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First, start with liquid
First let's define what we mean by liquid starting. When the refrigeration unit is shut down, there is refrigerant liquid in the compressor housing, and the refrigerant liquid will destroy the lubrication of the compressor when the compressor is started next time. Starting the compressor with refrigerant liquid is called starting with liquid. The culprit of starting with liquid is the large amount of refrigerant mixed in the lubricating oil!
When the compressor is started, the pressure is suddenly reduced, and these refrigerants will suddenly boil, causing the lubricating oil to blister. This phenomenon is very similar to the foaming phenomenon of Coke when people suddenly open a coke bottle in daily life. The duration of foaming is related to the amount of refrigerant, usually reaching a few minutes or more than ten minutes. A lot of foam floats on the oil surface and even fills the crankcase. Once the foam is sucked into the air chamber through the inlet, the foam will be reduced to a liquid, which is a mixture of lubricating oil and refrigerant, and it is easy to cause liquid percussion.
And how does the refrigerant that causes the liquid to start get into the crankcase?
First of all, it is not ruled out that the imperfect design or debugging of the system leads to the compressor being shut down, and part of the refrigerant is returned to the compressor under the action of gravity. Secondly, enter the crankcase in the way of "refrigerant migration".
The flow of refrigerant under gravity is easy to understand, but what about refrigerant migration? In order to solve the problem, we must first understand what "refrigerant migration" is, so as to eliminate the root cause.
Second, what is refrigerant migration?
When the compressor is shut down, the accumulation of liquid refrigerant in the compressor crankcase is refrigerant migration. As long as the compressor temperature is lower than the temperature inside the evaporator, this phenomenon will occur because the temperature difference will create a pressure difference between the compressor and the evaporator, driving the refrigerant to a colder place. This phenomenon is most likely to occur in cold weather, but for air conditioning and heat pump devices, the condensing unit is far from the compressor, even if the ambient temperature is high, it may occur.
Refrigerant migration is a slow process. The longer the compressor is down, the more refrigerant will migrate to the lubricating oil. This process will continue as long as liquid refrigerant is present in the evaporator.
The measures to prevent refrigerant migration resulting in liquid start-up are as follows:
1. Install the liquid pipe solenoid valve. After the unit is shut down, the solenoid valve is closed at the same time, and most of the refrigerant liquid is trapped on the high pressure side, which greatly reduces the possibility of starting with liquid.
2. Install the compressor heating belt. After the compressor is shut down, the electric heater is powered on to heat the oil pool, keep the heat input, maintain the high temperature of the oil pool, and evaporate the refrigerant liquid in the oil pool to avoid the influence of the liquid foam in the next startup.
3, the return pipe design inverted U-bend. After the shutdown, due to the height difference between the evaporator and the compressor, due to the role of self-weight, the liquid in the evaporator migrates to the compressor, and at the next start, the evaporator runs ahead of time, evaporating the liquid into a gas, which can ensure safety.
4. Install the gas-liquid separator.
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