what makes material sweat during heating?

[dmason83]

Hi, I'm curious as to why a normal hot rolled A-36 plate sweats during a burning process.

there are 2 ideas in the shop right now.
A) moisture from the atmosphere plays a role in the temperature transition of the material.

B) the is moisture in the plate(stored outside) and that is leaving the material when it is heated.

I personally agree more with answer B but am un-aware of the correct answer.

anyone know for sure?

 

[IRstuff]

Flame combustion creates water vapor, which contacting even room temperature materials, will condense.

I see the same thing on my large skillet on the stove. Until the skillet gets hot enough to vaporize water on its own, I see condensation on the sides of the skillet.

TTFN

Eng-Tips Policies FAQ731-376

 

[evelrod]

If you take (B) and steel starts absorbing H₂O then we are in deep do do!;-)

Rod

 

[amorrison]

1. The short answer is

The cold steel is acting like a air conditioning dehumidification coil which takes (flame produced)moisture out of the local air if it is cold enough.

2. The longer answer

The moisture (sweating) molecules are supplied by the burning process which produces a high amount of water vapor into the "local" air.

The steel local temperature - where sweating occurs -is below the condensing temperature for the amount of local water vapor in the air thus local condensation occurs.

When the burning process is stopped, no new water vapor is supplied. The local air is replaced by general ambient air which is usually more dry and therefore no new condensation occurs.

After say one minute . . .

The local steel is still "warm" - the condensed water vapor will now have enough vapor pressure to vaporize into the atmosphere - thus the steel surface "dries".

 

[dmason83]

Thank you all for your insight on the question

DM

 

[arto]

Like the moisture contrails on a plane up high

 

[pauljohn]

A car exhaust on a cold day.