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Function of a pressure switch
Let us begin with the function of the low-pressure switch in a heat pump. Low-pressure switches are usually installed close to the compressor on the suction side and are most commonly used as a protective cut-out for the compressor when the refrigerant is low. The reason for this is that losing refrigerant can cause a number of different problems for the system. For example, it reduces suction gas cooling, which is a major problem, particularly for compressors with 100% suction gas cooling. Such a leak will also always lose refrigeration oil – because this circulates through the entire cooling circuit together with the refrigerant – and this may result in inadequate lubrication. As a precautionary measure, to prevent the compressor being damaged or even failing completely, it is therefore highly advantageous to switch it off whenever the low pressure value drops sharply. To be sure, not every activation of a protective low-pressure switch necessarily implies low refrigerant. In such cases the service technician should always check the medium throughput at the evaporator. If some other fault is bringing this down there may be no lack of refrigerant. One simple example of this is a defective evaporator fan in an air to air or air to water heat pump, or a defective pump on the evaporator side in a water (brine) to water heat pump.
Types of high-pressure switches
For a start, on the high-pressure side we have the function of a conventional high-pressure switch. Such switches are a general requirement for all commercial heat pumps. They are used to switch off the compressor, which is (generally) the primary pressure source in a compression-type heat pump. If the permissible operating pressure is reached and the pressure threatens to exceed its threshold value, the high-pressure switch will switch off the compressor causing the pressure on the high-pressure side to fall again. The various types of high-pressure switches used for this purpose can be subdivided into pressure monitors, pressure limiters and safety pressure limiters. In the case of a pressure monitor, the system can always be restarted automatically when the pressure on the high-pressure side has dropped by a specific amount. In the case of a pressure limiter, such a protective switch-off must always be followed by a manual reset, which may, however, be activated by hand and without tools. Finally, when a switch-off is carried out by a safety pressure limiter, the necessary manual reset can be effected only using a special tool.
Types of pressure switches
There are two fundamentally different designs of pressure switches, standard adjustable wall-mount pressure switches and cartridge pressure controls. Wall-mount pressure switches such as the Danfoss KP are particularly popular with fitters. They enable trip thresholds to be adjusted and their weight is not entirely borne by the pipe. It is also possible to locate the pressure switch in a compartment at the front of the equipment, making it much easier to access and maintain. In contrast, cartridge pressure controls are the solution generally preferred by series manufacturers’ because their fixed settings cannot easily be modified by unauthorised persons in the field. Cartridge pressure controls are also usually extremely reasonably priced.
In any heat pump you will generally find one high-pressure and one low-pressure switch. Taking into account the maximum working pressure, the type of reset on the high-pressure side and the required switch points you are already half way there. The person constructing the initial system can choose between preset cartridge pressure controls and adjustable standard pressure switches. For service personnel the adjustable pressure switch will be the better choice.