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  1. #1
    Join Date
    Jun 2004
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    182
    I started this as a new thread so this procedure wouldn't get lost in lynn rodenmayer's post of 7/14/04. I think it is important to understand. On a Copeland refrigerant cooled compressor the following can be used to determine piston blow by.

    The oil check valve is NOT spring loaded. It is either a free floating ball or flat disc operated by the direction of oil flow. Any higher pressure in the crankcase is bad news, the ½ pound would be correct, and yes a ½ pound would be difficult to see.

    To check; put a gauge on the crankcase and another on the suction service valve. Let's say they both read 20psi. Start closing the SUCTION service valve. When the pressures begin to drop (pumping down), now close the suction slowing and watch both gauges. They should pull down together, all the way down, how ever far it will go.

    Let's say at 5psi the crankcase doesn't drop any more and the suction pulls lower. That is the point (5psi) where the crankcase pressure is overcoming the internal VENT valve and the oil check valve will now close. The check valve closes because of the reverse flow of oil direction.

    [Edited by basser on 08-09-2004 at 08:43 AM]

  2. #2
    In some Copeland models there is hole (always open) that provides a vent between the crankcase and motor. It is high on the firewall, and does not equalize oil level, only pressure.

  3. #3
    Join Date
    Jun 2004
    Posts
    182
    That hole is your crankcase vent. The oil check valve is located directly below it under the center bearing. The same procedure can be used to check these compressors also.

    Call me 2813.

  4. #4
    No contradiction intended- just meant to add there may be a gas vent with no check valve. There must be a vent of some sort as the oil check may be under oil on both sides and not allow gas to vent.

  5. #5
    Join Date
    Aug 2004
    Location
    Kansas
    Posts
    133

    To Basser and ECT

    OK for a blowby test we throttle the suction and watch for a difference in psi between suction valve and crankcase. How much is maximum psi differential recommended by Copeland indicating replacement?
    I have also wondered how many gallons or what percentage of crankcase oil capacity actually goes out of the compressor and has to be returned through the system on say an average new 5 hp as I understand that a high percentage of oil to refrigerant raises the refrigerant boiling point also

  6. #6
    Join Date
    Jun 2004
    Posts
    182
    If the crankcase is any amount higher then the suction, that is bad news. (That is where the ½ psi comes from.)That indicates blowby around the piston and the result will be an oil safety control trip. This is why you want to zero out your gauges as close as you possibliy can and use two low side gauges because of the smaller scale.

    As far as the circulation rate of the oil, it is approximately 1%. What ever amount of oil leaves the compressor must be returned. If the compressor pumps out more then is returning, that means you have excess oil out in the sytem somewhere. It will probably end up in the evaporator. Now your system may not achieve the desired temperature. If enough oil logs out in the system you starve the compressor and again the result should be an oil safety trip.

    System oil capacity depends on the length of runs (piping).
    After original start up be certain the compressor oil level is satisfactory. Semi-hermetics will have a sight glass.

    Raising the boiling point. Theoretically your point is probably true, because the refrigerant would have to boil through the oil. I really doubt if you could notice the difference. Think of oil in an evap. The oil reduces heat transfer and that is one reason you will reduce performance.

    Hope this helps.


  7. #7
    Join Date
    Jul 2004
    Location
    NJ
    Posts
    1,801
    Hello Basser.A while ago I went on a call.Oil safety was tripping.Barely seeing through sight glass the oil was very low.By the way this was a 17 door frozen food case.I believe it was 7.5 HP copelamatic.408a.Hot gas defrost.I added about a gallon of c3 oil given to me by my boss.couldn`t change the level.Then I ended up opening the 4 balanced port expansion valves all the way ,because they were starving the evaporator.Eventough it improved the situation dramatically I couldn`t read the temp.at the outlets of evaporators it was still high.So I concluded the excess oil in the system was hindering the heat transfer in the evaporator.I didn`t change the TXVs yet ( I think my boss ordered them and it takes time to get them after I opposed to install regular TXVs)Costumor is ok with 0*F for now.I am still apprehensive even if I put new TXVs that I may have oil problem.with the info I could provide anything you could suggest?
    thanks.

  8. #8
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    You might want to start a new thread, altan.

    This would be a good case to discuss.

    Do you have all of the particulars, such as pressures, temps, etc?

    I'll start off by saying that "barely seeing through the sightglass" is a bad situation that is easily remedied, by depressurizing the crankcase, removing the 3 bolts that hold the glass on, and wiping the inside of the sightglass with a rag dampened with Virginia #10 solvent, then reinstalling it. It would also behoove you at that time to remove and clean the oil sump strainer, and, if so equipped, the Sentronic sensor. A fresh charge of oil could then be added to 1/2 sightglass, and you start anew. Top it off with drier replacement, and you're on your way to Happytown.

    One more thing, if you added an entire gallon of oil to a 7.5 HP Cope, you most certainly should've seen a significant change in oil level!

