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  1. #1
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    flooded evaporator in screw chiller



    Hi sirs

    I've heard that some manufacturers apply flooded evaoprator design in their screw chillers. It's said that the COP is higher than DX system. But how about the pro and cons of flooded evaporator?


    regards
    LC


    I hear...I forget;I see...I remember;I do...I understand

  2. #2
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    Re: flooded evaporator in screw chiller

    Hi LC,

    For flooded units the following comments are applied:

    Pros
    1. Higher heat transfer
    2. Potentially lower approach temperature with same surface area
    3. Almost no evaporator superheat that can increase vapor density and mass flow, hence increased capacity
    4. Liquid/vapor separation incorporated into design
    5. Easier ability to reduce discharge pressure
    6. Less low load operating problems (better turndown)
    7. Cooled fluid flows through tubes (easier cleaning if needed)

    Cons
    1. More refrigerant is required
    2. Added cost for liquid/vapor separator
    3. Oil recovery method required

  3. #3
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    Re: flooded evaporator in screw chiller

    Hi Lc,

    Most of my equipment is screw compressor & flooded type,some are shell & tube eveporator, but mostly PHE.
    Better COP.

    Guapo

  4. #4
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    Re: flooded evaporator in screw chiller

    Hi,
    The item which you should be most concerned about is a loss of water flow condition. In flooded evaps, the tube bundle freezes and ruptures very easily if a loss of water flow condition occurs. We always suggested redundant proof-of-flow switches. We even had freeze-ups when the charge was blown, even though accepted practice [and instructions] was to operate the water pumps during this period.
    If it is a R134a machine, once water gets into EVERYTHING, huge trouble as the oil is highly hygroscopic. Very difficult to clean eveything out to acceptable levels.

    Good luck, bigJohn

    P.S. not to worry anyone, "Blow the charge" is a factory term in current usage. Of course, nowadays, it is recovered.

  5. #5
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    Re: flooded evaporator in screw chiller

    HI guapo
    Use plate heat exchanger as flooded evaporator? I don't know how to control the liquid level? pls teach me

    rgds
    LC
    I hear...I forget;I see...I remember;I do...I understand

  6. #6
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    Re: flooded evaporator in screw chiller

    LC,

    If you go to this post: http://www.refrigeration-engineer.co...9&d=1147285803

    You will find a sketch of a flooded PHE.

    The liquid level is maintained in the vessel above the PHE. A simple float switch controlling a solenoid valve will work nicely. The flow rate into the vessel is controlled by a hand expansion valve.

  7. #7
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    Re: flooded evaporator in screw chiller

    Hi Lc,

    US ICEMAN is correct, simple as that. You can also control the level by electronic level controller for better performance. I think most of the packaged chiller now they use PHE.

    Regards,
    Guapo

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    Re: flooded evaporator in screw chiller

    hi guapo,
    understand what you say
    Where is the key tech of this type machine? Oil return \ level control or liqiud vapour separator? Could you offer me a system sketch of your screw machine. How about the COP bigger than DX evap?

    thanks and regards
    LC
    I hear...I forget;I see...I remember;I do...I understand

  9. #9
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by Lc_shi
    hi guapo,
    understand what you say
    Where is the key tech of this type machine? Oil return \ level control or liqiud vapour separator? Could you offer me a system sketch of your screw machine. How about the COP bigger than DX evap?

    thanks and regards
    LC
    Hi Lc,
    The unit has oil separator right after discharge, and on the top of the PHE there is a what we call surge drum. Surge drum we maintain the level of refrigerant, around 1/4. Since the suction line is on the top of the surge drum, only vapor will go to the compressor.

    The PHE is full of refrigerant because the level is 1/4 on the surge drum which located on the top of the PHE, unlike the DX evaporator you have to left some space for your super heat.

    Regards,
    Guapo

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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by Lc_shi View Post
    HI guapo
    Use plate heat exchanger as flooded evaporator? I don't know how to control the liquid level? pls teach me

    rgds
    LC
    Is it possible to use PHE as flooded evaporator, I doubt it!

  11. #11
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by US Iceman View Post
    Hi LC,

    For flooded units the following comments are applied:

    Pros
    1. Higher heat transfer
    2. Potentially lower approach temperature with same surface area
    3. Almost no evaporator superheat that can increase vapor density and mass flow, hence increased capacity
    4. Liquid/vapor separation incorporated into design
    5. Easier ability to reduce discharge pressure
    6. Less low load operating problems (better turndown)
    7. Cooled fluid flows through tubes (easier cleaning if needed)

    Cons
    1. More refrigerant is required
    2. Added cost for liquid/vapor separator
    3. Oil recovery method required
    How we get the oil return to the compressor, that's what I concern.

  12. #12
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by wilsoncheung View Post
    How we get the oil return to the compressor, that's what I concern.
    That all depends!

