11. April 2017

Pressure switch for refrigeration systems

Pressure switches are used for control purposes and for protective shutdowns of refrigeration systems.

Task of a pressure switch

In addition to pressure regulators, there are also pressure switches in refrigeration systems. While the pressure regulators intervene directly in the main mass flow of the refrigeration circuit by opening and closing, the task of the pressure switch or pressostat is somewhat different. The information about the current pressure in the system at the point observed by the pressure switch is usually supplied directly to the pressostat through a stub or when the pressure switch is mounted directly on the piping. The pressure actuates a potential-free contact system, with which, for example, a compressor or a fan can be switched. But why are these circuits good or necessary? Let's start with the function of the low-pressure switch in a compression refrigeration system. Most common are low pressure switches, which are usually connected in the immediate vicinity of the compressor on the suction side, used for protective shutdown of the compressor in case of lack of refrigerant. The reason for this is the fact that refrigerant loss in the system can lead to various problems. For example, suction gas cooling is reduced, which is a major problem especially with a 100% suction gas cooled compressor.

Task pressure switch

In addition, refrigeration oil generally also emerges in the event of leakage, since this constantly migrates through the entire refrigeration cycle together with the refrigerant. This in turn can result in insufficient lubrication. It is therefore very advantageous to switch off the compressor as a precaution in the event of a sharp drop in low pressure in order to avoid damage or even complete failure of the compressor. It should be noted that not every protective shutdown of an LP switch must necessarily be due to lack of refrigerant.


The service technician should always check the flow rate of the medium to be cooled at the evaporator in such an accident. If this is too low due to another malfunction, then there is no lack of refrigerant. A simple example of this would be a defective evaporator fan or a defective pump in a chiller (refrigerant-water evaporator). In the latter case, however, the flow switch should already have struck in the water cycle, otherwise there is a risk of icing of the evaporator.


"Pump down" - and "pump out" circuits

The low pressure switch can also be used for control purposes. For example, in the "pump down" and "pump out" circuits used in refrigeration, it is used to shut down the system whenever the compressor shuts off for reasons of normality (eg when the shutdown temperature at the room sensor is reached) , This happens when the pressure is exhausted from the liquid solenoid valve via the evaporator to the suction side by closing the liquid solenoid valve and the initially continuing compressor. At a certain threshold, the ND switch will switch off the system. If the switch-on point of the room thermostat is reached again after a while and the solenoid valve is opened,


High pressure switch DWK -DBK -SDBK

On the high pressure side, there is the function of the classic high pressure switch on the one hand. According to EN 378, the use of high-pressure switches for commercial refrigeration systems is generally required. They are used for protective shutdown of the compressor as (usually) authoritative pressure generator in a compression refrigeration system. Now, if the permissible operating pressure is reached and there is an overrun, the compressor is switched off by the high pressure switch and the pressure on the high pressure side drops again. High-pressure switches for this purpose differentiate between pressure switches, pressure limiters and safety pressure limiters. With a pressure monitor (DWK) an automatic reclosure can basically take place after a certain lowering of the pressure on the high pressure side. For a pressure limiter (DBK), a manual reset must be performed after a safety shutdown. This reset may be triggered by hand and without tools. Finally, the safety pressure limiter (SDBK) requires a tool to initiate the manual reset.


Fan control the condenser

On the other hand, there is the possibility on the high pressure side to optimize the fan control of the condenser with a pressure switch. As an alternative to a fan speed control can be so cost-effective avoided that the condensing pressure falls too far, especially in winter too far. This measure is recommended for very small and therefore cost-sensitive commercial refrigeration systems but also for larger systems with three-phase fans, where speed control can prove to be very costly.



Basically, there are two general types of pressure switches. These are the adjustable standard pressure switch for wall mounting and the cartridge pressure switch. The pressure switch for wall mounting (eg type "KP" from Danfoss) is particularly popular with plant manufacturers. It allows a readjustment in terms of switching thresholds and does not sit with its entire mass on the pipeline. In addition, it is possible to put out the pressure switch in a machine compartment forward, which can increase the accessibility and ease of service significantly. The cartridge pressure switch, on the other hand, is the preferred solution of series manufacturers because its fixed settings can not easily be adjusted in the field by unauthorized persons.




Contact load

An important point when using pressure switches with potential-free contacts is the contact load. It can initially be confused that manufacturers usually specify three different values ​​for the contact load. What value should one address?

