Trane SYS-APM001-EN manual System Issues and Challenges Temperatures out of range

System Issues and Challenges

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

		60°F [15.6°C]
60°F [15.6°C]		35 gpm [2.2 L/s]
120 gpm
[7.6 L/s]			Mixing
		60°F [15.6°C]
			Valve
		85 gpm
		[5.4 L/s]
Chiller	70°F [21.1°C]		Bypass
			Pipes
	120 gpm
	[7.6 L/s]

85°F [29.4°C]

120 gpm [7.6 L/s]

Constant Volume

Pump

95°F [35°C]	Process
85 gpm [5.4 L/s]	Load

Constant Volume	95°F [35°C]	95°F [35°C]
Pump	35 gpm [2 .2 L/s]	120 gpm [7.6 L/s]
120gpm [7.6 L/s]

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.

SYS-APM001-EN Chiller System Design and Control 85