    The chillers I work on use a "gas pump" fitted under the evaporator that uses suction pressure to get the oil back to the compressor by opening and closing two solenoid valves.

  13. #13
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    Re: flooded evaporator in screw chiller

    Attached the principle of a screw chiller!
    Attached Files Attached Files

  14. #14
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    Re: flooded evaporator in screw chiller

    hI ICEMAN,

    CAN U TELL ME, HOW TO SELECT DX AMMONIA EVAPORATOR.If i select the pumped system evaporator and convert in to ammonia dx,will there change in the capacity?



    Quote Originally Posted by US Iceman View Post
    Hi LC,

    For flooded units the following comments are applied:


    Pros
    1. Higher heat transfer
    2. Potentially lower approach temperature with same surface area
    3. Almost no evaporator superheat that can increase vapor density and mass flow, hence increased capacity
    4. Liquid/vapor separation incorporated into design
    5. Easier ability to reduce discharge pressure
    6. Less low load operating problems (better turndown)
    7. Cooled fluid flows through tubes (easier cleaning if needed)
    Cons
    1. More refrigerant is required
    2. Added cost for liquid/vapor separator
    3. Oil recovery method required

  15. #15
    srini742's Avatar
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    Re: flooded evaporator in screw chiller

    can anyone tell me how to select DX ammonia evaporator. If i select Pumped system evaporator and convert in to DX System , will there be any changes in the capacity.

    srini

  16. #16
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by wilsoncheung View Post
    Is it possible to use PHE as flooded evaporator, I doubt it!
    It can be done, and works very well.
    Just needs a surge drum or LPR and you'll have a very efficient system.
    We have hundreds of systems like this in UK working off a high side float switch.

  17. #17
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by Lowrider View Post
    That all depends!

    The chillers I work on use a "gas pump" fitted under the evaporator that uses suction pressure to get the oil back to the compressor by opening and closing two solenoid valves.

    We use a similar system, but with 1 solenoid valve and a motorised valve.

  18. #18
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by srini742 View Post
    can anyone tell me how to select DX ammonia evaporator. If i select Pumped system evaporator and convert in to DX System , will there be any changes in the capacity.

    srini
    The best way to do this is donn't do it at all. Gravity flooded or pumped ammonia systems ar far more reliable and efficient. Unless you have a small medium temperature system, you're asking for trouble by using DX.

  19. #19
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by Lc_shi View Post
    hi guapo,
    understand what you say
    Where is the key tech of this type machine? Oil return \ level control or liqiud vapour separator? Could you offer me a system sketch of your screw machine. How about the COP bigger than DX evap?

    thanks and regards
    LC
    We are building flooded PHE chillers using R-22 and R-717. Our factory is bnear Beijing. Come and visit us some time.

  20. #20
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by Lowrider View Post
    Attached the principle of a screw chiller!
    Sorry for reviving this old thread. Can you possibly explain to me how this pump works. I can't understand the principle... Thanks

  21. #21
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by NoNickName View Post
    Sorry for reviving this old thread. Can you possibly explain to me how this pump works. I can't understand the principle... Thanks
    What's the problem? If you can read PID's you'll be able to locate the screw compressor, oil seperator, condensor and evaporator!

  22. #22
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    Re: flooded evaporator in screw chiller

    Of course I can, but what is inside what you call "GAS PUMP"? Are the solenoids interlocked and how are they managed? How is the geometry of the oil pipe in the evaporator vessel?

  23. #23
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    Re: flooded evaporator in screw chiller

    the dotted line in the evaporator is there to indicate the level,not a pipe, maybe that helps! The solenoids help to suck in and push out the oil.

  24. #24
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by Lowrider View Post
    the dotted line in the evaporator is there to indicate the level,not a pipe, maybe that helps!





    Quote Originally Posted by Lowrider View Post
    The solenoids help to suck in and push out the oil.
    Good to know.

  25. #25
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    Re: flooded evaporator in screw chiller

    As for the controls, it's managed by a CH530. (That's trane)

  26. #26
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    Re: flooded evaporator in screw chiller

    Quote Originally Posted by guapo View Post
    Hi Lc,
    The unit has oil separator right after discharge, and on the top of the PHE there is a what we call surge drum. Surge drum we maintain the level of refrigerant, around 1/4. Since the suction line is on the top of the surge drum, only vapor will go to the compressor.

    The PHE is full of refrigerant because the level is 1/4 on the surge drum which located on the top of the PHE, unlike the DX evaporator you have to left some space for your super heat.

    Regards,
    Guapo
    Hi Guapo,

    I've read this interesting topic. I'm just studying a ammonia chiller with flooded evaporator. I've understood the surge drum function but how can I design it or choose in a manufacturer's range?

    Many thanks in advance

    Ice Wombat

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