Thermostats for refrigeration systems

Tasks of a thermostat - room temperature control

One could almost go so far and claim that the unofficial fifth main component of a refrigeration system next to the compressor, the throttle body and condenser or evaporator is the thermostat. This is because even with the simplest compression refrigeration systems such as the refrigerator, a thermostat is always used for room temperature control. In this case, either the thermostat switches the compressor directly on and off again or a contactor or a relay. Alternatively, with the use of "pump down" or "pump out" circuits, the direct switching of the solenoid valve coil is possible. Thus, too high a contact load, as they would arise in the direct compressor load on the thermostat bypassed. If no solenoid valve is installed in the liquid line in the system, then this circuit variant can not be applied naturally. The use as a room thermostat assumes that it is a system in which the evaporator cools the air.

Water and brine temperature control

However, if the evaporator does not cool the air, but cold water or brine, the thermostat can regulate, for example, the water temperature. Such a control thermostat is usually installed in the return (the brine water comes back from the consumer, such as a chiller, back to the evaporator). In the flow you will normally find an additional thermostat, the antifreeze thermostat. It prevents - in addition to a flow switch - the falling below certain thresholds that lead to icing of the water cycle and thus frost blasting, for example. could result in the evaporator. This is therefore a danger, since the volume of water after the aggregate state transition to ice increases by about 10% and thus can quickly lead to greater damage in trapped water. Of course, it is still possible to use a room thermostat even in chilled water systems, even if the room is cooled with chilled water.

Defrost termination

Back to the actual refrigeration technology: Another standard application is the defrost limit thermostat. In order to avoid unnecessary energy consumption, the defrost length can be controlled very precisely by a thermostat whose sensor is ideally placed in the most stubborn iron nest of the evaporator. This offers a clear energy advantage over the time-controlled defrost, is always defrosted over a certain, defined time, regardless of whether the evaporator is already free of ice or not. Even better in this context is a demand-controlled defrost control, which, however, is no longer feasible with a simple thermostat. This requires more highly qualified electronics, which can decide on the basis of stored system characteristics whether a defrost must be initiated. However, care should be taken to ensure that certain time windows can be preselected despite the "Demand Defrost" function. This avoids the defrost being initiated just in or just before the loading phase. 

Compression end temperature monitoring

Somewhat rarer you will find Druckrohrthermostate. This application is intended to protect compressors from excessive compression temperatures. At the same time, the refrigerating machine oil is protected against excessive thermal stress and thus from denaturation. Especially with the use of scroll compressors, this protective measure is more common. But it is also recommended for reciprocating compressors.

sensor charge

Basically Danfoss thermostats of the "KP" series are delivered with two different sensor fillings. In order to be able to decide which filling and which devices are suitable for the specific application, it is important to know the difference. So it is on the one hand to a steam filling and on the other to an adsorption filling. The most important difference is that with steam filling, the sensor must always be placed colder than the "KP" housing. This fact stems from the fact that a certain amount of liquid is ready to evaporate in the interior of the sensor. If this is evaporated in the sensor, the pressure inside the sensor system increases and can push apart the pressure bellows, which in turn actuates the contact system. However, if this liquid moves towards the thermostat housing, it can no longer be vaporized in the sensor and the function will not work. The phenomenon is comparable to the classic refrigerant transfer, in which the refrigerant always moves to the coldest point. In the case of the adsorption filling, it does not matter if the temperature at the sensor is warmer or colder than at the associated housing.

Types Standard thermostat for wall mounting

Roughly speaking, there are two main types of thermostats in refrigeration: the adjustable standard thermostat for wall mounting and the refrigerator thermostat. The standard wall-mounted thermostat (such as the Danfoss "KP" type) with room sensor is often used by plant manufacturers for normal cold rooms. Thermostats of the same design are now also used with the remote sensor for the defrost limit function. It is important to pay attention to the appropriate sensor filling. It is recommended that the adsorption, as this does not shift, so that is switched reliably. 

Refrigerator thermostat

The second main design is the classic refrigerator thermostat. This serves as a room thermostat for refrigerators, but is also used in smaller counter cooling and the like. The advantages of a refrigerator thermostat are the very moderate price and the optimized longevity of the product. In case of replacement in case of service please note the following: Fridge thermostats are available in countless variants, which differ only in very few details. These are the length of the capillary tube sensor, the temperature switching points and the question of whether it is an automatic defrost or constant reclosure temperature in the temperature plus range and whether a signal should be connected in parallel or counter to the direction of action. For this reason, can be used to cover service cases on a few service thermostats, which greatly facilitates the task. At Danfoss, there are eight service thermostats that can replace the vast majority of refrigerator thermostats in the event of repairs, thanks to an extra-wide temperature range and a longer capillary tube. For the refrigeration system manufacturer, the service thermostat no. 3 and 8 are particularly important here. The service thermostat no. 3 is suitable for automatic defrost refrigerators or constant reheat refrigerators, which are currently used in the majority. For the small refrigeration, the service thermostat No. 8 is important, as it is often used for drinks vending machines and spirits cooling in the catering industry. He rather serves the upper evaporation temperature range. Saladettes are also easily adjustable with this thermostat. Of course, there are basically other types of mechanical thermostats. At this point, however, only the most important types, which are used in refrigeration, will be described.

