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jmcclellan
12-05-2006, 05:13 PM
Hey all

We just got back from repair a 0.5x0.5 HP cascade thermal chamber, which had the high stage compressor replaced. This is an R404/SUVA95 system.

Unfortunately, it appears the repair guys' test of functionality seems to be 'does it get to setpoint' not 'does everything work properly'. The chamber does acheive low temps, but slower than it should.

The low stage is now exhibiting a problem it didn't before - the overpressure switch is pulsing intermittently, which causes bypass of the evaporator and hence slow cooling. Can anyone give me any ideas of the most likely cause? I can think of a few possibilities, all of which involve poor workmanship. crap in the high system (partial blockage somewhere), improper charge of either system, etc.

Thanks in advance for any ideas!

can anyone confirm, to replace the high stage compressor, would (should?) they have discharged/recharged the low stage system also? their work order identifies only R404 suggesting they did not discharge the low system.. if this is the case, the charging should be the same as before, and since it worked fine before, that would mean that this pressure limit switch problem is probably related to the high stage providing insufficient cooling for some reason or another..

US Iceman
12-05-2006, 06:19 PM
Was the replacement high stage compressor the same capacity?

A 1/2 HP compressor for low temperature can be different from a 1/2 HP high temperature compressor.

jmcclellan
12-05-2006, 07:43 PM
I suppose so.. what can I say.. it was supposed to be, it was a special order unit we waited weeks to get from Copeland, P/N RS55C1E-PAA-909 and it looks identical to the old one in every respect..

when it came back and I noticed this problem, that was the first thing I looked at - did they put in a 1/3 instead of 1/2 HP? Of course I can't find that part number anywhere, so I don't know for sure.. but it certainly looks the same.. :(

jmcclellan
12-05-2006, 08:07 PM
just took a look.. and the low stage compressor is an RS55C1E-PAA-201.

Peter_1
12-05-2006, 10:09 PM
If the pressure switch trips on the low stage, then there's something wrong with the evaporator of the high stage.

If you measure SH on the evaporator of the HS, what do you have?

Not enough refrigerant?
Is the compressor displacement the same for the faulty and the new one? If so, then the fault must be something else.

If you have to change a compressor in a cascade, then you do'nt need to replace nor recharge the gass in the other stage.
But, both should work bery close to each other.

Is the LS waiting long enough before starting up?
Evaporator of the HS must be frist cooled down low enough before the LS can start cooling.

jmcclellan
12-05-2006, 10:56 PM
That's what I figured.. the low stage is not receiving sufficient cooling for one reason or another..

I don't think I have the ability to measure superheat.. both in tooling and skill.. I have no gauge set and have no actual refrigeration training, just alot of self-learning and observation from pros.

I can say that the new high stage compressor is the same displacement as the low stage compressor, but I have't got the old one to compare.. it's kind of hard to tell, Copelands' part numbers are somewhat cryptic and there's no data easily available

There seems to be sufficient startup delay, ~15 secs. same as before, same as the others of the same model we have.

US Iceman
13-05-2006, 12:48 AM
Peter,

Is R-404a one of those refrigerants that separate and have to be charged as a liquid?


...their work order identifies only R404

If the system worked before the compressor was replaced and the only thing that was replaced was the compressor, then something changed right?


The chamber does achieve low temps, but slower than it should

Sounds like the cooling capacity of the high stage compressor has changed. This is also confirmed by the low-stage compressor "switch is pulsing intermittently" and the above quote.

jmcclellan
13-05-2006, 05:34 AM
system worked fine as far as I know, until one day, literally over lunch hour, the high stage failed suddenly. The compressor would run, but not build pressure, so the low stage didn't ever start. The service guy said that these models contain a sort of spring-containing check valve and the springs can break sometimes (it did have a lot of hours on it) resulting in a compressor that runs but accomplishes nothing. anyway, all that was done was the compressor, filter/dryer, and of course refrigerant. They also said they did a nitrogen leak test, backflush and vacuum before charging.

It's behaviour is certainly indicitave of insuffient cooling of the low stage.. it gradually builds up pressure until hitting the limit switch, which pulses for 1/2-1 second until the pressure drops, and so on.

Interestingly, we have another chamber with similar symptoms. This one developed gradually over time rather than suddenly. They came and looked at it while this one was out for service. The low stage discharge pressure is about 280psi and suction is about 6. Guy wouldn't check the pressures on the high stage, saying it had nothing to do with the problem.. in this instance, they say there is a partial blockage in the capillary tube, as indicated by the high discharge and low suction pressures. I am unconvinced, seems like maybe it's the same problem, and maybe the low side is overcharged..? anyway, that's another question :rolleyes:

anyone know any cascade refrigeration expert repair shops in the Toronto/Vaughan area? :)

Andy
13-05-2006, 09:43 PM
system worked fine as far as I know, until one day, literally over lunch hour, the high stage failed suddenly. The compressor would run, but not build pressure, so the low stage didn't ever start. The service guy said that these models contain a sort of spring-containing check valve and the springs can break sometimes (it did have a lot of hours on it) resulting in a compressor that runs but accomplishes nothing. anyway, all that was done was the compressor, filter/dryer, and of course refrigerant. They also said they did a nitrogen leak test, backflush and vacuum before charging.

