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  1. #14
    Join Date
    Feb 2008
    Location
    Western NC
    Posts
    2,504
    enough already


    I gotta ask.....what are you needing that extreme low temp for??? Are you freeze drying dead people to bring back to life in the future???


    haha. so what ya doin here? My curiosity has killed my cat and is workin on me next
    I fully support the military and the War on Terrorism.


    If you don't know, then don't do. If you don't know and still do, then be prepared to pay someone else a lot to undo what you did and then do it right.

    If you do know, then do. But do it right. Otherwise, you may not be doing it long.

  2. #15
    Join Date
    Sep 2008
    Location
    Cedar Rapids, IA
    Posts
    486
    Have you thought about the compressor oil? I work on small lab low temp systems which are cascade systems. There are additives put into the refrigerants to help prevent the oil from gumming up. Typically propane or butane in the older systems and it comes premixed in the newer ones.

  3. #16
    Join Date
    Sep 2008
    Posts
    14
    Do U think we need special oil for -90F SST???

  4. #17
    Join Date
    Jun 2006
    Location
    Omaha, Nebraska
    Posts
    861
    Water on the condenser is a bad idea. It typically leads to scale buildup and corrosion. You may have to use a water cooled condenser with a closed-loop fluid cooler.

  5. #18
    Join Date
    Sep 2008
    Location
    Cedar Rapids, IA
    Posts
    486
    Quote Originally Posted by KOOLGURU View Post
    Do U think we need special oil for -90F SST???
    I don't see how you can use a compound compressor over such a wide temperature range and not have oil that is too thick at the evap temps yet too thin at the condensing temps. That's why cascade systems were developed. Oil is definitely a problem at ultra low temps. Even the smallest cabinets have an oil separator on the low temp compressor. They wouldn't add that if it wasn't needed.

    POE oils have a wider range but I would check into it with your compressor manufacturer as to what your compound compressor can handle. And what oil is recommended.

  6. #19
    Join Date
    Sep 2008
    Posts
    14
    I checked with comp manf and they recommend 30 viscosity oil for this temp..

    Also, we'll be using oil seperator in the system.

  7. #20
    Join Date
    Sep 2002
    Posts
    12,568

    I know a few things

    HG defrost would not be my choice with a system like this. Especially not with a single compressor. Too much concern for high suction temps. and pressures - plus the potential for running out of hot gas before the defrost job was done.

    I think the cabinet metals would be OK to -76 but call Alcoa for their alloy recommendations if you are nervous.

    I personally would prefer a cascade as the suction pressures could all be positive and it's just easier to design and control. I have gone that low with compound compression R-22 but have no experience with 404 that low. I can tell you that right off the bat the POE oil would make me real nervous for the first little while. <g>

    I think the accumulators and other components would be fine at -76F.

    I would want At Least 1/4" fin spacing. 3/8" or 1/2" would be better. You will need room for frost or have excessive time devoted to defrost.

    The problem with a very low density suction gas will be primarily oil return.

    Two-stage (compound compression) will work with one compressor and be slightly more efficient with the elimination of one step in the HX process. Although you may have to deal with interstage cooling, liquid injection, etc. Really watch your mass flow rates.

    Compound compression always feels to me like 'pushing the refrigerant envelope' whereas having one high stage compressor cooling one low stage compressor seems like a more 'relaxed' way of doing things. With each compressor and refrigerant operating inside it's 'normal' range, etc.

    One good oil seperator per compressor should be fine if you are controlling your suctions. With low temp work the pressure drops are always your big enemy. I would want to make damned sure that two in series wasn't going to lose me too much work.

    And watch your projected BTU numbers - don't work too close. With low temps like this BTU's have a way of slipping away on you. <g>

    PHM
    -------



    Quote Originally Posted by KOOLGURU View Post
    Hi davidd_danfoss,

    I need to know if someone has experience in designing low temp freezer for -76F (-60C).

    Also I am curious to know:

    1. What kind of defrost to be used -- Hot Gas or Electric. Will there be a big shock in the system with HG defrosting. This is a 1 evap system and during defrosting, there will be no cooling a/v for intermediate stage.

    Can we still use HG defrosting for 2 stage?

    2. Do we need to use any special metal for indoor unit enclosure -- Can alumnium/stainless withstand -76F temp?

    3. Will 2 stage or cascade will be better for the application?

    4. Accumulators and other components are rated for -40F -- can they be used for -76F?

    5. How many Fins/Inch are required for Evap?

    6. What are the other parameters needs to be considered for this low temp application.

    7. What will be the problems with comp running in vacumm?

    I need -76F room temp.

    -95F SST / 99F Condensing is the opearting limits for the 2 stage compressor I selected Bitzer 30 HP will deliver 20,000 BTU @ -95 SST.

    Do U think 2 stage will work -- it would be simpler than cascade?

    THANKS IN ADVANCE
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  8. #21
    Join Date
    Sep 2008
    Posts
    14
    I have gone that low with compound compression R-22 but have no experience with 404 that low. I can tell you that right off the bat the POE oil would make me real nervous for the first little while.
    POODLE HEAD MICKEY,

    Thanks for the detailed reply.

    As you've mentioned, as per the above quote, can you send me some details about the system you designed with R-22.

    Two-stage (compound compression) will work with one compressor and be slightly more efficient with the elimination of one step in the HX process. Although you may have to deal with interstage cooling, liquid injection, etc. Really watch your mass flow rates.
    Can you please explain this in details??


    I would want to make damned sure that two in series wasn't going to lose me too much work.
    Any disadvantages for using 2 oil separators in series?

    And watch your projected BTU numbers - don't work too close. With low temps like this BTU's have a way of slipping away on you
    Sorry about this-- can you please explain this in details???

    THANKS IN ADVANCE....

  9. #22
    Join Date
    Sep 2002
    Posts
    12,568

    They yell at me here for being too nice a guy -

    So before I go too much farther - I think I will need the OK from a Big Boss Moderator to help you. Get them to say it's not going to get me in trouble first. <g>

    I think I'm already on some kind of probation, or black-list, or something.

    PHM
    --------


    Quote Originally Posted by KOOLGURU View Post
    POODLE HEAD MICKEY,

    Thanks for the detailed reply.

    As you've mentioned, as per the above quote, can you send me some details about the system you designed with R-22.

    Can you please explain this in details??

    Any disadvantages for using 2 oil separators in series?

    Sorry about this-- can you please explain this in details???

    THANKS IN ADVANCE....
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #23
    Join Date
    Sep 2008
    Posts
    14
    Poodle Head Mikey,

    I Don't want you to face anything weird for me.

    Also, I don't want U to disclose me any of your secrets ot patents...

    I just only need an outline guidance and your experience with the Low temp freezer, you designed/ worked on.

    Any input from your side will be quite valuable for me.

    THANKS a TON

  11. #24
    Join Date
    Sep 2008
    Posts
    14
    Poodle Head Mikey,

    Did U check with "BIG BOSS", if you can able to suggest me?

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