Results 1 to 17 of 17
  1. #1
    Join Date
    Mar 2016
    Location
    IRAN
    Posts
    154
    Rep Power
    3

    Share an experience with you



    Dear friends,
    I leaned a lot form this forums and I want to share my experience with you.

    One of my customers had to make a small fridge with -50C low temperature for someone. First he bought small R404A (LBP) 1/2 Danfoss compressor and used it for this fridge.
    By decreasing size of increasing length of capillary tube finally he couldn't be able to go to lower than -42C.

    I suggested him use HBP R134a 1/2 Danfoss compressor and use it for low temperature with R404A.
    he used it and he could reach to -51C with 1 stage (No cascade and no auto-cascade).

    As you know for going to low temperatures we must have more compressed refrigerant at discharge of compressors (something happen that in 2 stage compressors). I think discharge valve of R134a compressor is tighter than R404A compressor and by this reason we can use R134a compressor (with R404A refrigerant) to reach to -51C.
    (Both of compressors was 1/2 horse power and I know displacement of these 2 compressors were different and R134A compressor had more displacement but I think tighter diameter of discharge valve helped us to go to lower temperature)

    Thanks.



  2. #2
    Join Date
    May 2012
    Location
    England
    Age
    53
    Posts
    1,281
    Rep Power
    29

    Re: Share an experience with you

    .

    Thank you for sharing.

    I might add s little to your theory, if I may?

    Piston compressors are actually quite inefficient because of their
    physical pump characteristics.

    Most piston compressors only have a pump ratio of about 8 to 1
    That means every bar in is potentially pumped out eight times higher.

    1 bar in 8 bar out 404A at -50 would have a suction pressure of -.25 barg
    or .75 bara, in a vacuum of -.25 barg your comp would theoretically only deliver
    about 7 or 8 barg discharge.

    R134a would be worse still so if you have got the 134a to work it must be one
    very efficient compressor.

    Regards

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

  3. #3
    Join Date
    Sep 2008
    Location
    Canada
    Posts
    770
    Rep Power
    12

    Re: Share an experience with you

    Displacement difference is the reason.

  4. #4
    Join Date
    Dec 2007
    Location
    australia
    Posts
    2,538
    Rep Power
    28

    Re: Share an experience with you

    Volumetric efficiency also severely effected.
    At 30deg C condensing 404A discharge pressure more than double.

  5. #5
    Join Date
    Mar 2016
    Location
    IRAN
    Posts
    154
    Rep Power
    3

    Re: Share an experience with you

    Quote Originally Posted by Segei View Post
    Displacement difference is the reason.
    Dear my friend,
    I don't agree with you. Bigger displacement means for example we could use 1 horse power R404 (LBP) instead 1/2 horse power.
    As you know 1 HP compressor definitely has more displacement. But twice with bigger compressor with bigger displacement we can not decrease temperature in evaporator more than catalogue permission.
    Definitely these two compressors R134a and R404A have difference in structure, Maybe in compression ratio or volumetric efficiency or as I told difference in diameter of discharge valve.

    Sincerely yours.
    Last edited by A.Mortezania; 22-12-2017 at 05:43 PM.

  6. #6
    Join Date
    Dec 2007
    Location
    australia
    Posts
    2,538
    Rep Power
    28

    Re: Share an experience with you

    I misread post as thought refrigerant was 134a in second compressor.

    Discharge valves unknown as compressor not running as designed, but expect valves to be bigger as for higher temp compressor.
    Would expect them to be flexible enough by design to handle gas flow though.
    Valves would be designed around gas densities for particular refrigerants & pressures.

    So why did you pick HBP compressor, what was your line of thought???
    Last edited by RANGER1; 22-12-2017 at 08:55 PM.

  7. #7
    Join Date
    Jul 2006
    Location
    USA
    Age
    67
    Posts
    727
    Rep Power
    19

    Re: Share an experience with you

    Quote Originally Posted by A.Mortezania View Post
    .
    Definitely these two compressors R134a and R404A have difference in structure, Maybe in compression ratio or volumetric efficiency or as I told difference in diameter of discharge valve. Sincerely yours.
    The compression ratio is not inherent to the compressor design in a piston compressor. It is determined by dividing the absolute suction pressure by the absolute discharge pressure, not by the compressor construction.

