Stack wall temperature

[zmis]

I'm trying to verify the temperature of a stack's steel (A36) structure.

8.33' OD, 2" of concrete insulation on the inside for an 8.0' ID.
I know the bulk fg temperature (529F), the ambient temperature (105F), thermal conductivity of the insulation (1.54 Btu*in/ft2*hr*F or 0.1283 Btu/ft*hr*F) and gas flow characteristics (163167 lb/hr @ 23.1 ft/s).

I've referenced ASME STS-1 and my old transport text. STS-1 indicates a free convection external heat transfer coefficient of about 1 to 1.1 (fig B2-1) and an internal heat transfer coefficient of about 2.7 (fig B1). I'm ignoring the contribution of the 1/4" A36 shell (k~24 btu/ft*hr*F).

I keep thinking this should be simple now, but it's not coming to me. I've set the three q values (internal film, conduction in the insulation, external film) to approximately equal by varying the hot and cold face temperatures of the concrete and stack shell. This gives me a q value and cold face temperature significantly different than the lightweight program we use to set refractory thickness (provided by the refractory manufacturers). I've tried tossing in radiative losses (and they *should* be in there at some point) but that gets me very far off once the 0.95 emissivity in the program is plugged in. If I use about 0.4 for the painted surface I get a lot closer but I'm still not certain I'm doing this right.

Isn't there a simpler calculation for the cold face temperature? STS-1 doesn't call it out explicitly and all of the formulae that are listed would require it to be a function of itself (which is what I'm doing iterating wall temperature changes until the q's are equal). Any suggestions? I really don't mind feeling like an idiot at this point, as I've got an expiditer hounding me for this documentation, so please feel free to tell me to "look it up" etc, just do so while pointing towards the solution please. (i.e. "look it up on page xx of spec API-yyy)

Thanks in advance.

 

[siretb]

Hi
I have not done detailed calculations, and even have not used all of your data.
I assume you want to verify the wall temperature to check the acid dew point, and avoid condensation/ corrosion

The external temperature is 105°F (40°C)
the heat tranfsfer coefficient should not exceed kout=30 W/m2/°C

the internal temperature is 529°F (276°C)
the velocity is 23 ft/s (7 m/s) that a bit low for a stack.
The heat transfer coeffficient should be around kin 45 W/m2/°C (inernal)

if (you would have no insulation the wall temperature would be
such as
kin(276-twall)=kout(twall-40) ki
that gives tw=180°C . because of insulation, this T will be higher.
Unless you have unusual compostion, you should be safe versus the dew point.

I am not sure what you call the cold face temperature (I assumed it was the internal, wall temperature). If it would be the external temperature (that you check for safety/ burns) , then it is highly likely that, with only 2 inches of insulation, as much as 105°F ouside and 529°F inside you will be too hot.

I can do more detailed calculations, but have very little time now. If you wish, I'll do it, but provide your dat in metric units. (espec thermal conductivities) and supply gas composition.

I hope this helps.

 

[ione]

zmis,

I don’t know ASME STS-1, but it seems to me your approach is correct.

You do have three thermal resistances in series as you’ve decided to neglect the conduction heat transfer through the stainless steel wall (this could also be taken into account as a fourth thermal resistance in series with the previous three one, just knowing the wall thickness and the thermal conductivity of the material). Now the process is iterative since the heat transfer coefficients are a function of the wall temperature, which is your unknown. Radiation must be taken into account.

Maybe you can find some further input in the thread below

http://www.eng-tips.com/viewthread.cfm?qid=268145

 

[trashcanman]

It is a trial and error solution. Heat loss due to convection + radiation = heat transfer from conduction. You assume an outside surface temp, say outside air (105) + 5 Deg. and calculate heat loss. Compare to conduction heat loss, adjust outside surface temp. and repeat until heat loss = heat conduction.

My solution is based on inputs given, assuming the stack wall thickness = .25" and still air (worst condition), and galvanized (dull finish).

Outside surface temp = 419.1 Deg. F, heat loss = 11685.41 BTUH/FT., Insulation (concrete) mean temp = 474 Deg.F.


Have fun

 

[siretb]

Naima's program (3EPlus4.1) gives, if I keyed in the input correctly
2 inches of insulating concrete 528°F T skin (outside)
T wall (inside) 168°F (no wind)
Naima"'s progarm is free and downloadable

 

[trashcanman]

sireth: There is something odd about the solution by 3ePlus4.1: How can the inner wall temperature be 168 Deg.F and the outer skin temp = 528 Deg.F? With that, the inner wall would be Absorbing heat from the outer wall.??

 

[siretb]

OOps. I messed up inside/outside when typing. Apologies