Cmpound cooling

[jpsmith1cm]

No, not those goofy carlyle pumps, this is an externally compounded rack...

Got 6, 10 horsepower carlyle 06Ds pumping into 3 more 10 horse 06Ds.

Kicked our azzes all weekend long until I was able to get there an stop the bleeding, but it's still an UGLY piece of equipment.

It is a early 90s vintage Hussmann Rack, R-22 low temp (-30 SST)

Sorry I didn't grab model/serial stuff, but at midnight on a Sunday after 15 hours work, little details like that tend to get ignored.

Anyone got any good reading info on this stuff. Read Hussmann's manual and there is something like 2 WHOLE paragraphs on it.

I understand the basics of what is happening in the compound process, looking for some good troubleshooting stuff if it's available.

[skpkey9]

I'm certain you can find the information you're looking for in the Green Book (Carlyle book) If you don't have it let me know and I can post it for you....

Hope this helps

[Phase Loss]

We have a few 2 stage externally compounded racks left. I don't work on them enough to be much use, but I do know.

interstage desuperheating is important

the low stage (booster stage) will not be able to run without the high stage. and should be electrically interlocked to prevent it from trying.

and when the racks go down, you must be very careful upon restart or else valve plates will get smashed.

If you are pumping the racks down for service, you need to pump down low stage first, then high stage.

If you are restarting racks, you start high stage, then low stage.

this style of 2 stage compression was big in industrial, i may be able to dig up some files that will relate somewhat to supermarket concepts.

[Dowadudda]

hussmans futile attempt to save energy by reducing compression ratio. Those racks are a big pile of $hit.

There is nothing to know other than what you do know now. All that is, is trying to get to saturated temp with two separate pumps. Nothing scary. But those are a pile of $hit, did I mention that? LOL.

[dirtyboy103us]

i had 2 of these beast with 9r's
and working subcooler is a must to keep the discharge/suction line to the second stage cool helps keep presure up also
once they are set they just run and run with very little trouble but all it takes is a little thing out of adjustment and boom its on fire again

[icemeister]

There never was much written up on the compound 2-stage systems as far as manuals and such. What did/does exist is also sadly lacking in any troubleshooting help.

If you want some interesting reading, I've attached copies of Hussmann's Thermoficient TD System manual from back in the 1980's (part 1 & 2). These were direct drive Carlyles, but the principle is basically the same.

What sort of troubles were you having? We're glad to help and/or learn.

[icemeister]

hussmans futile attempt to save energy by reducing compression ratio. Those racks are a big pile of $hit.

There is nothing to know other than what you do know now. All that is, is trying to get to saturated temp with two separate pumps. Nothing scary. But those are a pile of $hit, did I mention that? LOL.

If I remember correctly, the first to push the compound two-stage systems was Phoenix Refrigeration...back around 1989-1990 when Grant Brown was running the show. Hussmann-Atlanta jumped on the band wagon shortly after, along with just about everybody else at that time.

I was at the Food Marketing Institute (FMI) show in Chicago in 1991 and every rack manufacturer was showing off their "new" R22 low temp systems. As a result, most major chains were going all out with R22.

This rush to the market did cause some major disasters...one of which I believe involved Copeland Discus compressors running as boosters, or low stage pumps. As I recall, there were a rash of valve failures which were apparently due to valve flutter or float at very low pressure differentials such as when a LT system came out of defrost.

The solution was a redesign of the valves, essentially making them stronger to stand up to such conditions. In the interim, I know Phoenix was running around the country installing discharge pressure regulators on the interstage to keep the low stage pressure difference up.

At the time, the Copeland valve problem reminded me of what I had learned at Dunham-Bush some 10 years before. They had many years experience with two-stage systems and found (also the hard way) back in the 1960's that the booster compressors need heavier valve reeds, basically for the same reason.

Those who fail to learn from history are doomed to repeat it.

[jpsmith1cm]

There never was much written up on the compound 2-stage systems as far as manuals and such. What did/does exist is also sadly lacking in any troubleshooting help.

If you want some interesting reading, I've attached copies of Hussmann's Thermoficient TD System manual from back in the 1980's (part 1 & 2). These were direct drive Carlyles, but the principle is basically the same.

What sort of troubles were you having? We're glad to help and/or learn.

Thank you. I was actually warned about Hussmann's TD system.

The problem, as best as I know it so far, is that there were 3 (of 6) condenser fans down.

This caused the high stage compressors to overheat and go down on internal t-stat.

That then caused the low stage pumps to lock out.


The guys that went out before me didn't look at the condenser, only looked at the rack itself.

One guy just added oil, one other guy shut down a compressor that he said was blowing through (I couldn't prove it) and then later shut down one stage of interstage desuperheating.

In the cold light of day, those things clearly contributed to the problem rather than resolving it.

