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Thread: Cooling tower evaporation, blowdown and concentration cycles

  1. #1 Cooling tower evaporation, blowdown and concentration cycles 
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    Here are the governing relationships for the makeup flow rate, the evaporation and windage losses, the draw-off rate, and the concentration cycles in an evaporative cooling tower system:



    M = Make-up water in m3/hr
    C = Circulating water in m3/hr
    D = Draw-off water in m3/hr
    E = Evaporated water in m3/hr
    W = Windage loss of water in m3/hr
    X = Concentration in ppmw (of any completely soluble salts … usually chlorides)
    Xm = Concentration of chlorides in make-up water (M), in ppmw
    Xc = Concentration of chlorides in circulating water (C), in ppmw
    Cycles = Cycles of concentration = Xc / Xm
    ppmw = parts per million by weight

    A water balance around the entire system is:

    M = E + D + W

    Since the evaporated water (E) has no salts, a chloride balance around the system is:

    M (Xm) = D (Xc) + W (Xc) = Xc (D + W)

    and, therefore:

    Xc / Xm = Cycles = M / (D + W) = M / (M – E) = 1 + {E / (D + W)}

    From a simplified heat balance around the cooling tower:

    (E) = (C) (ΔT) (cp) / Hv

    where:
    Hv = latent heat of vaporization of water = ca. 2260 kJ / kg
    ΔT = water temperature difference from tower top to tower bottom, in °C
    cp = specific heat of water = ca. 4.184 kJ / kg / °C

    Windage losses (W), in the absence of manufacturer's data, may be assumed to be:

    W = 0.3 to 1.0 percent of C for a natural draft cooling tower
    W = 0.1 to 0.3 percent of C for an induced draft cooling tower
    W = about 0.01 percent of C if the cooling tower has windage drift eliminators

    Concentration cycles in petroleum refinery cooling towers usually range from 3 to 7. In some large power plants, the cooling tower concentration cycles may be much higher.

    (Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)

    Milton Beychok
    (Visit me at www.air-dispersion.com)


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  3. #2  
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    Spammers-Declare-War:
    suck me nerd
    Thank you for your reasoned and well-thought out response!! How old are you? I didn't think they let children post here.

    Milt Beychok


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  4. #3  
    Universal Mind John Galt's Avatar
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    Mybechok, your post is intriguing. My own relationship with cooling towers has generally been of the form "My, those are big." Myopic, perhaps, but it has never seemed important that I learn about their particulars.
    Might I ask, though, what exactly was the point of the post? Microscopic examination of the mechanics of tooling towers seems to be just information. Missing something, am I?
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  5. #4  
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    Ophiolite:

    The name of this forum is "The Science Forum" which I took it to mean that perhaps some people interested in or involved with Environmental Science or Environmental Technology or Pollution Control visited or participated in the forum. Such people would be interested in learning how to:

    (1) determine the amount of blowdown water from large cooling towers and how concentrated the dissolved solids were in the blowdown water.
    (2) determine the amount of windage loss to the ambient atmospheric air and how concentrated the dissolved solids were in the windage loss water.
    (3) reduce the concentration cycles of dissolved solids in a cooling tower.

    My posting was not a "microscopic examination of the mechanics of cooling towers". I did not go into how such towers are sized and designed because that has nothing to do with their environmental issues.

    As a retired chemical engineer, with over 45 years of experience in designing large oil refineries, power plants and petrochemical plants, I know that the sketch and the information in my posting about cooling towers are not well known nor are they easy to come by.

    I see that you are a moderator on this forum and I realize that my postings in this forum have been criticised as being self-serving ... which I deny. I also am a moderator of three engineering/science forums on the Internet. I have learned that it is very important to carefully define the purpose of a forum and the type of postings that will be accepted. If you don't want technical information and hard science discussed on this forum, then you should make that clear to prospective participants.

    Who is in control of everything?
    I'd like to know,was it coincidence or God was really behind all this???
    Come to think of it,why do we have time? Why the composition of water has to be h2o??
    Does anyone have any idea of this?
    ________________
    living like hell down here!!!!!
    Cracking your knuckles:
    i know its bad for me but i do it anyway, i would stop but i can't.

    why can some people do this while others can't?
    _________________
    mmmm...Babyfood.
    The above are two postings selected from other parts of this forum. Do you prefer them to mine?? Is that what the moderators of this forum call Science?

    Regards,

    Milt Beychok
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  6. #5  
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    First, a small point of clarification: my observations were entirely personal, unrelated to my role as a moderator.
    Secondly, I think you have entirely misunderstood the intent of my post. It is my perception, based upon observation on many forums, that simply posting information is of limited value.
    1. The chance that someone who specifically requires it will stumble across it is small.
    2. It is unlikely to generate discussion, which is arguably the principal function of such forums.
    These limitations can be overcome if the information is linked to a problem; even better, to the solution of a problem; or, open up new possibilities; or, suggest new comments. It seemed to me you may have had such a thought in mind so clearly that it seemed explicit, whereas it was either absent or obscure. So, I asked, ‘what was the point’?
    My observation that your post was interesting was not sarcasm, or irony, but a factual statement.
    Noting from your second post that you intended it purely for information, I suggest you have missed an opportunity. Surely there are some important issues that arise from your post that could have been explored, and that would have initiated much discussion. You would know better than I what these would be. Perhaps you could throw out some comments.
    If you don't want technical information and hard science discussed on this forum, then you should make that clear to prospective participants.
    We do. And the key word is discussed. I hope I have adequately explained how, as it stood, your post was not likely to directly induce such discussion.

    I see that you are a moderator on this forum and I realize that my postings in this forum have been criticised as being self-serving
    I am not sure how you would take from my post that you were being criticised for being self serving. I was not so doing. [I don't, frankly, see how simply posting some technical information could ever be seen as self serving.]
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  7. #6  
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    Ophiolite:

    It would appear from your last response that I have indeed misinterpreted your earlier remarks ... and I apologise for having done so.

    Since I included my website in my signature at the end of all my postings to this forum, I have received a number of emails from people who have accused me posting here so that I could advertize my site ... and I guess that is what led me to misinterpret your remarks.

    Happy Holidays and best wishes for the coming New Year,

    Milt Beychok
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  8. #7  
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    I suppose the inclusion of the website was towards the margins, so I can both understand the reaction of some, and why you would then interpret my remarks as you did.
    If I may return your apology: part of the 'terseness' of my post was a result of a whimsical notion. I noticed, as I was typing the post, that my first two sentences had begun with the letters m y. I then attempted to fit my thoughts into a structure where each sentence began with an m i or an m y. Silly, yes. But it is Christmas.

    Seasons greetings.
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  9. #8  
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    >>>1. The chance that someone who specifically requires it will stumble across it is small.

    OTOH I have just stumbled upon just the info I was looking for. Short, sweet, authoritative and I just registered merely to say thank-you.

    Rgds,
    TC
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  10. #9  
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    Delighted this was what you were looking for. Maybe you can stick around and fine some pleasure in the odd discussion or two.
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  11. #10  
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    Delighted this was what you were looking for. Maybe you can stick around and fine some pleasure in the odd discussion or two.
    Thanks, I wasn't sure if an engineer qualifies for a science forum. :wink:

    I'll have a look around and see if there's anything that catches my eye.

    Rgds,
    TC
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