Glossary Heating and cooling systems

Glossary heating and cooling systems


The actuator is an active component of the so-called control circuit. It is controlled through an adjusting command (see Control variable) from the controller (see Temperature controller) and triggers a measure that counteracts the control deviation.

Ambient temperature range

This is the admissible temperature range in which the unit can be operated in accordance with the regulations.


Directive 94/9/EC, also known as ATEX 100a, has been issued by the EC in order to harmonize fundamental safety and health requirements for devices, protection systems and components that are provided for use in accordance with the regulations for hazardous areas. This definition includes devices for safety, test and control which are required for use outside hazardous areas with regard to their explosion risks. Directive 94/9/EC is applicable to electrical and non-electrical devices, protection systems and components (electrical and non-electrical operating supplies).

Bus system

see Interface.

Cascade control

see External control.

Component tested safety facilities

As a rule all temperature control units fall within the pressure equipment directive (see DGRL 97/23 EC). Depending on the heat transfer medium and the temperature, the installations have to be equipped with special safety devices, for example safety valves, pressure, temperature or level switches. These devices serve as a protection for the temperature control unit when the admissible limits of the application are exceeded.

Compressor control

see injection control.


see coolant.

Control circuit

The control circuit is the control section (see Control section) and controller. Sensors and signals, their processing and transmission also form part of the control circuit. For example, transformers, clamps and electric lines.

Control deviation

is the deviation of the actual value from the preset setpoint (see control circuit and controller caracteristic).

Control section

Control section is the part of the control circuit to be influenced according to its application. Pipework, sensors and heat transfer areas with their walls and coatings also form part of the control section. Through distances from the test point to the final control element and through inertia at the heat transfer for example, a characteristic response time of a control section arises. Depending on the response time, a control section is classified from ‘good’ to ‘difficult’.

Controller characteristic

Different controllers are distinguished according to their so-called controller characteristic: proportional response (P-controller), integral response (I-controller) and differential response (D-controller). As for temperature controllers, combined characteristics have stood the test. Modern PID-controllers can be perfectly adapted to difficult control sections through freely eligible parameters. They are therefore widely used.

Cooling output

With medium-cooled heat exchangers the cooling output always depends on the temperature difference between the cooling medium and heat transfer fluid. The cooling output reduces at falling outflow temperature and tends to zero when it reaches the cooling medium temperature. With machine-aided cooling the characteristics are similar. The cooling output thus always relates to a definite outflow temperature. At LAUDA the performance data of refrigeration machines is based on cooling water or air with a temperature of 20 °C.

Cooling process

see coolant.

Cooling system

see coolant.

Delivery head

The delivery head is a theoretical value which is used within the pump characteristics (see Pump characteristic) instead of the feed pressure (see Feed pressure). The advantage is that the delivery head is valid for any given liquid. The feed pressure is then calculated for the respective feed medium by multiplying the density by the delivery head (see Feed pressure).


see refrigerant

Expansion volume/Expansion tank

When heating up the heat transfer medium to the desired operating temperature, the heat transfer medium expands. This expansion volume results from the expansion coefficient, the temperature change and the contents of the unit. The expansion tank installed in the temperature control unit has to collect the expansion volume safely. The assessment of the dimensions of the expansion tank for thermal oils depends on the medium The following applies for hot water: about 0.8 percent per ∆T = 10 K. The following applies to mineral oils: about 1 percent per ∆T = 10 K. The following applies to silicone oils: about 2 percent per ∆T = 10 K.

External control

If the outflow temperature is not the controlled variable, but it is instead a temperature measured outside the thermostatic unit, then the outflow temperature of the heat transfer fluid is tracked in such a way that the setpoint is reached at the external point. In practice this is achieved via a so-called cascade control (see Cascade control). The so-called “master controller“ produces as the control variable (see Control variable) the setpoint for the following slave controller (see Slave controller), which in turn controls the outflow temperature of the heat transfer medium liquid.

Feed pressure

Feed pressure is the manometric pressure measured at the outlet nozzle of a pump or a compressor. With pumps the feed pressure is also given from the pump characteristic (see Delivery head) and the density of the feed medium.

Filling volume

In connection with a LAUDA system, the recommended filling amount of the system (volume) which is required in order to guarantee trouble-free operation, and to allow for expansion or contraction of the system throughout its operating temperature range.

Film temperature

The maximum film temperature, which the manufacturer indicates with reference to organic heat transfer media, describes the temperature at which the heat transfer media begin to decompose. Especially when organic heat transfer media are heated up with the help of electric heaters, attention must be paid to the heat flux values because otherwise the heat transfer medium is destroyed by a high surface or film temperature.

