automotive air conditioning

8 Refrigerant Charge Methods You Should Know

Refrigerant charge is a very important issue. Insufficient or excessive charging can damage the compressor and affect the entire refrigeration system.

Therefore, we have summarized 8 kinds of refrigerant charge methods for your reference.

1. Weighing refrigerant charge

The most accurate charging method is to weigh the refrigerant added to the system. This method can only be used when the required charging amount of the system is known, and the system requires full charging. Generally, the integrated equipment meets the above requirements. In some cases, if the charging amount is small and the system needs to be repaired, the common practice is to discharge the refrigerant and add the amount required by the system after the repair is completed.

2. Current method

According to the rated current, install a clamp-type ammeter at the input end of the air conditioner power supply, connect the refrigerant cylinder to the pipeline of the low-pressure end liquid filling port of the air conditioner, drain the air in the pipe, start the compressor, open the valve of the refrigerant cylinder, and add refrigerant. And observe the position of the pointer of the clamp meter. When the pointer points to the rated current, stop adding refrigerant.

3. Pressure method

The saturated evaporation temperature of the refrigerant has a corresponding relationship with its pressure. If the evaporation temperature of the refrigerant is known, the corresponding evaporation pressure can check out. Converting evaporation pressure into gauge pressure and a pressure gauge can be installed in the high and low-pressure circuits to judge refrigerant charge. Install pressure gauges in the high and low-pressure sections, respectively, connect the liquid filling pipeline, start the compressor to add refrigerant to the refrigeration system, and observe the indications of the high and low-pressure gauges.

8 Refrigerant Charge Methods Worthy Of Your Collection

4. Mirror refrigerant charge

The most common way to determine whether the system’s refrigerant charge is appropriate is with the aid of a sight glass in the liquid line. Since the pressure of the liquid refrigerant is important for the control of the expansion valve, when the transparent flow of liquid refrigerant is seen from the sight glass, considered that the system is properly charged. If you see bubbles or flashes, it usually means not enough refrigerant. Remember, if there is no liquid and only vapour, it will appear clear in the sight glass. 

However, service engineers should know that air bubbles or flashes can also be seen in the sight glass when the system is properly charged. Because the resistance in the liquid line in front of the sight glass creates a pressure drop, causing the refrigerant to flash. Suppose the expansion valve is unstable or fluctuates with liquid. In that case, the expansion valve opens sharply, and the flow rate rises to generate enough pressure drop to cause flashing at the outlet of the liquid reservoir. Rapid changes in condenser pressure are also responsible for flash.

For example, a sudden fan speed change in a cold room can easily change the condensing temperature from 5.5°C to 8°C. The liquid temperature in the receiver will be higher than the saturation temperature corresponding to the changed condensing pressure. Evaporation will occur until the liquid temperature drops below the saturation temperature again.

The system may have different charging requirements under different operating conditions. When the air-cooled system operates in a low-temperature environment, the refrigerant is usually controlled to flood part of the condenser pipeline to ensure the required condensing pressure of the system. In this case, in summer, a system with a clear sight glass would require double the amount of refrigerant needed to operate properly at low temperatures. While the sight glass is useful in determining whether the system charge is appropriate, we cannot rely solely on the sight glass as the sole basis for judging the system charge. We must also carefully analyze system performance.

5. Liquid level gauge method

In some systems, the reservoir has a level test port. If the charging amount has been reached, open it slightly, and liquid refrigerant will appear in the test port. If only refrigerant vapour appears in the test port, the charging amount is insufficient. Larger reservoirs may be equipped with a float indicator to indicate the fluid level in the reservoir, similar to the construction of gasoline tanks used in automobiles.

Confirming the refrigerant subcooling method For small systems, if there is no other easy way to check the refrigerant charge, the refrigerant charge can be determined by measuring the liquid subcooling at the condenser outlet. When the unit operates under steady conditions, the temperature of a condenser outlet liquid pipe is compared to saturation, which compares the condensing temperature to the temperature of the liquid exiting the condenser. Continue charging until the liquid line temperature is 3°C lower than the condensing temperature at the maximum load. This inspection method is usually only used in the factory’s final assembly system, but it does give you an on-site emergency understanding of whether the system is normal.

6. Superheat refrigerant charge

Small unit systems equipped with capillaries can use the superheat method to determine the appropriate charge. Suppose there is a suction maintenance port that can measure the suction pressure. In that case, the difference between the temperature of the suction pipe 0.15 m away from the compressor and the temperature on the evaporator tube at the midpoint of the evaporator is measured as the superheat difference. Under normal operating conditions of the unit, continue charging until the superheat measured by the above method is approximately 20 to 30 degrees. If the superheat is close to 40 degrees, the charging is insufficient.

7. Liquid refrigerant charge

Add refrigerant to the system through the filling valve on the main liquid line. First, put the refrigerant cylinder bottle upside down on the scale, then connect the liquid filling pipe on the refrigerant bottle to the liquid filling valve, remove the gas in the liquid filling pipe, and then open the liquid bottle valve and liquid filling valve. The vacuum will cause the liquid material to be sucked through the filling port until the system pressure equals the pressure in the refrigerant bottle.

Then close the accumulator outlet valve and start the compressor. At this point, the liquid refrigerant will flow from the refrigerant bottle into the liquid line through the evaporator and accumulate in the condenser and receiver. To determine whether the charging flow has reached the system’s requirements, open the outlet valve of the accumulator, close the filling valve, and observe the system’s operation until the system has the specified refrigerant.

8. Gas filling

Weigh the refrigerated bottle before filling it. Connect the pressure gauge valve tube to the suction and exhaust service valves. Connect the public interface to the refrigerant bottle, flush out the gas in the pipeline, open the vapour valve of the refrigerant bottle, start the compressor, and use the pressure gauge tube valve to adjust the charge.

Please note that to determine whether enough refrigerant has been added, you can first close the valve of the refrigerant gas cylinder, observe the system’s operation, and see whether to continue adding refrigerant according to the situation.

These 8 refrigerant charge methods have advantages and limitations, such as gas charging, which is suitable for small refrigeration systems. Therefore, please choose the charging method suitable for your refrigeration system according to your specific situation.

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