Trane SYS-APM001-EN manual Primary System Components, Centrifugal chiller capacity control

Primary System Components

For more information about chillers, Trane Air Conditioning Clinics are available for centrifugal (TRG-TRC010- EN), absorption (TRG-TRC011-EN) and helical-rotary (TRG-TRC012-EN) chiller types.

In addition to monitoring data, it is vital that the chiller controls alert operators to possible problems. Diagnostic messages are necessary for the operator to respond to safety issues and data points that are outside normal operating ranges. While communicating these diagnostic messages is a requirement, some chiller controls include factory-installed programming that responds to the issue causing the diagnostic messages. For example, when the chilled-water temperature nears freezing, the chiller sends a diagnostic message and adapts its operation by reducing the compressor capacity, raising the chilled-water temperature to a safer condition.

Finally, the chiller controls should communicate with a system-level controller. There are many system aspects that are outside the chiller’s direct control, such as condenser-water temperature and the amount of fluid flowing through the evaporator and condenser. To minimize the system energy costs, the system controls must coordinate chiller, pump, cooling- tower, and terminal-unit controls. This can only be done if adequate information is communicated from each system component to the system- level controls. System-level control is discussed in detail in “System Controls” beginning on page 87.

Centrifugal chiller capacity control

The capacity of a centrifugal chiller can be modulated using inlet guide vanes (IGV) or a combination of IGV and a variable-speed drive (adjustable- frequency drive, AFD)(Figure 16). Variable-speed drives are widely used with fans and pumps, and as a result of the advancement of microprocessor-based controls for chillers, they are being applied to centrifugal water chillers.

Figure 16. Centrifugal chiller with AFD
	Compressor
Inlet Guide Vanes
Condenser		Adjustable
		Frequency
		Drive

Evaporator

ASHRAE 90.1 requires a chiller to meet both full and part-load efficiency requirements. Using an AFD with a centrifugal chiller degrades the chiller’s full-load efficiency. This causes an increase in electricity demand or real-time pricing charges. At the time of peak cooling, such charges can be ten (or more) times the non-peak charges. In return, an AFD can offer energy savings

16 Chiller System Design and Control SYS-APM001-EN