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Calculate humidity from dry/Wet bulb temp

  1. Nov 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Dry-bulb Temperature 46'C
    Wet-bulb Temperature 34'C
    P=101.3kPa

    with these, I have to 'calculate' Specific Humidity(w), Relative humidity(φ), Specific Enthalpy(h)


    2. Relevant equations

    Specific Humidity
    w=0.622*Pv/Pa
    when Pa=P-Pv

    Relative Humidity
    φ=w*P/[(0.622+w)*Pg]

    Specific Enthalpy
    h=ha+w*hg

    for this problem, I used approximated values based on temperature for ha.
    so
    ha=1.005[kJ/KgC]*T[C]
    ha[kJ/KG]

    3. The attempt at a solution
    P is 101.3
    T is 46'C

    from Table A-4, Pv is 10.14kPa at 46'C

    w=(0.622*10.14)/(101.3-10.14)=0.069
    6.9%

    φ=0.069*101.3/[(0.622+0.069)*10.14)=0.9976
    99.76%
    this is when I start to feel a bit weird


    h=1.005*46+0.069*2584.18
    =224.54kJ/kg


    Then I checked my Psychrometric Chart, and found out this makes no sense. Where did I go wrong? or am I reading the the chart wrong?
     
  2. jcsd
  3. Nov 26, 2015 #2

    rude man

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    Since your wet bulb temp is close to freezing the curves on the psychrometric chart are at one end so it's a bit hard to get accurate readings, but I got
    rel humidity ~ 33%
    enthalpy ~ 12.5 BTU per lb. dry air. Sorry these units are not SI.
    humidity mass ratio ~ .004 moisture/dry air.
    Don't know about your equations but I can tell from their simplicity that they are rough approximations to the chart numbers.
     
  4. Nov 26, 2015 #3

    SteamKing

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    Wet bulb temperature is 34° C. That's not close to freezing. In fact, it's rather balmy. I think you got your temperature units mixed up.
     
  5. Nov 26, 2015 #4

    SteamKing

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    It's not clear what Table A-4 is. Can you link to it?

    Here is a psychrometric chart in SI Units:

    http://www.uigi.com/UIGI_SI.PDF

    Your dry bulb/wet bulb temps are almost off this chart.

    I would say the enthalpy is about 122 kJ/kg of dry air and the relative humidity is about 45%
     
  6. Nov 27, 2015 #5

    rude man

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    Well you're absolutely right. I'll take another shot at it soon.
     
  7. Nov 27, 2015 #6

    rude man

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    With the correct temperatures I got about 38% rel. humidity, 0.025 kg moisture /kg dry air, and H ~ 56 BTU//lb dry air = 129 kJ/kg. On my chart I had to estimate enthalpy a bit.
     
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