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Thermonics Low Temp. Chillers - Glossary P

Packaged Chiller Systems

The term 'Packaged Chillers' refers to systems in which all the major components of the refrigeration system are located in one integral enclosure. Package configurations are popular for their convenience of installation and use, economy and smaller footprint. Water-cooled and Air-cooled systems are the most commonly 'packaged' chillers. See Split System, below.

PID (Proportional/Integral Derivative)

PID (Proportional/Integral/Derivative) is a Control Loop Feedback mechanism used in applications requiring continuously modulated control for increased accuracy. In Chiller technology, PID is used to maintain the set point as consistently and accurately as possible. The PID controller reads the signal from the Thermocouple/RTD sensor(s) to determine the current process temperature, subtracts that reading from the set point, then applies three predetermined constants working in conjunction with one another to act upon the error and maintain the set point as closely as possible. The Proportional Term (P) adjusts the output proportionally to the error by multiplying the error by a Proportional Gain Constant (Kp). The Integral Term (I) measures the degree and frequency of errors over time, and multiplies this accumulated offset by the Integral Constant (Ki), which represents a 'steady state error' for the process, to remove set point vs measured value errors. The Derivative Term (D) measures the scope/rate/ramp of the error and multiplies it by the Derivative Constant (Kd), which represents the amount of the derivative term that will be applied to the overall control output; predicting the behavior of the system and stabilizing the process accordingly. PID values must be correctly calculated for the particular process, or the process may become unstable and/or slow to respond. Many chillers therefore employ 'autotune' functions, using an 'autotune' period in which the rates and degrees of change, and response times of the process are monitored and used to establish the P, I and D values to be stored and used by the controller.


POA Oil is a Heat Transfer Fluid (See Heat Transfer Fluid) commonly used in Chiller Technology; a highly branched, compact, and extremely stable synthetic hydrocarbon used frequently in military and aerospace applications for its dielectric properties and wide range of operating temperatures. POA Oil offers exceptional heat transfer performance over a wide temperature range and has a thermal conductivity of 0.14 W/m°C. However, although dielectric fluids provide low risk liquid cooling for electronics, they generally have a much lower thermal conductivity than water and most water-based solutions.

Portable Chiller

Portable Chiller refers to any Chiller System that can reasonably moved from one application or location to another, usually by means of built-in Casters, in which all components are contained in a single integral enclosure; as opposed to large tonnage permanent chilling systems which are permanently installed in a fixed location.

Pressure Relief Valve

A Pressure Relief Valve (PRV) is a type of safety valve used in Fluid Systems to control or limit pressure in the system which could otherwise exceed the indicated or set level and result in the interruption of a process, damage to equipment, or a catastrophic event (fire, explosion, etc.). Pressure is relieved by allowing the pressurized fluid to flow from an alternate passage and out of the system. The PRV is set or pre-designed to open at a predetermined pressure.


In engineering, a Process is a series of interrelated tasks that, together, transform inputs into outputs, or result in a finished product. In Chiller Technology, the process is the activity (usually a manufacturing system), from which heat must be removed in order to protect the process and facilitate the result.

Process Cooling

The majority of Chiller applications are designed to, (in a variety of ways depending upon the application), provide temperature control for manufacturing (automotive, energy, chemical processing, industrial, semiconductor) and food/beverage production processes which require low temperature process cooling. Heat transfer fluid is circulated to the process in question, drawing the heat from the process back to the refrigeration cycle of the chiller where the heat is rejected, and the heat transfer fluid returned to the process again.


A pump is a device that uses reciprocating or rotary mechanisms to move fluids by mechanical action from one location to another, consuming energy to perform the mechanical work of moving the fluid. In Chillers, pumps are used to recirculate Heat Transfer Fluids to facilitate the process of removing heat from a load, and transferring that heat to the refrigerant.

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