});


Results 1 to 11 of 11

Thread: Compressor VI

  1. #1
    Join Date
    May 2009
    Location
    New Zealand
    Posts
    106
    Rep Power
    11

    Compressor VI



    Hi all,
    The MYCOM V series manual states
    Vi = Volume of suction refrigerant when compression begins
    Volume of same quantity of refrigerant at discharge port
    In other words, Vi is the ratio of the groove volume after competition of suction to the volume when the
    discharge port opens.
    Conventional screw compressors have three fixed Vi values, that is 2.63, 3.65 and 5.80, termed L port, M port and H port, respectively.
    The relationships are:
    Vi = (Pd/Ps)1/k or Vik = Pd/Ps
    For calculation purposes could I not use the Specific Volumes at the desired conditions eg:
    Ammonia Freezer
    -20degC = 0.6237
    +30degC = 0.1106
    VI = 5.6
    Ammonia Water Chiller
    0degC = 0.2895
    +30degC = 0.1106
    VI = 2.6



  2. #2
    Join Date
    Jun 2011
    Location
    Australia
    Age
    39
    Posts
    336
    Rep Power
    17

    Re: Compressor VI

    Hi

    You are missing K Ratio of Specific Heat

    Find K in table part way down this page
    https://www.engineeringtoolbox.com/s...tio-d_608.html

    I can put together a mock calculation a bit later if you like, uses to the power of X or indices (sorry cant do it just now, wash the dog & feed the child etc.)

  3. #3
    Join Date
    Jun 2011
    Location
    Australia
    Age
    39
    Posts
    336
    Rep Power
    17

    Re: Compressor VI

    Ps,

    I just re read your message
    Specific Volume probably already accounts for K
    I’m sure you’ve got the same result

    Cheers

  4. #4
    Join Date
    Dec 2007
    Location
    australia
    Posts
    2,689
    Rep Power
    30

    Re: Compressor VI

    Page 10 has a graph in this older manual if any help.

    http://www.midstatesrefrigsupply.com...ies_manual.pdf

  5. #5
    Join Date
    May 2009
    Location
    New Zealand
    Posts
    106
    Rep Power
    11

    Re: Compressor VI

    Thanks all,
    my math is rubbish so just looking for an easy way to calculate it, will check against a FRICK pannel.

    I have read if the VI is wrong for the application the oil in the oil separator can foam excessivly. Could someone please explain the reason for this? Over/under compression causes oil foaming? Cheers

  6. #6
    Join Date
    Dec 2007
    Location
    australia
    Posts
    2,689
    Rep Power
    30

    Re: Compressor VI

    Quote Originally Posted by Superfridge View Post
    Thanks all,
    my math is rubbish so just looking for an easy way to calculate it, will check against a FRICK pannel.

    I have read if the VI is wrong for the application the oil in the oil separator can foam excessivly. Could someone please explain the reason for this? Over/under compression causes oil foaming? Cheers
    Superfridge,
    Where did you read this, as never heard of it before, or experienced issues that I remember.
    Only issues is motor amps out of control, or possibly noisier operation with oil compression.
    Keep in mind though, a lot of things written out of USA nearly always have liquid injection oil cooling as well,.
    The only thing from link below that could possibly cause foam would be when over compressed, it re-expands to the lower pressure causing "foaming".

    https://www.irc.wisc.edu/export.php?ID=42

    https://www.irc.wisc.edu/export.php?ID=423
    Last edited by RANGER1; 10-05-2019 at 04:47 AM.

  7. #7
    Join Date
    Jul 2006
    Location
    USA
    Age
    69
    Posts
    743
    Rep Power
    20

    Re: Compressor VI

    I have always calculated VI this way.
    Discharge pressure + 14.7 lbs divided by Suction pressure + 14.7 lbs.
    Did I do it wrong all these years?

  8. #8
    Join Date
    May 2009
    Location
    New Zealand
    Posts
    106
    Rep Power
    11

    Re: Compressor VI

    Quote Originally Posted by RANGER1 View Post
    Superfridge,
    Where did you read this, as never heard of it before, or experienced issues that I remember.
    You got me there, probably read it in a FRICK manual. I see oil foaming in oil separators alot on compressors with fixed VI and running out side of their VI envelope. Early season, low loads and low discharge pressures, compressors noiser than normal.

  9. #9
    Join Date
    Dec 2007
    Location
    australia
    Posts
    2,689
    Rep Power
    30

    Re: Compressor VI

    Quote Originally Posted by Superfridge View Post
    You got me there, probably read it in a FRICK manual. I see oil foaming in oil separators alot on compressors with fixed VI and running out side of their VI envelope. Early season, low loads and low discharge pressures, compressors noisier than normal.
    Maybe machines unloaded. usually noisier in a lot of cases.

    It as also been stated that head pressure can drop rapidly causing oil foaming, that might be a winter or low load thing as well when condenser fans cycle.
    With most packages I work on has separate sight glass off side of separator, so not looking directly in lie with a bullseye.
    Most bullseyes there is usually a bit of foam on top, when separating 100-300 litres of oil a minute has to be something going on.
    Mycom "L" or "M" ports not a lot of difference.
    Usually pretty simple booster "L" port, but changes to "M" port if -40 deg C & below
    "M" port for highside compressor -10 deg C suction as a general rule.
    "H" port is single stage economised below - 30 deg C SST
    Even with Mycom "V" series, set & forget, as probably not going to change Vi seasonally.

    Is it just an observation, or do you have a problem?

    In the links I posted it shows in a table Vi for different seasons or pressures, also to check if correct Vi on auto Vi machine.

    Also don't believe everything you read, especially what I write.
    Last edited by RANGER1; 11-05-2019 at 10:50 PM.

  10. #10
    Join Date
    Jun 2011
    Location
    Australia
    Age
    39
    Posts
    336
    Rep Power
    17

    Re: Compressor VI

    Apologies, it’s only taken a week for me to get this together..
    Photos & Screenshot below,

    Vi = ?

    Pd Pressure Discharge in Absolute, equivalent +30 Celsius = 1166 kPa
    Ps Pressure Suction in Absolute, equivalent -30 Celsius = 119 kPa
    k Ratio of Specific Heat, Ammonia = 1.32

    Answer via screenshot of Google Calculator

    Cheers

    62323A49-C250-4D53-9DB0-AAED91CA9584.jpgA0911B11-975E-4132-99B1-6C7C4E0BB0FD.jpg

  11. #11
    Join Date
    May 2009
    Location
    New Zealand
    Posts
    106
    Rep Power
    11

    Re: Compressor VI

    Hey thanks for that, that makes the calculation look easier.

    Using the Specific volume dosn't give the same answer

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •