Humidity Calculation

nxdmsandkaskdjaqd

Hi

Wondered if someone could help with this calculation.

Temperature outside 10c and Humidity 80%

If this outside air is raised to 18c, via heat exchanger, what would the humidity be?

I have looked at wet and dry humidity tables but could not see the answer there.

Thanks

rdr

The Humidity ( the proportion of water in the air) will stay the same.
80% RH at 10C is 0.974% absolute humidity, (0.974% of the air is water) at 18C this is 47.8% RH

penrhyn

Relative humidity drops by a factor of two for every 10 c rise in temperature, according to wikipedia!!!

But are you talking about the same air inside and outside?

nxdmsandkaskdjaqd

rdr wrote: »

The Humidity ( the proportion of water in the air) will stay the same.
80% RH at 10C is 0.974% absolute humidity, (0.974% of the air is water) at 18C this is 47.8% RH

Could you please point me to a table or calculation that works this out for different values?

nxdmsandkaskdjaqd

penrhyn wrote: »

Relative humidity drops by a factor of two for every 10 c rise in temperature, according to wikipedia!!!

But are you talking about the same air inside and outside?

No I am using a Heat Exchanger so the actual air is not the same. House air goes out and outside air comes in (slightly warmer).

rdr

I did the calculation by hand using data from here.

This link explains how http://www.arca53.dsl.pipex.com/index_files/calcair.htm

Wolfram Alpha will probably do the calculation for you if you can get the enquiry right.

rdr

nxdmsandkaskdjaqd wrote: »

No I am using a Heat Exchanger so the actual air is not the same. House air goes out and outside air comes in (slightly warmer).

So you are heating the air without changing its water content, this is what my calculation assumes. The amount of water in the air (absolute humidity) does not change, the relative humidity will decrease because warmer air is capable of carrying more water.
See my link above for definitions.

nxdmsandkaskdjaqd

rdr wrote: »

I did the calculation by hand using data from here. QUOTE]

I had a look at the tables but was unable to work out how to do the calculation. Could you let me know how to complete.

rdr

Look up the saturated vapour pressure (SVP) of water at the first temperature, 10C - 0.0123 bar
The relative humidity is 80% so the partial pressure of the water is .8* this = .00984 bar.
At 18C the SVP is 0.0206 bar.
0.00984/0.0206 = 0.477669903
therefore relative humidity is 48%