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Furnace wall temperature
4

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.

RE: Furnace wall temperature

The worst case would be steady state with an inner temperature fixed at 1300C. If they've used that method then your observed temperatures would be much lower as they neglect the transient change of temperature through the layers of material. For insulating material it may take a considerable amount of time to approach steady state, and the maximum shell temperature.  

ex-corus (semi-detached)

RE: Furnace wall temperature

(OP)
They have stated in their emails that the calculation is based on steady state, but they have not provided details of the calcuation.

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

You have to equal the heat transferred by conduction throughout the multilayer from the inner to the outer surface (assume Tinner = 1300 °C and Touter is unknown) to the heat dissipated by convection and radiation from the outer surface. The paper I have attached before seemed quite clear.

RE: Furnace wall temperature

(OP)
Please forgive me if I'm not understanding something simple here?  I have had a good look at the attached paper, thanks for that, but does it not require an outside wall temperature for the equation for the heat dissipated from the outside surface?

RE: Furnace wall temperature

ProcessJim,

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

This is just a typical heat transfer application and calculation of multilayer wall found in text books but to get the results right you must get the conditions right first. For example, does the supplier assume some wind velocity of the 20 °C ambient air? This affects greatly the heat exchanging coefficient at the outer surface and shell's 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

Another factor is that the lining will expand and tend to crush the insulation board and so produce a lower thickness than as-built. Of course you could have some penetration of the molten material into the bricks to produce hot spots.

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

RE: Furnace wall temperature

Corus,

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

My bad, too.  

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

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Furnace wall temperature

ione, As I said, 10 W/m^2 K is an estimate, ie. for a rough hand calculation. It would include radiation as well as natural convection. Of course if the steel temperature is of the order of 300C then the value would be higher. You could produce a spreadsheet of values for natural convection plus radiation for various surface temperatures to improve this rough calculation.

ex-corus (semi-detached)

RE: Furnace wall temperature

IRStuff,
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

Thanks, that's why I had asked the question earlier.  However, steel's emissivity, unless it's painted or coated is substantially poorer than brick's.  So, with an emissivity of 0.38 and a thermal conductivity of 20 W/m-K, the surface temperature goes to 184°C.

But, the point was to get the OP to do the calculation himself, wasn't it?

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Furnace wall temperature

The value which appears in Irstuff's pdf seems more likely to be that typical of iron.

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

(OP)
Ione, no offence taken, I am always just wary that I am wasting people's time by asking for solutions to problems that are simple.

I thing I understand the iterative approach now.  Thank you all for your contributions.

RE: Furnace wall temperature

ProcessJim,

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

(OP)
I have now calculated a figure for the temperature of the shell following the procedures described above.  For an emmisivity of 0.94 and a h value of 2.5, I come to a shell temp of 260°C if the inside temperature is 1300 as in the suppliers assumption, the supplier calcualted 250.  

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

(OP)
The supplier has assumed a figure of 1300°C as a worst case scenario.  The temperature at which we tap our metal is 1050 - 1150°C (measured by dip thermocouple).  I assume that this is the interior wall temperature in my calculations.

RE: Furnace wall temperature

IRstuff,

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

My intent is not to solve the OP's problem, but merely to provide a tolerably close model that could be solved and used to evaluate potential issues in the parameters or assumptions.
 

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Furnace wall temperature

If you're dealing with molten iron (with a freezing temperature of 1150C) then you could have wear of the  brick lining reducing its thickness, as well as the reduction of the insulation board thickness. In general you should keep temperatures below 350C to prevent creep and significant loss of strength but hot spots can be acceptable (though worrying).

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

Corus,
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

(OP)
The shell temp is higher than predicted from commisioning.  We are trying a different liner than we usually use with a higher thermal conductivity.  The observed temperature is 320°C for the new liner whereas the supplier predicts 250 and my calcualtions predict 260 - 276°C depending on emmisivity.  I am currently using an emmisivity value of 0.76 for oxidised steel (Giving 276).

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

The lining life will depend on a number of factors, as well as wear resistance. Strictly speaking, if your new lining has the same strength as before, then a higher shell temperature will reduce the stresses in the lining as the lining is less restrained in thermal expansion. This would be an advantage towards lining life.
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

How have you measured the shell 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.
 

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