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
While communicating these diagnostic messages is a requirement, some chiller
controls include factory-installed programming that responds to 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.
In so-called constant flow systems, the pumps are either on or off, providing
relatively constant flow when in the on position. In practice, some flow
variation will occur as system pressure drop changes. In a variable-flow system,
pump control is most often performed by maintaining a pressure differential at
a selected point in the system. For example, a variable-speed drive will increase
its speed if the sensed pressure differential is too low, or slow down if the
pressure differential is too high. The control point is selected to minimize over-
pressuring the system and to assure adequate flow at all critical loads. Optimal
pumping control strategies are addressed in Critical valve reset, page 48.
1 2000 ASHRAE HVAC Systems and Equipment Handbook, Chapter 12,
Hydronic Heating and Cooling System Design and Chapter 36, Cooling
Towers, American Society of Heating, Refrigeration and Air-Conditioning
2 Schwedler, M., PE and Bradley, B.; "An Idea for Chilled-Water Plants Whose
Time Has Come...Variable-Primary-Flow Systems," Engineers Newsletter,
Volume 28, No. 3, The Trane Company, 1999.
3 Webb, R.L. and Li, W.; "Fouling in Enhanced Tubes Using Cooling Tower
Water, Part I: Long-Term Fouling Data," International Journal of Heat and
Mass Transfer, 2000.