Reasons for uneven liquid distribution in refrigeration systems and tragic cases!

Reasons for 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 bypasses and frost the evaporator. As a result, the evaporation is incomplete, which will affect the system's cooling capacity and may also cause 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 a colleague. 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 large vibration and noise;

　　 Compressor valve and connecting rod are broken;

　　 The compressor is short of oil;

　　 system ice block;

　　Lubricating oil deposits carbon and turns black;

　　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, there are inevitable causes. If the cause cannot be found and resolved in time, the practice of treating 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 failure was the blockage of the gas return hole, the blockage of the oil return hole came from system impurities, and the system impurities came from lack of oil operation (liquid hammer first). Leaving aside), the lack of oil comes from poor oil return (at this time the gas content is effective), and the poor oil return comes from insufficient air intake. A too low flow rate is not enough 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; (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 liquid distribution head 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 rate of the refrigerant entering the evaporation will be different; the amount of refrigerant liquid in 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, which produces a large degree of superheat 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, and the heat exchange area is insufficient. 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 that the amount of liquid is too much, the throttling device adjusts itself to reduce the amount of liquid; the amount of liquid is reduced, the mass flow rate is reduced, 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; 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 amount of oil in each branch is inconsistent.