Help finding insulation thickness needed for industrial furnace 1

[psheddy]

I'll start by saying I am on my first job since graduating, and it feels like forever since I took my heat transfer class, so I'm sitting here at my desk with 2 textbooks and still having trouble remembering how to do what feels like a fairly simple problem, haha.

My first assignment is to design a test furnace. The max temperature inside is 2000F, and the surface temperature on the outside needs to be less than 150F. I was wondering how to determine the insulation thickness needed to have the outside temp of 150F.

The equation if found was heat transfer rate, q = (T1 - T2)/[ (width of insulation/k insulation) + (width of steel wall/k steel wall) ]

I have the width and thermal conductivity, k, values for the outside steel walls, and my temperatures. I'm not really sure where to go from there, without the q value. I will have the k value for the insulation as soon as I decide on what I am using, I just needed help setting up the rest of the problem.

Like I said I feel like this is something I should easily know how to find, but it has been a while and I seem forgotten some of my heat transfer.

Thanks for the help

 

[IRstuff]

The critical part you are missing is the actual heat, obviously. One obvious workaround is to assume a class natural convection from the surface of the insulation into the air, so pick an air temperature, say, 90F, pick a reasonable heat transfer coefficient, say, 10W/m^2-K.

This gives you 333 W/m^2, by conservation of energy, that must be the same heat transferred through the insulation, so:
(333W/m^2)/(2000F-150F) = 0.324W/m^2-K That's the quantity that the insulation plus steel wall must provide.

You'll need to provide some sort of engineering margin, of course, since mileage may vary.

TTFN
faq731-376

 

[som1973]

The Thickness of Insulation is purely dependant on Economics, we need to provide optimum wall thickness so heat loss through furnance wall should be minimum, also cost of insulation is also kept minimum.

Hence you need to work out what should be allowable heat loss through furnance wall & work backwards to calculate minimum thickness required.

Generally heat loss through furnance wall accounts for 10-15 % of total heat input of the furnace.

so calculate the total heat loss through furnace wall ( BTU/HR) = 0.15 (Worst case) x Total Heat Input of furnace.

once you found Q (heat loss through furnace wall) then calculate the Insulation thickness required.

 

[chicopee]

What I would do is get some ME and Chem E handbooks such as Kent, Mark or Perry and study the relevant sections because there is going to be a lot more than calculating the insulation thickness of the test furnace.

 

[EdStainless]

Some of the high performance insulation guys have calculators on their web sites.
IR is on the money with how you do this.

Or you find real furnaces and see what they used.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[racookpe1978]

There is also the "cost" of the thicker insulation: First, because of its intrinsic (purchase) cost, but also the cost in supporting and hanging that insulation goes up as the weight (thickness) goes up. Second, because the surface area of the furnace losing heat increases as the outside area (proportional to insulation thickness) goes up.

Therefore, as indicated above, assume 15% "cost" of heat loss is industry standard. Don't make the insulation much thicker than that: unless the cost of your fuel goes down: Natural gas-fired furnace will be less expensive over time (now that fracking has increased supplies in most areas) compared to an electric-heated furnace.