Heat Transfer of Water Sprays on Hot Metal Strip
Heat Transfer of Water Sprays on Hot Metal Strip
(OP)
I work at a Steel Plant and have just been assigned to the Hot Strip Mill. For those of you not familiar, this mill takes slabs of steel, reheats them in a furnace, and then reduces the thickness through a rolling mill.
The problem we are currently having is not meeting our temperatures on the Run Out Tables. The ROTs are at the end of the line after all reduction is complete and use water sprays to cool the strip before it is coiled. I am not familiar with the thermodynamics that are involved with the heat transfer from the strip by the water sprays.
Right now there are two options; run one section of the tables with Industrial Water that is approx. 60 degrees F but by doing so this takes pressure away from other sprays that use the Industrial Water. The other option is turn these sprays to a Recirculated Water supply that is approx. 90F and allow there to be higher pressure in the other sprays. So the question is what will create a greater effect; the cooler water but at a lower pressure, or the slightly hotter water at a higher pressure? When we turn the 90F water on in this section the other sprays that are receiving more pressure are using the Industial Water at 60F.
I have been playing with some Fourier's Law stuff, but haven't been able to make heads or tails of anything yet.
Any help would be appreciated,
Thanks.
The problem we are currently having is not meeting our temperatures on the Run Out Tables. The ROTs are at the end of the line after all reduction is complete and use water sprays to cool the strip before it is coiled. I am not familiar with the thermodynamics that are involved with the heat transfer from the strip by the water sprays.
Right now there are two options; run one section of the tables with Industrial Water that is approx. 60 degrees F but by doing so this takes pressure away from other sprays that use the Industrial Water. The other option is turn these sprays to a Recirculated Water supply that is approx. 90F and allow there to be higher pressure in the other sprays. So the question is what will create a greater effect; the cooler water but at a lower pressure, or the slightly hotter water at a higher pressure? When we turn the 90F water on in this section the other sprays that are receiving more pressure are using the Industial Water at 60F.
I have been playing with some Fourier's Law stuff, but haven't been able to make heads or tails of anything yet.
Any help would be appreciated,
Thanks.





RE: Heat Transfer of Water Sprays on Hot Metal Strip
so I guess, if you increase the pressure (and reduce nozzle size),you can make smaller droplets, which would evaporate more easely
RE: Heat Transfer of Water Sprays on Hot Metal Strip
RE: Heat Transfer of Water Sprays on Hot Metal Strip
I do no believe we will be able to reduce the spray nozzles, and if we were it would be a much larger endevour and not an instant fix.
Mr. BTU, what is the "hfg" in the latent heat equation? Also, are these values that you're using examples or specific to this situation? i.e. where you have 0.12BTU/lb degF and 970 BTU/lb water.
I must admit the last time I've played with heat transfer equations was over 3 years ago, so I apologize if it seems I'm asking simple questions.
RE: Heat Transfer of Water Sprays on Hot Metal Strip
RE: Heat Transfer of Water Sprays on Hot Metal Strip
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RE: Heat Transfer of Water Sprays on Hot Metal Strip
RE: Heat Transfer of Water Sprays on Hot Metal Strip
However, there may not be as much quantitative difference as one might think, since the point of contact of the water with the hot metal would always have interstitial boiling and vaporized water, so the effective contact region might need to be modeled with a layer of steam, which drastically limits the heat transfer. Think of what happens when water is dropped on a hot sauce pan; the water drops skitter around on the pan, floating on a layer of steam, and never really experiences proper thermal transfer.
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RE: Heat Transfer of Water Sprays on Hot Metal Strip
RE: Heat Transfer of Water Sprays on Hot Metal Strip
RE: Heat Transfer of Water Sprays on Hot Metal Strip
Amongst the parameters involved I would list the following before water temperature :
1. Water velocity
2. Wettability
3. Pressure
4. Gases presence in the liquid
5. Metal surface geometry
6. Metal roughness
I anyway agree with posts above which state that a higher water droplet could somehow be useful.
In the link below it is possible to find a description of the role played by velocity on water droplet impact on hot surfaces (the higher the droplet velocity the lower the superheat required for boiling nucleation).
http:
In the link above it is also mentioned the phenomenon described by IRstuff, in one of his previous post. The phenomenon is known as Leidenfrost effect and it describes the vapour barrier formation between a water droplet and an impacted metal surface, when the metal surface is at a temperature above the water boiling point.