Vacuum pumping is an essential and important process before filling refrigerant in the process of refrigeration equipment production or maintenance. That is, the vacuum pump is connected with the refrigeration system pipeline (usually the high and low pressure sides are connected at the same time), so as to eliminate the non condensable gas and moisture in the system pipeline.
1. Remove the non condensable gas in the system: the existence of non condensable gas will increase the condensing pressure and exhaust temperature of the system, which will affect the refrigeration effect; it may also cause the lubricating oil to carbonize at high temperature, endanger the normal operation of the compressor, and even burn out the compressor motor.
2. Remove the water in the system: water is the biggest killer in the refrigeration system (especially the Freon system in the vapor compression refrigeration cycle); first of all, the interaction between lubricating oil and water will generate acid, which will corrode the system, and at the same time, it will cause “copper plating” phenomenon and damage the compressor; at the same time, water will cause ice formation in the expansion valve port or capillary tube, resulting in “ice blockage”.
Vacuum pumping operation of refrigeration system should be carried out after system cleaning, blowdown, pressure test and leakage detection. Vacuumizing can not only further check the air tightness of the system, but also remove the air, moisture and other non condensable gases in the system, so as to make preparations for filling the system with refrigerant.
Vacuum pumping should be carried out by vacuum pump. For the small refrigeration system using medium and small open and semi closed compressors, the compressor of the system itself can be used for air extraction without a vacuum pump, except that the use of this machine is prohibited in the product operation manual provided by the equipment manufacturer.
Operation method of vacuumizing with vacuum pump：
1) Close the valves (such as filling valve, vent valve, etc.) connected with the outside of the system, and open all connected valves inside the system.
2) Screw off the by-pass plug of the exhaust valve, open the bypass port, and connect the vacuum pump.
3) Starting the vacuum pump to pump air, the air in the system is difficult to be exhausted. In order to achieve a certain degree of vacuum, the vacuum pumping should be carried out several times, with an interval of about 10 minutes, so as to equalize the pressure in the system. The residual pressure of ammonia system should be less than 7.999 kPa (60mmhg) and that of Freon system should be less than 1.333kpa (10mmhg).
4) After the vacuum degree reaches the standard, close the bypass port first, then stop the vacuum pump, remove the suction pipe, and screw on the plug of the bypass hole of the exhaust valve
5) After vacuum pumping, it should be maintained for 24h, and the pressure rise in the system should not exceed 666.61pa (5mmhg). If the pressure rises rapidly, the cause should be found out and eliminated in time.
The operation method of vacuumizing with its own compressor
1) Close the suction valve and exhaust valve, screw off the screw plug of the bypass hole of the exhaust valve, install the exhaust pipe, and open the bypass port to discharge air.
2) Open all valves in the system, such as filling and venting valves.
3) If the water in the condenser is cooled completely, it will be difficult to drain the water from the condenser.
4) Force the contacts of the oil pressure controller and the low pressure controller to be normally open, start the compressor, slowly open the suction valve after the oil pressure is normal, and put the energy regulating device in the minimum gear. Due to the small diameter of the discharge valve of the refrigeration compressor, the suction valve can not be opened very large at the beginning, and the energy regulating device cannot be placed in the high gear. As the pressure in the system decreases, the suction valve can be gradually opened and loaded to increase the suction volume. In the process of air extraction, the minimum oil pressure of refrigeration compressor shall not be less than 50kPa.
5) The intermittent suction method should be adopted for vacuum pumping. When the compressor continuously exhausts air until no sound of air flow is heard, the exhaust pipe is immersed into the refrigerator oil cup to observe the bubbling at the pipe orifice. If there is no bubble in 5min, it can be considered that the gas in the system has been basically exhausted. If the exhaust pipe orifice has bubbles for a long time, it indicates that the compressor itself or the system has leakage, which should be checked and eliminated.
When checking, first close the suction valve of the compressor and check whether the compressor itself leaks. If the compressor does not leak, there will be no bubbles in the oil container, which also indicates that there is something wrong with the system; if the compressor has leakage, bubbles will be generated continuously, which is often caused by the shaft seal not sealing. If the bubble is larger at the beginning and then becomes smaller, the interval between bubbles is longer and longer, which indicates that the shaft seal is not closed to gradually. If it is found that the refrigerant oil is repeatedly sucked in and out of the pipe end (when the insertion surface is not deep), this phenomenon can not be seen when the pipe end is inserted into the oil depth. This situation is generally caused by the valve plate not tightly closed, which will be improved after heavy load use. After vacuumizing, the vent hole should be closed first, and then the machine should be shut down to prevent air backflow due to the valve plate not closing tightly.
The single-stage oil rotary vane vacuum pump is commonly used in the refrigeration industry. How to maintain the single-stage oil rotary vane vacuum pump.
First, the correct selection of rotary vane vacuum pump in refrigeration industry
The rotary vane vacuum pump with proper pumping speed should be selected according to the size of the system, so as to avoid the damage of the rotary vane due to the long working time and high temperature rise of the pump.
Second, the maintenance of rotary vane vacuum pump in refrigeration industry
The filter screen of rotary vane vacuum pump should be cleaned regularly to ensure smooth gas path.
Third, we must ensure that the oil in the rotary vane vacuum pump can reach the standard line and keep the oil clear
If the oil becomes dirty and saponified, it should be replaced; the new oil should be the special oil for the vacuum pump.