Compound the problem with a rack controller that none of the guys that went on the calls understood (NC-25) and that they couldn't really access even if they did understand it, and you have a recipe for a disaster, which is what I had.


I'll review that info on the TD system, but mostly what I'm looking for is pressure and temperature ranges for the interstage. Where does that need to be?

Also, you mentioned in your other post about a regulator in the interstage that prevents valve flutter. Can you give me a setting or a range where that needs to be? Assuming that it isn't in the TD info, that is.

[icemeister]

Thank you. I was actually warned about Hussmann's TD system.

The problem, as best as I know it so far, is that there were 3 (of 6) condenser fans down.

This caused the high stage compressors to overheat and go down on internal t-stat.

That then caused the low stage pumps to lock out.


The guys that went out before me didn't look at the condenser, only looked at the rack itself.

One guy just added oil, one other guy shut down a compressor that he said was blowing through (I couldn't prove it) and then later shut down one stage of interstage desuperheating.

In the cold light of day, those things clearly contributed to the problem rather than resolving it.

Compound the problem with a rack controller that none of the guys that went on the calls understood (NC-25) and that they couldn't really access even if they did understand it, and you have a recipe for a disaster, which is what I had.


I'll review that info on the TD system, but mostly what I'm looking for is pressure and temperature ranges for the interstage. Where does that need to be?

Also, you mentioned in your other post about a regulator in the interstage that prevents valve flutter. Can you give me a setting or a range where that needs to be? Assuming that it isn't in the TD info, that is.

An externally compounded system can be quite intimidating to the uninitiated...and it appears that have been the case there. Thanks for detailing that fateful night.

For a market rack, the interstage pressure is basically whatever is required for your high stage, or your lowest medium temp circuit. Set up the MT just like you would for a normal system.

As an aside, if this were a straight low temp system like for a freezer warehouse with no MT loads at interstage pressure, the ideal interstage pressure is one which gives you equal compression ratios for both the low and high stage. To determine this, there's a simple formula:

P interstage = SQRT [P high x P low]

(NOTE: Pressures are absolute pressures...ie, PSIA)

That pressure regulator was a differential regulator I think. I never got directly involved in setting one up, but as I understand it, the setting should be just below the normal low stage pressure differential. So if you have a -30ºF low stage suction (5 psig) and a +20ºF interstage (43 psig) the difference would be 38 psi. The regulator would probably be set around 33 psid or so.

[jpsmith1cm]

Ice,

This is a low temp only rack, so I'll grab that formula and apply it next time I'm there.

As far as the regulator, this is an A82 regulator, so I'm thinking holdback and not differential.

Currently, it's set to hold about 75# on the low stage compressors with a 5-7# suction. The high stage compressors are running a 55 ish suction (yeah, it floats around a little).

What concerned me the most, though, is the high stage suction temperature.

It would, for the most part, settle down around 50-60 degrees, maybe as high as 75 degrees. Not concerning.

On some occasions, and I can't determine yet, WHY, the interstage temperature runs up to about 150-160 with a corresponding rise in the actual discharge temperature.

I think that this is another contributing factor to the failure. Maybe an intermittent problem with the desuperheater??? Both are on the whole time while the temp jumps and drops back down.

[Dowadudda]

man, I do not miss this at all. Glad I lived that life. Can say I did it, but so happy I am out of it.

Seriously. I am so relaxed these days. Gosh.

[icemeister]

A plain A82 is an inlet pressure regulator. (The A82L would be a differential regulator.)

With a setting of 75# it would seem this valve is always holding back....and that would explain the wild ride on the high stage suction pressure. With that A82 in there, I doubt you could make the formula work. That may be best for another time.

I don't think the A82 is doing any particular harm at that setting (other than costing extra horsepower), so it's probably better to focus on the desuperheater performance and control.

What do this have for a desuperheater/subcooler or whatever? Is it's suction piped to interstage after the A82 or before? If it's after the A82, the fluctuating high stage suction will adversely affect the control of the subcooler/desuperheater.

Just a thought.

[jpsmith1cm]

The subcooler and desuperheater feed into the discharge line of the low temp compressors. Before the A8 valve.

Subcooler is a plate type HX and there are 2 "L1" type desuperheater TXVs controlled by interstage suction temperature. Stage #1 comes on above 45 and stage #2 comes on at 55 or 60, I think. I can't remember exact setpoints because I didn't write them down. Basically, the #1 desuperheater is never going to shut off and the #2 will very rarely shut off.


Now, the A8 valve was set to manually open when we got there. One of the guys kind of fixated on it until I told him to set the GD thing. I played around with it, but couldn't notice a significant effect on the system that I could determine. Given what you've told me, I think I'm going to go back to manually open. Thing that bugs me is that, even in manual open, there is about 23# across the valve. Maybe need to operate on it...