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16 Chiller System Design and Control SYS-APM001-EN
Primary System Components
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
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
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.
Adjustable
Frequency
Drive
Inlet Guide Vanes
Compressor
Evaporator
Condenser