Results 1 to 18 of 18
  1. #1
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Choosing the evaporator and condensator



    Hi

    I am currently in a project in which I have to design an AC unit, and since I am pretty new to this whole world (I am in my practical placement as an engineer), I have some questions regarding the choosing of components.
    When choosing the evaporater and the condenser, which elements influence the choice, as I understand it you have to choose it from the needed capacity, but almost everyone i look at is overdimensioned by a lot?
    How do you calculate what the necessary heating exchange area should be?
    Lastly, can you recommend any good suppliers of evap. and cond. coils?

    Kind regards

    Michell



  2. #2
    Join Date
    May 2012
    Location
    England
    Age
    53
    Posts
    1,234
    Rep Power
    28

    Re: Choosing the evaporator and condensator

    .

    The most important thing is knowing the product you are cooling.

    Water, milk, beer, beef, broccoli, Brussel sprouts, people, air
    all require different amounts of cooling so you need to know.


    Your talking of an AC system so you are dealing with a room
    full of people and for the most part AC deals in comfort cooling.
    Comfort cooling is the required temperature for a certain period
    in the year, so you can size the system to deliver comfort cooling
    for, say 300 days in a year, that means for 65 days when the temps
    get too cold or too hot the system starts to struggle.

    It depends on cost and

    How many people?
    Where is the room?
    How many light fittings, PC's, workstations and other such things?
    Windows, how big and facing what direction?

    When you know all these things they all add up to a heat load and
    that is how you start to calculate your requirements.

    Doing it on the cheap means you could put a system in a few Kw smaller
    than needed and it would work for the most part except when the weather
    gets too hot / cold or you could go the other way and put a system in that
    is a few Kw over sized and that would work 365 days a year but cost a lot
    more the install.

    It comes down to cost........

    Rob

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

  3. #3
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    Actually it is electronics cooling, i forgot to mention that earlier -.-
    I have gotten the cooling capacity from the customer and they calculated it to 300W.
    But I am struggling on how to decide the appropriate evaporator and condenser size.


    Ragards
    Michell

  4. #4
    Join Date
    May 2012
    Location
    England
    Age
    53
    Posts
    1,234
    Rep Power
    28

    Re: Choosing the evaporator and condensator

    .

    300w is nothing compared to even a small AC unit.
    Are you dealing with processor cooling because that is a whole
    different kettle of fish?

    Electronics in a room is more closely controlled and tends
    to use close control systems, not just AC systems.

    All rooms have a heat load and then find the heat loads
    of the electronics and then allow for them. Electrical
    equipment have wattage or Kw outputs on them so by using
    those values you can work out the total.

    There are quite a few free software packages online or
    you can use generic rules of thumb (all manufactures have them)
    and calculate from there.

    But be aware that when cooling electronics, redundancy tends
    to be built in.

    So for example if 15Kw of cooling was required instead of putting
    1 X 15 Kw system or 2 X 7.5 Kw systems in you might consider
    putting 3 x 7.5 Kw so if one fails you have a bit of redundancy.

    Rob

    .
    Last edited by Rob White; 22-08-2013 at 01:35 PM.
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

  5. #5
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    With the heat loads from the electronics the cooling capacity should be 300W it is inside a cabinet so it is a small confined space.
    The only thing I was actually wondering about is what factors are used when you are deciding the actual component :-)

    Regards
    Michell

  6. #6
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    And how to calculated the necessary heat exchange area of the evap. cond. :-)

    regards
    Michell

  7. #7
    Join Date
    Jan 2006
    Location
    Croatia
    Age
    62
    Posts
    2,121
    Rep Power
    23

    Re: Choosing the evaporator and condensator

    Hi, WootAC

    Welcome to RE forums ...

    Quote Originally Posted by WootAC?? View Post
    With the heat loads from the electronics the cooling capacity should be 300W it is inside a cabinet so it is a small confined space.
    The only thing I was actually wondering about is what factors are used when you are deciding the actual component :-)

    Regards
    Michell
    Not sure, but I believe if you check this forum maybe you'll find what you need .... or at least someone who can help you with equipment you need ...

    http://www.refrigeration-engineer.co...U-Overclockers

    here you can find such a small cooling unit ... I hope

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

  8. #8
    Join Date
    May 2012
    Location
    England
    Age
    53
    Posts
    1,234
    Rep Power
    28

    Re: Choosing the evaporator and condensator

    .

    Yeh there has been a lot of discussion re cooling
    PC boards and it is done in quite ingenious ways

    Regards

    Rob

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

  9. #9
    Join Date
    Jul 2013
    Location
    US
    Posts
    39
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    Cooling boxes are commonly available for electronic enclosures. I've worked on several machining centers that use them to provide cooling without admitting contaminated external air to the cabinet. Refrigeration capacities range from a few hundred watts to several kW.

    We have two of these cooling units on our main PLC cabinets. Basically, they are just glorified window shakers with absurd price tags. Cheaper than building from parts though.

