Maintenance Techniques for Air - conditioning Refrigeration Systems

2025-01-15

I. Leak Detection Techniques for the Refrigeration System (R22 System)

Leak Detection by Touching for Oil Stains
Most air - conditioners use R22 as the refrigerant. R22 has a certain degree of solubility with refrigeration oil. When there is a R22 leak, the refrigeration oil will seep out or drip. Utilizing this characteristic, we can determine whether there is a leak at a certain location by visually inspecting or touching for oil stains. When the leak is small and not obvious to the touch, you can wear white gloves or use white paper to touch the suspected area, and the leak can also be detected.
Leak Detection with Soap Bubbles
First, cut the soap into thin slices and soak it in warm water to make a thick soap solution. When detecting leaks, wipe the dirt off the inspected part with a gauze, dip a clean brush into the soap solution, and evenly apply it around the inspected part. Carefully observe for the presence of bubbles. If soap bubbles appear, it indicates a leak at that location. Sometimes, it is necessary to first charge the system with nitrogen at 0.
8 - 1.0 Mpa (8 - 10 kgf/cm²).
Leak Detection by Pressurization
After the refrigeration system has been repaired and welded, before charging the refrigerant, it is advisable to charge 1.5 Mpa of nitrogen near the end of the workday and close the three - way service valve (the valve itself should not leak). When coming to work the next day, if the gauge pressure has not dropped, it means the repaired refrigeration system is leak - free. If the gauge pressure has dropped, it indicates a leak, and then use the soap - bubble leak - detection method to find the leak.
Leak Detection in Water
This method is often used for leak detection of components such as compressors (note that the wiring terminals should have waterproof protection), evaporators, and condensers. The method is as follows: For the evaporator, charge 0.8 Mpa of nitrogen, and for the condenser, charge 1.
9 Mpa of nitrogen (for heat - pump air - conditioners, both should be charged with 1.9 Mpa of nitrogen). Immerse them in warm water at about 50 °C and carefully observe for the presence of bubbles. The purpose of using warm water is to reduce the surface tension of water. Because the lower the temperature of the water, the greater the surface tension, and small leaks may not be detected. The leak - detection site should have sufficient light and a calm water surface. The observation time should be no less than 30 seconds, and the workpiece is preferably immersed more than 20 cm below the water surface. The parts after immersion leak - detection should be dried before repair welding.
Leak Detection with a Halogen Lamp
As the flame color changes from light green → dark green → purple, the leakage amount changes from slight leak → severe leak.
Leak Detection with an Electronic Leak Detector
The main parts to be detected are: the welding joints of the suction and exhaust pipes of the compressor; the small elbows, inlet and outlet pipes, and the welding parts of each branch pipe of the evaporator and condenser, such as the connections of the dryer filter, stop valve, solenoid valve, thermal expansion valve, distributor, liquid receiver, etc.
Difference between Leakage and Blockage Judgment: For a leak, after leak - repair, evacuate and refill the refrigerant, and the air - conditioner can restore its refrigeration effect. If it is a blockage, even if refrigerant is added, the air - conditioner still cannot refrigerate, and the pressure is abnormal.

II. Techniques for Removing Air and Blockages in the Refrigeration System

The residual air containing moisture in the refrigeration cycle will cause an increase in condensing pressure, an increase in operating current, a decrease in refrigeration efficiency, or cause blockages (ice blockages) and corrosion, resulting in failures such as scoring and copper - plating of the compressor cylinder. Therefore, the air in the pipe must be removed. The methods are as follows:

Removing Air with the Refrigerant of the Air - conditioner Itself
Unscrew the rear - cover nuts of the high - and low - pressure valves and the refrigerant - charging nozzle nuts. Open the high - and low - pressure valve cores (rotate 1/4 - 1/2 turn), wait for about 10 seconds, and then close. At the same time, use an Allen wrench to push the refrigerant - charging needle upward from the refrigerant - charging nozzle nut of the low - pressure valve to open it, and air will be discharged. Stop evacuating when you feel cool air coming out.
The amount of discharged refrigerant should be less than 20 g.
Removing Air with a Vacuum Pump
First, unscrew the refrigerant - charging nozzle nut of the valve, and connect it with a vacuum - pumping connecting hose. Rotate the “LO” knob counter - clockwise to open it, then turn on the switch of the vacuum pump to start vacuum - pumping. After stopping vacuum - pumping, unscrew the rear - cover nut of the valve, and use an Allen wrench to rotate the valve core counter - clockwise to the bottom. At this time, the passage of the refrigeration system is opened. Then remove the connecting hose from the valve, and tighten the connecting nut and the rear - cover nut of the valve.
Removing Air by Adding Freon
Use an independent refrigerant cylinder. Connect the refrigerant - charging hose of the refrigerant cylinder to the refrigerant - charging nozzle of the low - pressure valve, and slightly loosen the connecting nut of the high - pressure valve on the outdoor unit. Loosen the valve of the refrigerant cylinder, charge the refrigerant for 2 - 3 seconds, and then close it tightly. When the refrigerant flows out from the connecting nut of the high - pressure valve for 10 - 15 seconds, tighten the connecting nut. Remove the refrigerant - charging hose from the refrigerant - charging nozzle, use an Allen wrench to push the refrigerant - charging valve core thimble, and the refrigerant will be released. When no more noise can be heard, release the thimble, tighten the refrigerant - charging nozzle nut, and open the high - pressure valve core of the outdoor unit.