It's behaviour is certainly indicitave of insuffient cooling of the low stage.. it gradually builds up pressure until hitting the limit switch, which pulses for 1/2-1 second until the pressure drops, and so on.

Interestingly, we have another chamber with similar symptoms. This one developed gradually over time rather than suddenly. They came and looked at it while this one was out for service. The low stage discharge pressure is about 280psi and suction is about 6. Guy wouldn't check the pressures on the high stage, saying it had nothing to do with the problem.. in this instance, they say there is a partial blockage in the capillary tube, as indicated by the high discharge and low suction pressures. I am unconvinced, seems like maybe it's the same problem, and maybe the low side is overcharged..? anyway, that's another question :rolleyes:

anyone know any cascade refrigeration expert repair shops in the Toronto/Vaughan area? :)

Hi:)
if the suction pressure is lower than normal, he may well be correct. Inadequate condensing of the low stage would usually mean high low stage suction also, unless hot gas was going thru a capillary instaed of liquid, this would cause low suction high head. Measurement of the liquid line temps and a look thru the liquid line sightglass would show this.

Kind Regards. Andy:)

jmcclellan
15-05-2006, 04:15 PM
measuring all the line temps on the first system (one with replaced HS comp) now.. in the process of stabilizing as we speak. using a 6-ch digital chart recorder with type-k fine wire tc's, so should have a pretty graph to show..

these don't have sight glasses on them, they are only 0.5HP Hermetic

jmcclellan
15-05-2006, 06:59 PM
Ok, here are the temps. Hope the picture attachments work.. again, this is a 0.5x0.5 1.2cu-ft chamber.

Ch1: chamber temp
Ch2: HS suction temp
Ch3: HS discharge temp
Ch4: LS suction temp
Ch5: LS discharge temp

http://exchange.itselectronics.com/comptemps.jpg


http://exchange.itselectronics.com/dataviewer.jpg

jmcclellan
16-05-2006, 10:05 PM
so yesterday it became apparant that the chamber could no longer even hold -55 setpoint, even empty.. the cycle time on the pressure switch was slowly increasing, as was the temp. so I left it run, set to -70 to see what it would do.. gradually the temp rose to about -51 (had acheived about -57) running full tilt. Until suddenly, while I was looking at it going 'hmm' it dipped for no apparant reason, almost like something let go and started working again.. but still not well.. check it out;

http://exchange.itselectronics.com/dataviewer2.jpg

Rob S
16-07-2006, 06:35 AM
Either the LS compressor has an issue or the charge is wrong. I have a feeling its the compressor. Very common to have to replace both compressors.

HS TXV is flooding... which caused the problem in the first place.

donato
16-07-2006, 08:16 AM
so did they charge 404a liquid or gas. I cannot find in your replies

jmcclellan
17-07-2006, 02:59 PM
so did they charge 404a liquid or gas. I cannot find in your replies

This I don't know for sure, when he was here his can was vertical (upright) but I guess it depends which valve he was using right? This was a couple months ago now.

jmcclellan
17-07-2006, 03:09 PM
Either the LS compressor has an issue or the charge is wrong. I have a feeling its the compressor. Very common to have to replace both compressors.

HS TXV is flooding... which caused the problem in the first place.

couple new pieces of info for you then..

the system is capillary tubes, both stages.

during extended run (ie down ramping, live load operation) LS compressor periodically cuts out on it's internal overload protection (thermal/current?) for ~2 minutes then comes back on.

Any more ideas? seems something is getting worse in that LS.. when I took those thermal graphs, it didn't have this cutting out issue, it would have been obvious in the graphs if only one stage cut out. when both stages cut out the controller has stopped calling for cooling. Of course with it like this the chamber is useless now.

I had given up on any more replies from this thread, so I would greatly appreciate any insights anyone might have..

Thanks

Rob S
19-07-2006, 06:50 AM
Wish you were in WA.... Love to come look at it.

If the LS is cutting out on it's overload.. then it is most likely overheating. What happens is when the HS compressor begins to fail. It's lack of cooling the LS system down puts extra strain on the LS compressor. With out getting to techy on ya... you end up with very high. I mean very high discharge temperatures, which burns out the LS compressors discharge valves or piston rings. Which causes the compressor to recirculate some of the hot gas back into the cylinder. And the overheating the compressor.

