Alloy for tubed fireplace heat exchanger ?

[bayislandsdiver]

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

 

[chicopee]

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.

 

[kenvlach]

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.

 

[bayislandsdiver]

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

 

[Sakols]

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

 

[pennpoint]

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