Filter
Top Products
System Issues and Challenges
SYS-APM001-EN Chiller System Design and Control 85
Temperatures out of range
A laboratory load requires 120 gpm [7.6 L/s] of water entering the process at
85°F [29.4°C] and returning at 95°F [35°C]. The accuracy required is more
precise than the cooling tower can provide. The selected chiller has adequate
capacity, but a maximum leaving-chilled-water temperature of 60°F [15.6°C].
Using a pair of bypass pipes allows the mixing valve to supply the proper
temperature to the process load and maintains acceptable water flow rates
and temperatures through the chiller. In the example shown in Figure 52, the
chiller and process flow rates are equal, although this is not a requirement.
For example, if the chiller had a higher flow rate, more water would bypass
and mix with the warm return water.
Figure 52. Temperatures out of range for equipment
Precise temperature control
An example of a process application layout where the required temperature
control tolerance is more precise than the chiller controls allow is shown in
Figure 53.
For example, when Chiller 2 is turned on, it takes a certain amount of time to
reach its supply-temperature setpoint. The dedicated control valve remains
closed and water is bypassed until Chiller 2 reaches its setpoint, which keeps
the water temperature supplied to the process within tolerance. When Chiller
2 reaches its setpoint, the control valve opens.
This design requires a different chiller-sequencing strategy than the standard
decoupled system. Water must not be allowed to bypass from the return side
to the supply side, since this will cause the temperature of the water supplied
to the process to vary outside the tolerance. Set the system controls to turn
on Chiller 2 before deficit flow occurs.
60°F [15.6°C]
120 gpm
[7.6 L/s]
60°F [15.6°C]
35 gpm [2.2 L/s]
85°F [29.4°C]
120 gpm [7.6 L/s]
Constant Volume
Pump
Process
Load
60°F [15.6°C]
85 gpm
[5.4 L/s]
Chiller
Constant Volume
Pump
120gpm [7.6 L/s]
Mixing
Valve
Bypass
Pipes
95°F [35°C]
85 gpm [5.4 L/s]
95°F [35°C]
120 gpm [7.6 L/s]
95°F [35°C]
35 gpm [2 .2 L/s]
70°F [21.1°C]
120 gpm
[7.6 L/s]