These have the most direct relationship with the refrigeration effect

The refrigeration system of a cold storage generally has the most common operational failures such as a slow drop in refrigeration temperature, abnormal operating conditions or inability to operate caused by system blockage, etc. A slow drop in the refrigeration temperature of the cold storage is mostly caused by improper operation and adjustment, among which the adjustment of the expansion valve is the most crucial.

If the opening degree of the expansion valve is small, the flow rate of the refrigerant passing through is low, and the pressure is also low; if the opening degree of the expansion valve is large, the flow rate of the refrigerant passing through is high, and the pressure is also high.

According to the thermodynamic properties of the refrigerant, the lower the pressure, the lower the corresponding temperature; the higher the pressure, the higher the corresponding temperature.

According to this law, if the outlet pressure of the expansion valve is too low, the corresponding evaporation pressure and temperature are also too low. However, due to the decrease in the flow rate entering the evaporator and the decrease in pressure, the evaporation speed slows down, the refrigeration capacity per unit volume (time) decreases, and the refrigeration efficiency is reduced.

If the outlet pressure of the expansion valve is too high, the corresponding evaporation pressure and temperature are also too high. The flow rate and pressure entering the evaporator both increase. Due to excessive liquid evaporation, over-wet gas (or even liquid) is sucked into the compressor, causing a wet stroke (liquid slugging) of the compressor, which makes the compressor unable to work normally, resulting in a series of poor operating conditions and even damage to the compressor.

The opening degree of the expansion valve should be adjusted according to the current temperature of the cold storage, that is, adjusted under the pressure corresponding to the temperature of the cold storage.

Example: When the temperature of the cold storage is -10 degrees, the evaporation temperature of the cold storage should be about 5 degrees lower than the temperature of the cold storage, that is, -15 degrees, and the corresponding evaporation pressure is about 0.3MPa (R22). Due to the factor of pressure loss, this adjusted pressure is basically reflected in the suction pressure of the compressor.

Each time the expansion valve is adjusted, it generally takes about 10 to 15 minutes for the adjusted pressure of the expansion valve to stabilize on the suction pressure gauge. The adjustment should not be too hasty. The suction pressure of the compressor is the main parameter basis for adjusting the pressure of the expansion valve.

The quality of the technical performance of the expansion valve directly affects whether it can be adjusted and operated normally. Common faults such as blockage of the filter screen and leakage of the temperature-sensitive agent often occur, resulting in insensitive adjustment response, out-of-control adjustment or inability to adjust, etc. When frost forms at the inlet of the expansion valve (or frost also forms on the valve cover), the temperature of the liquid inlet pipe is lower than the normal temperature, and even dew condensation occurs.

Example: If the suction pressure of the compressor is lower than the corresponding pressure at the temperature of the cold storage, the operating temperature and the discharge temperature of the machine are high, and the refrigeration temperature drops slowly or does not drop, it may be that the filter screen of the expansion valve is blocked, and there are phenomena of dirt blockage or ice blockage (or refrigerant leakage).

If the expansion valve itself is working normally, but the blockage is caused by excessive moisture or dirt in the system, resulting in severe ice blockage or dirt blockage, making the system work abnormally. The temperature of the liquid inlet pipe in front of the expansion valve is lower than the normal temperature, and even dew condensation occurs. There is no temperature response in the pipeline behind the valve. The suction pressure of the compressor is below 0, the operating temperature and the discharge temperature of the machine are high, and even knocking sounds occur. The refrigeration temperature does not drop, and the operating conditions are poor.

If gas is always discharged from the exhaust bypass port of the compressor, or lubricating oil is also carried out (sprayed out), it is sufficient to indicate that the compression performance of this compressor is poor, the airtightness of the air valve is not tight, and the wear clearance of the running parts of the cylinder and the oil ring is too large. It needs to be repaired.

This is the most basic, simplest and most practical method for testing the compression efficiency and airtightness of the compressor.

In addition, the operation of draining oil and releasing air from the system should be carried out regularly or frequently to improve the heat exchange performance of the heat exchanger and the refrigeration effect. Since the refrigeration oil has a relatively high viscosity, it is usually adsorbed on the inner surface of the pipeline or container to form an oil film layer. Especially on the low-pressure side (from the outlet of the expansion valve to the inlet of the compressor), due to the low temperature, the viscosity of the oil is even greater, and the formed oil film layer is of course larger. This increases the thermal resistance of the heat exchanger (evaporator and condenser), affects the heat transfer performance, and reduces the refrigeration effect. The more oil there is in the system, the greater this drawback will be, so it is very unfavorable for refrigeration.

The presence of air or other non-condensable gases in the system will cause an increase in the condensing pressure and temperature, an increase in power consumption, a high operating temperature of the compressor, a heavy load, and a reduction in refrigeration efficiency. The oil can be drained from the sewage outlet opened at the lowest point of the pipe evaporator; generally, the liquid outlet at the lowest point of the air cooler combined evaporator is equipped with an oil (dirt) drainage port by the manufacturer.

The operations of draining oil and releasing air should be carried out when the system is shut down and in a static state. Releasing air should also be carried out when the temperature is relatively low, so that the effect will be better. For systems without special air release equipment, the outlet at the highest point of the high-pressure side is generally selected for releasing air. The evaporating pipe and the finned tube of the air cooler in the cold storage should be defrosted in a timely manner (regularly) to ensure their good heat transfer effect.

During the use of the cold storage, some minor faults will inevitably occur. In order not to affect the use of the cold storage and solve the troubles quickly, learning some common maintenance knowledge can eliminate the faults of the cold storage in the shortest time and reduce losses. The following small pieces of knowledge are recommended for everyone, and I hope they will be helpful to you: