Civil Engineering Reference
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Coil
Coil
P
DPV
Bypass
Constant speed pumps
Chiller
Chiller
Chiller
Figure 9.2
Schematic of a constant primary-only pumping system with differential
pressure bypass valve.
way valves. Consequently, the differential pressure will exceed the set-point
level, and the controller will command the DPV to open wider so as to allow
more chilled water flow through the bypass pipe, thus the differential pres-
sure is relieved. The reverse happens when there is a drop in the differential
pressure until the flow control valve is fully closed. This constant primary-
only pumping system with a DPV control is a very common chilled water
system in constant water flow systems. To achieve higher energy efficiency,
variable water systems are commonly adopted, reducing the pump power at
off-design load.
9.3.4 Constant primary/variable secondary pumping system
For many years, the design of chilled water plants has been dominated by
the constant primary/variable secondary paradigm. The schematic diagrams
of this type of pumping system are shown in Figure 9.3. The rationale for
this system is straightforward. On the secondary system (distribution sys-
tem), constant chilled water flow wastes large quantities of pumping energy
at part load. On the primary system (production system), however, chiller
manufacturers recommend that the flow through the evaporator of a chiller
should be constant for steady control and safety.
The primary constant speed pumps require less power than the secondary
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