bashing compressors

[doesitall]

I've got a Blanchard condensing unit (located outdoors where ambient can go from -40 to +100f) with 2-27 hp copeland discus compressors and 3 evaps. I've replaced 1 compressor about 3 months ago and got a call-back saying it wasn't running. Checked and found all fuses blown (600v 3phase) so I isolate the compressor and open it up and find about a litre (just under a pint for my American friends) of oil in the heads, suction reeds completely smashed and discharge valves cracked (hard to believe!!) (will post pics once I get them downloaded from the camera) Obviously, I've got an oil problem, but can't quite figure out how to fix it. I've got separators on both compressors, a common oil reservoir, and pot feeders on the compressors. Reservoir maintains about half-full and pot feeders maintain about 1/2 glass on the compressors. I'm trying to
maintain about -25f space temp (freezing weenies) and it's a R409 system. This system is in addition to a separate 120 H.P. rack with 4 evaps which also cools the same space. I"m not sure what's happening on defrost as there's an aftermarket digital control system which I haven't figured out how to operate yet (I've inherited this abortion) I personally think I'm getting migration on the off cycle, but other than babysitting it, hard to tell. if I'm right, I'd like to pipe some discharge gas through my suction accumulator instead of the liquid that is currently piped through. Any bad implications to what I'm suggesting? Please help....i'm scratching my noggin as we speak!! Also, after re-reading the post, I guess it's important to note that it's a hot-gas defrost. Please help...any more info required, I got it.....

[Edited by doesitall on 02-23-2005 at 02:23 PM]

[stef]

I know that the compressor PUMPED liquid and broke all the valves. Then It ran until it died. That's the reason why you have a pint of oil on top of the head.

With the info you gave, I cannot tell you where to go. BUT

Run the system for at least 5 hours. Monitor superheat, subcooling.

Then force it to defrost and monitor superheat (very closely)

After running it through the test. If possible, turn it off overnight. Then start it back up again and monitor its superheat to see if it has freon migration.

Sit down and stare at it. Visual an ounce of freon is traveling thoughtout the system. I am sure you will catch it.

Have fun

[doesitall]

thanks for the input....I'm having a hard time having fun with it...customer is pissed!! Can't say I blame him as compressor is under warranty but crane and labor not. Gonna cost him for me to baby-sit and like most owners, can't see paying out $70./hr for me to take readings and no matter how much you talk to them, it's hard for them to open their wallets. Seems like most owners are like my wife and home improvement programs....we can "just" do this or that so easily!! NO PROB!! (anyone else have that problem??) I'll post the readings I took on start-up for sub-cooling and superheat when I post the pics.

[smilies]

Regarding your question about piping discharge gas through the accumulator, I would not. I wonder if the rack capacity includes the subcooling through that loop.

Besides, you'd only be band aiding, not fixing the real problem.

[rimek]

Hot gas defrost? Is there any thermobank or loading for that? How long for defrost, how many/day? Any working crankcase heat?

[kerndt]

Sounds like your compressors are shutting down without pumping down your system. Meaning either your defrost or temperature controllers are not pumping down the system before the compressors are shutdown, or some how your compressors are cycling off, on some auto resetting controls intermittenly. When any of the above happens your evaporators will flood with liquid and you will get a big slug of liquid back to your compressor. This only has to happen once or twice before it breaks.

The compressor should always operate until the system pumps down and cycles off on low pressure control. Make sure that the setting on your low pressure control is low enough, meaning below normal operating pressure. I seen some systems cycle off on the low pressure switch and the temperature control still calling for cooling. Again if these compressors cycle off without controlling the liquid flow to the evaporators, you will have heavy slugging of liquid back to the compressors when the compressors restart. Also the compressor that failed might be the 1st compressor on the system that starts, there must be some kind of delay on starting these compressors.

One last thing is make sure the compressors can maintain the pump down. Meaning during defrost cycles and off cycles of the temperature control, that the compressors are able to cycle on if suction pressure rises, and make sure your cut-in pressure is not set to high, you want your compressors to restart very soon after your liquid line opens, not allowing your evaporators to flood with liquid.

