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  1. #1
    Join Date
    Jun 2009
    wales, cardiff
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    equation for correct evap and cond pressures

    iv heard many different techs talking about many different ways to work out correct pressures for evap and cond pressures based on system design specifications and ambient temps but i cant seem to remember the the equation ...a bit of help here please guys
    many thanks.

  2. #2
    Join Date
    Feb 2012
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    Re: equation for correct evap and cond pressures

    If you want formulas:

    Supposing the main heat transfer is by convection.

    Q = U x A x TD

    Q = design load for the evaporator
    U = global heat transmission coefficient (if you generalize this coefficient you can take into account different forms of heat transfer as long as that mechanism uses the same TD)
    A = Heat transfer surface’s area
    TD = Temperature of air in – saturated temperature at outlet of evap. (though this is one of many differentials that can be defined and used)

    You can use the same formula for the condenser thought the new Q would be the previous Q plus the heat of compression + losses.

    In practice nobody uses these formulas. Manufacturers give you a relation between Q and whatever TD they think convenient that considers all forms of heat transfer to their equipment. So you end up working with their catalogs instead of formulas but the idea is the same.

  3. #3
    Join Date
    Oct 2006
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    Re: equation for correct evap and cond pressures


    There are equations but every system will work slightly differently to another.

    There are good rules of thumb.....

    Like the 10 degree rule.

    But again this rule only works in general and if you want specific calculations
    you realy need to go back to pressure temperature calculations using comparaters
    and maths.

    Most engineers agree that most evaporators do 10 degrees over the coil and most condensers
    are about 15 degrees above ambient.

    So if you have a room at 0 degC then the evap will be nearer -10 degC, if the system is a freezer
    at -20 degC then the evap will be nearer -30 degC but if its and AC and the air off the evap is
    +10 degC then the evaporating temp will be nearer 0 degC.

    All rough and easily proved wrong but they are agood starting point for most situations.



  4. #4
    Join Date
    Oct 2003
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    Re: equation for correct evap and cond pressures

    Important: DT for the evaporator determines also RH in you cold room! There's somehow a relation between DT and RH.
    If you want to reduce power consumption: max 10DT for the condenser.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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