Normally, these three values ​​are:

Pure ohmic (in this case classically the highest contact loading is possible), partial inductive and purely inductive loading case.

An example of an ohmic load (load designation: AC1) is the electric (resistance) heater for the defrost.

Partial inductor (AC3) is z. B. an electric motor , that is, a compressor.

In contrast, a coil (AC15), as used in solenoid valves, acts as an inductive load for a pressure switch contact system.


Electrical connection

In a standard pushbutton switch with changeover contact system, there are usually three connection contacts on which the wires of the electric cable can be placed. These three ports are "phase in", "fault" and "phase out (to the engine)". It makes no difference whether the two connections are changed or changed in a two-person assignment of "phase in" and "phase out". The connection "Fault" is rarely used. A conceivable fault message here is the activation of a red signal lamp or the forwarding to a remote control.

For a "KP1" from Danfoss (low pressure switch) "phase in" is set to contact 1 and "phase out" to contact 4. If the fault function is desired, it can additionally be clamped to 2. Please note that the connection with a "KP7" high pressure switch is not made analogue. While "phase in" remains on contact 1, "phase out" comes on contact 2 and contact 4 becomes a fault message. This seems confusing, but can be easily deduced by the service technician with the help of the "KP" pressure switch. If you remove the plastic cover, the contact system can be seen on the right. This is labeled "1", "2" and "4". In a ND switch "KP1" is the contact 4 above and 2 below. The relevant switch-off case results in a pressure reduction (eg refrigerant shortage). Since the Druckschalterbalgelement always with increasing pressure upwards (bellows "blows up") and when pressure reduction always moves down, the shutdown must take place during the downward movement. With "KP1" the connection 1 and 4 has to be selected for the main path network compressor. With a "KP7" high pressure switch, however, the shutdown takes place when the bellows starts up. Since the contact 4 is still at the top and 2 is still at the bottom, it must be set to "1" and "2" here. With a "KP7" high pressure switch, however, the shutdown takes place when the bellows starts up. Since the contact 4 is still at the top and 2 is still at the bottom, it must be set to "1" and "2" here. With a "KP7" high pressure switch, however, the shutdown takes place when the bellows starts up. Since the contact 4 is still at the top and 2 is still at the bottom, it must be set to "1" and "2" here.

In the case of a dual pressure switch, which is always a combined ND and HD switch at Danfoss with one exception ("KP7BS", two high pressure switches DBK + SDBK), there are variants with ND or ND and HD fault contact. By using a double pressure switch eliminates both an electrical installation and a mounting mounting. In addition, two pressure switches are usually more expensive than a duopressostat.


Refrigeration connection

When connecting a pressure switch for wall mounting, it should be noted that safety-related high-pressure switches (eg "KP7 W", "KP7B", "KP7S", "KP7BS") are always connected via a stub line with a minimum internal diameter of 4 mm. In other words, use a 6 mm copper tube.Of course, it is also possible to use special plastic connection pipes for refrigeration systems, as they have become fashionable in the last 15 years, with an inner diameter of at least 4 mm. Pressostats for the condenser fan control and low-pressure switches may continue to be connected with capillary tubes, whereby many plant manufacturers generally use 6 mm copper tubes or alternative plastic tubes for all pressure switches, in view of the optics and to prevent unwanted capillary closures.


Manual test

A manual trigger test should only be performed in exceptional cases. If this is absolutely necessary, this can be done with "KP" pressure switches by a screwdriver from the front side. For this purpose, the steel sheet, which is applied directly to the bellows element, pushed up to a bellows movement upwards (= pressure increase) simulate. Such a test should be done with extreme caution and remain the exception.



The level of IP protection may be another important issue, depending on location and environmental factors. The first digit of the two numbers, which make up the IP rating (eg IP54), stands for touch protection, and the second digit stands for water protection. An IP degree of IP 4 * means that with such a device, the penetration of a wire with 1 mm diameter may not be possible. IP * 4 also means that it is suitable for spraying water from all directions. All in all, the higher the degree of protection, the better the device is protected against dust, dirt particles and moisture. Danfoss' standard "KP" pressure switches have a degree of protection of IP33 against dust and moisture without a "head cover" installed. Is the included "head cover" mounted, the degree of protection increases to IP44. Using an accessory protective housing, IP55 is 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.


Stephan Bachmann,

Danfoss Refrigeration, Offenbach


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