Relationship between pressure, temperature and O2

shane70 · 20-01-2015 10:51am #1

I am trying to find out the relationship of the amount of oxygen in air and how it is affected at differing times of year, at differing temperatures and differing air pressures.

As temperature and pressure are directly related, with 20.9% of air being O2, rest mainly nitrogen:

1) Does this 20.9% vary between high pressure days and low pressures?
2) Does it also vary during hot and cold days (summer and winter)?
3) Is it more difficult for air consuming devices to extract the oxygen for combustion and how is it being affected, i.e. it is easier or harder due to pressure or due to temperature?

I have a particular interest in finding out the accurate answer and thanks in advance.

Tactical · 20-01-2015 7:51pm

Well, a number of internal combustion engines utilise an intercooler to cool the intake air (typically after a turbo)....

Cooling the air makes it more dense.....

So if you consider that for a given space containing air, a certain amount of oxygen molecules will be present. If the air is warm it will be less dense meaning greater spaces between the molecules. If you have cooler air, it will be more dense meaning less space between the molecules.

So, for a given space e.g. a combustion chamber or cylinder, if you introduce cold dense air you will have a greater number of oxygen molecules than if you introduce warm air.

More oxygen molecules for a set volume of fuel will result in more efficient combustion.

In this example the percentage of oxygen doesn't change, simply the density of the air.

A cold day will result in denser air and therefore more efficient combustion.

Others more knowledgeable may be able to offer a more scientific input to the query though.

shane70 · 20-01-2015 8:06pm

So should I expect to see a difference in the percentage of O2 from season to season?

When measured, I seem only to get a stable 20.9%. An odd time it may fluctuate perhaps 0.1 - 0.2% between seasons.

I am indeed measuring for a better stoiciometric combustion but not in an engine. When altering combustion CO2, I am leaving an amount of excess air to accommodate for seasonal variations but there is disputed amounts that should be catered for.
Would the seasonal variation in O2 content alter the CO2 by 1 - 1.5%?

Tactical · 20-01-2015 8:18pm

I don't know about variations in the actual percentage of oxygen in air.

Seasonal / temperature variations affect the whole parcel of air so the ratios of the gasses would remain the same.

What is the accuracy of your measuring instrument? Have you looked at its accuracy with respect to temperature? Is it a calibrated instrument? Have you consider polution in your measurements?

shane70 · 20-01-2015 8:36pm

Yes it is a calibrated analyiser, calibrated annually. It has a dedicated O2 cell and a CO cell and it calculates CO2 based upon the calorific value of the fuel that is being burned. I can only presume it is fairly accurate to within one decimal place.

Would the amount of O2 change from a low pressure day to a high pressure day or is it a matter of how easy it is for O2 to be extracted by the chemical reaction of the combustion process?

Thanks, I appreciate your input.

brianwalshcork · 20-01-2015 8:38pm

I'm with Tactical!

If you are measuring differences in the % of oxygen in the air then your more likely seeing local affects.

Higher pressures or temperatures won't change the % o2... But lower temperatures in winter could mean more coal fires burning, less leaves on trees so lower amounts of c02 being turned back into o2... whether this is a measurable amount or not, I don't know...we'll have to wait for the people who know what they are talking about arrive!

If your seeing variations of 1-1.5% over a short space of time (not years) then I'd say your seeing experimental errors...

shane70 · 20-01-2015 8:44pm

Tactical wrote: »

In this example the percentage of oxygen doesn't change, simply the density of the air.

Reading back over your post, I can see how the percentages would not change, but would that mean that there are more molecules of each property be present, just still in exactly the same proportions to each other?

If this is so, then there is more volume of O2 molecules within a 1m3 of air in winter than there is in summer. Am I correct in saying this? Being more dense, more can fit into the same space.

Don't get me wrong, 1m3 of air is 1m3 of air but if it is more dense, then we perhaps can fit more into it, just in equal proportions.

shane70 · 20-01-2015 8:56pm

brianwalshcork wrote: »

If your seeing variations of 1-1.5% over a short space of time (not years) then I'd say your seeing experimental errors...

Apologies, 1 - 1.5% is exaggerated.

Given a 20C differential, a 0.75% in combustion CO2 variation is experienced.

Tactical · 20-01-2015 9:26pm

shane70 wrote: »

Reading back over your post, I can see how the percentages would not change, but would that mean that there are more molecules of each property be present, just still in exactly the same proportions to each other?

If this is so, then there is more volume of O2 molecules within a 1m3 of air in winter than there is in summer. Am I correct in saying this? Being more dense, more can fit into the same space.

