Trane SYS-APM001-EN manual System design and control requirements

System Configurations

evaporator because its selection pressure drop is lower than that of Chiller 2. Load is proportional to flow rate and temperature difference, tons = (gpm × ΔT) / 24. Because Chiller 1 is asked to satisfy a load that exceeds its capacity, it cannot satisfy the chilled water setpoint when the return water temperature equals the design condition. Meanwhile, Chiller 2 is less than fully loaded.

Balancing the system at the design condition, for example, by installing a balancing valve in series with Chiller 1, reduces this problem and works well at design and part load conditions. Alternatively, you could increase the load on Chiller 2 by lowering its chilled water setpoint; however, this complicates system control. The simplest solution is to select chillers that have (nearly) equal pressure drops at their design flow rates, whether the capacities are the same or not.

Table 15. Effect of dissimilar evaporator pressure drops

	Capacity,	Flow rate, gal/min		Pressure drop, ft H20*
	tons
		Selection	Actual	Selection	Actual	Change, %
Chiller 1	500	750	819	12	14.3	+9.2
Chiller 2	300	450	381	20	14.3	-15.3

*Values shown here are based on the assumption that pressure drop changes with the square of the flow rate.

System design and control requirements

If experience has taught us anything about implementing variable primary flow, it’s this: The single, most important contribution of the engineer is to

provide written, detailed descriptions of the plant’s sequence of operation.

These descriptions should include control sequences for:

•Full- and part-load operation

•Minimum and maximum flow-rate management

•Transient flow-rate changes

•Starting and stopping chillers

Furthermore, this information must be shared early in the design process. Without specific, documented sequences of operation, it is unlikely that the controls provider will devise programs that operate the plant as intended. Bottom line: VPF plants that work result from close, early-on collaboration between the engineer, the chiller manufacturer, and the controls provider. Variable primary flow is a value-added option that can help your clients curb operating costs at a lower initial cost than traditional primary–secondary designs … but only if you select the right components, install them properly, and operate them in accordance with a well-thought-out control scheme.

SYS-APM001-EN Chiller System Design and Control 61