Causes of uneven liquid distribution in refrigeration systems and tragic cases!

Causes of uneven liquid distribution in refrigeration systems and tragic cases!

If the distribution head is not evenly distributed, it will cause too much refrigerant in some of the branches, which will cause the evaporator to frost. As a result, the evaporation will be incomplete, which will affect the cooling capacity of the system and may also cause the suction to carry liquid. There are many reasons for uneven dispensing, and the common reasons are mentioned in this article. Let's first enter this article through a cold storage maintenance case of our peers. This case is caused by a typical uneven liquid distribution. The final result is terrible!

　　Case: This system is composed of three parallel piston compressors, which provide cold sources for two cold storage and four air coolers, and the storage temperature is maintained at about -23 degrees. The customer's repair and maintenance records are very complete, and there have been many failures during the two years of operation. The summary is as follows:

　　 The evaporator freezes;

　　 oil pressure protection;

　　 Multiple evaporator fan motors burned;

　　 The compressor has high vibration and noise;

　　 Compressor valve and connecting rod are broken; 　　 The compressor is out of oil;

　　 system ice block;

　　Carbon deposits and blackening of lubricating oil;

　　The pistons of all compressors are pitted, and the valves and connecting rods are broken.

　　 was handled by the manufacturer many times before and after, but still can not prevent the bad luck of continuous maintenance of the compressor. This bad situation is worth pondering. Once a problem arises, it has its inevitable causes. If the cause cannot be found and resolved in time, the practice of remedying headaches and feet will bring about the above-mentioned vicious circle.

　　 Compressor damage can be summarized into three typical factors: lack of oil; liquid hammer; impurities. For a clean system, the impurity factor comes from the first two factors and causes the first two phenomena.

In this case, it was finally found that the gas-liquid separator failed. The cause of the gas-liquid separator failure was the blockage of the gas-return oil hole. Leaving aside), the lack of oil comes from poor oil return (at this time the gas separation is effective), and the poor oil return comes from insufficient air intake. The flow rate is too low to bring the oil in the evaporator back to the compressor. Insufficiency comes from insufficient liquid supply. Insufficient liquid supply comes from insufficient opening of expansion valve; insufficient opening of expansion valve comes from low pressure of low pressure and low temperature of expansion valve temperature sensor, that is, low superheat or no superheat ; Low low pressure comes from insufficient heat exchange of the evaporator, insufficient heat exchange comes from insufficient heat exchange area and insufficient air flow of the fan, insufficient heat exchange area comes from incomplete defrosting of the evaporator, and insufficient air flow also comes from the defrosting of the evaporator Incomplete; incomplete defrosting comes from uneven evaporation and frosting, partial defrosting is not complete; (this article source: Refrigeration Encyclopedia public account) partial defrosting comes from uneven refrigerant distribution or uneven frontal wind speed (this system The problem of uneven wind speed can basically be ignored), the uneven distribution of refrigerant comes from the uneven distribution of the liquid distribution head, and the uneven distribution of the liquid distribution head comes from the unreasonable design of the liquid distribution structure. (This article source: Refrigeration Encyclopedia Public Account) The unreasonable structural design comes from the gas content of the gas-liquid two-phase refrigerant, that is, the dryness of the refrigerant.

The change in dryness comes from the pressure difference before and after the expansion valve. Assuming that the high pressure is constant, that is, the condenser is normal, it depends on the low pressure pressure. The lower the low pressure, the greater the air content and the greater the dryness; as the dryness changes, the two The flow state of the phase flow changes.

　　If the gas-liquid mixture of the liquid splitter is not uniform, the mass flow of the refrigerant entering the evaporation will be different; the refrigerant liquid volume of each coil of the evaporator is different, which will produce different superheat at the outlet.

　　The refrigerant in the coil with insufficient liquid supply quickly evaporates, and a large degree of superheat is generated at the outlet, and the heat exchange area is not effectively used. The outlet superheat of the coil with too much liquid supply is very small, even with unevaporated liquid, (this article source: Refrigeration Encyclopedia Official Account), the heat exchange area is insufficient, and at the same time, the evaporator outlet header is due to refrigerant mixing or re-evaporation , The degree of superheat is lower than the theoretical value, the degree of superheat is reduced, the signal generated to the throttling device is too much liquid supply, the throttling device adjusts itself to reduce the liquid supply; the liquid supply decreases, the mass flow rate decreases, and the liquid separation effect Worse, a vicious circle is formed; eventually, the effective area of ​​the evaporator is drastically reduced, the cooling capacity is reduced, and the performance of the entire refrigeration system deteriorates...

　　 The final solution for this case: Refrigerant recovery; compressor replacement; liquid dispensing head assembly replacement; pipeline modification; system cleaning; filter replacement; system pressure maintaining and leak detection; vacuuming; refrigerant filling; startup and commissioning.

　　 Finally, back to the question at the beginning, what are the reasons why the dispensing head does not dispense liquid? There are many reasons for uneven liquid distribution in the evaporator, which are listed below:

　　The design of the liquid separating head is unreasonable, and the gas-liquid mixing is uneven;

　　The processing consistency of the dispensing head is poor;

　　 Dispensing head welding plug (this is known in general refrigeration);

　　 Improper installation of the liquid dispensing head, which is greatly affected by gravity;

　　The total length of each branch in and out of the evaporator is different;

　　The local resistance loss of the branch road is too large;

　　The wind speed on each branch of the evaporator is uneven;

　　 The evaporator shunt is unreasonable, and it is excessively affected by gravity;

　　The compressor has poor oil return, and the oil storage in each branch is inconsistent.