Hermetic compressor pumping or not? - Page 2
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  1. #14
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    Quote Originally Posted by Poodle Head Mikey View Post
    So what he is saying that the 'slippage' in the compressor is so great that it cannot pump down below a 40 lb. back pressure? Look; I am not TPCo - but I sure do find that hard to believe. Once the gas is shoved past the discharge valve on the upstroke - how does it get back to the suction side? Are there no rings on the piston? How does a positive displacement compressor Not pump at nearly 100% ?

    Anybody?

    PHM
    ------
    Hey, PHM
    I will hazzard a guess that it involves compression ratio (I don't mean ratio of the compressor).
    Refman tells 40/200 remember 200.
    That's 4:1 CR ok.
    Pumpdown to 15/200, now 7:1
    Get to 0/200, thats over 14:1 !!
    You reach a point where the gas trapped in the piston-to-valve area re-expands to totally refill the cylinder,... thus no new suction intake.

  2. #15
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    You're right

    And compound that compression / re-expansion dilemma by also holding the discharge valve closed with 200+ lbs. of pressure, eh? <g>

    PHM
    -------



    Quote Originally Posted by IRBH View Post
    Hey, PHM
    I will hazzard a guess that it involves compression ratio (I don't mean ratio of the compressor).
    Refman tells 40/200 remember 200.
    That's 4:1 CR ok.
    Pumpdown to 15/200, now 7:1
    Get to 0/200, thats over 14:1 !!
    You reach a point where the gas trapped in the piston-to-valve area re-expands to totally refill the cylinder,... thus no new suction intake.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  3. #16
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    [QUOTE=refmet;16091421]Exactly what i thought but Martin Rex at Tecumseh tech support says otherwise; he says that the increase from 68 to 210 is enough to say that this comp is working good mechanically. He says many models of fractionnal tecumseh compressor won't pull down to vacuum or even 0 psi. He says 40 psi suction with frontseated suction valve and 210 discharge does not mean comp has failed! Why would they say such thing? I /QUOTE]

    REFMET, you never put the fear into mr. Tecumseh by suggesting warrenty, did you? he he

    With listed op pressures, that compressor is hitting 《 2.7:1 CR!
    Next trip to roof, with new comp. I hope, grab that suction line while killing the compressor power. An immediate warmer line means suction valve leaking.
    Of course, inordinate hot discharge means discharge valve ng (recompressing)

    Refmet, you're topnotch to work with remotely, the best I've seen on here!

  4. #17
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    Were the solenoid valves recently replaced? (Backwards) Just a thought, If you really wanted to go into depth, you could complete a pressure enthalpy chart, find the density of the refrigerant entering and leaving and compare it to the amperage. Either way you slice it, if you close off the SSV and the compressor cannot pump down to at least 5 psig, replace the compressor. Thats why they made hermetic compressors, they are full of black magic and voodoo once the black smoke is released you will never get it back in lol.

  5. #18
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    [QUOTE=refmet;16084621 I checked pressures at condensing unit(205/68)I decided to check if the compressor was pumping by attaching a gauge to the service port of the suction rotolock valve and frontseating the valve to see if the compressor pulls down to 5 psi. The compressor pulled from 70 to 40psi rapidly but never went below 40psi.

    I'm reading here that you attached "a" gauge to the suction service port. No mention of a high side gauge at that time. I did read that you took initial pressures. Please clarify that you had 205 head pressure when you pumped it down. I say nothing in then nothing out. Normally the head will go down as the suction lowers. I am going with my #3 post BAD COMPRESSOR.

  6. #19
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    PHM, are we missing something?
    Why would Tecumseh say that this is acceptable?
    How can it pump-down (Refmet confirms that it CAN'T.)
    ??? over analysing?

  7. #20
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    I sure feel like I am missing something. <g>

    I agree with the logic that at some point the gas volume in the cylinder is just compressing and re-expanding with each piston stroke without ever developing sufficient pressure to open the discharge valve. But also I would expect that pressure-change-range in the cylinder to include a vacuum at some point. <g>

    I can't ever remember this phenomenon happening under my hands. And I have a hard time picturing that it can occur with a 40 lb. suction pressure.

    PHM
    ------







    - .
    Quote Originally Posted by IRBH View Post
    PHM, are we missing something?
    Why would Tecumseh say that this is acceptable?
    How can it pump-down (Refmet confirms that it CAN'T.)
    ??? over analysing?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  8. #21
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    At top dead center of the piston stroke only 80% of gas passes past the reed valves 20% stays in the cylinder and on the down stroke and pressure drop in the cylinder the 20% gas expands, all this is normal but does not explain not pulling down past 40 psig.

  9. #22
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    Compressors faulty, heres a copeland in a similar condition i found, luckily there was a good one next to it to compare it to. Unluckily the vid was before i learned to talk so no commentary... I have the low side gauge line on the duffer and the high side on the goodun. Note how the pressure rises quickly in the faulty one when it trips on the klixon.



    Can't see there being 20% of refrigerant left in the cylinder of a normally working compressor as stated, every one i have took apart the piston pretty much just touches the valve plate at the top of the stroke, in fact i've had comps with worn bearing where the piston did hit the valve plate when doing a pump test. the 80/20% thing sound more like a combustion engine to me but could be wrong!

  10. #23
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    I took 80%\20% right out of the r a/c theory 6th edition text book, but the book is not always right.

  11. #24
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    Quote Originally Posted by RACMT View Post
    I took 80%\20% right out of the r a/c theory 6th edition text book, but the book is not always right.
    Its probably correct then! Thinking about it again, when the pistons at the bottom you have 100% filled with suction pressure, and when its at the top you have a tiny gap and the volume of the hole in the valve plate to the discharge valve filled with discharge pressure so it could well expand back to fill 20% of the cylinder volume depending on operating conditions.

    One of these days i'm gonna have to do some book learnin'

  12. #25
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    On a Discus compressor that space is much less. One reason they are more efficient.

  13. #26
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    You are correct and for this reason scrolls became more popular because they move 100% so to combat this compitition cope land came up with a new valve setup for recip that took tighter tolerances, better metallic relationships for expansion and contraction and they were able to narrow the gap between the piston head and valve seat. I can remember the name for this setup I will get back to you on that, I know when they were in the developmental phase they were having problems with piston knock with the extremely small tolerances, that 80/20 relationship is there for compromise of best performance for cheapest manufacturing cost while still being reliable and competitive in today's markets.

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