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How to Calculate Flammability of gas mixture 9
- Thread starter Leclerc
- Start date
There is relatively recent research by Wierzba and others from University of Calgary which covers this in a strongly technical manner for those so inclined....
Journal Inst Energy Sept 1996 V69 pp 122-130
Journal of Energy Resources technology 1994 V116 p 181
Transactions of ASME 1999, V121 p 74
The upper limit is a little complex to estimate because it is a factor of the energy content of the intermediate radicals which are released by partial compbustion. The low limt is a breeze to estimate because it is only a factor of the thermal properties of the components.
All gases have a limiting heat transfer characteristic at the low limit which inhibits flame propagation. This is a feature of the radiant properties and adiabatic flame temperature. As the radiant properties at the limit are largely independent of inert content, we only need to take account of the varying heat-sink ability as we change the inert from Nitrogen (=Air) to something else. This means replacing the excess nitrogen at the low limit with an amount of Nitrogen which has the same heat sink propeties as the actual inert.
For a given mixture of Flammable in inert an estimate of the true low limit concentration of flammable within the mixture [CLF] and of the complete mixture in Air may be obtained by use of the familiar Le Chatelier procedure modified by the Nitrogen equivalent of the Inert portion.
NE = S { NEi * I%i }
where I%i = fractional quantity of ith Inert component
NEi = nitrogen equivalent of ith Inert component
CLF = Sigma { F%n } / Sigma { F%n / CLn } = low limit of Flammable portion in Air
CLM = Sigma { I%i + F%n } / Sigma { F%n / CLn } = low limit conc of total Mixture in Air
where F%n = fractional quantity of nth Flammable component
CLn = lower limit of nth Flammable component
There are relatively few common Inerts in gas mixtures and the following suggestions are made for values of NE
N2 = 1
H2O = 1.35
CO2= 1.82
SO2 = 2.1
He + Ar = 0.65
Remember that this is an estimate and use an approriate factor of safety depending on whether you are looking to be flammable or non-flammable.
Other cautions - if you use the flammable limits in tables they are based on 25 degC for the gas and the air. Flammable limits get closer together as the temperature goes up towards the Auto Ignition temp and get a little fuzzy close the the AIT. However, if the air is always ambient, the gas temperature itself has less of an effect. It's all about total energy in the mixture.
Check the Wierzba ASME paper.
If you have already tried my excel sheet (noted above) and it doesn't work let me know.
Happy calculating
Journal Inst Energy Sept 1996 V69 pp 122-130
Journal of Energy Resources technology 1994 V116 p 181
Transactions of ASME 1999, V121 p 74
The upper limit is a little complex to estimate because it is a factor of the energy content of the intermediate radicals which are released by partial compbustion. The low limt is a breeze to estimate because it is only a factor of the thermal properties of the components.
All gases have a limiting heat transfer characteristic at the low limit which inhibits flame propagation. This is a feature of the radiant properties and adiabatic flame temperature. As the radiant properties at the limit are largely independent of inert content, we only need to take account of the varying heat-sink ability as we change the inert from Nitrogen (=Air) to something else. This means replacing the excess nitrogen at the low limit with an amount of Nitrogen which has the same heat sink propeties as the actual inert.
For a given mixture of Flammable in inert an estimate of the true low limit concentration of flammable within the mixture [CLF] and of the complete mixture in Air may be obtained by use of the familiar Le Chatelier procedure modified by the Nitrogen equivalent of the Inert portion.
NE = S { NEi * I%i }
where I%i = fractional quantity of ith Inert component
NEi = nitrogen equivalent of ith Inert component
CLF = Sigma { F%n } / Sigma { F%n / CLn } = low limit of Flammable portion in Air
CLM = Sigma { I%i + F%n } / Sigma { F%n / CLn } = low limit conc of total Mixture in Air
where F%n = fractional quantity of nth Flammable component
CLn = lower limit of nth Flammable component
There are relatively few common Inerts in gas mixtures and the following suggestions are made for values of NE
N2 = 1
H2O = 1.35
CO2= 1.82
SO2 = 2.1
He + Ar = 0.65
Remember that this is an estimate and use an approriate factor of safety depending on whether you are looking to be flammable or non-flammable.
Other cautions - if you use the flammable limits in tables they are based on 25 degC for the gas and the air. Flammable limits get closer together as the temperature goes up towards the Auto Ignition temp and get a little fuzzy close the the AIT. However, if the air is always ambient, the gas temperature itself has less of an effect. It's all about total energy in the mixture.
Check the Wierzba ASME paper.
If you have already tried my excel sheet (noted above) and it doesn't work let me know.
Happy calculating
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1
- #22
Laclerc:
It might be useful to read this tutorial from a Professor Ludlow at Arizona State University:
web.umr.edu/~dludlow/classes/che258/Flammability.ppt
I am not sure if it will help you, but I hope that it does.