Flow rate

Volume flow of a pump or a compressor. The flow rate depends on the respective operating point and the characteristics (see Pump characteristic) of each component.


In processes in which the reaction temperature or the respective temperature control represent a critical quantity, the temperature control unit has to be qualified and validated. Qualification takes place according to the “Good Manufacturing Practice“ (GMP). If, for example, pharmaceutical products are produced for the American market, the manufacturing process – and with it the process equipment including the temperature control unit – is subject to the requirements of the American “Food and Drug Administration“ (FDA).

Hazard evaluation (HAZOP)

As far as hazard evaluation is concerned, which has to be generated by the operating company, the information relevant for safety from the operating manual of the manufacturer has to be considered. This especially includes: mounting and integration of the temperature control unit into the total system, commissioning, operation, maintenance and inspection and indications for possible improper use, as far as such an application has not already been prevented by the rating/technical measures.

Heat transfer fluid

These are liquids which supply or dissipate energy at the consumer. The heat carrier is transported to the consumer by the circulating pump on the thermostatic unit. The larger the circulating quantity, the lower the temperature difference at the consumer. The less the temperature difference, the better the control accuracy.

Heater power

With electric heating, the heater power is identical to the power consumption of the built-in electric heaters. It is always the maximum possible heater power and, in the case of electric heating, it remains invariable at all operating temperatures. With medium-heated heat exchangers, the heater power always depends on the temperature difference between heating medium and heat transfer fluid. The heater power reduces at increasing outflow temperatures, and tends to zero when it reaches the heating medium temperature. The applied pump output is helpful but not taken into account.

Injection control

With LAUDA process cooling units, the cooling power (see Cooling power) is permanently controlled by adjusting the injection flow (0 - 100 percent). The final control element is a continuously working control valve that is placed in the refrigerant feed line before the evaporator. If several compressors are operated in parallel, an automatic step switch (see Compressor control) ensures energy-saving partial-load operation.

Interface (electrical)

Always serves to exchange data and may be set up on an analogue basis (mostly standard signal 4-20 mA or 0-10 V) or on a digital basis. In the digital area you find the simple serial interfaces (RS 232) or the more powerful, parallel addressable systems for many users (RS 485 or the different industrial bus systems).

Master controller

see External control.

Modular engineering

This describes the modular kit from which any conceivable thermostatic system can be planned and assembled according to a recurring pattern. It saves costs in the planning, implementation, commissioning, documentation and servicing, because each module is itself well-proven. By using modular engineering, a maximum safety standard is achieved.

Monofluid (single fluid) system

This is a heat transfer system which works with only one heat transfer liquid. It is useful when heating, cooling and freezing must be done simultaneously and heating and cooling units are working together.

Overlay pressure/Inert gas covering

Through an inert gas covering (nitrogen) on the expansion tank, oxidation of the heat transfer medium and seeping of water vapor from the air can be prevented. If the heat transfer medium is operated below its evaporation point, overlay pressure should be as low as possible (about 0.1 bar), so that when heating up through the reduction of the gas space the pressure increase is not too high. If the heat transfer medium is operated above its evaporation point and atmospheric pressure (1.013 bar), an overlay pressure of at least the respective vapor pressure is necessary in order to safely prevent cavitation. In both cases a safety valve has to be installed on the expansion tank.

Peripheral wheel pump

Peripheral wheel pump is a centrifugal pump with a running wheel that has a so-called ‘peripheral’ shape. The almost linear characteristics of a peripheral wheel differ fundamentally from the characteristics of a radial running wheel. At highest feed pressure and lowest feed flow rate the highest drive energy is required with the peripheral shape. Peripheral wheel pumps are especially suited for small feed flow rates and high pressures.

Permanent control deviation

In the case of a purely proportional controller there is always a „remaining control deviation“. The control variable will always be proportional to the deviation. There is no adjusting command without a deviation.

Plug & Play modules

Temperature control units and modules arrive at the construction site with clearly defined interfaces, ready for connection, complete with pipework and insulated. They only have to be docked (see Modular engineering).

Pressure Equipment Directive (DGRL 97/23 EC)

Temperature control units are in the control of the pressure equipment directive considered as modules, which are assembled from several pressure devices (expansion tank, pipelines, fittings, safety valve, etc). The fundamental safety requirements for this are described in Annex I of the pressure equipment directive (DGRL). The process of conformity assessment for the module to be applied depends on the category in which the module is classified. This category is determined through the highest category of the respective built-in pressure device. In this connection, parts of equipment with a safety function are ignored. The category that describes the hazard potential depends on the maximum operating pressure, heat transfer medium, content and type of the built-in pressure device. Before the temperature control unit is commissioned, the manufacturer has – according to the operating conditions – to classify the module into a category and to submit it to a process of conformity assessment. The temperature control unit has to be labeled with the CE label and, starting from category II, has to be labeled with the identification number of the named testing laboratory.

