A Comprehensive Explanation in One Article: How to Recover, Evacuate, Leak Test and Fill Refrigerant!

2025-05-06

Discharge of Refrigerant
The discharge of refrigerant generally refers to directly releasing the refrigerant into the air. When discharging, the surrounding environment must be well-ventilated and should not be close to an open fire, otherwise toxic gases will be generated.

Close the high and low-pressure manual valves on the manifold pressure gauge, and connect its high and low-pressure hoses to the high and low-pressure service valves of the compressor respectively. Place the free end of the middle pipe on a working cloth.
Slowly open the high-pressure manual valve to let the refrigerant discharge from the middle hose. The valve should not be opened too wide, otherwise the refrigeration oil in the compressor will flow out along with the refrigerant.
When the reading of the pressure gauge drops below 0.35MPa, slowly open the manual valve again so that the refrigerant is discharged from both the high and low-pressure sides simultaneously.
Observe the reading of the pressure gauge. As the pressure drops, gradually open the high and low-pressure manual valves wider until the readings of the high and low-pressure gauges indicate zero.
Recovery of Refrigerant
There are mainly the following three methods for refrigerant recovery:
The cooling method, the compression method, and the adsorption separation method. During the recovery process, the quality of the refrigerant will be impure. Therefore, it is best to regenerate the recovered refrigerant.
Recovery steps:

Connect the manifold pressure gauge to the recovery machine and the automotive air conditioning system. Note that before connection, all connection hoses must be flushed with the refrigerant of the air conditioning system to remove the air in the hoses.
Connect the recovery cylinder to the recovery machine, and also remove the air in the hoses.
Connect to the power supply, turn on the switch, and perform the recovery operation and shutdown according to the instruction manual.
Use a vacuum pump to evacuate the system.
Charge the refrigerant that has been dried, filtered, and degreased after recovery into the recovery cylinder. Turn off the machine, cut off the power supply, and remove all connection pipes.

Recovery by the cooling method
Cool and liquefy the refrigerant vapor. Dry ice or the like can be used to cool and liquefy the refrigerant for recovery. This method is suitable for clean refrigerants.
The cooling method has an independent refrigeration cycle system, and the refrigerant in the recovery container condenses into a liquid in the evaporator. The refrigerant discharged from the automotive air conditioning system passes through a filter dryer to remove moisture and impurities; the lubricating oil in the refrigerant is removed through an oil separator and enters the recovery container. For occasions where the requirements for refrigerant purity are not too strict, the recovered refrigerant can be refilled into the refrigeration system.
Recovery by the compression method
Use the compression method to evaporate the refrigerant into a liquid: The refrigerant discharged from the air conditioning system passes through a filter dryer to remove moisture and impurities. Controlled by the suction pressure regulating control valve, part of the liquid refrigerant remains in the liquid storage tank. The gaseous refrigerant enters the compressor and is compressed into a high-temperature and high-pressure gas. When passing through the oil separator, the refrigeration oil mixed with the refrigerant is separated and flows back to the compressor. The refrigerant enters the condenser to be cooled. Through the gas-liquid separator, the condensed liquid refrigerant flows to the recovery container, and part of the gaseous refrigerant in the recovery container will be sucked into the compressor through the capillary tube.
Evacuation of the Refrigeration System
Evacuation is to remove the air and water vapor in the refrigeration system, which is an extremely important procedure in air conditioning maintenance. Because when the refrigeration system is repaired or components are replaced, air will enter the system, and the air contains a certain amount of water vapor (moist air). When evacuating the system, the longer the time, the less water remaining in the system. In order to extract the air and moisture in the system to the greatest extent, the repeated evacuation method must be adopted, that is, after the first evacuation is completed, continue to evacuate for more than 30 minutes.
Connection method of the evacuation pipeline:

Connect the two high and low-pressure hoses on the manifold pressure gauge to the high and low interfaces of the compressor respectively; connect the middle hose on the manifold pressure gauge to the vacuum pump.
Open the high and low-pressure manual valves on the manifold pressure gauge, start the vacuum pump, and pay attention to the two pressure gauges. Evacuate the system to 98.70—99.99kPa.
Close the high and low-pressure manual valves on the manifold pressure gauge, and observe whether the indicated pressure of the pressure gauge rises. If it rises, it means that the system is leaking, and at this time, leak detection and repair should be carried out. If the pressure gauge needle remains stationary, open the high and low-pressure manual valves, start the vacuum pump, and continue to evacuate for 15—30 minutes to make the pointer of the vacuum pressure gauge stable.
Close the high and low-pressure manual valves on the manifold pressure gauge.
Turn off the vacuum pump. First, close the high and low-pressure manual valves, and then turn off the vacuum pump to prevent air from entering the refrigeration system.

