How to calculate thermal exchange rate of Air?

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Discussion Overview

The discussion focuses on calculating the thermal exchange rate of air as it relates to varying humidity levels, particularly how this affects radiant heat absorption at a specified temperature (10°C). Participants explore the implications of humidity on heat capacity and the mechanisms of heat transfer, including radiation, conduction, and convection.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about calculating the thermal exchange rate of air with different humidity levels and expresses a desire to create a graph illustrating radiant heat absorption.
  • Another participant suggests that heat capacity changes with humidity and points to available formulas online.
  • A participant mentions that the difference in heat capacity due to humidity is not significant for most ambient conditions, providing a specific value in English units.
  • There is a question about the specific conditions under which the calculations are being made, such as standard atmospheric pressure at mean sea level.
  • One participant provides a formula for the specific heat capacity of humid air, noting that it is slightly greater than that of dry air, but emphasizes that the difference is minimal.
  • Another participant raises the point that dry air and humid air absorb radiation differently, suggesting that a formula could be developed to illustrate this relationship across varying humidity levels.
  • Concerns are raised about the dominance of conduction and convection over radiation in practical scenarios, questioning the relevance of radiation in the calculations.
  • Participants discuss the need for measurements or calculations regarding how heat transfer mechanisms change with water vapor content in the air.
  • One participant expresses confusion about the original question and suggests looking up the water vapor infrared absorption spectrum for relevant data.

Areas of Agreement / Disagreement

Participants express differing views on the significance of humidity's effect on thermal exchange rates and the relative importance of radiation compared to other heat transfer mechanisms. The discussion remains unresolved, with multiple competing perspectives on how to approach the calculations.

Contextual Notes

There are limitations regarding the assumptions made about ambient conditions, the specific definitions of heat transfer mechanisms, and the mathematical steps involved in deriving the thermal exchange rate.

AntiSpark
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How would I calculate thermal exchange rate of Air for different amounts of humidity?

I'd like to create a graph to show how much radiant heat air would absorb, (at some temperature (10°C) changes depending on the humidity amount, 0 - 100%
 
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Air in which setup?
Heat capacity changes as function of humidity, you can find formulas somewhere in the internet.
 
The difference is not significant for most ambient temperatures/humidities. In English units, it is about 1.082 BTU/CFM-F
 
For air at SAP at MSL ?

I've looked in the internet and haven't found it yet...
 
Are you looking for the Specific Heat Capacity of humid air?...

See reply 4...

http://physics.stackexchange.com/qu...s-how-much-time-is-needed-for-heating-the-air

The heat capacity of humid air is approximately given by:

C p =1.005+1.82H

where 1.005 kJ/kg°C is the heat capacity of dry air, 1.82 kJ/kg°C the heat capacity of water vapor, and H is the absolute humidity in kg water vapor per kg dry air in the mixture. So the specific heat capacity of humid air is greater than dry air and humid air will take more energy to heat by a given amount.

But the difference is quite small. I think 100% RH at 25C is only about 2% water, and if you need to heat the room the temperature, and therefore the water content, is presumably even lower. Taking the 2% water content only increases the specific heat by about 3.6%.
 
CWatters said:
Are you looking for the Specific Heat Capacity of humid air?...

As I understand it, dry air is heated by radiation by some small amount and 100% humid air by a greater amount?
So I should be able to find or create a formula to make a graph showing the change from 0 to 100% humidity (or find a graph)?
 
You are forgetting about other, more important, heat transfer mechanisms like conduction and convection. In most practical situations, these will dominate over radiation.
 
Chestermiller said:
You are forgetting about other, more important, heat transfer mechanisms like conduction and convection. In most practical situations, these will dominate over radiation.

If there are measurements or calculations to show how much they change (if any) depending on the amount of water vapor in the air? I'd take that as well.

But I was looking for how much the air would be warmed by IR radiation, and then that heat would be transferred by conduction and convection.

Seems as tho if I knew what amount of IR radiation would be absorbed by a sample of air that was 1% water vapor, that I would just double that to estimate it for 2% water vapor?
 
AntiSpark said:
If there are measurements or calculations to show how much they change (if any) depending on the amount of water vapor in the air? I'd take that as well.

But I was looking for how much the air would be warmed by IR radiation, and then that heat would be transferred by conduction and convection.

Seems as tho if I knew what amount of IR radiation would be absorbed by a sample of air that was 1% water vapor, that I would just double that to estimate it for 2% water vapor?

Sorry. I was confused about what you were asking for. For what you are asking, Google
Water Vapor Infrared Absorption Spectrum. Of course, this is just the basic property data. You still need to know how to apply the data in modeling atmospheric heating.

Chet
 

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