Pump down - a way to diagnose bad valves

[r2]

Recipocating compressors you close the liguid service valve and pump down to a vacuum, shut the unit off and if it holds valves are good and if not they're bad.

scrolls tend to slowly equalize.
i'm not very familiar with the desing of the scroll compressor so, what kind of things would you check to diagnose internal mechanical problems?

[r2]

at this point anything would help.
how about compression ratios?
amp draw?
discharge temp?

[maxster]

if their is a suction service valve pump the compressor down from there cans,semi hermetics don' need to go into a deep vac or the oip will start to move out.on the scrolls the seal on the orbiting plate to the stationary is the critical point....DO NOT pump them into a vac at any time could ark the winding sensors 10 PSI to 0 PSI and isolate it.check copelands site http://www.copeland-corp.com/cp_rf/prod_sol/cp_rf_products_scroll.htm for diagrams of scrolls

[Diceman]

Originally posted by r2
Recipocating compressors you close the liguid service valve and pump down to a vacuum, shut the unit off and if it holds valves are good and if not they're bad.




Like Max said be careful, but if you shut the liquid valve and are sure it is closed and no HGB or whatever and watch the suction it should pull down deep below 0 pretty quick, if it doesn't, bad valves.

[pcscott]

Still figuring the quote system. Thank you maxter ,learned somthing new.

[basser]

Pumpdown is not a sufficient test for "bad valves". Use amp draw to verify if the compressor is pumping correctly. Do a search many threads on this subject.

Think about it, you close the "liquid service valve and pump into a vacuum". How long is the liquid line and how much time will it take to pump the liquid refrigerant from the liquid line into the receiver and/or condenser?

Let's think about a 6 cylinder compressor. One of the cylinders has broken suction reeds, so we loose 1/6 of our pumping rate. That means we won't pump down as fast (but I would question what is a good time for pumpdown). However, it will still pull down to the same amount of vacuum, as long as the discharge reeds are not damaged. By amp draw I would expect to loose about 1/6 of proper amp draw.

Once the compressor is pumped down, power the compressor off. Watch the low side gauge. If the gauge rises in pressure, what does that indicate (assuming the service valve is holding)? Not broken suction reeds! It would indicate broken discharge reeds, but not suction. Suction reeds are not the seal point. The discharge reeds are the reeds that are designed to separate the pressures.

For troubleshooting first look at amps, then you may decide to pump it down and look for leak back, next take a few minutes to pull the heads. If one head is hotter then the others start with that one first.

Hope this helps, Basser

[basser]

On early Copeland scrolls, if you tried to pump down you would short the compressor out for reasons stated above. On later versions of Copeland scrolls they have an internal protective device that does not allow a deep vacuum to occur. The depth of the suction pressure would be restricted by the head pressure, compression ratio limits. You would still use amp draw for troubleshooting.

[Paul Pippin]

the best way to determine bad valve was mentioned by checking amp draw. Each compressor manufacturer should be able to get you a curve on each compressor ( copeland has them on their website ) that curve will give you different operating conditions for that compressor and tell you what your amps and btu output should be. That is the most accurate way to determine what is going on inside that compressor.

[r2]

thank you for all the good points. that particular compressor has an amp draw of 6.4 of 14.1 rla. 3ph.
38ckc048 carrier. '98. 72 inside with 70 outside.
74/143 psig, 20 superheat. i know everybody's going to want to know the subcool but I don't have it.

it had a leak in it and had been pumping air for a while.

i still don't have a good understanding on how to diagnose valve problems but i just want to ask, how do you get these curves in the middle of the day? that really seems to be the only sure fire way to tell if a compressor is performing to it's capacity or not.

can't find them from the copeland web site either...can somebody post a link?

[absrbrtek]

It will be hard to determine a compressor problem with that kind of a load. If the compressor is bad you won't see it until the first hot day. This following has worked for me many times over the years.

