this section for simplicity.
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