# dry cold air vs a wet hot air

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### #1 MatsNorway

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Posted 22 January 2014 - 12:55

Is it possible to get a dry cold air mixture to be equal in weight with a wet hot air mixture?

I was looking through Mollier diagrams but they all talk about energy so now i do not know what to search for.

Lets assume same pressure.

Edited by MatsNorway, 22 January 2014 - 13:09.

### #2 blkirk

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Posted 22 January 2014 - 14:07

What you need is a psychrometric chart.

http://www.fao.org/d...0e/S1250EEV.GIF

It looks really complicated, but once you look at it a while, it'll start to make sense.

Lines of constant temperature run vertically and are labeled across the bottom axis.

Lines of constant specific volume (inverse of density) are straight and run up and to the left like this \.

Lines of constant moisture content are horizontal and are labeled on the right axis.

Lines of constant relative humidity curve up to the right.

The simple answer to your question is, no.  Cold dry air is heavier than hot wet air.  Cold wet air can be the same density and hot dry air, though.

The reason this happens is because H2O molecules weigh less (18) than N2 (28) and O2 (32) molecules.  So when you add H2O to your air and keep the pressure the same, the lighter H2O molecules displace the heavier N2 and O2, and the weight of your air goes down.

Edited by blkirk, 22 January 2014 - 17:29.

### #3 gruntguru

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Posted 23 January 2014 - 02:47

Is it possible to get a dry cold air mixture to be equal in weight with a wet hot air mixture?

I was looking through Mollier diagrams but they all talk about energy so now i do not know what to search for.

Lets assume same pressure.

Mats, if your question relates to aero (downforce or drag) then air density is important. Note that Specific Volume on the psychrometric chart does not include the mass of the water vapour so that will have to be added in.

If your question relates to engine performance, it is oxygen per unit volume that is important. This is inversely proportional to specific volume on the psychrometric chart since this is expressed in m^3/(kg of dry air) so moisture is not included in the mass term, but is included in the volume term.

Moisture content is also useful if the engine is detonation limited (allows more ign adv or boost) but is generally detrimental because of the oxygen it displaces.

To illustrate all this, look at 3 points on the chart, starting with 25*C - 20% relative humidity. From the chart, the specific volume is 0.85 m3/kg and water vapor content is 4g/kg. Dry air density is the inverse of SV ie 1/0.85 = 1.176. Actual air density is 1.176 x (1000 + 4)/1000 = 1.18

If we move to 100% RH at the same temp, we see that SV is now about 0.87 so engine power will be down by about 2.3% just because of the humidity. Water content is up to 20g/kg. Dry air density is 1/0.87 = 1.149. Actual air density is 1.149 x (1000 + 20)/1000 = 1.172

If we look at a cold day - say 8*C but still 20% RH, the SV is now 0.8, so engine power will now be up by 6.25% from point 1 and 8.75% from point two.

Edited by gruntguru, 23 January 2014 - 02:55.

### #4 JAW

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Posted 23 January 2014 - 02:52

Fuel type/fractional composition also figures into data set variables.. ..molecular catalytic synergistics interactivity-wise..

### #5 gruntguru

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Posted 23 January 2014 - 02:57

Too technical for this forum JAW.

### #6 JAW

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Posted 23 January 2014 - 03:07

Well, then..

Maybe M.N. would like to tip a can of I.C. model Methanol-Nitro-meth' mix in to his turbo tank - for a bit more boost?

### #7 MatsNorway

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Posted 23 January 2014 - 07:52

I and others do not mind getting explained in great detail what molecular catalytic synergistics interactivities is.

The question was related to air compressors and their performance in relation to air temp and humidity. Then i went all crazy because i could not answer what was the heaviest air mixture. I was teorising that perhaps the hydrogen atom lifts the oxygen atoms..

Working on the frame today perhaps. Perhaps some pictures gets shown.

Edited by MatsNorway, 23 January 2014 - 07:56.

### #8 JAW

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Posted 23 January 2014 - 07:57

Burnout pix?.. Bit of drift action.. ..maybe? ..c'mon..