Over capacity Reheating rotary Furnace 1

[partaab]

We have an over-capacity rotary hearth furnace for reheating steel billets. the furnace capacity is 15 tons /hour but the required output is only 3 tons per hour due to to limitation of our rolling mill. This low capacity utilization is resulting in more burning losses typically around 6 percent along with a higher gas consumption. Can I place the billets with more space between each other, say 4-5 times the current space to place less steel in the furnace and expect less burning losses? Kindly suggest is it the right approach?

 

[MintJulep]

No.

 

[partaab]

would you mind explaining the reason for this No.

 

[MintJulep]

Because the losses that you are concerned with are dominated be the geometry of the furnace.

You can't change that by changing the placement of the product inside.

 

[partaab]

My idea was to place fewer billets in the furnace, which means they would stay for less time, so the burning losses would be less than the current number, which is currently 6%. The other thing was that less amount of energy was required to maintain the billet's temperature when there were fewer billets

 

[Compositepro]

It requires no energy to maintain the temperature of the billets. It is the furnace itself that requires energy to maintain temperature (due to heat loss through the walls). Energy is required to heat the billets to temperature, but not to maintain their temperature. Obviously, though, heating fewer billets per hour will use less energy.

 

[MintJulep]

If the billets are in the furnace for less time then they will (probably) become less hot.

Unless the new spacing dramatically shifts hear transfer towards radiation instead of convection.

 

[EdStainless]

say that you load every other slot.
You can't double the furnace speed and still reach temp.
The heating and soaking times are related to heat transfer, and they take finite time.
At half load you might be able to increase the furnace speed by 20-25% If you have very good heat transfer.
But if you also try to reduce the firing rate you may find that you need all of the original time.

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P.E. Metallurgy, consulting work welcomed

 

[partaab]

what, about the burning losses i.e. scale formation at the surface of the billet, which at current is 6%. The more the billet remains in the furnace the more the formation of the scale. Kindly shed some light on this phenomena too.
regards

 

[MintJulep]

what, about the burning losses i.e. scale formation

Ah! "Burning losses" means something different to you than it does to those of us who have replied.

Still, scale formation seems likely to be a combination of time, temperature and atmosphere.

For any given billet size and shape you are probably constrained in time and temperature to get acceptable product at the furnace output.

That leaves atmosphere for you to fiddle with if you want to reduce scale. Inert or vacuum, but neither of those happen for free.

 

[partaab]

I will have to provide the maximum details so my point will be clear.
The capacity of the furnace is 15 tons per hour.
Rolling Mill capacity is 3 tons per hour.
The billet size is 180mm in diameter, the length of the billet is 2000mm( roughly 400 kg) and the furnace can hold 75 Billets at a time, which is 30 tons.
The time required to reach the rolling temperature is around two hours including soaking time. So with 3 tons per hour mill capacity with a furnace of 15 tons per hour, each billet will stay in the furnace for 10 hours instead of 2 hours. So the extra 8 hours will result in more burning losses( scale formation due to oxidation). Charging and discharging of the billets are manually done by the furnace operator. There is a gap of around 4 meters between the discharging and charging of the billets and the distance between the billets is adjustable if we increase the distance we can hold 15 billets of the above size instead of 75 billets. So 15 billets at a time mean 6 tons of billets will stay in the furnace and the capacity of the furnace will be reduced to 3 tons per hour. We know that there is a huge mismatch between furnace capacity and production rate but until we change the furnace to the right size, I think the above arrangements will reduce the burning losses significantly. The rolling temperature is 1200 degrees Celsius. The furnace is rotary hearth type.
So kindly advise on the above-detailed information.

 

[partaab]

The furnace hearth only rotates when the system gets a manual command from the furnace operator.

 

[MintJulep]

Ah!

Fewer billets in the furnace at any time.

I was reading your original post as same number of billets, just arranged differently.

Yes, that should work.

 

[partaab]

oh, thank you very much. Something In my head kept telling me that this concept of mine was not being communicated properly.
Regards

 

[partaab]

Could someone please tell me the relationship between billets staying extra time in the furnace and scale formation on the surface of the billet? Like in my case above the billet heating heat for the rolling is 2 hours but the billets stay in the furnace for 10 hours that is 8 hour extra. The average scale on the billet is 6% and the rolling temperature is 1230 degree Celsius.
Regards

 

[EdStainless]

Teh scale formation goes in stages, but generally the longer you heat the heavier the scale gets.
I doubt that you could see the difference in scale formation between a 2hr cycle and a 4hr cycle.
To some degree the scale formation is helpful as it makes the surface more absorbent of radiant heating.
If your furnace is really 5x too large for your current demand I would load every 4th slot and aim for a 2.5-3hr cycle time.
You want a little buffer in the furnace so that the rest of the mill is never waiting for material.
What alloy are you running this hot?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[partaab]

some medium allow steel-grades