trouble with liquid return after defrost in rack

[coldkeeper1]

Good evening to everyone. Hope you can help me out with this problem.
This is a Tyler reverse cycle (two pipe)hot gas defrost R-22 rack system. There is suppose to be a solenoid operated throttling valve with the purpose of slowly allowing the remaining hot gas in the suction line to get into the suction manifold (bypassing the still closed EPR) right after defrost but before refrigeration re-starts. The problem is that when this throttling valves opens, it is allowing a lot of liquid refrigerant into the suction manifold: the valve gets heavily frosted, and the frost goes all the way down to the suction manifold and even to the compressor suction service valve.
This surely means that there is a lot of liquid refrigerant left in the evaporators right after defrost. But how could this be? I have checked the system during defrost, and I can feel the cool liquid returning through the liquid line, check valve, liquid return manifold, and entering into the condenser inlet.

[Gary]

Does the system have discharge reduction or liquid reduction? What is the differential pressure?

[Gary]

Let's elaborate on this:

On a reverse cycle hot gas defrost system a stop valve closes on the suction line at the suction manifold.

Then hot gas enters the suction line on the evaporator side of the stop valve.

The hot gas travels backwards through the suction line into the evaporator where it condenses to a liquid.

The liquid is pushed backwards through the liquid line and into the liquid manifold.

In order to do this the hot gas pressure needs to be at least 20psi higher than the liquid manifold pressure.

This pressure differential can be accomplished by either raising the hot gas pressure (discharge reduction) or by lowering the liquid manifold pressure (liquid reduction).


Note: Hot gas can use either strategy, but cool gas (from the top of the receiver) defrost systems can only use liquid reduction.

[joe magee]

Is it possible your defrost is too long? What type of strategy are you using for termination. Time or temp? Is this an equalizer rack? Are you using a controller? As far as I know only Hussmann uses cool gas defrost.

[friocold2008]

Thank you very much for your interest in my thread.
To Gary: the pressure differential needed for the hot gas-converted-to liquid to get into the condenser is by means of a large throttling valve installed in the compressor discharge manifold. As far as I know, during defrost this valves closes a little bit, causing a pressure reduction downstream, so that the condener pressure reduces as well. Since the hot gas supply is uptream from this valve, it is at a higher pressure than the condenser, so that the liquid flows into it.
To Joe: I believe this is a Tyler Enviroguard rack. It does have a label that reads "Equalizer" ( I guess that means it is equalized). The rack is about 10 years old and uses an RMCC controller. Defrost is time terminated. Defrost hot gas supply comes from a "tee" in the compressor discharge manifold.
Best regards.

[Gary]

There is suppose to be a solenoid operated throttling valve with the purpose of slowly allowing the remaining hot gas in the suction line to get into the suction manifold (bypassing the still closed EPR) right after defrost but before refrigeration re-starts.

Is the liquid line solenoid closed during this throttling cycle?

If the liquid line solenoid is open there will be additional pressure to push the liquid refrigerant through the throttling valve as well as an additional supply of liquid.

If the liquid line solenoid is closed while the throttling valve is open, the pressure wiil drop, any remaining liquid will boil off and be sucked (rather than pushed) into the suction manifold.

[coldkeeper1]

Hi Gary:
in this rack both the liquid line solenoid valve and epr remain closed as long as the throttling valve remains opened. There is an adjustable time delay between both actions.

[Gary]

Is the suction line insulated? Does it travel overhead or under the floor?

[coldkeeper1]

Gary:

It took me some time to ckeck the entire length of both suction and liquid line piping, and now I can inform you of the following:
Suction line runs above ground and is insulated all the way up to a point where the installer entered the freezer walk-in. From that point on, there is no insulation neither in the suction line nor in the liquid line, just the bare copper pipe ( I guess he wanted to save some money)
I was curious about your question in regards to the insulation of the suction line, but I haven´t been able to figure how it is related to my problem. I know that during defrost, as the hot gas travels through the suction line all the way to the evaporators, if the line is not insulated and goes across cold ambients, some of the hot gas could convert into liquid prematurely and might not be able to taw the frost. But I guess that as long as there is a pressure difference between the hot gas supply and condenser, even this liquid should get back to the condenser and not get logged in the evaporators, am I right?

[Gary]

Suction line runs above ground...

What does that mean? Laying on the floor? Above the freezer? Above the ceiling? Across the roof? I can't see it from here and my crystal ball isn't working.

I'm thinking the hot gas is condensing somewhere in the suction line with gravity feeding it back to the rack.

That suction line should be insulated all the way to the coil.

[dirk]

Are you only experiencing this fault on the one circuit

[Phase Loss]

Here is my 2 cents.

Gas defrost is a function of latent heat transfer (vapor to liquid). During defrost you are sending vapor to the case and it is changing state to a liquid as the ice melts. Once the ice is melted you no longer have a medium to transfer heat. So towards the end of defrost you should be having vapor return and not liquid.

My suspicion is you are running too short of a defrost on this loop. and leaving condensed liquid in the coils.

[oldtimer]

your problem is the systems on defrost to long try setting 5 minutes less see if coil stills get to / 10 on defrost and if it does drop it 2 mins at a time and it will be ok then