Types of sensors

For the various applications of the thermostats, there are also various sensor designs, which are optimized for the appropriate purpose. To measure the room actual value, a fixed spindle is usually mounted under the thermostat (type "KP" or "RT"). For use as an evaporator thermostat, a simple capillary tube sensor or a cylindrical remote sensor with capillary tube connection to the main unit can be used. For mounting to a pipeline - e.g. When monitoring the compression end temperature at the discharge port of a compressor - the cylindrical remote sensor is recommended. Finally, to regulate or monitor the temperature in a ventilation duct, there are special duct sensors.Kontaktbelastung

An important point when using thermostats with potential-free contacts is the contact load. The three different values ​​for the contact load, which manufacturers usually state, have already been mentioned in the article of the last issue "Pressure Switches". These load cases are also valid for thermostats, so here again a short summary. These three values ​​are the purely ohmic value (in this case the highest contact load is classically possible), the partially inductive and the purely inductive load case. An example of an ohmic load (load designation: AC1) is an electric resistance heater for defrosting. This load case is to be selected when selecting a defrost limiting thermostat, via which the heating is switched directly. Partial inductor (AC3) is for example an electric motor. Of course, this also includes a compressor, which is switched directly from the thermostat. By contrast, a coil (AC15), as used in solenoid valves, acts as an inductive load for a thermostatic contact system.

Electrical connection

A standard thermostat "KP" with changeover contact system usually has three connection contacts on which the wires of the electric cable can be placed. This is similar to the pinout on "KP" pushbuttons. The three connections are "phase in" (contact designation "1"), "thermal sense" (contact designation "2") and "cold acting" (contact designation "4"). It does not matter if, for a two-person occupancy, "phase in" ("1") and "cold effect" ("4") are confused with each other. The connection "heat sense" ("2") is rarely used in refrigeration. An exception, for example, is when it needs to be heated for certain reasons. Conceivable here is the control of an electric heater. Specifically, a "KP61" from Danfoss (room thermostat) places "phase in" on contact 1 and "cold acting" on contact 4.

Settings

The thermostats for wall or sheet-metal console mounting "KP" offer the following setting options. On the front side of the device two separately adjustable scales can be seen. It is on the left to the setpoint and right next to the difference setting (hysteresis). On the left scale the upper switching value is set and on the other side the difference. Switching value minus difference results in the switch-off value. With simultaneous electrical connection on the contacts "1" and "4" is now switched on at the left set value (for example, the compressor switched) and switched off at this value minus the difference set to the right. Example: Value left "-10" ° C, value right "6" ° C - corresponds to a cut-off value of -16 ° C. Since this is a mechanical component, the value for the difference is not always exactly the same. It differs depending on the set switching value. The exact difference value can either be determined empirically at the plant by readjustment or determined exactly by a corresponding nomogram (part of the instructions for "KP"). In practice, the procedure "left value minus difference on the scale right" is usually sufficient.

IP-Protection

The level of IP protection may be another important issue, depending on location and environmental factors. IP crash course: The first digit of the two digits that make up the IP rating (IP54, for example) describes protection for touch protection, and the second for water protection. An IP degree of IP 3 * indicates that a wire with a diameter of 2.5 mm may not penetrate into the device so certified. IP * 3 also means suitability for falling water spray up to 60 ° from vertical. All in all, the higher the degree of protection, the better the device is protected against dust, dirt particles and moisture. Standard thermostats "KP" have a degree of protection of IP33 against dust and moisture. If an accessory protective housing is used, IP55 has already been achieved. If an even higher degree of IP protection is desired, the "RT" series, designed for particularly inhospitable environments, is available. These have IP degrees of protection from 54 to 66, depending on the version.

Electronic thermostats or refrigeration controller

So far, all versions relate exclusively to mechanical thermostats. With regard to reliability, robustness and simplicity of use, these thermostats will be indispensable in refrigeration even in the future. Nevertheless, it may be useful in some cases to use electronic thermostats or refrigeration controller. Especially by the bundling of functions like e.g. Room temperature control with defrost limitation and display of the actual room temperature, which is available today with all common cold store controllers, often offer these devices advantages. In addition, the desire for specific hysteresis values can lead to the selection of an electronic thermostat, as mechanical devices do not always allow any hysteresis or switching differential value to be realized.

Information

The content is based on the booklet series of the same name by Danfoss (www.danfoss.de/kaelte), which deals with the basic relationships in compression refrigeration systems and the associated basic components. The series is aimed at refrigeration engineers in service and plant construction, at new entrants in refrigeration technology, at apprentices to the refrigeration plant manufacturer and at all those who would like to gradually incorporate the practice-oriented basic knowledge of the cold again.