Milton Beychok
(Contact me at www.air-dispersion.com)
It might be useful to read this tutorial from a Professor Ludlow at Arizona State University:
web.umr.edu/~dludlow/classes/che258/Flammability.ppt
I am not sure if it will help you, but I hope that it does.
Milton Beychok
(Contact me at www.air-dispersion.com)
- Thread starter
- #24
Interested? I am always interested! I, personally, was pleased to receive some useful replies within the first week which got me going, for which I posted my thanks.
Since then I have been happy to see the thread unravel, but I suspect that Guidoo is wondering how long should a thread run? I don't know what the protocol is here: does the thread initiator remain a "Threadminder" and has he a responsibility to stop a thread along the lines of, "This correspondence is at an end: Threadminder"?
I now know that I have one duty, of which Guidoo has gently reminded me, and that is to regularly thank contributors. I apologise for not doing so in recent years.
As to ending the thread, who am I to do this deed? All I can do is to remind people (as if they need reminding!) that if a contribution looks and feels like a new topic, then start a new thread.
thank you
Leclerc
Since then I have been happy to see the thread unravel, but I suspect that Guidoo is wondering how long should a thread run? I don't know what the protocol is here: does the thread initiator remain a "Threadminder" and has he a responsibility to stop a thread along the lines of, "This correspondence is at an end: Threadminder"?
I now know that I have one duty, of which Guidoo has gently reminded me, and that is to regularly thank contributors. I apologise for not doing so in recent years.
As to ending the thread, who am I to do this deed? All I can do is to remind people (as if they need reminding!) that if a contribution looks and feels like a new topic, then start a new thread.
thank you
Leclerc
There is nothing that prevents a thread from running ad infinitum or even ad naseum, particularly if there is something new to be added.
The only thing to be vigilant about are people that hijack old threads with off-topic questions or people that are simply reviving old threads for some sort of self-aggrandization.
In general, if the new additions are not contributions, but separate questions, the poster should be gently reminded to RF their post and start a new thread, so as to not confuse others with unrelated material.
TTFN
The only thing to be vigilant about are people that hijack old threads with off-topic questions or people that are simply reviving old threads for some sort of self-aggrandization.
In general, if the new additions are not contributions, but separate questions, the poster should be gently reminded to RF their post and start a new thread, so as to not confuse others with unrelated material.
TTFN
The calculated value from the stoichiometric reaction would be 12.5*0.5 = 6.25% vol of oxygen. Where 12.5 is the published LEL, and 0.5 represents the moles of oxygen per mol of CO.
However, published MOC values call for 5.5%, which is lower than the "estimated" MOC.
Although in theory, below the MOC, the reaction couldn't generate enough heat to get a self propagating flame, it should be applied with a sizable safety factor as been repeatedly said in this and other threads.
I'm always leery of saying MOC. Does it mean minimum or maximum?
The Lower explosive limit for CO (upward propagation) is very roughly 12.8% CO in 87.2% air which gives an oxygen figure of 18.3% for the lean condition (ie. too much air to burn).
Similarly at the UEL of 74% (upward propagation) the air is 26% = 5.5% O2 (too rich to burn).
These are the experimental limits at which, some reaction will almost/just propagate through a mixture (in the right conditions)
Apply a factor of safety.
If you want it to burn, apply a factor of safety of 1.5 to the air/gas ratio to find
- the richer mixture at LEL (O2= 17.2%)
- the leaner mixture at UEL (O2 = 7.3%)
If you want it to NOT BURN use
- 0.25* LEL (O2 = 20.3%)
- O2 at UEL - 3% (O2 = 4.3%)
The Lower explosive limit for CO (upward propagation) is very roughly 12.8% CO in 87.2% air which gives an oxygen figure of 18.3% for the lean condition (ie. too much air to burn).
Similarly at the UEL of 74% (upward propagation) the air is 26% = 5.5% O2 (too rich to burn).
These are the experimental limits at which, some reaction will almost/just propagate through a mixture (in the right conditions)
Apply a factor of safety.
If you want it to burn, apply a factor of safety of 1.5 to the air/gas ratio to find
- the richer mixture at LEL (O2= 17.2%)
- the leaner mixture at UEL (O2 = 7.3%)
If you want it to NOT BURN use
- 0.25* LEL (O2 = 20.3%)
- O2 at UEL - 3% (O2 = 4.3%)
Using purely stoichometric theory is the MOC (min) for Deuterium 3.3%?????
Big thanks to Mr Beychock for the link to the ludlow article.
Excellent insight on the theoretical determination of Flammability limits and MOC.
Has made it quite straight forward for me to spreadsheet MOC's using the chemical composition.
Am still very wary of Acetylene and Ethylene Oxide as I understand they "misbehave". Are there any/many others that don't obey the MOC convention?
Big thanks to Mr Beychock for the link to the ludlow article.
Excellent insight on the theoretical determination of Flammability limits and MOC.
Has made it quite straight forward for me to spreadsheet MOC's using the chemical composition.
Am still very wary of Acetylene and Ethylene Oxide as I understand they "misbehave". Are there any/many others that don't obey the MOC convention?
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