Results 1 to 6 of 6
  1. #1
    Join Date
    Aug 2012
    Location
    Tawan
    Age
    27
    Posts
    4
    Rep Power
    0

    Calculation of the cooling tower evaporation



    Hi everyone!

    Do you know whether it is possible to estimate the rate of evaporation in a wet cooling tower? A rule-of-thumb indicates that the evaporation rate is approximately 1% of the water flow-rate per each 10F of cooling range. Otherwise, this rule-of-thumb doesn't take into account the meteorological conditions as the dry-bulb temperature and the relative humidity of the inlet air.

    If you have heard about some studies about this, I would be pleased to know more about them.

    Thank you.



  2. #2
    Brian_UK's Avatar
    Brian_UK is online now Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    69
    Posts
    10,383
    Rep Power
    49

    Re: Calculation of the cooling tower evaporation

    Brian - Newton Abbot, Devon, UK
    Retired March 2015
    Please support http://the100project.net/Home

  3. #3
    moideen's Avatar
    moideen is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    Apr 2008
    Location
    u.a.e
    Age
    42
    Posts
    417
    Rep Power
    16

    Re: Calculation of the cooling tower evaporation

    To find out the evaporation loss in percentage, the formula is
    Evaporation loss rate I =Absolute humidity at cooling tower exit-absolute humidity at tower inlet*1/ (L/G)*100.
    L/G= LIQUID GAS RATIO.
    OR
    Evaporation loss (m3/hr)=0.00085*1.88*circulation rate (m3/hr)*(T1-T2)

  4. #4
    Join Date
    Aug 2012
    Location
    Tawan
    Age
    27
    Posts
    4
    Rep Power
    0

    Re: Calculation of the cooling tower evaporation

    Thank you for your answers Brian_UK and moideen.

    The formula from: www.eng-tips.com, www.retscreen.net and the second one of moideen are almost all the same, only the significant digits differ. I know this formula and I think it gives a good idea of the evaporation loss rate, but it's not really accurate because the meteorological data isn't used.

    The formula from www.jutzi.com may be more accurate: ER (gpm) = F * DT (F) * RR (gpm) / 1000, where ER=evaporation rate and RR=recirculation rate. F depends on the meteorological data. But do you how the function to determine F? Because I looked for it a couple of weeks ago and I've never found it. I only found a graph with no accuracy, so I can't used it.

    I know a formula similar to your first one moideen. First you determine the flow rate of air:

    ma = ( mw(h3-h4) ) / ( (h2-h1) - (w2-w1)h4)

    where:
    ma: air mass flow rate
    mw: water mass flow rate
    h: enthalpy
    w: specific humidity
    1: inlet air
    2: outlet air
    3: warm water
    4: cool water

    Then, the evaporation loss can be determined by using this equation: EL=ma(w2-w1).

    But there are two many unknowns, we need to know the air outlet temperature to determine the air mass flow rate. But this temperature depends on the air mass flow rate. I'm lost.

  5. #5
    Brian_UK's Avatar
    Brian_UK is online now Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    69
    Posts
    10,383
    Rep Power
    49

    Re: Calculation of the cooling tower evaporation

    I agree, 'F' does seem to be a factor that is varied according to who is using it and for what purpose.
    Brian - Newton Abbot, Devon, UK
    Retired March 2015
    Please support http://the100project.net/Home

  6. #6
    Join Date
    Jan 2006
    Location
    Croatia
    Age
    62
    Posts
    2,117
    Rep Power
    23

    Re: Calculation of the cooling tower evaporation

    Hi, Kakolio

    Quote Originally Posted by Kakolio View Post
    Hi everyone!

    Do you know whether it is possible to estimate the rate of evaporation in a wet cooling tower? A rule-of-thumb indicates that the evaporation rate is approximately 1% of the water flow-rate per each 10F of cooling range. Otherwise, this rule-of-thumb doesn't take into account the meteorological conditions as the dry-bulb temperature and the relative humidity of the inlet air.

    If you have heard about some studies about this, I would be pleased to know more about them.

    Thank you.
    I do not know why do you need precise values of evaporation loss .... because I think that is not possible to calculate neither with simple nor with complex equation .... we can only estimate the rate of evaporation ....

    why!?!

    .... because cooling system is alive system and input values change every second ... we can only know how much water we utilize for i.e. one month period ... it is possible to install metering devices on fresh water inlet pipe and another one on blow-down pipe (still we have possible leaks and droplets removed by draft - unknown values) to calculate approximately how much water evaporate and that is that ... we need those data to determine equipment for chemical treatment of cooling water ....

    ... in another words rule-of-thumb or newest available formulas for new systems are quite applicable (old systems are another story, but still OK) and my warm suggestion to you is ... don't feel lost


    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.

Posting Permissions

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