  8. #8
    Join Date
    Aug 2006
    Location
    england
    Age
    44
    Posts
    3,795
    Rep Power
    38

    Re: Share an experience with you

    Look up compressors in manufacturers data, they have a set range of displacements and just match the motor to suit the temperature range and refrigerant.
    Mostly found in Oxfordshire, UK :)

  9. #9
    Join Date
    Sep 2008
    Location
    Canada
    Posts
    770
    Rep Power
    12

    Re: Share an experience with you

    What catalogue you are talking about? You take BHP R134A compressor and tried to use for LBP R404 application. The same compressor can have different HP motors based on application (LBP,...HBP) and refrigerant. You need more powerful motor for R404a because of higher discharge pressure.

  10. #10
    Join Date
    Mar 2016
    Location
    IRAN
    Posts
    154
    Rep Power
    3

    Re: Share an experience with you

    Quote Originally Posted by Segei View Post
    What catalogue you are talking about? You take BHP R134A compressor and tried to use for LBP R404 application. The same compressor can have different HP motors based on application (LBP,...HBP) and refrigerant. You need more powerful motor for R404a because of higher discharge pressure.
    Dear Segi,
    1 - Yes you are right. For a compressor working with R404A , When a motor is HBP it means this compressor must have more power winding instead LBP compressor with R404A. It is clear.
    2 - Also when a compressor work with R404A it means it must have more power winding instead compressor that working with R134a as result of high discharge pressure.

    Now you can merge these two items together. It means you can use HBP compressor (that works with R134a) as LBP compressor with R404A.

    You can see it very clear on most of catalogues of semi-hermetic compressors such as DWM EMERSON , COPELAND , BITZER and etc.
    For example Bitzer suggest a compressor for using at low temperature (LBP) with R22 and on other pages you see also Bitzer suggest this compressor for R134a but at HBP situation.
    Why ? As result that I explained above.

    Sincerely yours.

  11. #11
    Join Date
    Mar 2016
    Location
    IRAN
    Posts
    154
    Rep Power
    3

    Re: Share an experience with you

    Quote Originally Posted by NH3LVR View Post
    The compression ratio is not inherent to the compressor design in a piston compressor. It is determined by dividing the absolute suction pressure by the absolute discharge pressure, not by the compressor construction.
    Dear My friend,

    Please check valve plate of a semi-hermetic Copeland compressor that company suggested for LBP situation instead a same power compressor that company suggested for HBP situation.

    When you check discharge valve of these two compressors, Compressor that suggested for LBP situation has tighter discharge valve.

    Find my photo below.


    Also I think when compressor works with different refrigerants beside of power of winding maybe valve plates also could have little different in structure. I,m not sure about this and I wanted to share my experience with you and know your opinion.

    Thanks.
    Attachment 15070

  12. #12
    Join Date
    Mar 2016
    Location
    IRAN
    Posts
    154
    Rep Power
    3

    Re: Share an experience with you

    Quote Originally Posted by NH3LVR View Post
    The compression ratio is not inherent to the compressor design in a piston compressor. It is determined by dividing the absolute suction pressure by the absolute discharge pressure, not by the compressor construction.
    Dear Friend,

    For low temperatures (LBP) with R404A we have not compressor that can prepare for us 51C in 1 stage.
    Maximum is -45C evaporator and with 5 Delta T maximum room temperature would be -40C.
    By this situation we used a trick (used R134a HBP compressor with R404A refrigerant for LBP situation and reached to -51C room temperature).

    Sincerely yours.

  13. #13
    Join Date
    Jul 2006
    Location
    USA
    Age
    67
    Posts
    727
    Rep Power
    19

    Re: Share an experience with you

    Quote Originally Posted by A.Mortezania View Post
    When you check discharge valve of these two compressors, Compressor that suggested for LBP situation has tighter discharge valve. Attachment 15070
    Your attachment did not come through.
    I do not know what you are referring to when you speak of a "tighter discharge valve'.
    This is not a complicated issue, and I understand it well.
    I cannot transmit this understanding to you on this forum.
    Here is a bit of the information.

    31.35 Motor Size CalculationsHorsepower of a motor required to drive a refrigerationcompressor can be calculated, The usual methodUses the mean effective pressure(MEP) in the cylinder'This is the pressure that is bearing down on the pistonhead and which must be overcome by the electric motorwhen driving the compressor. The MEP is determined by a formula. Basic variables of the formula arethe low side pressure, the high side pressure, and the ratioof the specific heat at constant Pressure to the specificheat at constant volume Cp/Cv= K (for the kindof refrigerant).

    Study! Download a capacity chart for open industrial compressors if you do not have one. See how the horsepower requirements change due to changes in suction and discharge pressure.

    That is how I learned.

