VAPOR COMPRESSION CYCLE CHILLERS
As introduced in Section , a secondary refrigerant is a substance that does not
change phase as it absorbs heat. The most common secondary refrigerant is water
and chilled water is used extensively in larger commercial, institutional, and
industrial facilities to make cooling available over a large area without
introducing a plethora of individual compressor systems. Chilled water has the
advantage that fully modulating control can be applied and, thus, closer
temperature tolerances can be maintained under almost any load condition.
For very low temperature applications, such as ice rinks, an antifreeze
component, most often ethylene or propylene glycol, is mixed with the water and
the term brine (left over from the days when salt was used as antifreeze) is used to
describe the secondary refrigerant.
In the HVAC industry, the refrigeration machine that produces chilled
water is generally referred to as a chiller and consists of the compressor(s),
evaporator, and condenser, all packaged as a single unit. The condensing medium
may be water or outdoor air.
The evaporator, called the cooler, consists of a shell-and-tube heat
exchanger with refrigerant in the shell and water in the tubes. Coolers are
designed for 3–11 fps water velocities when the chilled water flow rate is
selected for a 10–208F range.
For air-cooled chillers, the condenser consists of an air-to-refrigerant heat
exchanger and fans to provide the proper flow rate of outdoor air to transfer the
heat rejected by the refrigerant.
For water-cooled chillers, the condenser is a second shell-and-tube heat
exchanger with refrigerant in the shell and condenser water in the tubes.
Condenser water is typically supplied at 70–858F and the flow rate is selected for
a 10–158F range. A cooling tower is typically utilized to provide condenser water
cooling, but other cool water sources such as wells, ponds, and so on, can be used.
