Aug 12, 2021 
Refrigeration Compressor Return Air Inlet Frosting
The frost on the refrigeration compressor return air inlet shows that the return air is in extremely low temperature. So what will cause this failure?
As is known to us, if the liquid refrigerant volume and pressure are changed, its temperature would be different. In this sense, the return air in low temperature, the pressure would generally decrease and the amount of refrigerant would be higher with the same volume. The root cause is that the refrigerant flowing through the evaporator cannot absorb enough heat to expand to a predetermined level. This causes relatively low return air temperature, pressure, and volume.
*Reasons
The liquid refrigerant supply of the throttling valve is normal, but the evaporator fails to absorb heat, causing the refrigerant to expand.
The evaporator of the refrigeration unit works normally, but the refrigerant supply in the throttling valve is extensive, resulting in more refrigerant. We usually say that there is too much fluorine, which means that excessive fluorine will also cause lower pressure.
Less Fluorine Causing the Return Air to Frosted
* Less Flow of Refrigerant
Because of the extremely small flow rate of the refrigerant, the refrigerant will start to expand after flowing out of the first expandable space at the rear end of the throttling valve. Most of the frost on the back end of the expansion valve is caused by the lack of fluorine or insufficient flow of the expansion valve.
The especially small amount of refrigerant expansion will not affect all the evaporator area, but will only form several low-temperature parts in the evaporator. In these parts, due to the rapid expansion of the small amount of refrigerant, the local temperature will be extraordinarily low and the evaporator will become frosted.
After partial frosting, due to the heat insulation layer on the surface of the evaporator and low exchange heat in this area, the refrigerant expansion will be transferred to other areas, and frost or icing will spread, causing the entire evaporator to form a heat insulation layer. So the expansion will spread to the compressor return air pipe leading to the frosting of the return air.
* the Small Amount of the Refrigerant
Due to a small amount of refrigerant, the low evaporation pressure of the evaporator leads to low evaporation temperature, which will gradually cause condensation in the evaporator to form an insulating layer and transfer the expansion point to the compressor return air to cause frosting on the return air.
* Solutions
In both of the above two cases, the evaporator frosting would occur before the compressor return the air frosting. In fact, in most cases, adjusting the hot gas bypass is enough to deal with the frosting. If there is no hot gas bypass on the compressor, and the frosting is serious, you can appropriately increase the take-off pressure of the condenser fan pressure switch.
First, find the pressure switch, remove the small piece of the pressure switch adjusting nut, and then use a Phillips screwdriver to rotate it clockwise. The entire adjustment needs to be carried out slowly. After adjusting a half circle, you should decide whether it needs adjustment.
Frosting on the Compressor Head or the Crankcase
Frosting on the head is caused by a large amount of wet steam or refrigerant sucked into the compressor.
* Excessive Thermal Expansion Valve Opening
The opening of the thermal expansion valve is adjusted largely, the filled-bulb temperature sensor is installed incorrectly or is loose, leading to high temperature and the abnormal open of the valve core.
The thermal expansion valve of the cooling unit uses the overheat signal at the outlet of the evaporator as the feedback and compares it with the given heat value, thus generating a deviation signal to adjust the refrigerant flowing into the evaporator. It is a direct-acting proportional regulator integrating the transmitter, regulator, and actuator.
When the parameters measured by the transmitter deviate from the given value, the physical quantity changes, and it generates enough energy to directly push the actuator to move. The position change of the actuator is proportional to the adjusted parameter. Thermal expansion valves can be divided into internally balanced thermal expansion valves and externally balanced thermal expansion valves according to their balancing methods.
The liquid refrigerant evaporates and cools the evaporator, and when it flows to the outlet, it has been completely vaporized and in a certain degree of overheating. The filled-bulb temperature sensor of the thermal expansion valve is closely attached to the device outlet pipe and feeling the heat. If the liquid filled in the bulb is the refrigerant, the pressure of the liquid above the diaphragm of the thermal expansion valve would be higher than that below the diaphragm. And the higher the temperature of the evaporator outlet, that is, the severe the degree of overheat, the higher the liquid pressure above the diaphragm.
This pressure difference is balanced by the tension of the ejector rod and the adjusting spring under the diaphragm. If you change the tension of the adjusting spring, you will change the top force of the ejector rod, thereby changing the opening degree of the needle valve.
Obviously, the degree of overheating of the evaporator will also lead to the change of the needle valve opening. When the spring is adjusted to a certain position, the expansion valve will automatically change the needle valve opening according to the temperature of the evaporator outlet to keep the evaporator outlet overheat degree at a certain value.
The extreme opening of the thermal expansion valve, the incorrect installation of the filled-bulb temperature sensor, or the loose installation, will make the felt temperature extraordinarily high and the valve core opened abnormally. As a result, a large amount of wet steam is sucked into the compressor, causing the head to frosted. The thermal expansion valve is used to adjust the overheating degree when the evaporator is running.
The larger overheating degree at the outlet of the evaporator causes a long overheating section at the rear of the evaporator so that the cooling capacity will be significantly reduced. If the outlet overheating degree is small, it may cause the liquid strike or even frost the head. Generally, the overheating temperature of the evaporator outlet should be adjusted to 3°C - 8°C.
* Solenoid Valve Leaking & Opened Expansion Valve
The liquid supply solenoid valve leaks or the expansion valve is not closed completely when the machine is shut down, causing a large amount of liquid refrigerant to accumulate in the evaporator before starting. The temperature relay is used with the solenoid valve for control.
The temperature sensor of the relay is placed in the refrigeration storage. When the temperature in the cold storage is higher than the upper limit, the contact of the relay is turned on, the solenoid valve coil is energized, and then the valve opens, with the refrigerant entering the evaporator for cooling.
When the temperature of the storage is lower than the smallest setting value, the relay contact is disclosed, cutting off the solenoid valve coil current, and the solenoid valve closes, and the refrigerant stops entering the evaporator so that the storage temperature can be controlled within the required range.
* High Liquid Level in the Condenser
When there is too much refrigerant in the refrigeration system, the liquid level in the condenser will be higher, the condensation heat exchange area reduced, and the condensing pressure increased. That is, the pressure on the expansion valve increases, and more refrigerant will flow into the evaporator. There the liquid refrigerant cannot evaporate completely, so the compressor will suck in wet steam, the head will be cold or even frosted, which may cause liquid strike and higher evaporation pressure.