Heres what I normaly try, it takes maybe 10 minutes and some plastic or trash bags. If the compressor has an unloader make sure that its loaded up. Get the head pressure up to between 250# to 300# by blocking off the condenser and/or turning off condenser fan motors. Watch the suction pressure and compressor amperage. If the suction pressure starts rising as the head pressure goes up and doesnt return to a normal suction pressure, I am suspect of the compressor. IE: 275# head with 80# to 100# or higher suction pressure.




Originally posted by r2
thank you for all the good points. that particular compressor has an amp draw of 6.4 of 14.1 rla. 3ph.
38ckc048 carrier. '98. 72 inside with 70 outside.
74/143 psig, 20 superheat. i know everybody's going to want to know the subcool but I don't have it.

it had a leak in it and had been pumping air for a while.

i still don't have a good understanding on how to diagnose valve problems but i just want to ask, how do you get these curves in the middle of the day? that really seems to be the only sure fire way to tell if a compressor is performing to it's capacity or not.

can't find them from the copeland web site either...can somebody post a link?

[r2]

point well taken. simulate desing temperatures and you'll get more accurate readings.

so if 80 - 90 and above suction pressures with 75 indoor temps and 250 ll psig. tell you that you might have a problem with your compressor. your sh would be high and sc low?

also heard about grabbing the suction line after shutting off compressor and feeling if it gets hot, because of valve leakage. does this work for scrolls?

how about the claim that scrolls seldom just lose capacity and if they'll beak they'll break completely.

one more, comp curves are mostly used with large semihermetic compressors to diagnose individual discharge reeds.

i'm trying to learn as much as i can on this threat and i'm sure there's more info that can fit in here but every opinion is appreciated.

[atmosphere]

I block my coils with cottonwoods instead of bags.
Works very well.

[speer007]

I have actually seen a scroll compressor torn apart, and how it works is there is a stationary scroll and a orbiting scroll. The orbiting action draws the gas into the chamber as the scroll rotates the gas opening is sealed off because of the close clearance between the stationary and rotating, and as it keeps turning the gas gets caught in pockets between the two scrolls, this is where the compression occurs, then the gas is forced into a smaller pocket at the center and then is discharged. So a scroll compressor actually doesnt have valves. As far as the pressures go, i'm not quite sure, i have only experienced the scrolls pulling high amps from bearing wear. I have heard if you run them backwards they will not make as high of discharge pressure and pull low amps, but i dont think it would run long enough for you to even check pressures.

[basser]

[QUOTE]Originally posted by r2
"also heard about grabbing the suction line after shutting off compressor and feeling if it gets hot, because of valve leakage. does this work for scrolls?"

If the discharge reeds are allowing gas past them and heat is present then possibly, yes. On scrolls, there is a discharge check valve to hold out the discharge pressure. If the check valve leaks then the same scenario would apply here (amp draw would not be affected).

"how about the claim that scrolls seldom just lose capacity and if they'll beak they'll break completely."

If a scroll member breaks, then the compressor will not compress at all.

"one more, comp curves are mostly used with large semihermetic compressors to diagnose individual discharge reeds."

Compressor curves can and should be used on any size of compressor.

Earlier you stated you had 6.4 amps of 14.4 RLA. Remember operating amps is not a percentage of the RLA. 6.4 amps may very be proper amp draw, depending on what your pressures are at that period of time.


[Edited by basser on 01-30-2006 at 12:34 PM]

[r2]

great info guys, i got to go to a compressor class today and asked them the same questions. like somebody mentioned earlier, shouldn't pump scroll into a vacuum and to properly diagnose capacity problems should run the diagnostics tool....here's the link: emersonclimatecontractor.com
still downloading so no opinions on it yet.
just go a head and register.

and about running the scroll backwards, it will pull low amps and not pump at all. doesent sound that bad either.

[joeywpittman]

link to download
Copeland´s Compressor Performance Calculator
https://opi.copeland-corp.com/documents/clc/SetupCpCalcFull.EXE