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Originally posted by jsnmrphy
Hey guys,
I am in need of some assistance on an old supermarket rack, I wonder if anyone can help. The model# is DA4A000RCM, am having trouble with it flooding the compressors when it comes out of defrost. should mention that only 2 circuits on this one have hot gas defrost, everything else is off cycle. I have a sporlan head pressure control in the liquid line but it does not seem to be working therefore my liquid line pressure is exactly the same as the dicharge line pressure which I believe is not allowing the condensed liquid refrigerant to come back to the system until I come out of defrost. That is what I think but I do not have any literature on this equipment so I am not really positive that I fully understand the sequence of operation. If anyone can help either with printed material or with advice it would be greatly appreciated. My fax#is 256-341-0616 & I am willing to pay for any info. Thanks Jason

So you have a hold back on your drop leg. And with it I bet you have an ord off disharge line that dumps into receiver. Neither one of these means squat until winter. That drop leg wants to be the same right now. As the pressure falls below a certain point, she will throttle and hold back liquid, reducing effective condensing, raising head, and since I am doing that with all my liquid on roof, I need to keep enough pressure on receiver to push what liquid is in there out. These two in conjunction are nothing more than a headmaster. Doesn't have anything to do with Hot Gas.

You should be seeing a Parker big ole blue valve, piped right off your dishcharge line with a solenoid on it. What it does is, it will drop regular dishcharge gas, by redirecting it to hot gas manifold. Hot gas goes out to case and does it's thing, it comes back and dumps back into liquid. That pressure should exceed normal liquid pressure so as to have differential and so we got flow of hot gas.

I totally misunderstood you, and I probably even confused myself.

That drop leg valve your seeing is precisely what you said it is, a valve to control head pressure. Head pressure if it falls below a certain point. It holds back. It stacks your liquid to increase head pressure. But doesn't do anything for you unless were talking low ambient.

Dave was explaining that a check valve might be seeping. Generally the hot gas goes out after the suction stop, flows out to case and exits the coil before txv through a check valve and back into liquid line at rack. If the check aint holding, the liquid can enter coil and on back into coil and back to rack.

Here is a IOM of Heatcraft and their rack systems.

http://www.heatcraftrpd.com/Maintena...s/H-IM-72B.pdf

Originally posted by condenseddave
Every manufacturer in the world uses that type of defrost differential regulator, or one of it's relatives.

That should be an OLDR.

That should be deenergized when in defrost. When deenergized, it is creating a differential.

In conjunction with the OLDR, there should be an ORI in the condenser dropleg, and an ORD from the discharge line to the top of the receiver.

You most certainly CAN defrost cases with hot gas, with no differential. The pressure will find it's way back through the liquid line. I take care of several racks that are twenty years old and never had any type of defrost differential regulator on them. They would be nice, but they work without them. (Very short runs, BTW.)

The OLDR, the ORI, and the ORD are important when operating in low ambients, although these old pigs that I'm talking about were never told that they wouldn't work, so they just do.

[Edited by Dowadudda on 08-16-2004 at 09:03 PM]

Every manufacturer in the world uses that type of defrost differential regulator, or one of it's relatives.

That should be an OLDR.

That should be deenergized when in defrost. When deenergized, it is creating a differential.

In conjunction with the OLDR, there should be an ORI in the condenser dropleg, and an ORD from the discharge line to the top of the receiver.

You most certainly CAN defrost cases with hot gas, with no differential. The pressure will find it's way back through the liquid line. I take care of several racks that are twenty years old and never had any type of defrost differential regulator on them. They would be nice, but they work without them. (Very short runs, BTW.)

The OLDR, the ORI, and the ORD are important when operating in low ambients, although these old pigs that I'm talking about were never told that they wouldn't work, so they just do.

I didn't know Hill used that type of valve. The results would be the same though.

Originally posted by davidchsw
It sounds like there was no hot gas flow through the evaporator in defrost. When the defrost ends the evaporator goes from the high side pressure to the low side pressure right away causing liquid to rush out of the evaporators to the compressors. When you got the discharge modulating differential valve to work that caused hot gas to flow through the evaporators in defrost. After a little while the case warm up and the defrost thermostats close shutting off the hot gas valve. Now the cases sit for a few minutes doing nothing, called drip down time. When the cases go back to refrigeration the evaporator pressure is at case temp, say 55. The refrigerant doesn't flood back to the compressors.

David

Dave, thats what I was thinking he was saying also, but he's talking liquid and not hot gas. I have like 5 racks set up with hot gas with the flo con dishcharge mod valve. So that was what I thought he was talking about. He's got the liquid differential, where the liquid mainfold will drop in pressure, to get the hot gas flowing out to coil, coming back.

It sounds like there was no hot gas flow through the evaporator in defrost. When the defrost ends the evaporator goes from the high side pressure to the low side pressure right away causing liquid to rush out of the evaporators to the compressors. When you got the discharge modulating differential valve to work that caused hot gas to flow through the evaporators in defrost. After a little while the case warm up and the defrost thermostats close shutting off the hot gas valve. Now the cases sit for a few minutes doing nothing, called drip down time. When the cases go back to refrigeration the evaporator pressure is at case temp, say 55. The refrigerant doesn't flood back to the compressors.

David

Is that a LDR or OLDR valve?

Okay now I gotchya. I thought we were talkin hold back off drop leg. Sorry. My mind works, it just sometimes needs a rubber maleet upside the head.

It aint prehistoric. I can't remmeber, Tyler maybe, they loved that way of doing it.

