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TRG-TRC016-EN 45
period two
Chilled-Water System Design
notes
The distribution pump circulates water from the supply tee, through the load
terminals, and back to the return tee. Although the same water is pumped twice
by different pumps, there is no duplication of pumping energy. The production
pumps overcome only the pressure drop through the production loop, and the
distribution pumps overcome the pressure drop through the distribution loop.
The distribution pump(s) should be capable of varying the flow through the
distribution loop. Typically this is accomplished by using a pump with a
variable-speed drive to modulate the flow of water through the pump.
In a properly-designed and -operating system, distribution-pump energy
consumption will decrease significantly at part load. The pump power reduction
approaches the theoretical cubic relationship to flow. That is, when the load is
50 percent of design, requiring 50 percent of design water flow, the energy
consumed by the variable-flow distribution pump is 12.5 percent of full load:
The total installed pump capacity required in a primary-secondary system is
typically less than in a system not designed for primary-secondary pumping.
This is because the total system head (production plus distribution) is divided
between pumps. Each pump is more efficient because it works against a lower
head. Furthermore, the distribution pump is sized to meet the diversified (block)
system load, not the sum-of-peaks coil loads. This can represent a 20- to
25- percent reduction in the size of the distribution pump.
Distribution Loop
two
two
-
-
way valve
way valve
distribution
distribution
pump
pump
b
y
p
a
s
s
p
i
p
e
b
y
p
a
s
s
p
i
p
e
return
return
tee
tee
supply
supply
tee
tee
Figure 51
0.50()
3
0.125=