Leak Detection of the Refrigeration System
Pressure leak detection
Add nitrogen to the refrigeration system, and then use soapy water for leak detection. If there is a leak, soap bubbles will appear at the leak point. When using pressure leak detection, it is strictly prohibited to use compressed air for leak detection because compressed air contains moisture, and the moisture will cause ice blockage at the expansion valve after entering with the air. Industrial nitrogen is non-corrosive, moisture-free, and inexpensive, but bottled high-pressure nitrogen must be filled with a pressure reducing gauge.
When conducting a pressure leak test, first connect the manifold pressure gauge correctly: Connect the high-pressure hose to the exhaust pipe (high-pressure side), and connect the low-pressure hose to the suction pipe (low-pressure side). Connect the hoses to the high and low-pressure service valves of the compressor, open the high and low-pressure service valves, and add dry nitrogen to the system. The pressure is generally about 1.5MPa. When the system reaches the specified pressure, apply soapy water to all connections and welding points of the system, and carefully observe for any leaks.
At the place with a large leak, there will be a slight sound and large bubbles will appear. At the place with a small leak, small bubbles will appear intermittently. Therefore, leak detection must be carried out carefully and checked 3 to 5 times repeatedly. Mark the leak point and repair it in a timely manner, and then test for leaks at other joints until the leak is completely eliminated. After the leak repair is completed, conduct a leak test. Let the system maintain the pressure for 24 to 48 hours. If the pressure does not drop, the leak test is qualified. If the pressure drops significantly, the leak detection must be carried out again until the leak point is found and eliminated.
Vacuum leak detection
Use a vacuum pump, and the vacuum degree should reach 0.1MPa. It is acceptable if the vacuum degree does not increase significantly within 24 hours. Evacuation is used to remove the residual nitrogen in the system, check whether the system is leaking, and dry the system. The refrigerant can only be charged after the system is evacuated.
Visual leak detection
Refrigeration oil often oozes out at the refrigerant leakage site. If oil stains are found oozing out at a certain place, it can be further wiped with a clean white paper or directly touched by hand for inspection. If there is still oil coming out, there may be a leak.
Charging of Refrigerant
Charging from the high-pressure side
Charge through the side hole (multi-purpose passage) of the compressor discharge valve (high-pressure valve), and the charged refrigerant is in a liquid state. Its characteristics are safety and speed, and it is suitable for the first charging of the refrigeration system, that is, the charging of the system after leak detection and evacuation. However, when using this method, it must be noted that the compressor should not be started during charging (the engine is stopped), and the refrigerant cylinder is required to be operated upside down.
Operation steps:

After the system is evacuated, close the high and low-pressure manual valves on the manifold pressure gauge.
Connect one end of the middle hose to the joint of the refrigerant cylinder injection valve, open the refrigerant cylinder opening valve, then unscrew the nut at one end of the manifold pressure gauge hose, let the gas escape for a few minutes, and then tighten the nut.
Unscrew the high-pressure side manual valve to the fully open position and invert the refrigerant cylinder.
Inject the specified amount of liquid refrigerant from the high-pressure side. Close the refrigerant cylinder injection valve and the high-pressure manual valve on the manifold pressure gauge, and then remove the instrument. When charging the refrigerant into the system from the high-pressure side, the engine is in a non-starting state (the compressor is stopped), and do not unscrew the low-pressure manual valve on the manifold pressure gauge to prevent hydraulic shock.

Charging from the low-pressure side
Charge through the side hole (multi-purpose passage) of the compressor suction valve (low-pressure valve), and the charged refrigerant is in a gaseous state. Its characteristic is that the charging speed is slow, and it can be used when replenishing the refrigerant in the system.
Operation steps:

Connect the manifold pressure gauge to the compressor and the refrigerant cylinder.
Open the refrigerant cylinder, loosen the nut of the middle injection hose on the manifold pressure gauge until the sound of refrigerant vapor flowing is heard, and then tighten the nut. Thus, the air in the injection hose is discharged.
Close the manual high-pressure valve, place the refrigerant cylinder upright, start the engine, make the air conditioning compressor operate, open the low-pressure manual valve, and let the gaseous refrigerant enter the compressor from the low-pressure side. When the pressure value of the system reaches 0.4MPa, close the low-pressure manual valve and the refrigerant cylinder switch valve.
Start the engine, turn on the air conditioning switch, and adjust the blower switch and the temperature control switch to the maximum.
Open the manual valve on the manifold pressure gauge again to let the refrigerant continue to enter the refrigeration system until the charging amount reaches the specified value.
After charging the specified amount of refrigerant into the system, observe from the liquid sight glass to confirm that there are no bubbles and no excessive refrigerant in the system. Then adjust the engine speed to 2000r/min, turn the cold blower air volume to the highest gear. If the air temperature is 30 to 35°C, the low-pressure side pressure in the system should be 0.147—0.192MPa, and the high-pressure side pressure should be 1.37—1.67MPa.
After the charging is completed, close the low-pressure manual valve on the manifold pressure gauge, close the injection valve installed on the refrigerant cylinder, stop the engine, and remove the manifold pressure gauge from the compressor. When removing it, the action should be rapid to avoid excessive refrigerant leakage.

Charging of Refrigeration Oil
Utilize the suction effect of the compressor itself: suck the refrigeration oil in from the low-pressure valve. At this time, the compressor must be kept running at a low speed.

Evacuate the refrigeration system.
Select a graduated cylinder and fill it with more refrigeration oil than the amount to be charged.
Unscrew the low-pressure hose connected to the compressor from the manifold pressure gauge and insert it into the graduated cylinder containing the refrigeration oil.
Start the vacuum pump and open the high-pressure manual valve on the manifold pressure gauge. The charged lubricating oil will enter the compressor from the low-pressure side of the compressor. When the amount of refrigeration oil reaches the specified amount, stop the suction of the vacuum pump and close the high-pressure manual valve.
After charging the refrigeration oil according to the evacuation method, the refrigeration system should continue to be evacuated and charged with refrigerant.