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TRG-TRC016-EN 55
period two
Chilled-Water System Design
notes
When the flow of chilled water from the production loop exceeds the demand of
the distribution loop, the direction of flow in the bypass pipe reverses. Chilled
water flows from the supply side of the production loop, through the bypass
pipe, and mixes with warm water returning from the distribution loop. This is
called excess flow.
In this example, the pumps operating in the production loop are supplying
2,000 gpm [126 L/s] of water, while the distribution pump is pumping
1,800 gpm [114 L/s] to meet the demand of the cooling coils. The result is that
200 gpm [13 L/s] of supply water flows through the bypass pipe to be mixed
with the water returning from the production loop. The temperature of the
water returning to the chillers decreases to 54.6°F [12.6°C], reducing the load
on the operating chillers.
Some excess flow is normal in the operation of a primary-secondary system.
The amount of excess flow is almost always less than the flow of one
production pump. The energy consumed by pumping this excess water through
the production loop is typically very low, because the production pump only
needs to produce enough head to push the water through the chiller evaporator
and the bypass pipe.
If a pump-and-chiller pair is turned off as soon as this excess flow condition
occurs, deficit flow will result and the pump and chiller will be turned on again.
To prevent this from happening, a production pump and its respective chiller
are not turned off until the excess bypass flow exceeds the capacity of the next
production pump that is to be turned off.
Some systems are designed with variable flow also in the production loop.
Although this minimizes excess flow in the bypass pipe and further reduces
production-energy consumption, it results in a significantly-more-complex
control system. This type of system will be discussed in Period Three.
Excess Flow
1,800
1,800
gpm
gpm
at 56°F
at 56°F
[114 L/s at 13.3°C]
[114 L/s at 13.3°C]
2,000
2,000
gpm
gpm
at 54.6°F
at 54.6°F
[126 L/s at 12.6°C]
[126 L/s at 12.6°C]
2,000
2,000
gpm
gpm
at 42°F
at 42°F
[126 L/s at 5.6°C]
[126 L/s at 5.6°C]
1,800
1,800
gpm
gpm
at 42°F
at 42°F
[114 L/s at 5.6°C]
[114 L/s at 5.6°C]
2
0
0
2
0
0
g
p
m
g
p
m
a
t
4
2
°
F
a
t
4
2
°
F
[
1
3
L
/
s
a
t
5
.
6
°
C
]
[
1
3
L
/
s
a
t
5
.
6
°
C
]
Figure 61