Don't get me wrong, 1m3 of air is 1m3 of air but if it is more dense, then we perhaps can fit more into it, just in equal proportions.

Yes, that is exactly what I am saying.

FWVT · 21-01-2015 8:21am

Taking two different weather scenarios (at sea level) shows how much density can be affected.

1. A cold and frosty winter's day. Temp 0, dewpoint -5, pressure 1040 hPa.

Air density: 1.324 kg/m3

2. A warm and muggy day in summer. Temp 25, dewpoint 20, pressure 990 hPa.

Air density: 1.156 kg/m3

Air density is 12.7% less in the summer example. This means a 12.7% reduction in combustion output.

shane70 · 21-01-2015 9:27am

FWVT wrote: »

Taking two different weather scenarios (at sea level) shows how much density can be affected.

1. A cold and frosty winter's day. Temp 0, dewpoint -5, pressure 1040 hPa.

Air density: 1.324 kg/m3

2. A warm and muggy day in summer. Temp 25, dewpoint 20, pressure 990 hPa.

Air density: 1.156 kg/m3

Air density is 12.7% less in the summer example. This means a 12.7% reduction in combustion output.

But the density of 12.7% less is total density and taking into account all the other properties of air, such as nitrogen if the proportions remain constant.

Therefore would it not infer the affect to combustion (as the only useable property for combustion is O2) would only be 20.9% of 12.7%, thus a net affect of only 2.65%?

shane70 · 21-01-2015 9:38am

Or is my last post a stupid reply and everything within air is reduced equally by 12.7%?

I am leaving an amount of air additional to the amount of air required for stoichiometric combustion of approx 35% (or approx 7% additional O2) to allow for weather seasonal variations. Perhaps this is too low or I need to adjust the CO2 lower in winter.

FWVT · 21-01-2015 11:09am

The relative stochiometry of the air will not change. I m3 of air will still contain 20.9% O2, it's just that there will be less molecules overall.

For things like combustion engines (aircraft piston engines, for example) the power generated per cylinder cycle will be reduced by 12.7% if taking off on the same runway in on these two days.

Changes in elevation will also affect your system. For every 100 m rise in altitude the density will fall by about 1.2% (below around 1500 m), so something to bear in mind if designing systems for different locations.

shane70 · 21-01-2015 4:21pm

Thanks. It all makes perfect sense.

I can adjust the air intake via a damper whilst measuring the combustion products, i know the required CO2 at 20C. The problem I face is the calculation to adjust for a different temperature such as between winter/summer and for differing air densities.

b318isp · 21-01-2015 4:26pm

shane70 wrote: »

Thanks. It all makes perfect sense.

I can adjust the air intake via a damper whilst measuring the combustion products, i know the required CO2 at 20C. The problem I face is the calculation to adjust for a different temperature such as between winter/summer and for differing air densities.

This is why many cars have MAF (Mass Air Flow) sensors - they will take account of the density of the air and use this to calculate fuelling.

Could you do a few tests over a range of temperatures and come up with your own rough curve? Alternatively, google a graph of air density v temperature...

shane70 · 21-01-2015 7:28pm

I'm afraid I don't have the ability to continuously monitor and adjust the damper according to specific times. It can only be adjusted once per annum so has to be set to suit the condition at that particular time, thus allowing an excess air requirement to maintain good combustion.

FWVT · 21-01-2015 8:39pm

shane70 wrote: »

I'm afraid I don't have the ability to continuously monitor and adjust the damper according to specific times. It can only be adjusted once per annum so has to be set to suit the condition at that particular time, thus allowing an excess air requirement to maintain good combustion.

Set it for summer conditions and that will ensure a slight excess in winter

shane70 · 21-01-2015 8:48pm

FWVT wrote: »

Set it for summer conditions and that will ensure a slight excess in winter

That would grossly over-air the combustion which would produce high levels of sulphur in the chamber. It could also cause flame impingement on the chamber surfaces which would lead to high levels of CO.

I know the setting for ambient temperature of 20C and a graph adjustment for temps between -25C and +25C. So I just need to make adjustment based on current temp and adjust in proportion to required setting.

whizbang · 31-01-2015 2:13am

Depending on the fuel in use of course, the guys in the heating forum would have graphs/ tables already published to take into account the temp and altitude variations.

I.e. given a specific CO2 result, the effect of temp and altitude, can be plotted to see the variation, and hence estimate the correct setpoint.

Relationship between pressure, temperature and O2

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