# How to calculate thermal exchange rate of Air?

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%

mfb
Mentor
Air in which setup?
Heat capacity changes as function of humidity, you can find formulas somewhere in the internet.

russ_watters
Mentor
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...

CWatters
Homework Helper
Gold Member
Are you looking for the Specific Heat Capacity of humid air?...

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%.

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)?

Chestermiller
Mentor
You are forgetting about other, more important, heat transfer mechanisms like conduction and convection. In most practical situations, these will dominate over radiation.

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?

Chestermiller
Mentor
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?