Compressor suction icing up on cold room set up

[Leviathan]

Hi guys,


I'm currently trying to fault find on a 9 room cold store set up consisting on 2 large freezers, 2 small freezers 3 fridge rooms, and 2 defrost cupboards all supplied by a variable load 3cyl carrier comp.

The system is running on R-22 with a suction temp of between -24 and -30 and a condensing temp of +35.

I am getting icing up back to the compressor and on the compressor body itself! i know all to well this is not good and have pretty much hit a brick wall with it.

I had a compressor failure recently and metallic fines contaminated the system. Since the i have cleaned out all the orifices and renewed the filter/driers.

If i shut any one of the rooms off the icing up stops and the system seems to rum normally, but...

I have checked all the evap. fans, and all are ok.

I'm pretty much stumped, but i know i'm missing something!

any suggestions would be great! thanks in advance.

[lana]

Hi there,

You have to describe your system more clearly.
Is it a DX system?
Is one compressor connected to all the rooms?
If so, is there any regulating valve installed?



If i shut any one of the rooms off the icing up stops and the system seems to rum normally, but....

When you shut down one room, Does the solenoid valve close the liquid flow to that room?
When one room is shut down then, does to compressor reduce the capacity (capacity control)?

Need more info.


From what I understand is this : compressor capacity might be higher at full load for the rooms, so you have liquid flood back. This is just a first thought.

Cheers

[NoNickName]

If the suction is -30°C, of course the compressor is icing up. This is not a symptom of bad operation. It would be in case of an AC system for comfort application.

Just make sure the superheating is correct (if it is a dx system).

[Leviathan]

the one compressor is connected to all the rooms, and i have thought about the back pressure valve tonight, but not got round to checking it yet.

When the room is shut down the solenoid valve closes, i've checked all these and they are holding, the compressor is already running on its lowest capacity. The installation is already 15 years old and at least for the last 6 i've not had any ice up problems.

[US Iceman]

If the suction temperature is between -24 & -30C, I would expect to see some ice somewhere on the suction line or all the way back to the compressor.

The ice forming simply means the suction line is below 0C. That does not mean it might be liquid slugging. Check the superheat at the evaporators during operation and also at the compressor suction.



If i shut any one of the rooms off the icing up stops and the system seems to rum normally...


This is somewhat concerning. Is this only one room you are shutting off?

[LRAC]

the one compressor is connected to all the rooms, and i have thought about the back pressure valve tonight, but not got round to checking it yet.

Just remember one thing the back presure valve( i hope you mean crankcase pressure regulator?) cannot distinguish between liquid or vapour and purely does what it says and regulates pressure.

Is the installation new if not any previous history in its life of crompressor failures other than the one you've changed?

Is the refrigerant charge ok if the tev's are not getting a full liquid delivery, your valves might be wide open until any one of the systems is shut down and the liquid is then enough to supply all systems with liquid at the tev's.

Not sure if the theory's correct but might be a starting point.

Kind regards
Lrac

[lana]

Hi there,

If one compressor is connected to three rooms then how can you control the liquid flow to each evaporator? When one solenoid valve is closed then you have all the liquid going into the the remaining evaporators???!!!! If there is capacity control then what is the controlling signal, temperature, pressure,...?

What kind of design is this?

Appreciate any comment.
Cheers

[thebigcheese]

have u expansion valve trouble? also is it liquid back or just cold vapour?

[slingblade]

I would be iclined to lift the head temp from +35 c to +45 c (assuming fan speed control/pressure switches or an ata valve if water cooled) and then check the superheats of each evap under full load to identify any problems.

[Leviathan]

Ok, sorry for the late reply... Right then the system finally seems to be running normally now, after replacing one of the regulating valves and resetting the superheat on the two larger cold rooms. Also found the system was low on refrigerant, and dont know why I didnt see it before! like you said... wood for the trees.

Lana, the capacity is controlled by means of a hydraulic switch taking its signal from the suction pressure in the crankcase, it is a two stage switch which when activated lifts the suction valve on the cylinder loading it up. As for the design, its Italian and so hughly over-complicated, and theres not really a lot I can do about it! just work with what i've got.

Anyhow thanks for all the suggestions