seagiant Posted November 11, 2008 Share Posted November 11, 2008 Hi, I'm looking at building a new blown forge but would like to use a air preheat feature somewhat like the Sandia Lab forge. What I"m thinking is to cut a couple of holes on the lower side of the forge then build a box against this, line with K-wool then run my piping through the box to pick up heat then the pipe runs to the intake of my blower and is used to mix with the propane to the other side of the forge to the burner. I could even run the pipe doubled in the box a couple of times to pick up more heat. Of course the actual air is coming from outside the box and is only heated as it runs through the pipe in the heated box to the blower. The question is how hot is enough and how hot is to much,ie: blower burnt and or pre ignition of propane??? Was wondering if anyone has done this before? Preheating is suppose to save up to 25% on fuel so I see this as a good thing! Any ideas? Thanks! Quote Link to comment Share on other sites More sharing options...
Jymm Hoffman Posted November 12, 2008 Share Posted November 12, 2008 While preheating air can increase efficiency, it is not always practical in application. I did run preheat tubes on my first gas forge for several years. The costs due to wear and tear on the piping, plus my time to maintain, replace etc. plus the loss of pressure due to extra bends in the pipe etc. did not pay. My system actually worked better by getting rid of the preheat piping that wrapped around the forge, etc. I used regular pipe, that did not last long, heavy wall that did last longer, as well as stainless, including heavy wall stainless. It is necessary to get the pipe really hot (red to orange,) to effectively heat the air for the short distances my system had. I found it better to use really good insulation, and good refractory materials and stop messing around with preheat tubes. Quote Link to comment Share on other sites More sharing options...
seagiant Posted November 12, 2008 Author Share Posted November 12, 2008 Hi Jymm, Thanks for the comeback on this. You seem to have strong ideas on this and from a look at your site seem to have the work history to back it up! I actually never saw a downside to preheating the air but what you come up with in your mind and what actually works is sometimes very different!!! I want to heat treat knives in my forge (1500 degrees F) and thought the preheat would help with this? On the other hand may be able to do it anyway without it. Just want to do a little research before starting, to make the best one I can. Thanks again! Quote Link to comment Share on other sites More sharing options...
Jymm Hoffman Posted November 12, 2008 Share Posted November 12, 2008 You're welcome. Now that I know you are wanting it for heat treat, I highly recommend not messing with it. I was trying to get not only more efficiency, but higher heat since I do a lot of forge welding. When some one brought out some sort of heat measuring device from an old steel mill, one of my forges that he tested was going above 2300 degrees F. That is one of the newer non preheat forges. Without pyrometers, I still work by color. Is sparking white hot hot enough? To best control my systems for running cooler, I use a water gate valve on the air pipe before the gas is introduced. Quote Link to comment Share on other sites More sharing options...
seagiant Posted November 12, 2008 Author Share Posted November 12, 2008 Hi Jymm, Yea,well I've been smithin for about 20 years, and all so have a coal side draft forge. I weld also for knives mostly but all so for items like fireplace sets and such. Modern steel dosen't weld as nice as the old wrought iron but you already know that I'm sure. Maybe using 2" of K-wool and satinite and then some ITC-100 I can do with out the preheat. It would be easier to build and maintain. I built a Don Fogg type vertical forge which was excellent and made a blown forge fan out of me,but the only thing I didn't like was the no floor aspect of it. Sometimes I like to "soak" the steel and if you wanted to do that you had to stand there and hold it! I had a friend that wanted to forge so gave it to him and now want to build another. I noticed people are using the gate valve like you mentioned for the air throttle,looks good to me also. I've looked at Indian Georges site and Tim Zowada's so have a few ideas. Did you do anything special on the burner end of things. I was thinking of using 1.5" pipe to the side and then going up to3" of 2" and then back to 3/4" into the forge to creat a mixer. Any ideas on that? Quote Link to comment Share on other sites More sharing options...
Frosty Posted November 12, 2008 Share Posted November 12, 2008 Preheating the propane actually does more to increase efficiency than preheating the air. Around 1,000f and the air:fuel mix will ignite. For a while there were a number of guys experimenting with air preheaters (recuperative) designs and I don't think anyone still uses them. As Jymm says the improvement doesn't balance well with the increased maintenance. An additional inch of Kaowool, etc. will provide far more efficiency gain. However, if you really want to experiment with a heat exchanger on your intake you might consider making it a flat plate rather than a tube in a chamber configuration. SS is a poor conductor of heat but will last somewhat longer in the forge exhaust invironment. Iron or steel will exchange heat much better but will corrode pretty quickly. Ti will last a LONG time but is an even poorer conductor than SS and is expensive. I don't know anyone who's tried a ceramic heat exchanger so that's a possibility. Another idea that is almost undoubtably more trouble than it is worth on a small scale is the "recuperative wall" furnace. Searching Google Patents with that name will give you plenty of info on them. Frosty Quote Link to comment Share on other sites More sharing options...
seagiant Posted November 12, 2008 Author Share Posted November 12, 2008 Hi Frosty, Thanks for the info! You are probably right a few more bucks spent on K-wool might get better results and be easier to do. Being a Marine Engineer, I deal with heat exchangers on the engines,evaporators,watemakers,ect. I guess I had to look into it trying to find that free ride so to speak! Jymm, sent me some photos of a burner that he uses that looks promising. I used the Don Fogg vertical for years and it worked great and all it was,is a 2" pipe with a reducer on the end as a burner! I think as long as you get the air and fuel mixed well you will have a forge! Thanks again! Quote Link to comment Share on other sites More sharing options...
philip in china Posted November 13, 2008 Share Posted November 13, 2008 Also remember that as you heat the air it expands. That means that a given volume of air actually contains less oxygen (and less nitrogen of course as well) so the advantage of the preheat is partially offset by this. Quote Link to comment Share on other sites More sharing options...
patrick Posted November 13, 2008 Share Posted November 13, 2008 At work we are switching all our forge furnaces to a recuperative design that is commercially available through a company called North American. The principle is the same as that used in a blast furnace heat exhanger. In our case, we have multiple burners arranged on opposite sides of the furnace. When one burner is on, the other is not and vice versa. When not on, ceramic media just below the burner is being heated. Once the buriner turnes back on, the air passes over the medie before entering the burner. This has saved us around %40 on natural gas. Note that some of our furnace will easily hold 300,000 lbs or more in a load. Patrick Quote Link to comment Share on other sites More sharing options...
seagiant Posted November 13, 2008 Author Share Posted November 13, 2008 Hi Patrick, That sounds pretty wild! I think the difference comes in at commercial size and hobby size. Like I said we use the heat exchange principle all over the ship where I work and it saves money no doubt,but for a small forge,may not matter that much. I am going to use ITC-100 this time on top of the satinite and look for that to make the difference! Quote Link to comment Share on other sites More sharing options...
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