Pressure control device
In refrigeration and air conditioning system pressure controls divide into two main related categories: control function and protection function. Example for control functions is compressor cycling, pump down or condenser fan control which are most using pressure transducers and electrical controllers but protection functions is depend on pressure limiting and include pressure switches.
Pressure switches
Pressure switches are included High pressure switch, Low pressure switch and differential pressure switch.
High pressure switch
On the high-pressure side, there is the function of the classic high-pressure switch. 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. Basically, there are two general types of pressure switches; adjustable standard high pressure switch for wall mounting and the cartridge or fixed high pressure switch.
Low pressure switch
On the low pressure side, using of low pressure switches are most common. This switch is used for protective shutdown of the compressor in case of lack of refrigerant. Lack of refrigerant in the system can lead to various problems and installing a low pressure switch on refrigeration systems is necessary. Some type of low pressure switch is used for oil pressure too. Low pressure switches are manufactured in adjustable and cartridge one.
Differential pressure switch
Differential pressure switches are usually used as a safety switch for oil pressure or air cooled condenser fan control in refrigeration systems.
Functions of pressure switches contacts
Pressure switches can be active on pressure rise above or pressure drop below the setting. It can be manual or automatic reset. Anyway, the function of the pressure switch contacts is as follows:
Function of contacts SPDT
On pressure rise above setting, 1-2 opens and 1-4 closes. On pressure drop below setting 1-2 closes and 1-4 opens.
Function of contacts SPDT with manual reset Max.
On pressure rise above setting, 1-2 opens and 1-4 closes and latches. The switch can be manually reset when the pressure has dropped below setting.
Function of contacts SPDT with manual reset Min.
On pressure drop below setting, 1-2 closes and 1-4 opens and latches. The switch can be manually reset when the pressure has risen above setting.
