6 replies to this topic

[slwy]

Hi, I'm a student designing a furnace unit.

 

My radiant tubes will be holding Heat Transfer Fluid and my convection tubes will be holding corrosive chemicals such as acetic acid. Flue gas temp in the furnace is about 1500 F.

 

Is it reasonable to assume that I can use carbon steel for my radiant tubes and stainless steel 316 for the convection tubes?

 

[Problem arises because I read that 'carbon steel has possibility of conversion from carbide to graphite if exposed to heat for a long time'. Therefore I'm not sure if it is possible to use carbon steel in the furnace.]

 

Would also appreciate if someone could guide me to material selection for a furnace unit.

 

Thanks.

[thorium90]

1500F is 815C. Carbon steel is not used at such high temperatures. What HTF are you using? I see mostly HTF like the dowtherm types is used only up till about 400+C.

Attached Files

•  Excerpt Coulson & Richardson - Chemical Engineering Vol 6.pdf   76.1KB   69 downloads

Edited by thorium90, 20 March 2013 - 10:10 AM.

[slwy]

HTF I'm using is Duratherm. Similar to Dowtherm types. The HTF will only be heated to 300C. 815C is the flue gas temp in the furnace. which temp should I look at to choose the tube material?

[thorium90]

The fluid in the tubes is only 300C but you have flue gas temperatures at 800+C, a difference of 500+C. Do you require such high heat fluxes? I guess the choice of material would be influenced by the temperature in the tubes and the amount of heat flux it would be experiencing, though I could be wrong as Im not too familiar with selecting materials.

 

The reason I ask about the heat flux is because since you only need to heat up to 300C, there is no need to heat the flue gas to 800+C. High heat fluxes are necessary if the fluid in the tubes degrade or decompose etc upon heating. To reduce this undesirable effect, a high heat flux reduces the residence time in the furnace and therefore less degradation products.

I suppose a furnace with flue gas temperatures <500C and using carbon steel tubes would easily satisfy your process requirements.

Edited by thorium90, 23 March 2013 - 10:02 PM.

[ramsingh_mech]

Material selection is very first important thing in any design for any industry, this step needs lot of study, experience & standard references...

 

But one basic thing , you cannot and should not assume to use Carbon Steel for High temp services.Remember , when your temp is crossing 400-450degc you should opt for Alloy steel . there are many types of alloy steels based on the diffrent type of resistance requiremens such as corrosion resistance, creep resistance,etc..

 

And , your flue gas temp is about 800 degc (Inlet), can you tell me the outlet temp of this?? 

 

However , it is just for your project work , i suggest you to go through the attachment which helps you to understand the basics of Material Selection.

 

Regards

RAM

Mechnical Engineer.

Attached Files

•  proc_heat_tech_brief.pdf   468.72KB   68 downloads

[Raj Mehta]

You can also check this out, in conjunction with the excerpt of RC6 which thorium has shared. 

 

http://encyclopedia2.thefreedictionary.com/Radiant+Tube 

 

Thanks.

[Dacs]

CS at convection section is just asking for trouble.

 

SS might be possible but at the temperature you gave, you're bordering on 316SS limits.