Why evaporative cooling is a constant wet bulb temp process?

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Discussion Overview

The discussion centers around the concepts of wet bulb temperature and dew point, particularly in the context of evaporative cooling processes. Participants explore the definitions, differences, and implications of these terms, as well as their application in adiabatic saturation processes and evaporative coolers.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants clarify that wet bulb temperature is defined as the lowest thermodynamic temperature achievable at 100% relative humidity, while dew point is the temperature at which air must be cooled to reach 100% relative humidity without evaporation.
  • There is a question regarding the difference between dew point and wet bulb temperature, with some noting that they appear the same at 100% relative humidity but differ significantly at lower humidity levels.
  • Participants discuss the use of the term "approximately" in textbooks when comparing wet bulb temperature and adiabatic saturation temperature, suggesting that the two are not exactly the same despite both reaching 100% relative humidity.
  • One participant raises a concern that in the adiabatic saturation process, the air is saturated, while in the wet bulb process, only the cotton wick is saturated, implying ongoing evaporation from the wick.
  • There are multiple inquiries about the heat transfer mechanisms in both the adiabatic saturation process and the wet bulb process, with participants suggesting the need for control volume analysis to understand temperature and heat transfer dynamics.
  • Some participants express understanding of the individual concepts but question whether the similarities between wet bulb temperature and thermodynamic wet bulb temperature are coincidental or fundamentally linked.
  • Another participant suggests that the difference between wet bulb temperature and thermodynamic wet bulb temperature is minimal, indicating that the former is close to saturation but not fully saturated.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and interpretation regarding the definitions and implications of wet bulb temperature and dew point. There is no consensus on the exact relationship between these concepts, and multiple competing views remain regarding their application in different processes.

Contextual Notes

Some discussions involve assumptions about humidity levels and the conditions under which wet bulb temperature and dew point are measured. The conversation also reflects a dependence on specific definitions and the context of heat transfer in different processes.

Ravi Singh choudhary
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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.
 
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Ravi Singh choudhary said:
(i) What is the difference between dew point and wet bulb temperature; both appears same to me?
Att 100% relative humidity they are.
One would notice a big difference at 0% relative humidity.

Ravi Singh choudhary said:
In textbooks it is quoted as wet bulb temperature is approximately same as wet bulb temperature
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?
 
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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.
 
256bits said:
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?

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?
 
Ravi Singh choudhary said:
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?
Makes sense.
 
256bits said:
Makes sense.
I understood both individually. My doubt is; is it a coincidence of both things are same or what?
 
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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