Relative humidity calc at high temps

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Discussion Overview

The discussion revolves around the calculation of relative humidity (RH) and wet bulb temperature at high temperatures, specifically up to 500 degrees Fahrenheit. Participants explore the limitations of existing psychrometric charts and the applicability of steam tables for these calculations, as well as the implications for industrial applications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks equations for calculating RH based on wet bulb and dry bulb temperatures at high temperatures, noting a lack of resources for temperatures above 250F.
  • Another participant suggests that steam tables can provide RH from partial pressure and saturation pressure, but expresses uncertainty about deriving wet bulb temperature from these values.
  • A participant mentions that the ASHRAE manual may contain relevant psychrometric data but questions its usefulness given the challenges at high temperatures.
  • Concerns are raised about the meaningfulness of RH at high saturation pressures, with one participant noting that at 500F, the saturation pressure is 680 psi, complicating the concept of RH.
  • Discussion includes the idea that at high temperatures and pressures, the concept of RH may not apply in the traditional sense, especially in scenarios with varying proportions of air and water vapor.
  • One participant indicates that the pressure in their application could range from 0.5 to 2 atmospheres and emphasizes the need for dynamic inputs in calculations.
  • There is a recognition that while wet bulb temperature may still provide some information, its accuracy diminishes significantly at higher temperatures, with a participant noting it could still be within 5-10% accuracy.

Areas of Agreement / Disagreement

Participants express differing views on the applicability and accuracy of RH and wet bulb temperature calculations at high temperatures. There is no consensus on the best approach or the validity of existing methods in this context.

Contextual Notes

Limitations include the lack of psychrometric charts for high temperatures, the complexity of determining partial pressures of water vapor, and the potential inaccuracy of wet bulb temperature measurements in industrial applications at elevated temperatures and pressures.

Ivan Seeking
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I am looking for equations valid at temps up to 500 degrees F, that allows one to calculate the relative humidity for air based on wet bulb and dry bulb temps. Everything that I can find is only accurate at lower temps. For that matter I can't even find a psychrometric chart that goes that high. Am I missing something here?

Any ideas?

I need to calculate the wet bulb temp based on the rh, and Tdry, over the same range as well, but I can work around this if needed.
 
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Psychrometric charts don't go that high, but steam tables do. You can get RH from partial pressure and saturation pressure, though I'm not sure offhand how to get from that to wet bulb temp.
 
russ_watters said:
Psychrometric charts don't go that high, but steam tables do. You can get RH from partial pressure and saturation pressure, though I'm not sure offhand how to get from that to wet bulb temp.

thanks Russ.

I did find one chart that goes to 250F [dry bulb].

Have you ever looked at the psychrometric data and equations in the ASHRAE manual? I am thinking that it may be worth buying it, but then again it may not have any more information that I have now.

I know that we can get the saturation line at a given temp and press, but to dynamically determine the partial pressure of water is apparently a real problem at high temps. I had a long talk with the NIST this morning, and according to them, wet bulb temps are virtually meaningless once we get up around 100C. In fact they are working on this very problem right now for a 200C test, and they have no realible way to get a measurement as yet.

I suspect that there are going to be a few raised eyebrows when I pass along what I have found. It seems that there has been 50 years of reliance on meaningless data.
 
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Well, since the saturation pressure at 500F is 680 psi, it is a little meaningless to talk about "relative humidity" - there could be 50 times more water than air in the sample (unless the air is highly pressurized in the sample too).

I remember doing calculations on this sort of thing (not RH or wet bulb temp, though, just reproducing the steam tables) in thermo, but that was one of those this-is-how-its-done-we'll-never-do-it-again type things.

May I ask what this is for?
 
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Still a fraction though... I expect that the rh is always very low at those temps.

Its for an industrial application. As usual I can't say much more or someone might break my legs. :biggrin:
 
Ivan Seeking said:
Still a fraction though... I expect that the rh is always very low at those temps.

Its for an industrial application. As usual I can't say much more or someone might break my legs. :biggrin:
I didn't mean about the application, just the conditions. Can you at least tell me if the pressure you are dealing with is atmospheric? I guess it is ok if you can't, it doesn't really change my point:

The point I was trying to make (and I was slightly wrong in what I said...) is that you could have a vessel that is completely filled with water vapor (no air) at 500F and atmospheric pressure. Without having any air, is there even such a thing as "relative humidity"? Or you could have a mixture of half air and half water vapor. I would think that in that situation, you don't want to say you have 1.1% relative humidity (7.35/680), but rather that you simply have a 50% volume fraction of water vapor in your mixture.
 
The pressure could range from about 0.5 to 2 atmospheres. This also needs to be a dynamic input for the calculation.

It seems that the industry standard is rh, but either way the problem is to dynamically determine the moisture content present in air up to 500F, to within 0.5% accuracy. IR beam attenuation is one approach, but contaminants in the air stream make this impractical.

Also, earlier I said that web bulb is meaningless at these temps, but what I should have said that it is far too inaccurate for this application at the higher temps. Apparently it is still good to within five or ten percent, even at these temps and pressures.
 
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