×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Alloy for tubed fireplace heat exchanger ?

Alloy for tubed fireplace heat exchanger ?

Alloy for tubed fireplace heat exchanger ?

(OP)
I am going to build a grate for a wood burning fireplace (home brick fireplace) out of tubing.

I will bend, weld, cap, and connect the tubes at the points where they make contact to each other in such a way that I can force air through the center of the tubes while the fireplace is use and the burning wood is sitting on top of the tubed grate, and as the forced air is heated, send it back into the room to increase the amount of heat  transferred from the burning wood.     

As the cold air entering into the grate is of positive pressure, if any of my welds crack or develope leaks, only air will be vented into the fireplace and no carbon monoxide will be sent into the room.

In the past I was building stainless steel exhaust for my trucks with alloy 420 stainless steel using a low carbon 316L rod so that the welds would not work harden and crack.

I did have some very light surface rusting of the 420 stainless but it did not seem to affect the integrity of the system.

What would be a good alloy to use for this application and what alloy rod should I weld it with?

To build it with 316 alloy and weld it with 316L rod would seem to work well, look very good, but would be pricey.

Any comments are welcome,

bayislandsdiver

RE: Alloy for tubed fireplace heat exchanger ?

Hi, I would use low carbon steel tubes for your idea. Stainless steel is expensive and I would be concerned about chromium contaminant mixed in with the heated air. Your idea is already in use as I have seen it sold and also exhibited at the Big E in West Springfield, Massachusetts. Those units appeared to be of carbon steel tubing.  Stainless steel in truck exhaust system is undoubtely preferred due to its resistance to carbonic acid that forms with water vapor being a by-product of combustion.  Any chromium contaminant in the exhaust systewm would have very little health effect in the open air.

RE: Alloy for tubed fireplace heat exchanger ?

I suggest using 409 SS (UNS 40900) as it is less expensive than 316 and should be available as it  is used for auto exhaust systems. 420 is a bit more expensive as it requires a bit more Cr due to a higher C limit (which you don't need since not high strength application) and has a lower scaling limit.  Also, if 405 or 409Cb or 430 are available, they are preferable to 410 since don't need preheat for heavy or complex sections.
SS Alloy         Scaling Temperature    weld filler
420                        800 F         ER420 or E/ER309
405, 409Cb, 410*  1200 F        E/ER430 or 409Cb or 309/309L
430                      1500 F         E/ER430 or E/ER309
443*                    1600 F         similar composition
*preheat & post-anneal recommended.

I wouldn't worry about Cr contamination of the air, but rather, possible CO poisoning if CS pipes leaked. Remember VW Beetles & Porsche 914 HXers? CS rusted through, so SS replacements were sold.

RE: Alloy for tubed fireplace heat exchanger ?

(OP)
kenvlach,

Thanks for the info.

I know there are some systems out there in mild steel, but I want to offer a quality product.

My system will work under positive internal pressure for the exact reason that you pointed out, if the welds or the piping leaks for any reason, air from inside the system will enter the fireplace through the cracks under positive pressure and be carried up and out with the exhaust and smoke from the burning wood.

Another reason for the blower mounted to the inlet side of the system is that the temperatures will be much lower for the blower to operate in.

By solving two problems with one solution I want to streamline it for simplicity, cost, and durability.

This is the process that I approach all of my projects.

This is also why it is hard for me to "complete" my projects as I am constantly improving them.

bayislandsdiver


RE: Alloy for tubed fireplace heat exchanger ?

Hi,

I am interested in what design you came up with for the fireplace heat exchanger, as I am interested in putting something similar together

dan

RE: Alloy for tubed fireplace heat exchanger ?

bayislandsdiver
You'll find this device will work very well at heating and drying the room air.  So think about a humidifier or always keep the teapot going on the stove.

pennpoint

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources