Furnace wall temperature
Furnace wall temperature
(OP)
I have a rotary furnace with a new lining (270mm brick, 70 mm brick, 10mm insulation board and 15mm steel liner. The supplier has given us some figures on what the shell temp should be at our tapping temp, 1300°C.
I am trying to work out how they have calcualted the shell temp as our observations don't match their calculations. I am struggling to understand how you can calculate the shell temp using heat transfer calculations. Can anyone help. Outside of vessel is at ambient temp 20°C or so.
I am trying to work out how they have calcualted the shell temp as our observations don't match their calculations. I am struggling to understand how you can calculate the shell temp using heat transfer calculations. Can anyone help. Outside of vessel is at ambient temp 20°C or so.





RE: Furnace wall temperature
RE: Furnace wall temperature
ex-corus (semi-detached)
RE: Furnace wall temperature
The temperature we are observing is higher than they predict. I expected this as the new liner has a higher conductivity (lower resistance) than the previous liner.
The bit I am struggling with is how to calculate the shell temperature. Do you have to assume a heat flow figure?
RE: Furnace wall temperature
RE: Furnace wall temperature
RE: Furnace wall temperature
I didn't mean to offend you but if you've taken my previous post as an offence please accept my apologies.
Not only the heat dissipated from the outside surface does require the outside wall temperature, but also the heat transferred by conduction.
The outside wall temperature is your unknown in the equation that equals the heat transferred by conduction through the multilayer wall and the heat dissipated to ambient (convection + radiative heat transfer).
You have to proceed by iteration until the two amounts of heat (Qcond and Qdissipated) converge, as the outside surface temperature reaches an equilibrium when the two amounts of heat are equal.
This is the approach I would follow if I had to face this problem, anyway you'll find in this forum people more qualified than me and prone to help you with valuable inputs.
RE: Furnace wall temperature
One other possibility I can think of is that most suppliers assume some fouling factors on the inside surfaces which adds heat resistances but foulings might not immediately happen to new furnaces.
The liner (refractory layer, I guess) heat resistance that you suspected could also be the reason. The actual properties of the material might be different from what's given to the engineer to calculate. I would suspect more about the moisture content being still high inside the liner. The initial baking and curing procedure cannot normally drive all the water out resulting a lower heat resistance value. If this is the case you should see the shell temperature coming down slowly after using the furnace for a while.
For your particular application I would recommend a CD from refractory supplier Harbison-Walker and it contains a software to calculate outer surface temperatures with both data from their standard products and mannual inputs. I just don't remember if it contains applications for more layers than just insulation/refractory plus outer metal liner/jacket. You should be able to request this CD by contacting H-W.
RE: Furnace wall temperature
To calculate the wall temperature, calculate the resistance of the layers by summing x/k. The flux across the layers is the temperature difference between the shell and 1300C divided by this resistance. This is equivalent to the heat loss to the ambient which is h.(Tshell-20). As an estimate, take a value of about 10 W/m^2 C for h and solve for Tshell.
ex-corus (semi-detached)
RE: Furnace wall temperature
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RE: Furnace wall temperature
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RE: Furnace wall temperature
Have you deliberately omitted radiative heat losses or have you accounted for this component assuming 10 W/m^2 C for h? I think radiation should not be omitted in this case.
RE: Furnace wall temperature
If you add radiated loss, the surface temp drops to 116°C. Note, however, the temperature could be higher, depending on the emissivity of the brick. Ratio of radiated to convected is about 2.5:1
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RE: Furnace wall temperature
ex-corus (semi-detached)
RE: Furnace wall temperature
I'm not sure where you get your material properties from to calculate your temperatures, but for sure the conductivity of steel is about 50 W/m K, decreasing with higher temperatures. The steel shell would be on the outside, radiating to ambient, with the brick seeing the 1300C.
ex-corus (semi-detached)
RE: Furnace wall temperature
But, the point was to get the OP to do the calculation himself, wasn't it?
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RE: Furnace wall temperature
I agree with corus that steel shell is probably on the outside. Anyway note that if it is a stainless steel shell conductivity is lower than that indicated by corus.
RE: Furnace wall temperature
I thing I understand the iterative approach now. Thank you all for your contributions.
RE: Furnace wall temperature
I think no one is wasting time here: we are not forced to give answers and hints.
Back to your problem, both corus and IRstuff suggested an approach aligned to what I have suggested. What matters, in the end, is that you have earnt some useful answers to solve your problem.
RE: Furnace wall temperature
Our observed shell temp is still higher (with a lower internal temperature), so further investigation required. Will start with compression of the insulation board as Corus suggests.
Thanks for all the advice!
RE: Furnace wall temperature
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RE: Furnace wall temperature
RE: Furnace wall temperature
Please correct me if I am wrong. You have approached the conduction heat transfer through the multilayer wall as you were dealing with a flat multilayer wall.
But as the OP spoke about a rotary furnace I think it would be better to consider a cylindrical shape and so evaluate the thermal resistance of each layer accordingly.
RE: Furnace wall temperature
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RE: Furnace wall temperature
In general your calcuations are good enough (and simpler) using simple flat wall geometry as opposed to assessing a cylindrical shape as at large radii the heat transfer equations converge for the two coordinate systems.
ex-corus (semi-detached)
RE: Furnace wall temperature
I see your point. In effect my mind was much more "pipe" oriented (small diameters) than rotary kiln oriented (big diameters), if you know what I mean.
ProcessJim,
At this point it would be opportune to know if the discrepancies you've noticed between calculated and measured temperature appeared from the very beginning (after the first commissioning) or after a period of operation. I am asking this because I presume thickness reductions, due to wear issues, require time to take place.
RE: Furnace wall temperature
The concern is that the liner will not last as long as the previous liner due to the higher shell temp. We are trying this liner as it is supposed to have better wear resistance. Obviously there are significant cost implications to reduced liner life.
RE: Furnace wall temperature
Future problems may occur in the shell as you may have a greater differential thermal expansion to cooler parts of the shell and hence increased stresses in the shell, possibly leading to fatigue failure or gross distortion as stresses exceed design limits. In addition, as you now see temperatures of 320C then, as the lining wears, temperatures may exceed the accepted limit of 350C for carbon steel. Above 400C you might get distortion of the shell due to creep or general lack of material strength as the modulus and yield strength of the material decrease rapidly thereafter. Distortion of the shell would have implications on the lining as the bricks may no longer be held in place or hot material could penetrate between the bricks.
It's all a bit of a balancing act in keeping reasonable shell temperatures whilst minimising the stresses in the lining.
ex-corus (semi-detached)
RE: Furnace wall temperature
If you have used the same type of thermo-couple you've used to measure your internal temperature, this could have led to misleading values as it couldn't be suitable for the shell's temperature range.