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54 TRG-TRC016-EN
notes
period two
Chilled-Water System Design
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 supply-and-demand relationship in order to
restore the temperature of the chilled water supplied to the distribution loop.
Deficit Flow
1,200
1,200
gpm
gpm
at 56°F
at 56°F
[76 L/s at 13.3°C]
[76 L/s at 13.3°C]
1,000
1,000
gpm
gpm
at 56°F
at 56°F
[63 L/s at 13.3°C]
[63 L/s at 13.3°C]
1,000
1,000
gpm
gpm
at 42°F
at 42°F
[63 L/s at 5.6°C]
[63 L/s at 5.6°C]
1,200
1,200
gpm
gpm
at 44.3°F
at 44.3°F
[76 L/s at 6.8°C]
[76 L/s at 6.8°C]
2
0
0
2
0
0
g
p
m
g
p
m
a
t
5
6
°
F
a
t
5
6
°
F
[
1
3
L
/
s
a
t
1
3
.
3
°
C
]
[
1
3
L
/
s
a
t
1
3
.
3
°
C
]
Figure 60