Trane TRG-TRC016-EN manual Variable-Primary-Flow Systems

period four

Chiller-Plant Control

notes

Variable-Primary-Flow Systems

variable-flow
pumps
check
valves
control
valve	two-way
	valve
	optional bypass
	withthree-wayvalve	Figure 97

The variable-primary-flow system, introduced in Period Three, is designed to operate with variable flow through the chiller evaporators. Sequencing chillers in this type of system cannot be based solely on temperature, because in a properly-operating system the supply- and return-water temperatures will be nearly constant. Determining when to turn chillers on or off is not a simple task. For control stability and chiller reliability, the flow rates through the chillers, and the rate of flow change, must be kept within allowable ranges.

Therefore, control of a variable-primary-flow system must:

nInclude a method to determine system load. Many systems measure flow rates and temperatures.

nEnsure that flow rates through the chillers are within the allowable minimum and maximum limits. Modulation of a control valve in the bypass pipe is commonly used to ensure minimum flow rates through the chillers.

nControl the rate at which the system flow rate changes, to ensure that it does not change more rapidly than the chillers can adapt. This is especially critical when turning on additional chillers.

Adequate time must be spent designing the control sequence and commissioning the system after installation, to ensure proper operation of a variable-primary-flow system.

TRG-TRC016-EN 89