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Up Topic Welding Industry / Technical Discussions / Moisture from Preheating
- - By dasimonds (**) Date 10-18-2001 12:07
Does anyone know what percentage of the moisture visible during oxy-acteylene preheating is actually coming out of the carbon steel base metal?
I have been told that it's around 70%.
I know oxy-acetylene(C2H2 + O2) produces 2CO(Carbon Monoxide) + H2(Molecular Hydrogen) + Heat as the primary reaction, and the (2CO) + H2 combines with the O2 in the air to produce 2CO2(Carbon Dioxide) + H2O(water) + Heat as a secondary reaction.(Taken from Modern Welding Technology by Howard B. Cary, first Edition, 1979)
Moisture can be seen as circle radiating outward from the heat source.
Could the radiating circle of moisture be an indication of the dew point( ie... the temperature at which moisture will condense from the atmosphere) for local atmospheric conditions?
Dale Simonds
Parent - - By CHGuilford (****) Date 10-18-2001 15:29
I am sure that many won't agree with me and I haven't seen results from testing to back up my opinion. I think the percentage of moisture from the metal is negligible, that the bulk of of H2O comes from combustion of the fuel gas. The circle of moisture you see is from water condensing on the surface because the metal is colder than the dew point of the local atmosphere. Local meaning the immediate vicinity of the flame and the combustion byproducts mixed with air. As you get further from the flame the water vapor is dispersed into ambient air which lowers relative humidity and dew point, so that usually condensation is only seen in the circle around the flame. It appears that water is coming out of the metal but it actually comes from the flame.
To show this you can heat a vertical plate on one side and watch the other. You will see moisture on the flame side but not the other side. If a significant amount of water was in the steel and was being driven out by the increasing temperatures, you should see water on both sides.
Parent - By G.S.Crisi (****) Date 10-18-2001 17:31
Congratulations, Chester, you've given a 99% perfect lesson and nobody will even imagine not to agree with you.
To reach 100% perfection, you should have said that the percentage moisture coming ouy from the metal is ZERO, for metal doesn't contain any humidity, and that the WHOLE H2O comes from combustion of the fuel gas.
Giovanni S. Crisi
Sao Paulo - Brazil
Parent - - By GRoberts (***) Date 10-18-2001 17:55
Another way to look at it is this: If you were "driving out" moisture from the steel as it is heated with a flame produced by combustion, why do you not get any moisture from the steel if you heat it with non-combustion means such as electric heaters? I have to agree with Mr. Crisi, the percent of moisture coming from the steel would be 0%, and the percent coming from the flame would be 100%.
Parent - By dasimonds (**) Date 10-18-2001 18:39
Thank you gentlemen,
It's as I suspected, although I wasn't aware that 100% of the H2O is from the combustion reaction. And as Mr. Guilford, I haven't seen any testing data on the subject.
Again, thanks for the clarification.
Dale Simonds
Up Topic Welding Industry / Technical Discussions / Moisture from Preheating

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