[smilies]

Can't pump down; hot gas defrost.

[kerndt]

thanks jerry, i missed the last sentence about the hot gas defrost. but everything else still applies. hot gas defrost -20 ambeint thats was a good idea?

[icemeister]

A few things about this bother me:

MY CONCERNS:
1. R409A as in an old R12 retrofit job??? At -25ºF box temperature? That's going to put your suction pressure off the bottom of the charts! Is it an old R502 system retro'd to R408A maybe? That would make more sense.

2. It's an outdoor system. If those units aren't protected somehow from the extremely low ambients there could be lots of funky stuff happening that could potentially take out a pump or two...with refrigerant migration at the top of the list. Factory crankcase heaters just aren't enough. I've seen heat tapes wrapped around just about everything including the low pressure controls just to stay one step ahead of the cold.

3. Since it's mentioned that there's another big rack on this box and they're freezing product, I assume that it's a blast freezer application. Not much is said about the lowsides on this job except for the HG defrost aspect, but I suspect that it's a production freezer where the load is huge when the freezing line is running but drops off to near nothing afterward when they only need to maintain temperature. If all the evaps are always on line trying to operate at both load extremes, you're probably going to see some serious floodback conditions during those low load periods (like at night). The TXVs just can't control well over such a range. They're sized for the freezing load.....not the holding load which could be less than 10% of the max summertime design load.

4. How big is that accumulator and what does it have for oil return? Typically accumulators are very undersized to provide any real protection from a major slug of refrigerant (or oil) coming down the line. Most are merely bumps in the suction line.

ACCUMULATOR COMMENT:
Piping the discharge through the accumulator boilout coil isn't a good idea. With liquid to the coil it is actually a suction/liquid heat exchanger which can be beneficial to system efficiency. If there's discharge gas going to it the heat you dump in there that was supposed to be rejected at the condenser now becomes part of you cooling load........similar to what a hot gas bypass would do.

SIDE NOTE:
This job reminds me of Zero's post from a short while back. He's wrestling with a 7 compressor Bohn rack also on a blast freezer. He's losing pumps regularly as well.

[doesitall]

Hi guys, and thanks for all the quick feedback!! I posted without reading my post...my bad!! It's r404, not 409....apparently a 502 changeover. I'm still waiting on a compressor for the Blanchard, and got a call today for the rack that is also doing the blast-freezer. Another compressor bites the dust!! Opened the head...cylinder full of oil and valves also gone. Head, suction, super-heat and sub-cooling all acceptable...found it strange though that with a common suction header, all the take-offs were piped off the bottom. Haven't done much with 502...does it have bad oil return hence piping off the bottom of suction header? I think I'm getting oil carry-over when-ever compressor is off. Am having an engineer look at it (I know....wussy way out of it, but customer a bit irate!!) on Monday. Again thanks for the input and I'll post once Mr. Engineer has his input. I think icemeister has hit it on the button regarding low-load, off-cycle migration. Accumulator is pretty large, shouldn't have any trouble flashing gas off it's about 5ft tall and 3 ft rnd.

[Edited by doesitall on 02-24-2005 at 04:12 PM]

[jimp]

I've seen nothing but bad when changing a old 502 system to 404, I would have used 408 and not worried about the mineral oil mixing with the POE oil.

[basser]

I agree with you and icemeister on the migration. If the suction reeds are broken look for an off cycle migration problem. Your accumulator will not protect against off cycle refrigerant migration. The oil on top of the pistons MAY be the result of the failure and not the cause.

Please post what the engineer says so we can all see and learn.

Jerrycoolsaz, good to meet you last week. I hope our paths cross again. If I can ever be of assistance please don't hesitate to contact me. Basser

[doesitall]

Good point Basser!! Never thought of it being the result as opposed to the cause....opens up a new can of worms though. I most certainly will post the results....I'm heading there tomorrow to check on things...I'll swallow the time just to learn.



Cheers