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 (http://www.refrigeration-engineer.com/forums/showthread.php?46233-Refrigerant-R134a-Normal-condensing-pressures).

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.

[Brian_UK]

Perhaps you should reread your previous thread as various suggestions for the noise were given.

Not agreeing with the answer does not make the answer incorrect.

http://www.refrigeration-engineer.com/forums/showthread.php?46233-Refrigerant-R134a-Normal-condensing-pressures&highlight=

[mrr]

Hi Brian_UK,

Thanks for your answer. Could you please let me know to which of the answers I got you refer? I agreed with all the previous answers and actually you can see my last answer in the linked post. I still do not see the reason of this phenomena, even though I have been several weeks logging data of the system.

Regards.

[RANGER1]

Mrr,
Possible cause if I understand correctly could be when lowering condensing pressure refrigerant boils in liquid receiver & liquid line to adjust itself to new saturated condensing pressure.
With any Tx valve the more constant condensing pressure, the better control overall.

[Brian_UK]

Also sizing of liquid line.

[mrr]

Mrr,
Possible cause if I understand correctly could be when lowering condensing pressure refrigerant boils in liquid receiver & liquid line to adjust itself to new saturated condensing pressure.
With any Tx valve the more constant condensing pressure, the better control overall.

That might be the cause RANGER1, actually I have seen that phenomena everytime the condensing pressure is lowered. We can control the ambient temperature but it inevitably oscillates +- 0.5 ºC, this makes at the same time the condensing pressure to oscillate and thus the refrigerant flowrate becomes unstable. Then, all the variables are affected, such as the superheat which also fluctuates. When we simply do not control the ambient temperature this effect seems to disappear and an stable point is reached.

Still, why refrigerant would boil to adjunst itself when lowering the condensing pressure? I am thinking about the Mollier Diagram and entering temporary in the liquid-vapor region.

In addition, in every experiment I have done so far I see a subcooling of about 13 K, the delta T of temperature in the condenser is also about 13 ºC.

What can be the possible troubleshooting of an oversized condenser? I am trying to find resources to understand all this troubleshooting but so far with no luck. Specifically, when a condenser is oversized I would expect a low delta T (i.e. ambient temperature - condensing temperature) but a high subcooling. Am I right?

Regards and thank you.

[RANGER1]

mrr,
not necessarily high subcooling unless there is liquid in the condenser that can be further cooled below saturated condensing pressure.
If you lower condensing pressure any liquid in high side of system will boil to find new saturated condensing pressure.
How much it boils depends on how much pressure changes.
It will boil in liquid receiver, liquid line to expansion device & anywhere there is liquid unless it is subcooled below new set of conditions.
Even the cange of degree of subcooling upsets expansion valve.
If you can draw out period of oscillation, then maybe more control on superheat will be less drastic.
You have to expect some sort of disruption to system when conditions change.

[RANGER1]

Someone from forum developed this, may be of some help.


14197

[mrr]

Hi Ranger,

Thank you for your help. I will think about it and I will save that pdf file.

Best regards.