    Now, you say opened the TXVs all the way up. Let me ask you this:

    Did you pull and inspect/clean the TXV inlet strainers before doing this? This should be done before any TXV adjustment is ever made, on a case that is starving for refrigerant. Often, the system crap (That you saw staining the oil sightglass on that pump) collects in these 100 mesh strainers, and restrict the flow of refrigerant. Opening the superheat stem all the way sometimes fools you into thinking you've made it better, but you didn't...

    Those TXVs need a solid column of liquid at their inlets to operate properly. Clogged strainers prevent a solid column of liquid from entering the valve inlet.

    It would take a quantity of oil on a rather grand scale to oil log 38' of low temp evaps! I'm talking many gallons. Also, oil logging with hot gas defrost cases is relatively rare.


    I will say that I prefer a synthetic alkylate (Alkylbenzene) like Zerol 200TD with the HCFCs, but often we just have to deal with what we're handed...



    [Edited by condenseddave on 09-18-2004 at 10:39 PM]

  9. #9
    Join Date
    Jul 2004
    Location
    NJ
    Posts
    1,801
    Originally posted by condenseddave
    You might want to start a new thread, altan.

    This would be a good case to discuss.

    Do you have all of the particulars, such as pressures, temps, etc?

    [Edited by condenseddave on 09-18-2004 at 10:39 PM]
    408a.head 265-270 psi,water outlet 83*F valve all the way open.,back 16-18 psi,box; two sections 0-5*F,one section 16-18*F,one section 25-30*F couldn`t take the stem cap off to open valve, was leaking too strong.
    didn`t check the strainers because didn`t wanna warm the box more it was fully loaded.but on a balanced port txv thought because it responded to adjustment might not be the strainer?didn`t wanna mingle with sight glass was affraid could crack it.definately this job needs more than me but I am the only one to steer this ship in this water.

  10. #10
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    ALWAYS check the strainers before you make an adjustment.


  11. #11
    Join Date
    Jan 2004
    Location
    Earth
    Posts
    4,879
    99% of the time adjusting the TXV will only confuse the situation. It should be the absolute last resort. TXV’s are one of the most reliable devices in refrigeration. When they do start having problems, adjusting them won’t help.
    A Diamond is just a piece of coal, that made good under pressure!

  12. #12
    Join Date
    Aug 2004
    Location
    Kansas
    Posts
    133

    Never walk away from a wide open TXV!

    You have a problem many of us have lost alot of sleep over before and I hope your customer stays with you until it's fixed.

    If you have a dirty sightglass you have dirty screens too it's showing you what's inside the system.
    Valve salesmen told me when balanced port came out they had tighter clearances than standard valves and required a finer screen and I have had those screens rupture and fill the valve with crap and stop it from working.I do not sell balanced port valves for this reason. You will find none on my truck. I have had many plugged or ruptured screens after retrofitting to the new refrigerants. I would never open up a valve and walk away as it might blow the crap out at midnight and flood the compressor. Some cases even come with a C-083 ahead of the strainer because of this fine screen or they request added filters. Assuming this system was changed from 502 the 408a or the oil may have a higher detergent action and washed a truck load of crap lose into the screens. I have watched a 1/2 ton valve frost a 10 hp when coming out of defrost because someone left it open and walked away.Always check the txv screen and change the main driers before setting superheat

    Do you have pressure limiting power elements? I'm guessing you do on a freezer or just low temp elements with a CPR
    Sometimes a mop power element will starve the coil if the compressor can't pump the correct amount and if and when the valve comes out of mop range (if you have opened the valve wide open) it will immediately flood the comp so you put another compressor on and it will destroy another and another. Check the compressor capacity against Copeland charts. Read Basser's post and check for blowby. In the past I have taken out over three gallons of excess oil out of 5/7hp comps that other serivce men had addded trying to keep oil in the crankcase. You can't correct blowby by adding oil and if you put a new comp on you will need to remove the excess oil before you leave and I would change the TXV's too and check the superheat at case design temp only.

    Well I should throw another what if in here. What if different refigerants have been mixed in the system? I have seen multiple compressor failures running 50% 502 and 22 after liquid lines got cross piped on a med/low temp rack they fixed the mistake but left the mixture in the system for financial reasons and that was when gas was cheap and the replacement comps were under warranty.
    If they have mixed they will never admit it due to EPA laws.
    It happens and maybe more than we think about. Some mechanics are tempted to top off to help the customer but how many times can you top off and maintain correct superheat?

    If your customer stays with you through this hang on to him he's a good one.

  13. #13
    Join Date
    Jun 2001
    Location
    Michigan
    Posts
    12,077
    Subcooling Measurment would have helped to see if she even had the ability to provide TXV with a good column. A superheat measurment would tell you if your oil was logging in coil. Starving. How do you know whats going on with out a superheat measurment.

    Never add oil till your reasonably sure that it left the building.

    You just caused even more problems by cranking on those txvs and adding oil.

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