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  1. #1
    Join Date
    Oct 2004
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    9
    What is considered to be a good range for compressor compression ratio? High end and low end as well.

  2. #2
    boneman,

    It depends on the application and refrigerant used. the R-12 replacements under typical operating conditions will have a lower compression ratio than the r-502 replacements. But even within the same refrigerant, the compression ratio will vary based on the application.

    Is this a theoretical question (no real system involved), or can you tell me about the specific system you would like to evaluate?

  3. #3
    Join Date
    Jun 2004
    Posts
    182
    Compression ratio's will vary for different application ranges and different compressors. Generally speaking for the Copeland discus, maximum is 20:1 refrigeration and 7.5:1 air conditioning, minimum is 3:1 for either.

    The Scroll will actually unload itself if the compression ratio gets to high. This unloading will occur at 11:1 air conditioning and 26:1 refrigeration.

    Ideal compression ratio, keep the suction and discharge pressures within the approved operating envelope of the compressor. These pressure limits will be found on the compressor data sheet.

    To figure compression ratio take the suction and discharge pressure readings and add atmospheric pressure (14.7 sea level) to both, then divide the discharge by the suction.

    [Edited by basser on 11-18-2004 at 03:39 PM]

  4. #4
    Join Date
    Oct 2004
    Posts
    9
    Thanks. The system is an R 12 semi hermetic freezer that has been changed over to 134 a. We covered up the condenser and brought the head pressure uo to 185 psig. The suction pressure was 15 psig. Compressor amps were exactly at RLA rating of 5 amps.

  5. #5
    boneman,

    15 psig on 134a (thats a +15 evap temp) for a freezer seems too high unless you are holding ice. Why did you block off the condenser?

    The lower the compression ratio, the more efficient the system is. There are practical limits to lowering compression ratios however. Your evap temp can only be raised so much before you have no temperature difference left to work with. The condensing temp can only be lowered so much since there is generally not a reliable/constant source of a colder medium to ultimately transfer heat into. Also, even if there were a reliable source, the system must be designed properly to run at a much lower condesning temp.

  6. #6
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579

    You are far better off looking at the "Effective Stroke" or "Volumetric Efficiency" (Two names for the same thing) than concerning yourself with compression ratio. Volumetric Efficiency tells you a great deal more about the system operation than compression ratio.


  7. #7
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,582
    I have personally never seen a properly operating refrigeration system run over 10:1 ratio, where and how do you get a 20:1 ratio?

    R-22 low temp never get close to 10:1?

  8. #8
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579
    Originally posted by Freezeking2000
    I have personally never seen a properly operating refrigeration system run over 10:1 ratio, where and how do you get a 20:1 ratio?

    R-22 low temp never get close to 10:1?
    Even 8 to 1 is usually pretty bad and 10 to 1 is almost a sure failure. Normal AC ratios are down in the 3 to 1 area while low temp not a great deal higher.

    Cryogenic cascade systems break up a huge 20 to one or higher between three or four stages. That is why there are several stages in cascade systems. To break up the compression ratio into workable units and still achieve the super low temperatures at very low suction pressures.


  9. #9
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,364
    Aren't we really talking "pressure ratio" here?.... the term compression ratio to me has to do with the ratio of the max and min volumes in the cylinders as in automotive work.....and when talking refrigeration, it's the ratio of the absolute pressures. Am I wrong?

    Anyway......a compression ratio of 20 isn't unheard of. The old timers here may remember R22 low temp before R502 where compressors were rated for -40F SST and 130F SDT.

    I'll bet anything installed under those conditions didn't last until dinnertime.

    When I worked for a market chain in New England, we did a remodel of the store on the island of Nantucket. There was an air-cooled R22 LT ice cream freezer system there that had been running for over twenty years....with HG defrost......on the original Copeland semi.

  10. #10
    Join Date
    Oct 2004
    Location
    Annapolis, MD: ragboat capitol of the world
    Posts
    358
    THANK YOU NORM & ICE !

    Can we line up all the guys who use the term "compression ratio" when they mean pressure ratio or effiency and shoot them?

    [Edited by boat racer on 11-19-2004 at 09:34 PM]
    Sam

    Know-it-all? Nope, not even half!

  11. #11
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,582
    Even r-22 low temp at 6 psig and 250 high side will give you?

    6+15=21
    250+15=265
    265/21= 12.61 compression ratio

    If i see a rack running like this i raise the suction to 10 or 12 psig and wash the condenser.

    12+15=27
    240+15=255
    255/27= 9.44

    This is something that all supermarket men should watch or you will be changing comp oil,cleaning oil sump strainers and compressors alll the time!

  12. #12
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Originally posted by boat racer
    THANK YOU NORM & ICE !

    Can we line up all the guys who use the term "compression ratio" when they mean pressure ratio or effiency and shoot them?

    [Edited by boat racer on 11-19-2004 at 09:34 PM]
    Absolutely not. They're fun to watch.

    Just like the negative superheat crowd, although, I think I might've run them all out of town a year or so ago.

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