There are many industrial processes and applications that require a continuous supply of a process fluid used for cooling purposes. The devices used to furnish this process fluid supply are ordinarily called "water chillers", even though the process fluid is most usually a mixture of water and other liquid, usually ethylene glycol, and that term is used in this specification to refer to controlled chillers using any appropriate coolant.
A water chiller must maintain the temperature of the process fluid essentially constant within a very limited tolerance range; in some instances the critical parameter may be selected as the return temperature from the process apparatus, but more frequently the supply temperature should be selected as the basis of control. The water chiller is often a self-contained unit that can be transported from one location to another, literally constituting a heat transfer system on wheels. Water chillers of this kind are most frequently employed in processes involving heat transfer where the coolant must be maintained constant at some temperature between -30.degree. F and +60.degree. F.
For the most part, the controls for water chillers have constituted conventional thermostats, freezestats, and pressure-actuated switches connected directly in the electrical actuating circuits of the pumps, compressors, and other components of the water chilling apparatus. A further control that is often used is a float switch, connected to a coolant reservoir, employed to interrupt operation whenever the water supply is inadequate. Another control frequently used is a hot-gas bypass valve, used to control cooling capacity during intermittent or partial load conditions.
For many applications, these direct acting controls are adequate. However, they do not afford the precision control essential to some sensitive manufacturing and other industrial operations. Furthermore, controls of this nature do not provide comprehensive protection for the water chiller and the process apparatus it serves relative to possible malfunctions in the chiller itself or in the process apparatus. Moreover, the inherent inertia of these conventional controls makes it difficult to control chiller operation on the basis of coolant supply temperature; it is usually necessary to use the coolant return temperature as the control parameter.