  10. #10
    Join Date
    Oct 2008
    Location
    newbury
    Age
    56
    Posts
    1,349
    Rep Power
    29

    Re: Choosing the evaporator and condensator

    rittal make small coolers

  11. #11
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    I am actually more interested in how you buildt a unit from scratch, as I wrote before I am an engineering trainee and are very interesed in how the whole system works and how to decide on which components are right for the given job. I must admit that I am still unsure on some of the basics ex. what happens physical and theoretical with the fluid during startup, and how do the system responds if you use a capillary tube and run the sytem below the disigned ambient temperature.

    An example could be :
    ambient temperature 55 celcius and target temperature in the enclosure is 25 celcius.
    A capillary tube is used to regulate the pressure
    I know the ambient temperature is pretty high but this kind of device may need to be deployed in desert/jungle enviroment so that is the reason why it is this high.
    How does the fluid react (physical and theoretical) if the "start supply" temperature on the evaporator side is equal to the ambient temperature?
    And how will the system then respond if it is used in, lets say 30 celcius ambient suddenly?

    Regards
    Michell

  12. #12
    Join Date
    Mar 2009
    Location
    KZN, South Africa
    Age
    57
    Posts
    2,212
    Rep Power
    13

    Re: Choosing the evaporator and condensator

    Actually it is electronics cooling, i forgot to mention that earlier -.-
    I have gotten the cooling capacity from the customer and they calculated it to 300W.
    But I am struggling on how to decide the appropriate evaporator and condenser size.
    Your rough logic pathway:

    1. Known : Target cooling capacity : Q',e.
    2. Add in safety factor => Lock design cooling capacity : Q',evap.

    3. Determine
    3.1 Evaporator end
    3.1.1 Cooled part - operating temp : Tp,m
    3.1.2 Allowable DT,e for cooled part - say DT,e=Tp,m - Te,sat (nominal point)
    3.1.3 Te,sat = refrigerant saturation temp in evaporator - determined

    3.2 Condenser end
    3.2.1 Sink temperature (where the heat goes to) - Th,m
    3.2.2 Allowable DT,c for condenser - say DT,c=Tc,sat-Th,m
    3.2.3 Tc,sat = refrigerant saturation temp in condenser - determined

    3.3 Compressor
    3.3.1 Assume an operating efficiency (other approaches also work).
    3.3.2 Plug into Coolpack - run

    3.4 Determine
    3.4.1 Q',cond = condenser heat load
    3.4.2 W',comp = compressor power input
    3.4.3 m',ref = refrigerant mass flow-rate around circuit

    Known - to this point: Q'evap & Q'cond

    Head to Coolit software site, to see if their coil programs will assist for the condenser end of your design - if air-cooled.

    The evaporator part will, in all likelihood, be a specifically-designed component. Here you'll need to understand a little more about two-phase flows & forced evaporation. A fair bit of reading up for you here. A useful book to look for is :
    Heat Transfer and Fluid Flow in Minichannels and Microchannels - Kandlikar S.G, et al - Elsevier

    Iterate a few times, with incremental changes in one variable at a time. Refine solution.
    If this is a research project - I'm happy to collaborate. If commercial - I'm happy to consult on the specifics.
    Last edited by desA; 24-08-2013 at 10:25 AM.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

  13. #13
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    56
    Posts
    5,519
    Rep Power
    28

    Re: Choosing the evaporator and condensator

    Use a Peltier element for such a small capacity.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  14. #14
    Join Date
    Mar 2009
    Location
    KZN, South Africa
    Age
    57
    Posts
    2,212
    Rep Power
    13

    Re: Choosing the evaporator and condensator

    Peter:
    Use a Peltier element for such a small capacity.
    Nice & simple.

    Have COP's improved much over the years?
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

  15. #15
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    Thanks for the answer desA, we needed some sort of plan on how to begin the project!

    Peter_1
    We have already looked at Peltier devices, but the efficiency is way too low for the customer, so we quickly turned away from that again.

  16. #16
    Join Date
    Mar 2009
    Location
    KZN, South Africa
    Age
    57
    Posts
    2,212
    Rep Power
    13

    Re: Choosing the evaporator and condensator

    Absolute pleasure, WootAC. You'll learn a great deal along the development pathway.

    What compressor type have you in mind?
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

  17. #17
    Join Date
    Aug 2013
    Location
    DK
    Age
    26
    Posts
    27
    Rep Power
    0

    Re: Choosing the evaporator and condensator

    I hope so, it is pretty complex to understand every aspect of the AC unit so we have broken it down into the different components and are now on our way to designing one for our self.

    We are currently looking at micro rotary compressors (aspen, Purswave etc.), because we have a limit regarding the dimensions of the whole unit. But we have also looked at the slightly larger compressors from SECOP and Masterflux :-)


    Regards
    Michell

  18. #18
    Join Date
    Mar 2009
    Location
    KZN, South Africa
    Age
    57
    Posts
    2,212
    Rep Power
    13

    Re: Choosing the evaporator and condensator

    You are on the right track. Let us know how you get on.

    Remember, at all times : The evap/compressor/cond/restricter/piping/fans/pumps are all part of a system. One small change in one component will change the system equilibrium point.
    Last edited by desA; 26-08-2013 at 03:08 PM.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

Posting Permissions

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