  14. #14
    Join Date
    Dec 2007
    Location
    australia
    Posts
    2,538
    Rep Power
    28

    Re: Share an experience with you

    I think different compressors have different valve plates & this MAY be the case here.
    If compressor is for low temp application it's valves could be smaller as volume of gas or mass flow is lower.
    In high temperature compressor larger mass flow so bigger valves.
    If displacement of compressors are the same valves from high temperature compressor may be able to allow compressor to operate more efficiently, not starving cylinder of gas.
    Non some older industrial compressor that is the case for Mycom & Sabroe, as ***** compressor valve plates are different to ammonia due to refrigerant densities.
    Mao they used to have valves with more "lift" to get gas into cylinder.
    Min this article they indicate it in reciprocating ompressor description.
    If valves were smaller then they could restrict gas flow.
    Makes you wonder if compressor could overheat with low vacuum on suction.

    https://www.airah.org.au/Content_Fil...s-Workshop.pdf

    I can only speculate.
    Last edited by RANGER1; 23-12-2017 at 09:37 PM.

  15. #15
    Join Date
    Jul 2006
    Location
    USA
    Age
    67
    Posts
    727
    Rep Power
    19

    Re: Share an experience with you

    Good afternoon RANGER1!
    I think you are trying to disrupt my lazy afternoon by sending me back to the parts books.

    We do not know what is inside a hermetic compressor. I have cut a couple open over the years, just for fun, but have no idea what difference there is in the valves between HBP and LBP machines.
    Please bear in mind my career was mostly spent on NH3 machines.

    There are differences in the Mycom A and B ring arrangement between NH3 and Fre*n machines. I do not recall any valve differences, nor do I remember and with Vilter. I could be wrong on that.
    I have worked on a few Carriers (5H and 5F) over the years and do only remember one valve plate.
    I do not have a Copeland parts book at hand, but I think that could be quite telling as to differences in valves between machines.

    Again I doubt the manufacturers of Hermetics are going to let us in on their secrets.

    Why cannot I type in Fre*on?

  16. #16
    Brian_UK's Avatar
    Brian_UK is offline Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    70
    Posts
    10,553
    Rep Power
    52

    Re: Share an experience with you

    I thought that it depended on whether your system had a capillary or a TXV whether you used an LBP or HBP compressor.

    It is only the starting of the motor that is the difference in the compressor, do you need a strong starting effort to overcome the pressure difference in the compressor or not.
    Brian - Newton Abbot, Devon, UK
    Retired March 2015
    Please support http://the100project.net/Home

  17. #17
    Join Date
    Mar 2016
    Location
    IRAN
    Posts
    154
    Rep Power
    3

    Re: Share an experience with you

    Quote Originally Posted by RANGER1 View Post
    I think different compressors have different valve plates & this MAY be the case here.
    If compressor is for low temp application it's valves could be smaller as volume of gas or mass flow is lower.
    In high temperature compressor larger mass flow so bigger valves.
    If displacement of compressors are the same valves from high temperature compressor may be able to allow compressor to operate more efficiently, not starving cylinder of gas.
    Non some older industrial compressor that is the case for Mycom & Sabroe, as ***** compressor valve plates are different to ammonia due to refrigerant densities.
    Mao they used to have valves with more "lift" to get gas into cylinder.
    Min this article they indicate it in reciprocating ompressor description.
    If valves were smaller then they could restrict gas flow.
    Makes you wonder if compressor could overheat with low vacuum on suction.

    https://www.airah.org.au/Content_Fil...s-Workshop.pdf

    I can only speculate.
    Dear Ranger,
    Thanks for participation in this topic.

    Please note, I told you LBP compressor just has tighter discharge valve ! If you focus on suction valves on HBP and LBP compressor they are same ! Just discharge valve is tighter on LBP compressor.

    If we want to focus on mass flow yes your are right at LBP compressor as result of lower mass flow we could have suction valve also tighter same as discharge valve but we see just discharge valve is tighter.
    Please note for reaching to lower temperature at evaporator we need more compressed gas at outlet of compressor, Thing that happen on 2 stage compressors on blast freezers.

    For example 30 HP compressors 2 stage capacity is same as 2 * 30 HP standard compressor capacity, It means we can use 60 HP compressor instead 30 HP compressor 2 stage, capacity is same but with 2 stage compressor we can go down to -50C R22 but whit 1 stage compressor we can not reach to this temp.

    I think company when make LBP compressor, They make discharge valve tighter to reach to more compressed refrigerant at outlet of compressor.

    Sincerely yours.

Posting Permissions

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