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Thermonics Low Temp. Chillers – Glossary

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.


Ramp is a term used in temperature generation to describe the amount of time it takes for the system to reach Set Point. This Ramp Speed can be set in the programming of the system to be more or less rapid, according to the needs of the application.


Refrigerants (Primary Fluids) are used in compression refrigeration cycles that require compression, condensation, expansion and evaporation sequences to phase transition from a liquid to a gas and back again, thereby removing heat from the load. They are direct expansion fluids that evaporate in the portion of the process that requires refrigeration/cooling. Refrigerant types include chlorofluorocarbons and non-halogenated hydrocarbons.

RTD (Resistance Temperature Detector)

Resistance Temperature Detectors (RTD) are Temperature Sensing Probes constructed of finely wound platinum wire in which temperature changes produce corresponding, predictable linear resistance changes; Resistance Increases as the Temperature Rises. RTDs are a common device used in Chillers to obtain the Output and DUT temperatures readings necessary for precise temperature programming, generation and control. RTDs are superior to Thermocouples in that their readings are more accurate, stable, and more repeatable, and receive more robust signals. They are substantially more expensive than Thermocouples, however, leading to a cost vs. accuracy/durability decision when choosing which type of temperature sensing device is appropriate for a particular chiller.


SEMI-S2 is a set of performance-based Environmental, Health and Safety (EHS) Standards and Guidelines applicable to equipment used to manufacture, measure, assemble and test Semiconductor Products, including the design, manufacture and documentation of state-of-the-art Chiller Equipment. SEMI-S2 is a product of SEMI, a global Electrical Industry Association, and is developed from the consensus of its voting industry-representative and scientific members worldwide. Related standards are SEMI F47 (Line Drop) and Semi S8 (Ergonomics).

Set Point

The desired temperature, Hot, Cold, or Ambient to which a Temperature Generating Device is programmed to Run (achieve). When the system has reached that Set Point, the System is said to be 'At Temperature'.


Soak is a term used in temperature generation to describe the amount of time that a set point is maintained before the system moves on to the next segment of a cycle. This Soak Time can be set in the programming of the system to be longer or shorter, according to the needs of the application.

Split Chiller Systems

The term 'Split Chillers' refers to systems in which some portion of the chiller's system, such as the condenser, is located remotely, apart from the controller cabinet. Split chiller systems are most commonly used in applications in which the load being chilled must be stored separately, as in the beverage and hospitality industries, or where equipment weight and/or footprint are concerns. Split chiller configurations are most commonly found in the air-cooled category. See Packaged Chiller System, above.
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