III. Refrigerant Charging Techniques

For hermetically sealed compressors, the low - pressure charging method is often used to charge Freon.

Transferring Refrigerant from a Large Cylinder to a Small Cylinder before Charging
First, place the small repair cylinder in a container with ice cubes to cool it down. Then connect the large and small cylinders with a rubber hose, but do not open the valve of the large cylinder for the time being. Loosen the valve of the large cylinder and the joint of the small cylinder, use Freon gas to expel the air in the hose, then close the valve of the large cylinder and tighten the hose joint of the small cylinder. Open the valves of the large and small cylinders to charge the refrigerant. When it is charged to 80%, close the valves of the large and small cylinders and remove the hose.
Charging Refrigerant from the Cylinder into the Refrigeration System
The cylinder can be connected to the repair valve, or it can be charged through the middle joint of the compound pressure gauge. Open the small cylinder and invert it. After expelling the air in the connecting pipe, tighten the joint and charge the refrigerant. Close the straight - through valve when the gauge pressure does not exceed 0.15 Mpa. Start the compressor to suck in the refrigerant, and at the same time observe the frosting situation of the evaporator. Stop charging when the evaporator is fully frosted or dewed. There are several methods for determining the amount of refrigerant charged:
- Weight Measurement: When charging Freon, prepare a small platform scale in advance. Place the refrigerant cylinder in a container, and then pour warm water below 40 °C into the container (suitable for low - pressure charging of refrigerant vapor in air - conditioners). Record the weights of the cylinder, warm water, and container before charging. Observe the pointer during the charging process. Stop charging when the reduction in the weight of the refrigerant in the cylinder is equal to the required charging amount. You can also directly weigh the cylinder without adding warm water.
- Pressure Measurement: The temperature and pressure of the saturated vapor of the refrigerant have a one - to - one correspondence. If the evaporation temperature of the refrigerant is known, the corresponding evaporation pressure can be found. The gauge pressure value of this pressure is displayed by the high - and low - pressure gauges. Therefore, according to the pressure value on the gauges installed on the system, it can be judged whether the amount of refrigerant charged is appropriate. For example, if the evaporation temperature of the air - conditioner is 7.2 °C, the condensing temperature is 54.5 °C, and R22 is used. Check the saturation temperature and saturation pressure correspondence table of R22 to determine its evaporation pressure value and condensing pressure value.
  From the table, it can be seen that for R22 at 7.2 °C, the corresponding absolute pressure value is 0.53 Mpa (5.3 kg/cm²), and at 54.5 °C, the corresponding absolute pressure value is 2.11 Mpa (21.1 kg/cm²). Convert these pressures to gauge pressure values. Use high - and low - pressure gauges or a compound pressure gauge to test the refrigeration system during refrigerant charging. If the gauge pressure values of the high - and low - pressure gauges are within the above range, it indicates that the amount of refrigerant charged is appropriate. If both the high - and low - pressure values are low, it indicates insufficient charging. If both the high - and low - pressure values are high, it indicates over - charging. The pressure measurement method is relatively simple and is often used in maintenance, but its disadvantage is that it is relatively rough and not very accurate.
- Temperature Measurement: Use a semiconductor thermometer to measure the temperatures at various points such as the inlet and outlet of the evaporator and the outlet of the accumulator to judge the amount of refrigerant charged. The temperature difference between the inlet (150 mm in front of the capillary) and the outlet of the evaporator is about 7 - 8 °C, and the temperature at the outlet of the accumulator should be 1 - 3 °C higher than that at the outlet of the evaporator. If the temperature difference between the inlet and outlet of the evaporator is large, it indicates insufficient refrigerant charging. If the frost - covered section of the suction pipe is too long or there is a frosting phenomenon near the compressor, it indicates over - charging of the refrigerant.
- Operating Current Measurement: Use a clamp - on ammeter to measure the operating current. During refrigeration, when the ambient temperature is 35 °C, the measured operating current corresponds to the current on the nameplate. The higher the temperature, the larger the current; the lower the temperature, the smaller the current. Compare the measured current value with the rated current value of the air - conditioner under normal conditions of the fan and good heat dissipation of the two heat exchangers.