period two
Chilled-Water System Design
notes
Distribution Loop
return tee
two-wayvalve
supply tee
distribution pump
Figure 51
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:
(0.50)3= 0.125
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.