Primary side

Denominates primary energy carriers such as vapor, cooling water, air, brine, liquid nitrogen, etc., that have to be connected to the temperature control unit by the customer. These primary energies can be gaseous, vaporous or fluid.

Protection type IP

Pursuant to EN 60529 two numerals assess the electric degree of protection. The first numeral represents the quality of the touch and foreign body protection (dust). The second numeral assesses the protection against water. For example, IP 54 protection signifies dust protection and sealing against splashed water on all sides.

Pump characteristic

This is a diagram that shows the function of the delivery head in relation to the flow rate.

Radial pumps

Radial pumps are centrifugal pumps with a running wheel that has a so-called ‘radial’ shape. The non-linear characteristics of a radial wheel differ fundamentally from the characteristics of a peripheral wheel. At lowest feed pressure and highest feed flow rate the highest drive energy is required with the radial shape. Standard pumps for the chemica industry are radial pumps. Radial pumps are especially suited for small feed pressures and high flow rates.


Operating material of the cooling process that is located in the closed refrigeration system. The compressor sucks it from the evaporator, where it changes to the gaseous condition under heat absorption. On the warmer side of the evaporator the medium cools down due to heat extraction. On the high pressure side of the compressor the cooling medium is liquefied in the condenser/ heat exchanger under heat transfer. The condenser/heat exchanger is water or air-cooled.

Risk analysis

The risk analysis has to be generated by the manufacturer of the temperature control unit. The analysis serves to determine the dangers in connection with the unit with reference to the provided operating conditions. Rating and construction of the temperature control unit take place in consideration of the risk analysis. As to the remaining dangers which cannot be covered through technical solutions, as well as measures required from the operator, the manufacturer has to inform the operator through indications in the operating manual, and if necessary through mounting alert labels on the temperature control unit.

Secondary side/Heat transfer medium side

Designates the side of the temperature control unit from which the heat transfer medium flows. Heat transfer media (thermal oil, water) are chosen according to the operating temperature range and their respective application.

Set value

Adjusting command of the controller that acts on the so-called actuator.

Slave controller

see External control.

Sound pressure level

Quantity for the acoustic emission according to DIN EN ISO 11200. In contrast to the acoustic power level, the pressure level is always assigned to a defined distance.  In practice both quantities are stated in dbA.

System pressure

This is the pressure which is produced by pump pressure, vapor pressure at operating temperature and overlay pressure in the heat transfer medium system. Please pay special attention to the maximum system pressure because all components that have the heat transfer medium flowing through them have to be suitable for the maximum system pressure (see Pressure equipment directive).

Temperature control

Temperature control is taken to mean the controlled supply or extraction of heating or cooling energy to achieve a constant temperature on the consumer.

Temperature control unit (TCU)

This is a comprehensive term for differently designed heating and cooling systems which in a defined working temperature range can control temperature of the consumer by means of a liquid.

Temperature controller

A temperature controller is an active component which compares at least one actual temperature value with one setpoint, and depending on the deviation (see Control deviation) outputs an adjusting command (see Control variable). This so-called “adjusting command“ acts on the so-called ‘final control element’ which also actively triggers a measure that acts against the deviation. Temperature controllers can be of purely mechanical design (for example the so-called ‘radiator thermostat’) or electronic, i.e. analog or digital. Often several operating modes are combined.

Through-flow cooler

A through-flow cooler is an electrically cooled or otherwise cooled heat exchanger, where a fluid is forced to flow through the system. The cooler mainly serves as a cooler for the fluid that is flowing through. Usually the forced flow is generated by a pump.

Through-flow heater

A through-flow heater is an electrical or otherwise heated heat exchanger, where a fluid flow is forced through the system. The heater mainly serves as a heater for the fluid that is flowing through. Usually the forced flow is generated by a pump.

Two-circuit cascade refrigeration

This is a cascade connection of two refrigeration systems with cooling mediums (see refrigerant) of different thermodynamic properties. Two-circuit cascades with compression refrigerating processes are used at working temperatures below -50 °C.  The first cascade (high-temperature stage) produces temperatures of about -35 °C in the evaporator. On the warm side of the evaporator the cooling medium of the second stage (low-temperature stage) condenses at about -30 °C and vaporizes at about -90 °C, and cools the heat transfer fluid to about -80 °C.

Working temperature range

The temperature range that can be run in the heat transfer medium flow line to the process equipment (outflow).