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Thread: Wetbulb formula

  1. #1

    Wetbulb formula

    I'm looking for a formula that I can write into my control basic that calculates wetbulb temperature. I have drybulb, humidity, dewpoint, and enthalpy to work with. This will be used for cooling tower fan speed control for my Carrier 19XRV centrifugals. Any advice would be greatly appreciated. Thank you.

  2. #2
    Join Date
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    http://hvac-talk.com/vbb/showthread....hlight=wetbulb

    osyio is da man. See the last post on the thread above.
    Propagating the formula. http://www.noagendashow.com/

  3. #3
    Thx!

  4. #4

  5. #5
    A quick technique that many forecasters use to determine the wet-bulb temperature is called the "1/3 rule". The technique is to first find the dewpoint depression (temperature minus dewpoint). Then take this number and divide by 3. Subtract this number from the temperature. You now have an approximation for the wet-bulb temperature.

    Here is an example: suppose the temperature is 42 degrees Fahrenheit with a dewpoint of 15 degrees Fahrenheit. The dewpoint depression is 42 - 15 = 27. Now divide 27 by 3 = 9. Now subtract 9 from the original temperature of 42. 42 - 9 = 33. If the temperature was 42 with a dewpoint of 15 and it started raining, the temperature and dewpoint would wet-bulb out to a chilly 33 degrees Fahrenheit. As dewpoint depression or temperature increase, the evaporational potential increases.

  6. #6
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    Quote Originally Posted by charity1218 View Post
    A quick technique that many forecasters use to determine the wet-bulb temperature is called the "1/3 rule". The technique is to first find the dewpoint depression (temperature minus dewpoint). Then take this number and divide by 3. Subtract this number from the temperature. You now have an approximation for the wet-bulb temperature.

    Here is an example: suppose the temperature is 42 degrees Fahrenheit with a dewpoint of 15 degrees Fahrenheit. The dewpoint depression is 42 - 15 = 27. Now divide 27 by 3 = 9. Now subtract 9 from the original temperature of 42. 42 - 9 = 33. If the temperature was 42 with a dewpoint of 15 and it started raining, the temperature and dewpoint would wet-bulb out to a chilly 33 degrees Fahrenheit. As dewpoint depression or temperature increase, the evaporational potential increases.
    First, thanks for the link. Informative stuff there. Much of which I've read before (I went through all his articles) but the particular writer explains certain aspects of the subject of weather in ways that make my fuzzy knowledge a little clearer. I always enjoy learning a little more than I knew. Or being a little more clear about something than I was.

    The short cut method you cite is pretty decent. I ran a quick calculation using a somewhat more accurate method and numbers you used, and the shortcut method was only about 1'F off. That's holding elevation constant. If I compare between a low point in a coastal city and someplace like Denver, the error would be around 2 degrees.

    I haven't a clue as to how accurate the OP is trying to get, or needs to be.

    In most cases, as long as you are talking dry bulb temps between 30 and 60 degrees, given the usual expected accuracies if the sensors involves, the shortcut method is probably close enough. But I ran a few calculations at higher dry bulbs ... i.e. 70 and 75 degrees ... and the errors are quite significant.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  7. #7
    Thanks for all the replies. I believe the formula i need is below (from Osiyo's post), but I'm not familiar with all of the language in the equations. I am trying to incorporate this into the control basic of WinCntrl XL Plus software. Any advice on how to accomplish this?

    Function psychro_wb(db, h, atm)
    wbtest = db
    Do
    htest = 0.24 * wbtest + (1061 + 0.444 * wbtest) * psychro_w(wbtest, wbtest, atm)
    wbtest = wbtest - 1
    Loop Until htest < h
    wbtest = wbtest + 2
    Do
    htest = 0.24 * wbtest + (1061 + 0.444 * wbtest) * psychro_w(wbtest, wbtest, atm)
    wbtest = wbtest - 0.1
    Loop Until htest < h
    wbtest = wbtest + 0.1
    psychro_wb = wbtest
    End Function


    Thanks!

  8. #8
    Join Date
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    Here is another post where he has his program in RapidQ which may be closer to Control Basic

    http://hvac-talk.com/vbb/showthread.php?t=73144

    Wetbulb is an iterative calculation so first you will have to insert the code he has in other functions to calculate it once properly and since there are no "loops" in Control Basic you will have to just copy and paste your code multiple times in a row to get the final number you need.

    In reading other sites about calculating in Excel it seem about 10 times through the calculation usually provides a pretty accurate number for most basic uses.

    It will take some time and plying with the code but all the information is there to do it.

    -Jeremy

  9. #9
    Thanks for all your help, but I can't decipher through it. I'm having a hard time putting it together properly because I don't know what some of the the letters refer to, but I will keep at it. Currently, I'm using the 1/3 rule, but it is way off when the dry bulb temp gets into the 80's and 90's. This is how I have the 1/3 rule written in Control Basic, which works fine in lower temps, but I live in Texas. If you'll notice the divisor changes from 3 to 2 when the OAT gets above 60, but that doesn't seem to be accurate enough.


    100 REM WetBulb Calculation
    105 IF 8-VAR20 > 60 THEN GOTO 150 ELSE 110
    110 D = 8-VAR20 - 8-VAR19
    120 E = D / 3
    130 8-VAR17 = 8-VAR20 - E
    140 GOTO 180
    150 D = 8-VAR20 - 8-VAR19
    160 E = D / 2
    170 8-VAR17 = 8-VAR20 E

    8-var20 = outside air temp
    8-var19 = dewpoint
    8-var17 = wetbulb temp
    D and E represent variables

    Thanks, Mark

  10. #10
    Join Date
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    Location
    Northern NY
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    I would have to take some time to look at the algebra and determine a calculation that could be used in Control basic BUT..


    I had another idea, I know control Basic can calculate dewpoint & enthalpy automatically so I did a quick google search for finding wetbulb from enthalpy and found lots of info.

    http://wiki.answers.com/Q/How_do_you...lb_temperature

    Some asked a question and got a really simple calculation but I'm not sure if it works but you could try and see if it helps.

    Wetblb= -0.015991*enthalpy*enthalpy+2.374*enthalpy+7.5089

    -Jeremy

  11. #11
    That simple calculation seems to be giving me the correct wetbulb temp at these higher ambient temps. I'll have to see how it does at colder temps and lower humiditys when the time comes. I will continue to monitor. You da man!

    Thank you!


    Mark

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