I catch what your getting at. Your saying she was not energizing, creating a 20 lb diff at liquid manifold so the hot gas that get's shipped out aint flowing back to liquid manifold, rather dumping back into suction. Yeagh I guess, maybe. Something like that I can't be sure unless I am there on it.

Hill

Dow,
Sorry about the confusion, the valve that I was referring to is a liquid differential valve and is positiond at the inlet to the liquid manifold, it is used in conjunction with an electric solenoid valve so that it only operates when one of the circuits is in defrost.Its only job is to maintain a 20# pressure differential so that the liquid that condenses in the evaporator during the defrost cycle can be returned to the system.I know this is primitive but it seems to work ok as long as everything is set up & operating properly, this thing is so old I doubt that they had computers back then. Thanks again for all the help.

I am not getting your thing here. Are you saying your drop leg Hold back is cranked all the way in and your flashing to your liquid manifold?? If you got no liquid leaving the rack out the liquid manifold, how then hell can you come back with out any superheat. Plus you should have a check valve leaving your separator.

As far as HOT GAS, did I read where your not redirecting hot gas to hot gas manifold during a call for hot gas defrost with an increased hot gas by use of a diff valve.

Or are you saying that in hot gas, your coming back and flooding suction manifold.

I aint sure if this might be your problem but I have a few racks where I had to ressurect. And they would flood suction. The PID on the controls was keeping my pumps off too long to react quick enough to change in suction manifold. The PID on my puters was real low. All I did was ask the controller to respond faster. Off cycles on a rack should be dealt with as well, at a minimum a cch when pump is off so your can try to prevent migration.

Hill Rack

Seems as though the liquid pressure regulator was the whole problem. After I got that working I started getting my refrigerant back to the liquid manifold and have not been able to see any flooding. tink I will go back one more time after I give enough time for evaps to ice up and see it defrost again just to be sure. I really appreciate the info & technical advice from all of you.Even though I dont post very often, I read here almost daily & it has been well worth the few dollars that it takes to become a supporter. Thanks to all.
Jason

Originally posted by jsnmrphy
dave
Thanks for the reply, when I went on this rack for the first time the hot gas solenoid was disconnected so I do believe that there was a problem for awhile.

What was the reason that you were called to the store in the first place. If the hot gas valve was disconnected when you got there and the cases were not iced up I would suspect that maybe the cases have been changed from original and maybe they are no longer gas defrost cases. How long was the defrost time before ypou started messing with things. Open up 1 case and look for a check valve around the txv. If there is one then the cases are probably gas defrost. If no check valve then the cases are off cycle.

If the cases really are gas defrost then the first order of buisiness is to get the differential regulator working. Without this valve there is nothing to move the refrigerant out of the cold evaporator and back to the main liquidline where the refrigerant is then used by the rest of the systems. Refrigerant condenses in the coil and all comes back at once when defrost is over. The differential should be set for a minimum of 35 psig across the regulator.

Once you get the regulator working then if the problem persists then you might look for a check valve in the case that is sticking closed While this is not common it does happen. Since the flooding seems to be occuring only after defrost I would rule out any txv problems and I would rule out a check valve that is sticking open.

Good luck. Hope you get to the bottom of the problem before you have to start replacing compressors.

Sounds like you only have a suction stop and someone opened up a txv all the way allowing liquid to FILL the entire system when in defrost. Was this happening when the hot gas valve was disconnected or when you re-connected it?

If you had no differential your cases would be icing. Some differental valves are de-energized for differential flow. So you would always have differential, even when the case is in refrigeration.

Put it in defrost and do what Dave told you, look at those check valves real hard. They only need to be a little slow in reacting to the pressue change to flood you.

Then get that Differential system working properly. The electric company will hate you for it, but the store manager may ask for your hand in marrage.

dave
Thanks for the reply, when I went on this rack for the first time the hot gas solenoid was disconnected so I do believe that there was a problem for awhile. Also, I am very sure that the compressors are being flooded because the heads are frosting in a spot about 2"x2" for the first couple of minutes after either system comes out of defrost.After about 2-3 minutes the system settles down & works normally. This one circuit serves approx. 60 feet of meat case,about half of it is coffin style & the other half glass front. I have checked the check valves & did not find any that were sticking.I know for sure that the pressure regulator on this rack is not working & I will have to replace it anyway but I just honestly dont know if that will cure this other problem.

I'd be more inclined to look directly at the cases that ARE hot gas defrost, and see if there is a check valve stuck open in one of them. Another possible cause could be a loose TXV bulb in one of the T.O cases.

No defrost differential is not a really ideal situation, but it's unlikely to be the cause of the problem. Possible, but rare.

Does this flooding occur only when one or two particulkar systems come out, or does it happen after every single defrost?

Also, are you 100% positive that it's flooding? (Oil failures???)

Hey guys,
I am in need of some assistance on an old supermarket rack, I wonder if anyone can help. The model# is DA4A000RCM, am having trouble with it flooding the compressors when it comes out of defrost. should mention that only 2 circuits on this one have hot gas defrost, everything else is off cycle. I have a sporlan head pressure control in the liquid line but it does not seem to be working therefore my liquid line pressure is exactly the same as the dicharge line pressure which I believe is not allowing the condensed liquid refrigerant to come back to the system until I come out of defrost. That is what I think but Ido not have any literature on this equipment so I am not really positive that I fully understand the sequence of operation. If anyone can help either with printed material or with advice it would be greatly appreciated. My fax#is 256-341-0616 & I am willing to pay for any info. Thanks Jason