Flood back issue?

[Arc]

Have recently done some work on an industrial refrigeration unit which is having an issue with liquid flood back. When the unit runs there is a constant build up of ice from the accumulator (whole thing) to the compressor. This then runs all the way up to the evap and freezes up.
The system has four evap coils each with a solenoid valve used for staging, water cooled condenser, recipricating compressor, is on R22 and runs a cooling tunnel at about 8 degrees C.
I believe this flood back issue has been a problem in the past as the suction accumulator appears to have been replaced recently.
On further inspection one of the txv's was faulty and replaced, however this did not solve the problem.
Unfortunatly I was only filling in at this site and will have little opportunity to work on it for some time but am very interested in helping solve the issue.

[Brian_UK]

A few points/questions - is the compressor a fixed speed or does it have any unloaders for when the solenoids shuts down?

Is the accumulator large enough for the system.

Does the system have a liquid receiver to take the surplus refrigerant on low load?

[Arc]

The system has two unloaders on 3 heads, when the thing unloads ice begins to go but not totally. It seems the biggest problem is caused on start up, fully loaded for some time. There is no liquid receiver.
I had the thought after the original post that accumulator could have been undersized as it is new.
Unortunately the tech who is full time on the site is secretive/deceptful and I don't believe I will share informaton or even deal in this matter again. However the theory behind the problem intersets me.
Thanks for the reply.

[Grizzly]

Arc
Is there any form of liquid level control in this system. (say on the accumulator)
Also am I correct in thinking the compressor is controlled on Suction Pressure?
Have you considered that the system may be overcharged?
Grizzly

[abet_meneses]

arc,

maybe a regulator control problem.check capacity of evaporators and and check if the installed txv rating match the evaps.maybe an ice build up cause by busted heaters,inadequate defrost cycle,defective fan motors to name poor product stacking to name a few.

regards

[Gary]

I'm betting gross overcharge... and no receiver.

[US Iceman]

... cooling tunnel at about 8 degrees C.


If the suction line is +8°C how can it have ice on it?

Secondly, just because the line has ice on it does not mean liquid is present.

[Gary]

When the tunnel is not in use, do they pump the system down or just shut off the condensing unit?

If they shut down the condensing unit and the solenoids are still energized, the tunnel will warm up, the solenoids will open and all of the refrigerant will migrate to the accumulator.

[US Iceman]

Based on the information presented, I'm not convinced the compressor is flooding (yet). It may very well be experiencing this, however, no data has been presented yet to suggest this is occurring.

[Gary]

Based on the information presented, I'm not convinced the compressor is flooding (yet). It may very well be experiencing this, however, no data has been presented yet to suggest this is occurring.

I agree. I have seen no reason to believe liquid is flooding back to the compressor.

[Arc]

As mentionedI am unable to get back to this unit until who knows when. Overcharge is a possibility, no receiver. I was under the impression ice build up on pipe work means liquid, is this incorrect.
To answer other questions it is control on air temp, pumps itself down when not in use, no defrost cycle. No info on background, could be a design fault.

[Gary]

I was under the impression ice build up on pipe work means liquid, is this incorrect.


Frost on the line indicates line temperature and nothing else. Without the saturation temperature to compare it to (superheat) we cannot accurately tell if there is liquid in the line. It could be, and probably is, cold vapor in the suction line.

[Gary]

What controls the unloaders?

[Gary]

If the suction line is +8°C how can it have ice on it?


A very good question.

The tunnel (air on?) is at 8C. The suction line must obviously be at or below 0C. The saturation temperature must then be considerably below 0C.

Therefore the unloader controls are set way too low for the application.

[US Iceman]

The suction line must obviously be at or below 0C. The saturation temperature must then be considerably below 0C.


Now we are getting somewhere.

If we assume the air temperature is +8°C, then obviously the saturated evaporating temperature has to be below this temperature to produce cooling. So, if by chance the evaporating temperature is at or below 0°C (and probably not too far below this temperature) then the suction line temperature would probably be at or slightly above 0°C because of the vapor superheat!

Edit: Remember this system is using TXV's.

Around these temperatures you might expect to see a lot of condensation or light frost. Neither of which indicate floodback.

It is only an indication of pipe temperature.

[Gary]

When the unit runs there is a constant build up of ice from the accumulator (whole thing) to the compressor. This then runs all the way up to the evap and freezes up.


Assuming the TXV is adjusted correctly, the saturation temp must be about -4C or lower. This gives us an evap TD of at least 12C/21.6F. This seems excessive.

And for the frost to continue all the way to the compressor, we may be looking at a much lower saturation temp and much higher TD before the compressor decides to unload.