Refrigerant flowrate problem in liquid line (fluctuations)

[mrr]

Hi,

I have a refrigerant flow rate problem in liquid line. I recorded a video here: https://www.youtube.com/watch?v=3WkcFul2HvA. The refrigerant flowrate should be constant but you can see something happens (flash gas?).

Problem description: 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 (the one shown in the video), 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 at the ouput of the condenser. In general, 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. Can you tell me what can cause this problem?

I can of course provide more data since this is an experimental plant. I really would like to understand what causes this problem.

Best regards,

-Mario

[Brian_UK]

I understand that this is an experimental test rig.

You 'think' the expansion valve is correctly sized, is it or isn't it? Wrong size can cause problems.

What is the subcooling at the condenser?

What is the liquid level in the receiver?

Do you have enough refrigerant in the system? Most important item.

Do you a high enough differential pressure at the expansion device?

Is you flow measuring device adding too much resistance when you are at reduced temperature/pressure?

[Tycho]

Looks like it might be a solenoid clicking on and off at 1 sec intervals.

is the stepper valve fitted with a solenoid or is there a solenoid on the liquid line?

The changes in flow rate seems to consistent to be related to anything other than electronic controls.

[mrr]

Hello again,

Thank you for your answers. I had this problem since two years in the test rig, and actually I posted the description of the test rig in this forum. Please read it here (http://www.refrigeration-engineer.com/forums/showthread.php?46446-Non-constant-refrigerant-feeding-in-the-liquid-line)

I know it is difficult to find the cause. However, given the expertise of some people in the forum, what I'd like to know is if you have seen this problem before. I also have to say that the test rig simply works and do the job, but I think something is not properly working.


In any case, let me ask each of you:
@Brian_UK:
You 'think' the expansion valve is correctly sized, is it or isn't it? Wrong size can cause problems.
I can select two valves, the electronic valve and a AKV pulsed valve. I can see the effect in both, even taking into account the AKV is a pulsed valve.

What is the subcooling at the condenser?
Subcooling is always about 15 K

What is the liquid level in the receiver?
I do not really know. Is there any way to measure it?

Do you have enough refrigerant in the system? Most important item.
I am planning to reload the refrigerant. However, I think there might be an overcharge of refrigerant, but I am not sure

Do you a high enough differential pressure at the expansion device?
I think so, I have about 9.5 bar of delta P in the compressor.

Is you flow measuring device adding too much resistance when you are at reduced temperature/pressure?
I'd say no since it is built to measure refrigerant in liquid state, but could be a possible cause.

One important note: I have an electrical resistance that acts as the thermal load in the evaporator. I have seen the oscillations dissapear in some cases, specifically when the load is high.

@Tycho. Thank you. There is no solenoid valve in the liquid line.

I can do any experimental and suggested test to try to find the cause.

Kind regards,

-Mario

[Brian_UK]

How is the thermal load applied?

Is there a solid state relay switching as the load stabilises?

You say the readings are steady when under load. Try - when it starts pulsing, switch load off. Does the pulse also stop?

[RANGER1]

Mrr,
You mention delta P of 9.5 bar, but also 7 bar discharge pressure?
15 deg C of subcooling is very high for air cooled condenser, so condenser must be banked up with liquid (overcharged).
Can you look at raw data from stepper motor, as expect it must be modulating.
With 15 deg of subcooling & a suction/ discharge pressure currently unknown, but low delta P, TX valve selection could be at its lowest end to control correctly.
Remove some refrigerant to see effect, would expect improvement.
Check TX valve capacity at minimum load, is it really correct size.
When designing a system, normally select a condition to run & try to maintain that condition for optimum performance.
Stepper motor TX valve have very good turn down, but with high subcooling, might be on the edge. subcooling

[chemi-cool]

Something is pulsating, could be a SV, unload, or EXV.

Check your controls, I think your answer is there.

[mrr]

Hi,

Bear in mind that I can see this problem dissapear (not always) when the condensing pressure is high and the thermal load is high too. Another important note is that (dont know if in the video can be appreciated), when this effect happens, I can hear a sound, like an evaporation.

@Brian_UK:
The secondary fluid in the evaporator is a brine of around 40% ethylene glycol aqueous solution, which circulates through a small tank. An electrical resistance is inserted inside the tank. The power supplied by the resistance is controlled by a solid state relay. The secondary circuit in the evaporator also contains a hydraulic pump that controls the secondary flow rate to the evaporator. As the heat load can be modified by the resistance, the cooling demand can be consequently imposed.

I will try what you mentioned, but I think that I would not get a stable point, i.e., if I switch off the thermal load the brine temperature will decrease and I will have to stop the test rig (I have programmed some lower and upper limits for safety of course).

@Chemi-cool:
Thank you. I am 99.9% sure that the problem are not the controls since I have done several tests in "open-loop" (i.e., I manually set the actuator) and the problem remains.

@Ranger:
Thank you for your detailed answer. When I back from holidays I will post some data (I can log several variables). I think the problem could be in the refrigerant load. If you remember we talked about this problem 2 years ago (the link is in the first post), and I think the valve is not the problem since we have two VEX in parallel and I can see the commented effect when using one or the other, just in one of the VEX it is more difficult to appreciate it given the valve is a pulsating AKV VEX from Danfoss. I also can see the driver of the VEX stabilized about 50-70 % and also the compressor and the discharge pressure. If the VEX was small I think the superheat could not be controlled. The VEX can control up to 3.5 kW and the thermal load is no more than 1.5 kW (for the stable points I work).


Kind regards,

-Mario

[Tycho]

Is the discoloration in the sightglass at 0:17-0:19 in the video from shadow/liqhting? to me it looks like there is flashing or not enough liquid, that's why I suggested it was a solenoid cycling on/off

[RANGER1]

Also subcooling is said to be 15 deg, assume celsius.
At what ambient & system pressures was this calculated.
Where wee readings taken from?

[seanf]

Would seeing how much an affect the flash gas has on the flow meter reading be of any use? You could run the unit on the stable high load/condensing temp, and then dump discharge gas into the liquid line to try and mimic what your seeing in the sight glass on the video.