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I Why evaporative cooling is a constant wet bulb temp process?

  1. Jun 9, 2016 #1
    Wet bulb temperature is the lowest thermodynamic temperature of air that can be achieved at ambient condition, by creating 100% relative humidity in the immediate surrounding of the thermometer using cotton wick.

    As Wikipedia says "By contrast, the dew point is the temperature to which the ambient air must be cooled to reach 100% relative humidity assuming there is no evaporation into the air; it is the point where condensate (dew) and rain would form."

    So I am clear with the definition of both the terms.
    Now come to the adiabatic saturation process; where I understand how it is a constant enthalpy process; just redistribution of molecules within system.

    My doubt:
    (i) What is the difference between dew point and wet bulb temperature; both appears same to me?
    (ii) In textbooks it is quoted as wet bulb temperature is approximately same as wet bulb temperature; why this approximately term is used. Both should have exactly same, as both have final condition of 100% relative humidity and both are achieved by evaporation of water molecules to the unsaturated air.

    (iii)Now come to evaporative coolers; say a sand pot, it is also said to be constant wet bulb temperature process, HOW?; Wet bulb temperature is defined at the point where 100% relative humidity is achieved not before same as adiabatic saturation temperature.
  2. jcsd
  3. Jun 11, 2016 #2


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    Att 100% relative humidity they are.
    One would notice a big difference at 0% relative humidity.

    I believe you mean "adiabatic saturation temperature" for one of the terms.
    Put a control volume around the long chamber for the adiabatic saturation process, and how is heat transferred. What temperature and of what are we recording at the output?

    Put a control volume around the wet bulb process, and how is heat transferred. What temperature and of what are we recoding?
  4. Jun 11, 2016 #3
    But adiabatic temperature is measured when air is saturated. In case of wet bulb only cotton wick is saturated, not the air flowing nearby, that means all the time water from cotton wick will evaporate there.
  5. Nov 5, 2016 #4
    Putting control volume around the long chamber for a adiabatic saturation process; there is no heat transfer across the boundary. Inside the boundary; water is evaporating and humidity is increasing. While in the case of wet bulb thermometer there is a heat transfer to the cotton wick and water is also at the same time evaporating from cotton maintaining equilibrium temperature.

    Does that make sense?
  6. Nov 5, 2016 #5


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    Makes sense.
  7. Nov 6, 2016 #6
    I understood both individually. My doubt is; is it a coincidence of both things are same or what?
  8. Oct 9, 2017 #7
    Bro as per my understanding WBT and thermodyanamic WBT has very little difference .Simply WBT is not at saturation of air but close to saturation of air but thermodyanamic WBT is for 100 %RH or fully saturated air .
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