Venturi burner question

KnuckleDownKnives

KnuckleDownKnives

Time to make the doughnuts..
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Feb 12, 2015
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I've built a venturi burner modeled just like the one from high temp tools. My forge is roughly 395 cubic inches in volume. I'm running a .030 mig tip as the fuel jet. I'm having issues dialing my forge less than about 1520°. would running a smaller jet help me get a lower temp? I'm currently running it at 1 psi and have the air inlet gated almost all the way closed. Any lower and I can't really get a stable flame.

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If you step down to a .025 mig tip, that will possibly be too much- the burner works best with a small range of orifice size. Go too big or too little, and it will huff, burn back, choke the forge out, or be fine when cold but display some of these behaviors as it warms up.
A better solution may be to go to a smaller burner overall, or to go with a solenoid on the gas line with PID etc so the burner actually cuts out intermittently.
If I was running a PID forge, I'd go with a blown burner however. They are just more adjustable.
If you are concerned about heat treating in your small venturi forge, all I ever did with a rig like that was to just have a magnet, turn the shop lights down, and pump the blade in and out carefully to build up that even temp.
But, try that smaller tip if you have one. Not a very hard thing to swap out. Might work... or, if it doesn't, then drill that .025" mig tip out a bit with the next size of wire drill bit until it does work.
 
Salem, thanks for the reply.

John, no, I haven't gotten the thick walled pipe yet for that, and I didn't want to put my other one in the forge and possibly contaminate any of it with the old one, as it was galvanized pipe. I have pretty much burned all of it off, but like I said I don't want to contaminate this one. Now, I did try an experiment, and placed a piece of insulation in front of the thermal couple to block it from direct flames, but could still get full heat, checked the temp, then after it was fully soaked at temp for about 15 mins I removed the piece of insulation, and it made zero difference in temp.



This thing really seams to be heating very uniformly. The flame swirls inside like a little vortex. It probably makes 5 or 6+ complete revolutions before it gets to the front exhaust port.
 
That's good. I need to build a cylindrical one. My box is like the bright / dark side of the moon.
 
Hi Marc,
I have a very similar issue with my gas forge even with a 2x2.25 thick square walled muffle in place. I can run mine down to about 2 Lbs pressure yet I still get very hot (closer to 2000 per my TC) I only heat treat at night. One of the reasons that I was questioning you a while back about the PID build is that once I get my toaster oven working on one, I want to get one on my gas forge. I have my PID built but have to go back over it. I plugged it in last PM and didn't get any smoke (good) but I didn't get any power through everything (bad) I have something bass ackwards but my buddy who works on complex electronics is going to take a look at it for me. I used the diagram from the stickies but my PID has 18 possible connection points vs the 10 shown on the diagram from the stickies.
 
im sure I'll get it tuned in. I was tying to see how it's going to run and get approx settings. When I plumb for the solenoid for the PID to run I'll prob just let it run without a low bypass. And just the the heat of the forge light it back up as it cuts on and off. So I'm not too worried about it. The temp is very stable when I set it manually. A am going to out in a second thermal couple tube closer to the front.

PA, do you have a diagram for your exact PID? I'll be glad to offer some help as well. I've gotten to know these things pretty good as this is the third setup I've done in just a short couple months.
 
Somewhat counter-intuitively, you are likely to find you get finer, more progressive control by going UP a jet size, not down.

Going up a jet size will mean that less air is drawn in relative to the gas. More importantly, it will mean that less "suck" is being generated at any given gas flow and that a larger movement of the choke is required to cause a given change in airflow and mixture. This provides finer temperature control.

At present, you are throttling the gas with a rather insensitive choke plate. I imagine it is difficult to get accurate and precise movement. I would try to arrange for some sort of screwed adjustment and I'd try to make it slow and progressive.

I use commercial Venturi mixers in my burners, largely because I can't build anything that performs half as well for twice the money. These have an axial screw-adjustable choke which is very progressive and can provide temperature control approaching that available from an electric oven (albeit not a particularly good electric oven). I tend to find that around 750 degC, 1382 degF is about the lower limit for keeping things burning reliably.

For HT, I tend to change the jet for one with about twice the area, or one-and-a-half times the diameter, of the one I use in the same burner for forging. I'd suggest you try a .045" mig tip.
 
Timgunn, thanks for replying. I'll give it a try. Bigger and smaller tips and see where I get. The choke plate actually works rather well with the screw set at just the rights tension in able to move the choke plate very smoothly and precisely. I'll be sanding the edges down to smooth it out. I cut it out early in the morning while my wife and daughter were still sleeping. I'm going to scribe some lines where the outer tangent meets the outer ring of the inlet and number erery few of them on the outside with some number stamps so I can keep track of different settings.
 
Thanks Marc, I'll have to go look at it after work. Thanks for the help! One thing I do find curious is on Dwg ion the stickies, the SSR on the AC side shows a black (hot) line running from the Hot side of the female plug and a hot black wire to the positive bus? Even though the SSR shows the LH terminal of the AC as a (-) terminal, yet it's running to the positive power buss? Is that because it is switching the power to the female plug on and off as it receives DC input from the PID?
 
Can you post a link here to the exact schematic you're referring to? There are multiple wips in the stickies and I'm not sure the exact one you are talking about and want to give you an accurate answer.
 
I did find one small problem. My SSR is a 240v not 110v......DOH! I'll have to order a 110v SSR. Dang it, one step forward 2 steps back. My PID is rated from 90v to 240v so it should be good.
 
Now I wonder if the 240v is just the limit on my SSR? All the ones I was looking at seem to have a range of voltages they work at?
 
Ok so the SPST switch (lower left of diagram) is the switch turns the power on and off to the buss. The buss just distributes the power to the fan, PID and SSR. The wire coming out of the SSR is the power that goes to the oven for plug and play if you haven't modified your oven. The other 2 wires going to the SSR (top) from the PID are the SSR signal wires. those wires are what controls the SSR to turn on and off completing the circuit for the wire coming from the solid black wire coming from the power distribution buss.

Yes, your PID is good, but you will need t get a 110v SSR if you plan on running 110v.
 
Thank you Marc. I'll have to order one. Do you have one that you particularly like? Thanks again! Don
 
PaMtnBkr said:
Thank you Marc. I'll have to order one. Do you have one that you particularly like? Thanks again! Don
Click to expand...

Don, I just ordered 2 of mine is kits from ebay that had the PID, thermal couple and ssr in one package and not sure of a brand. The one i'm going to get for my forge i'll probably get from uberins as that is where I got the PID and thermal couple for it. I'd say just make sure it's good for 20-25 amps. I'm not sure how much a oven draws but that should cover it.

If you can post some pictures of the diagram on the side of your PID and SSR or post the makes and models. That'll help make sure I can help determine what pins are for what and just to double verify if you need a different SSR.
 
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perfect! Thank you! I may do exactly the same thing and save my current PID and SSR for when I convert my gas forge. Thank you so much!!
 
Hi Marc,
Thank you for all the help. Got my PID fired up this weekend. Just have to replace the transformer for running the fan but it's up and running (for a short time, did not want to overheat the SSR) Thanks again for all the help!!! PID gas forge will be next!
 
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