1. The opening of expansion valve is too small:
If the expansion valve is opened too small, the refrigerant charge in the refrigeration system is normal. In this fault, the whole system is short of circulating refrigerant. When other working conditions remain unchanged, it cannot fully satisfy the gasification and heat absorption of the liquid preparation agent at low pressure. Therefore, the superheat of the return air is very high, and the suction and exhaust temperature and the air-conditioned room temperature rise back, while the suction and exhaust pressure will drop.
While the temperature of the refrigerant at the outlet of the condenser will drop (that is, the refrigeration capacity of the system will be increased), because the heat exchange capacity of the condenser has not changed, but the circulating refrigerant flow in the whole system is very small, although the refrigeration coefficient and the unit refrigeration capacity are increased at this time. But the total cooling capacity is reduced, so the temperature of air-conditioned room will rise.
There is a small opening in the expansion valve (where the yellow arrow is). Adjusting the opening of the expansion valve means adjusting the size of that small opening. The larger the opening, the larger the caliber, and the more liquid flows through.

2. Insufficient refrigerant:
Too little refrigerant is usually caused by two reasons: first, the product is not filled enough before leaving the factory, which generally occurs less; Second, the unit runs all the year round, and the leakage of system valves or welding parts causes too little refrigerant, which is a relatively common fault. There is no obvious difference between the causes of refrigerant shortage and expansion valve opening too small.

3. Excessive refrigerant:
Excessive refrigerant comes from blind refrigerant charging, which is not uncommon in refrigeration system maintenance. This will reduce the effective heat exchange area in the condenser, reduce the heat exchange effect, increase the condensing temperature, increase the condensing pressure and increase the high pressure.
Too much refrigerant enters the evaporator and cannot be completely gasified, which causes low pressure and high suction by the compressor, condensation or frost on the cylinder head of the compressor, and in severe cases, a liquid impact cylinder flushing accident occurs. Part of the liquid refrigerant enters the compressor, which increases the load of the motor, makes it difficult to start, overloads the current during operation, and burns down due to overload of the motor in severe cases.

4. The suction and exhaust valves of compressor are damaged:
For reciprocating compressors, it is a common mechanical fault that the compressor suction and exhaust valve plate is damaged. This experiment is simulated by the method of compressor suction and exhaust valve bypass. When a fault occurs, the suction temperature and pressure will rise, and the exhaust temperature and pressure will fall, which is equivalent to a decrease in the actual gas transmission capacity of the compressor, so the refrigeration effect will be poor, and the temperature of the air-conditioned room will rise quickly. In severe cases, the low-voltage relay will stop the compressor.