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I've been designing a thermocouple/PID controlled forced air/propane forge with the hopes that I can get +/- 10 degrees F accuracy over a range of about 1400 F to 2300 F.

As an overview what I'm attempting to do is have an idle pathway each for the propane and the air at just above the lowest settings that will produce a stable flame in the burner.  At this point I'm leaning towards a ribbon burner.  The main pathway for both would be tuned to a high heat with the atmosphere adjusted how I want it.  These pathways would be off by default until the PID sends the signal to the relay which would then open both simultaneously.  Hopefully the designs of both will show up well enough on here to make sense.  What I'm asking for is any details I've overlooked or any known reason why my design won't work.  I already know that to close a rectangle with pipe I'll need a union or at least one fitting and pipe nipple with left-handed threads on one end. The section of 3 inch pipe was included because I read that it makes a good mixing chamber, but I have no experience with that. If you see anything else that may be a problem please let me know. 


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I think it is unlikely that the thermocouple will be able to read accurately better than +/- 5 deg. F, and certainly not for the full volume of the forge, so that will be your first control hurdle.  Not sure why you want that kind of accuracy, but assume it is for heat treating high carbon steels.  Make sure you get a PID controller that is programmable and has an Autotune function, or you will regret it later.  Electric heat treatment ovens work quite well for this.

From the narrative it appears that you wanted the 3" section of pipe to be the mixing chamber.  You need to add the propane inlet to that location if that is the case, not where the air first branches.

The 1" air piping may end up being too small for the amount of air you want to push, unless you have a blower that can develop a lot of static pressure (you are going to want a high pressure blower, not a squirrel cage blower for this).  I would recommend upping that to a 2" pipe (at least) and 1 1/2" for the two branch lines.  If you can get butterfly valves for the air side instead of the gate valves they will work better for balancing your system without as much static losses.

You are approaching the kind of industrial configuration I designed for my temperature controlled glass furnaces.  I used high/low pressure safeties, a pilot, and UV flame sensor, zero pressure regulators and an Eclipse proportional mixer to make this work with a proportional valve for the air metering (make sure the solenoid valves you get for your system are rated for gas!).  The industrial assembly worked like a charm as the zero pressure regulator opens and closes the gas supply in response to the airflow upstream of the mixer.  The mixer could be used to set the proportion of air/gas over the range of heat output.

I recommend looking at industrial solutions rather than this home fabrication.  Expensive, but lots safer.  I wouldn't turn my back on this system.

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Thanks for the reply latticino.  There are a couple reasons I want to fairly accurately control the temperature.  One is heat treating and the second is to help me with recognizing the color of different steels at certain temperatures.  This is still a hobby for me, so I can't dump a lot of money on things like this.  I already have an old portable air tank, ceramic fiber blanket, kiln wash, kiln shelf, and the blower.  The blower is from a bouncy house type of toy and it will easily blow material on the floor around 15 feet away if it's not restricted.   Since it was made for maintaining a static pressure I don't think that it will be any problem.

I'm curious as to why wait until the 3 inch pipe to dump the propane in.  Won't it mix somewhat through the rest of the system and then even more when the velocity is slowed by the larger opening?  I'm not challenging your recommendation; I just want to understand it.

The reason for the 1 inch pipe to begin with is mainly cost.  A normally closed 1 inch solenoid valve is around 40 bucks.  A 2 inch valve is over 200 dollars.  I share your concern about whether or not I can get enough air through the 1 inch pipe though.

I'll have to look into the zero pressure regulator, but my guess is that's another relatively expensive component.

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I'm going to quote a portion of one of your own posts back to you:  "Pick a burner design that is known to work well and then *follow the build instructions exactly*"

You are beginning the stages of experimentation in the design of a temperature controlled forge burner assembly (which includes the design of the gas train).  Experimentation implicitly includes the possibility for failure.  If you are just trying to save cost of construction you need to be prepared to have the first couple of iterations of your design fail, possibly even catastrophically. Your rebuilds and testing time may end up costing you more than going with a vetted design, not to mention the safety issue.

The reason I recommend having the propane connection at the mixer is that I don't like having any obstructions to the propane/air mixture before it gets to the ignition point (other than the ribbon burner ports or flame retention nozzle, if used).  God forbid that your 1" air solenoid valve on the high fire side is closed and the gate valve on the low fire side is close to closed as well (say if you were trying to fine tune the low fire setpoint).  With the nominal pressure that a bouncy castle blower (BCB) can develop through a 1" pipe (aren't the outlets on those blowers closer to 2.5"?) I can easily see the propane pressure overcoming the blower and backing the mixture up into the BCB.  Bet the BCB isn't spark proof  :blink:

I really think that your combustion air piping is too small.

Working on a budget, in your shoes, I would just get a cheap type K thermocouple and readout only pyrometer.  I would construct a basic forced air burner with only one air path and one gas path. I would then learn to adjust my propane regulator and air damper to the correct points to maintain the temperatures I was looking for at various forge door openings.  Get a log book setup for different damper and regulator positions cross-referenced to different door openings.  Then I would build a PID controlled electric heat treat oven (old enameling kilns can be easily modified and are often available cheap).  Note that the Type K thermocouples aren't really all that accurate above around 1800 deg. F, and won't last that long above 2,100 deg. F, so get the thickest one you can and put it in a ceramic thermowell if possible.  You could go for a Type R thermocouple, which would work better and last longer, but they are quite expensive.

BTW, the only reason I was able to put together my temperature controlled system is that I got all the gas train components for free when I dumpster dived after a large glass blowing studio was being renovated.  They threw out two complete assemblies, including sophisticated safety controls.  Probably my best score as far as "it followed me home".

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Back in the day................ Actually not that long ago  to me but 1964 to you  I worked running annealing  and heat treating furnaces for a company that made boiler tubing for nuclear plants, and steel tubing that was used to manufacture bearing races and gun barrels. 

We used thermocouples and manual control to do the whole thing.     So Latticino's   suggestion is a very good one.

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Thanks again for the advice.  This is exactly the kind of thing I was looking for.  The point about the propane potentially backing up is especially appreciated.  Somehow that thought didn't even occur to me. You've given me several good points to think about and with the weather starting to be reasonable for forging outside again this project will likely be delayed anyway. 

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It's mostly a question of funds.  I already have a fair number of the components I'd need to go with propane, so in theory the additional cost would be far less. Besides that I like to build things for myself rather than purchase something already made.  I believe Frosty once asked, "Why buy something for 10 dollars when you can build it for 20?"  Some of the points latticino brought up have me questioning whether that would be my end result.  I'm going to ponder on the whole thing and do a bit more research before going any further with it.   I don't want to waste all the time I could be forging only building yet more things for forging.  That's kind of like working a lot to afford an expensive car to drive to work. 
I posted this mainly to have other sets of eyes finding the flaws with the design, but also with the overall concept.  I got what I asked for, but welcome additional comments and suggestions as well.

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I am sold on the ribbon burner.  I have a pyrometer and adjust the flame to obtain the desired temp for critical for the particular metal that I am treating.  I find it much better than judging just by color or a magnet.  I can also get good long soaks at the desired temperature.  When you finally build a forge with a Ribbon Burner you will kick yourself for waiting so long.  BTW, I am welding with 1/2# of propane pressure.

Check out the attachments on the Forge Supplies page at www.WayneCoeArtistBlacksmith.com.

Let me know if I can help you,



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