According to its principle, compressors can be divided into positive displacement compressors and speed compressors. Volumetric type is divided into reciprocating compressor and rotary compressor; Speed compressors are divided into axial compressors, centrifugal compressors and mixed-flow compressors.
At present, the compressors of household refrigerators and air conditioners are all positive displacement, which can be divided into reciprocating and rotary types. Reciprocating compressors adopt piston, crank and connecting rod mechanism or piston, crank and sliding tube mechanism, and rotary compressors are mostly rolling rotors. Commercial air conditioners are mostly centrifugal, scroll and screw.
According to the scope of application, it can be divided into low back pressure type, medium back pressure type and high back pressure type. Low back pressure type (evaporation temperature -35 ~- 15℃), generally used in household refrigerators, food freezers, etc. Medium back pressure type (evaporation temperature -20 ~ 0℃), generally used in cold drink cabinets, milk freezers, etc. High back pressure type (evaporation temperature -5 ~ 15℃), generally used in room air conditioners, dehumidifiers, heat pumps, etc.
principle of operation
Used in air compressor, mainly used to adjust the start-stop state of air compressor. By adjusting the pressure in the gas storage tank, the air compressor can stop and rest, which has a maintenance effect on the machine. When the air compressor leaves the factory for debugging, adjust it to the specified pressure according to the customer's needs, and then set a pressure difference. For example, when the pressure reaches 10kg, the air compressor stops or unloads, and when the pressure reaches 7kg, the air compressor restarts, and there is a pressure difference here.
The compressor is directly driven by the motor, which makes the crankshaft rotate and drives the connecting rod to make the piston reciprocate, causing the volume change of the cylinder. Due to the change of pressure in the cylinder, air enters the cylinder through the intake valve and the air filter (muffler). In the compression stroke, due to the reduction of the cylinder volume, compressed air enters the air storage tank through the exhaust pipe and the check valve (check valve). When the exhaust pressure reaches the rated pressure of 0.7MPa, it is automatically stopped by the pressure switch. When the pressure of the gas storage tank drops to 0.5-0.6MPa, the pressure switch is automatically turned on and started.
construction of switch
For different temperature measuring ranges, temperature switches with different structures should be selected. In the temperature range of 0℃ ~ 100℃, solid expansion temperature switches are generally used, and in the temperature range of 100℃ ~ 250℃, gas expansion temperature switches are mostly used. For the temperature range above 250℃, only thermocouple or thermal resistance thermometer can be used.
The working principle of solid expansion temperature switch is to use the difference of length change of different solids after heating to produce displacement, so as to make the contacts act and output the temperature switch signal. For example, a temperature switch is made of bimetal (brass sheets are stacked on indium steel sheets). Because the linear expansion coefficient of brass sheet is larger than that of indium steel sheet, bimetal will bend when heated. When the specified temperature is reached, the free end of the bimetal (the moving contact of the temperature switch) generates enough displacement to disconnect from the fixed static contact and send a switch signal.
The gas expansion temperature switch works according to the principle of gas pressure thermometer. It has a temperature measuring bag filled with nitrogen, which is connected with the measuring element of the pressure switch through a sealed capillary. When the measured temperature reaches the specified value, the inflation pressure in the temperature bag makes the pressure switch act.