I work on alot of chambers.... Ultra-low temperature refrigeration is the "Dark Arts" of refrigeration. Only about 3% of all the techs have every worked on one.

Peter_1
19-07-2006, 07:57 AM
Mmmmmmmm, thought it was 4.2% :p

wambat
19-07-2006, 09:38 AM
I believe that at 75*F condensing temperature on the low side, that exceeds the critical pressure for suva 95 and cannot condense the hot gas to liquid. This in effect would starve the back pressure on the lo side.The problem seems to lie in the hi side not able to lower the low side head pressure.

jmcclellan
19-07-2006, 03:47 PM
Wish you were in WA.... Love to come look at it.

If the LS is cutting out on it's overload.. then it is most likely overheating. What happens is when the HS compressor begins to fail. It's lack of cooling the LS system down puts extra strain on the LS compressor. With out getting to techy on ya... you end up with very high. I mean very high discharge temperatures, which burns out the LS compressors discharge valves or piston rings. Which causes the compressor to recirculate some of the hot gas back into the cylinder. And the overheating the compressor.

I work on alot of chambers.... Ultra-low temperature refrigeration is the "Dark Arts" of refrigeration. Only about 3% of all the techs have every worked on one.

That seems to make sense.. I can tell you, that LS compressor is DAMN hot. so basically when you lose the HS comp, your LS is fukered too? :eek: So why ever do a HS-only compressor job?

The more I deal with refrigeration, the more I learn, and the more I think this repair house has no clue.. the last repair job, *I* had to tell *them* what the problem was.. (stuck artificial loading valve) cause they couldn't figure it out. When brazing stuff, anything within 3-4" of the joint gets torched too. Insulation doesn't get put back around pipes. Panels come back with missing screws. I can say for sure, I won't be calling them again.. but I need somewhere for service..

anyone know a RELIABLE cascade refrigeration service place in the Toronto/Vaughan area?!?

jmcclellan
19-07-2006, 03:58 PM
PS.. dilemma now.. these chambers cost about $4700 USD NEW, $5320 CAD. We just spent $2400 replacing the HS comp. Of course most of that was labor and incidentals. If we now have to go and change the LS comp it'll prolly be like another $2400. We could have almost had a freakin new one by then. If they had changed them both in the first place, it might have just been $3000 or something like that I don't know, but a lot less than $2400+2400.

Advice from you experts; should they have KNOWN the LS compressor needed replacing as well? Should we beat them up over this, rake them over the coals to get them to do the second compressor replacement for 'free' (ie. parts-only cost, no labor is what I would expect, or maybe just a couple hours)

This thing is useless right now, we either have to get it repaired MORE, or trash/sell it and buy a new one. Either one sucks given what we just spent on it. Speaking of which, is $2400 CAD reasonable for a HS compressor job (all inclusive total, labor, materials, taxes, delivery)

Thanks for your input

US Iceman
19-07-2006, 04:35 PM
I have been following this thread. Rob S has pretty much nailed your problem:



Ultra-low temperature refrigeration is the "Dark Arts" of refrigeration


A lot of people who think they understand refrigeration, don't. After reading your description of their work:



When brazing stuff, anything within 3-4" of the joint gets torched too. Insulation doesn't get put back around pipes. Panels come back with missing screws.


Their abilities seem a little suspect.



should they have KNOWN the LS compressor needed replacing as well?


Maybe the LS compressor was not at fault (originally). It may be now though, with the problems experienced by the HS.

Cascade systems can be rather complicated, much like a two-stage system can be. While they are both "refrigeration" systems, they can be quite a bit different than a supermarket system or beer cooler. Even though they use most of the same components.

With some systems you have to look for problems that are caused by another problem. This is what I call cascading failures (no pun intended). One event leads to another event, which leads to another event. If all of the problems are not dealt with, changing the obvious parts does not correct anything.

Now, you are in a difficult situation of having spent money to get this far and need to spend more. And, maybe by the time your are finished the old box will cost more than a new one.

Personally, I would not be too fond of letting the same people work on it again. But that's just me.

Rob S
20-07-2006, 01:19 AM
I will say this. If I'm correct. And even if I'm not they should have informed you the LS compressor could go out. Or may not alrdy be working correctly.

How it works in my part of the world...
All small medical cascade freezers. If I have do anything mechanical or refrigerant related to the HS. I tell them up from. There is an 80% chance the LS comp has been weaken by this. Which will only mean replacing the LS comp. Then I urg them to just buy a new freezer. 50% of the time when its all said and done.. they just paided for a new one.

Now large Envo chambers (what I usually work on) its a diff market. Becuase the chambers alone cost 30,000 to 120,000 dollars.