Non-constant refrigerant feeding in the liquid line

[mrr]

Hi,

I am having a problem I still do not understand the cause. I have a vapour compressor experimental setup, described here .

I have a stepper expansion valve which I think is properly sized for the evaporating temperatures I am working with. I also have a flowmeter in the liquid line, and I can control the ambient temperature at the condenser. I am working with R134a. I also have a condenser fan and a liquid receiver.

The problem is the following: when I have relatively low ambient temperatures, e.g., 20 ºC, the refrigerant flowrate is not constant and fluctuates a lot. I even can hear a refrigerant sound similar to a boiling sound, though I do not see bubbles in the liquid line viewer. However, I can observe how the refrigerant level within the pipe changes. It happens when I have low condensing pressures, e.g. 7 barg (about 31 ºC). In general, I'd say this effect happens when I lower the condensing pressure either lowering the ambient temperature or by rising the speed fan. It is a instability in the liquid line. I could do something with the fan condenser to raise the condensing pressure but that's not the point. The point is why this effect happens, because 7 barg for R134a is (I think)a not a very low condensing pressure.

What is the reason of this? I read in the forum that a non-constant feeding of refrigerant to the expansion valve might be due to an oversized condenser. I'd like to solve this problem and I'd really like to understand the cause of this.

Best regards.