period two
Chilled-Water System Design
notes
Deficit Flow
1,000gpm at 56°F
[63L/sat 13.3°C]
1,000gpm at 42°F
[63L/sat 5.6°C]
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1,200gpm at 44.3°F
[76L/sat 6.8°C]
1,200gpm at 56°F | Figure 60 | |
[76L/sat 13.3°C] | ||
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If production supply is inadequate to meet the load demand, a “deficit” of supply water exists. To make up for this deficit, the distribution pump will pull water from the return pipe of the distribution loop through the bypass pipe. This is called deficit flow.
In this example, the pumps operating in the production loop are supplying 1,000 gpm [63 L/s] of water while the distribution pump is pumping 1,200 gpm [76 L/s] to meet the demand of the cooling coils. The result is that 200 gpm
[13 L/s] of system return water flows through the bypass pipe to be mixed with the supply water from the production loop. The temperature of the mixed water supplied to the distribution loop is 44.3°F [6.8°C].
Control of the water temperature supplied by the distribution loop is compromised due to this mixing. When this deficit flow condition exists, starting an additional chiller and pump increases the supply water flow from the production loop. It also changes the
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