Heat Transfer of Water Sprays on Hot Metal Strip 3

[mechnick9]

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.

 

[CH5OH]

the most heat you can take away is through evaporation
so I guess, if you increase the pressure (and reduce nozzle size),you can make smaller droplets, which would evaporate more easely

 

[MrBTU]

Yes, the bulk of your heat transfer, and hence cooling, is by the latent heat of evaporation, not by the sensible heat from 60 or 90 to 212. The total heat input to the water steam is sensible plus latent. Your sensible is MASS Flow of water x specific heat of water (1) x delta T (60 or 90 to 212), your latent is mass flow X hfg from your steam tables - 970 btu/lb of water. This is a direct contact heat transfer problem. You've got to take the sensible heat out of the steel plate and put it into the water. You can calculate your Q from the plate by pounds of plate times specific heat of the plate (.12 BTU/lb deg F) times delta t of the plate. Then you convert this to a rate by knowing how many pounds per hour go over the line. Then, the heat that you want to take out of the steel must go into the water, so you use your sensible plus latent on the water.... good luck!

 

[mechnick9]

Thank you for the responses!

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.

 

[MrBTU]

The hfg is the latent heat of vaporization of the water. Yes these are actual values that apply to this situation.

 

[IRstuff]

But, what temperature are you trying to achieve, and what have you achieved? Running cooling water at 90F limits the exit temperature to >90F

TTFN

FAQ731-376

 

[mechnick9]

The temperatures are in the range of 1200-1600F, so being below 90F is not an issue. We typically are looking to drop no more than 100F on the ROTs.

 

[IRstuff]

OK, then going with the 90F water should make more sense, since that puts you 30F closer to the boiling point.

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.

TTFN

FAQ731-376

 

[mechnick9]

IR, you are correct and that is also another one of our issues. There are two sets of sprays, top and bottom. Because of this problem you stated, we get the majority of our cooling from the bottom sprays. These are the sprays the I am trying to get more pressure to in hopes to provide more cooling. Down the road we will need to have side sweep sprays that will blast the water off the strip preventing the steam build up. This is common practice in the industry but our mill is quite old.

 

[Compositepro]

For your process the water temperature is almost immaterial. Almost all the heat transfer is due to boiling (not evaporation which occurs below boiling, so small droplets will not improve cooling). Warmer water could possibly improve cooling because the surface tension of water decreases with temperature. Lower surface tension means less beading of water and better spreading.

 

[ione]

There are many factors involved in this direct contact heating process and probably water droplet temperature is not the most significant.
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://rspa.royalsocietypublishing.org/content/462/2074/3115.full

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.