Recuperative Gas Forge Furnaces

[knots]

There have been a few references in past threads regarding recuperative gas forges/ forge furnaces. The Sandia recuperative forge furnace seems to be the example most referenced. However so far as I can determine from the posts no one seems to have one. If anyone actually has and uses a recuperative gas forge I would like to hear about your first hand experience. Seems like if there were tangable advantages they would be common. Who knows what ?

[ThomasPowers]

The Sandia forge works very well I have seen it in use several times here where it originated.  However it is more expensive and technically more difficult to build.  Perhaps you should develop the Sandia-Lite version that doesn't require the refractory board and can be built from an easily available shell.

[kubiack]

I have used one once and watched demos with one several other times. I don't have much gas forge experience so keep that in mind but I thought it worked very well. It seemed to get as hot as any other gas for I have used but it was quiter and had less dragons breath. Another thing I like about the design is that it has a front door which is mostly closed while taking a heat and all of the exhaust comes out of the back and away from the user. As Thomas noted the construction is more complex the average forge and it uses refactory boards which are more expensive than wool blanket. I think these two things keep people who smith for a hobby from building one. However if you were using it daily I think it would be worth the trouble and expense.

[Herb Upham]

I have one I built 10-15 years ago, and it works well with lower fuel consumption than my other gas forges. The front door and the chimney that control internal back pressure are largely responsible for the fuel efficiency. The refractory board is more difficult to obtain, but it last well and can be repaired much more easily than fiber blanket if it is accidentally "wounded". I use ITC 100 on the refractory board and kiln shelving rather than the as-designed cast floor.

The down sides are design complexity and the tools and skills necessary to construct the forge. Mine does not have a pass thru for long stock which my other gas forges have. My experience is that the Sandia design uses about 20% less fuel for the same job compared to my forced draft gas forge and my 2 burner atmospheric forge. The majority of the savings is due to better back pressure control and less wasted heat through "dragon's breath". If you have access to a machine / sheet metal shop, can weld stainless steel, and have a forging press to form the heat exchangers then the build is easy. I did all the processes on mine except for forging the stainless heat exchangers and it took me about 8 hours to build it.

[knots]

Here is are pictures of one of mine.  Made from 10" sched 40 pipe, with weld bends for the aiir tube.  The portion of the air tube that passes through the chimney is stainless steel .  Used 1 1/2" fiber blanket.   The floor is grogged refractory repair mix rolled flat in a 3/4" food form and laid in place leather hard, then fired in place.  The fiire brick at the back closes a pass through opening. The gas tube is still packed from my move.    

 

There is another one as well that is made from a section of 12 " pipe that was split down the axis of the pipe and had 4" wide plate welded into the top and bottom to form a wide bodied forge for larger pieces.  That one doesn't see much use because it sucks 20 tank of propane dry in about four hours.  Both were built in 1993-94.  Both use a small blower bolted to the forge manifold, which were sourced at Grainger.

 

Next time I reline these I will use 2" blanket.  They both get hot fast but really cook after the ceramic floor comes up to heat.

 

 

 

 

 

 

 

 

 

[knots]

Kubiac and Herb U  -  So the forges that you have and have used are built to the Sandia design .  I have never seen one in service but it seemed such a good idea when I was building mine that i included the feature in my hybred build.  I have been surprised that they are not common since it didn't seem that dificult to build.  

 

I do not have a way to measure the actual temperature in the forge chamber so I have been at a loss as to how to carry on a meaningful investigation/discussion of the benefits or lack of benifits and was hoping that some one here has a way of measuring results and could offer some sort of hard data comparing a recuperative forge to a non-recuperative forge of similar size.

[kubiack]

Herb is more qualified to answer than I am since he has actually built one and uses it on a regualr basis. I almost always use coke to forge with and have not used a gas in several years. However the few times I used/watched a recuperative it was built to the Sandia design. I would like to see some hard numbers as well but even without them I can say that I was very impressed with the amount of heat they generated and the lack of dragons breath and noise. I don't think it would be hard to build either but if you follow the plan you will have to cut and bend sheet metal, forge and weld stainless, and cast a floor. In my mind its a lot more work then the typical pipe lined with blanket forge.

 

What temperate is the air in the pipe after it passes through the exhaust? That would give some indication to the effecency of your forge.

 

A few things to note about the Sandia design opposed to yours. The heat exchanges are insulated after they pass throught the ehaust and before they enter the forge. The Standia forge has two tubes that pass through the exhaust and they are forged into an oval shape so the exhaust takes longer to pass over them. The Sandia forge is also naturally asperated.

 

I remember reading a post where, I think, HWooldridge said he had built one and was not very pleased with the preformance. It would be nice if he would weigh in on the subject.

[ThomasPowers]

I'd suggest contacting Robb Gunter and asking---I'd bet they did all sorts of testing and data collecting on it when it was first designed. (and he was working at Sandia National Labs...)