Copeland Scroll compressor flooding in sub-zero temperatures

[Bartek Celary]

Fellow engineers,

I made a heat pump with ZH Copeland Scroll working on R410A. The problem is as follows: when evaporation temperature drops below ~ 0 deg. C, liquid refrigerant floods the compressor. I checked superheat, amount of R410A in the system and EEV.

Do you have any idea what may have gone wrong?

Best regards.

[joe-ice]

Are you sure its liquid coming back and not just cold gas which will also sweat/frost the suction line as its below the freezing point of the moisture in the air

[Brian_UK]

What is the superheat?

Did you include a suction accumulator in your design?

[Rob White]

.

As the others have said don't confuse cold or frosted
pipes with liquid flooding back to the compressor.

If the evaporation temperature is below freezing the vapour
coming back down the suction will be below freezing
and therefore your suction pipe will sweat or freeze.

If you have superheat you should have vapour.

Regards

Rob

.

[Bartek Celary]

Thank you for your replies!

There is this peculiar sound coming from the compressor. I have never heard it in situations other than flooding. If you ever worked with a flooded compressor, you know what I'm talking about. I can upload it if you think hearing the sound would help.

I usually run it with superheat SH = 2K. However, the heat pump has been running on SH = 5 K for about an hour and it didn't change anything. I raised SH up to 10 K, still the same.

The compressor is warm on the bottom (engine), cold near the inlet and hot near the outlet. I have spoken to Copeland engineer and he said it shouldn't look like that. Copper lines freezing are acceptable, but as far as I know, it should not happen with the compressor.

Brian, I didn't include an acumulator. Just basic components.

Regards,
Bartek

[NewmanRef]

If the compressor body is frosted then you do have liquid at compressor. Is the sound similar to the misses washing up pots and pans in the kitchen? Are you sure EEV is operating correctly? Maybe a faulty evaporator in/ out probe giving false superheat readings?... If you are positive this is correct and you still have liquid perhaps fit a suction accumulater as Brian mentioned. I would definately spend a bit of time at the valve first though!

[RANGER1]

2k superheat on even EEV is a bit
fine, no room for any error.
Try to confirm pressure transducer reading etc &
see if it matches up with actual pressure & suction line temperature
using old conventional measurements.

[Bartek Celary]

I used two different sensors - Carel and Testo, they both show same results. SH is confirmed, as well as pressure. I opened the valve by manual control and set it in place to achieve steady superheat. Tried different degrees of opening and every time I experienced the sound of liquid.

What troubles me is - why does it happen only in low temperatures? why don't I see it anywhere over let's say 5 deg. C?

[Brian_UK]

What type of evaporator are you using?

Are you controlling the evaporator load correctly in reaction to the heating load and the compressor loading?

Is the PID control of the EEV set to maintain a good response speed?

[Bartek Celary]

I have SWEP Brass Plated Heat Exchanger. Type and size recommended by SWEP itself. Select program confirms the choice.

I am not certain I understood the second question, could you please rewrite it?

PID does not matter, I have a set heat load ensured by a large water buffer. Therefore I can work in a constant evaporation temperature and constant superheat by manually settig the EEV position.

The PID alorithm, however, is working a bit slowly, but correctly.

[RANGER1]

Bartek,
I think you are going to have to give us more information as we are only guessing, trying to read between the lines.
I think you need to give us a sketch of system with all components.

Water temperatures in/out heat exchangers or air on/off.

If you are using water through a heat exchanger then there is possibility of spot freezing, especially if water flow is down & below zero. Normally flows switch or evaporator pressure regulator.

A few other temperature readings along suction line would be helpful to determine flood back is occurring. if superheat immediately out of evaporator is example 5 degC at evaporator outlet,
what is it at compressor? is there anything else in the suction line.

Maybe discharge & sump temps when running"normal" compared to "flood back " condition.