boiler breeching heat analysis

[sabotage]

Is it possible to calculate the surface temperature of boiler breeching with only the boiler flue temperature, the construction of the boiler breeching, and the ambient temperature?

The client has requested us to perform these calculations, but I dont think they have given us enough information.

 

[macmet]

Why can't you just measure it?

 

[sabotage]

this is for a future installation

 

[MintJulep]

Sounds like a simple one-dimensional heat transfer problem, as found in any heat transfer text.

 

[sabotage]

could you walk me through it?

the typical equations i have seen have you solving for the total heat flux given the interior and exterior surface temperatures, but in my case i only have the interior surface temperature. i have checked my heat transfer textbook but have so far been unable to figure this out.

 

[MintJulep]

I guess you might need the flow rate too.

Given that plus what you have, work with a 1 degree temperature drop through the breeching system.

Maybe someone with more experience will jump in a suggest a better temperature drop, but solving for 1 will simply let you multiply later.

Flow rate x 1 degree x heat capacity = heat flux

That should get you to a single unknown to solve for.

 

[IRstuff]

Do you have a drawing of what the system looks like, i.e., something that shows what 1-D heat flow should be, and what temperatures you know, htc/thickness of the materials involved?

TTFN

FAQ731-376

 

[chicopee]

What type of a boiler is it?

 

[rmw]

Does the "construction of the breaching" include all the information like insulation thickness, type and/or R value as well as any other impediments to heat transfer?

rmw

 

[sabotage]

please see the attached sketch for what im talking about. with the information i was given, is it possible for me to solve for the surface temperature of the flue?

 

[sabotage]

anyone?

 

[sabotage]

any information that can shed any light on this whatsoever would be greatly appreciated

 

[davefitz]

This calc is usually completed to confirm the surface metal temperature is below 140 F , for OSHA safety reasons, or to confirm the "radiation heat loss" listed in ASME PTC 4.1 .

You will also need to compute the inside film heat transfer coefficient , outside heat transfer coefficient ( which is combined effects of wind , natural convection, and radiation). The project spec should define the desing windspeed used for this calc if the flue is outdoors. One trick to help lower the surface temp for OSHA purposes ( for beaching that is within a distance that can be accessed by workers) is to use an anodized aluminum lagging; it has a emmissivigty of 0.95, compared to plain aluminum with an emmissivity of 0.10 .

 

[vpl]

sabotage

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Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[sabotage]

davefitz,

that is exactly what i need to do, i need to show that the surface temperature of the flue will not cause any harm to workers or tenants. however as indicated by my first post and my subsequent attachment, i dont think the client has given me enough information to make this determination.

can i calculate the surface temperature with what i was given? if not, what other information would i need to get an answer? are there any spreadsheets or programs that makes this any easier? thanks in advance.

 

[davefitz]

You will either need to take a course in heat transfer or find a similar problem solved for another project. The add'l information needed is Tambient and the outdoor windspeed,usaully defined in the contract spec.

q(btu/hr/ft2) = (Tg-Ta)/ R,total

R, total =sum{R1+R2+...}

R1= 1/h,i where h,i= inside film convective heat transfer coef for gas flowing inside the flue, from heat transfer text, or estimate as 7.0 btu/hr /ft2/F

R2= steel casing resistance to heat transfer = t/k (ft2-F-hr/btu) , t= thickness(ft) k= conductivity

R3= insulation resistance to heat transfer =t/k

R4= lagging resistance to conductionheat transfer =t/k

R5 = 1/(h,r+ h,c),, h,r= apparent radiation heat transfer coefficient = e*g*(Ts^4-Ta^4)/(Ts-Ta); iterative value, asume an initial value, solve the problem, then update estimate and solve again. e= lagging emissivity, see prior discussion a= 0.173E-8 btu/hr/ft2/R4, T in degrees rankine, H,c= outside film convective heat transfer coef due to either widnspeed or natural convection- see heat transfer text or use h,c=4. btu/hr/ft2/F Please note that it is easy to cheat by assuming a high e and a high H,c, so your ethics will be questioned if the client detects this.