Another Gas Forge And Burner (Pic Heavy)

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A good while back, I was playing around with some burners and built a quick-and-dirty test forge from IFBs. It was used to test a 1" burner based on an Amal atmospheric injector (a British commercially-made Venturi mixer. Amal were once a big name in motorcycle carburettors). The intention was to get it to run over a range of temperatures all the way from heat-treating to welding. I showed it to some of the guys from the BritishBlades forum at a hammerin and one asked if they made smaller injectors. The 1" had seemed quite small to me, as I'm used to using the 2" ones at work, but he knows what he's talking about and I thought it might be worth trying an even smaller one.



These are 1/2", 3/4", 1", 1 1/2" and 2" atmospheric injectors


A quick word on burner sizing here; the Amal injectors are threaded to fit standard pipe, but have the flare as part of the injector assembly and a throat diameter that is considerably smaller than the nominal pipe size. Most of the online Naturally Aspirated burner write-ups I have seen, that incorporate the Venturi principle, effectively use the nominal pipe size as the throat and add a larger-diameter flare onto the end. The 1” Amal has a throat measuring 0.67”. The throat on the 1/2” is 0.350”. My initial assumption was that a 1” Amal mixer with its 0.67” throat would probably be similar to a 3/4” burner built to one of the better-known designs.






I was not entirely convinced it would be big enough to do any real hot work, so my first try with a 1/2" burner was a quick-and-dirty Heat-Treat forge. Broadly speaking, the plan was to scale down a Don Fogg-style HT forge (normally made from a full-sized 45-/55-gallon oil drum) to something that would fit the space a hobby knifemaker might have available. After a fair bit of messing about, I thought I had something 10” diameter and under 2' long that might just about do the job. As a bonus, it should also be usable for forging.








791 degC is 1455 degF, the lower end of the Austenitizing temperature range for O1.






1183 degC is 2161 degF, more than hot enough for forging Carbon steels.


I then built another forge, about the size of a 2BF, with a 1/2” burner. This would just about manage the low temperatures for HT and would easily reach welding temperature, but at about 2 1/2” x 10” the chamber seemed too small to actually weld anything useful in.




A few weeks ago at work, we had a couple of small compressors to scrap, fitted with 25 litre (about 7 gallon) receivers. These looked a good size for forge shells at around 9” diameter and 20” long. I thought they'd work ok with 1” burners.



I built one; cut off an end and holesawed an opening in it, lined it with Ceramic Fibre Blanket, coated the hot face with a mixture of Rigidizer, Zirconium Silicate and porcelain clay powder, then welded the end back on. The burner port was cut to take the 1” burner. After a couple of days, the inside was still wet and I was getting impatient. I stuck in the 1/2” burner and fired it up at low pressure to dry out the porcelain mix. I also stuck in a thermocouple to see how the temperature went, expecting it to start low, rising as things dried out and stabilizing once dry.


After an hour or so, I looked at the temperature display and it was saying 1146 degC/2095 degF; a pretty decent forging temperature. There was still plenty of adjustment on the burner, so it got fiddled about with. HT temperatures seemed no problem. Nor did welding temperatures; I've seen online sources giving values of 2300 degF/1260 DegC, but 1300 degC/ 2372 degF seems to be a nice welding temperature taken from my actual measurements at hammerins. At 50 PSI and with a fully open airgap, the 1/2” burner maxed out a type K thermocouple, which is “only” good to 1370 degC/2500 degF.


Being primarily interested in the burner, rather than the forge, I wanted to see whether the temperature was being limited by the gasflow or the airflow. I stuck in a type S thermocouple (Platinum-based and expensive) instead of the type K and measured the temperature at around 1410 degC/2538 degF.


The size 30 gas jet was then transferred to a 3/4” burner and tried again for maximum temperature in the forge, reaching about 1420 degC/2588 degF, so pretty much the same. The Amal jets are calibrated for petrol/gasoline but the 30 jet is, as close as I can measure, about 0.5 mm/.020” (a 0.500 mm drill fits, a 0.508 mm drill does not).


Next, the 1” burner was tried, fitted with a MIG tip sized for 0.6mm (.023”) wire and around 0.725 mm/.029” diameter (0.7mm drill fits, 0.75mm does not). This gave a maximum temperature of 1552 degC (2825 degF) and pretty much kakked the forge lining. I grudgingly decided this was rather more burner than necessary.






The other receiver was lined, this time with only a 1/2” burner port, and fired up to test.


The gas pressure was set at 20 PSI and the only thing that was adjusted initially was the air gap. First impressions were that it would probably make for an adequate beginners forge. Forging and Heat-Treat temperatures seemed OK and maximum steady temperature was 1345 degC, still at 20 PSI. Winding up the pressure to 40 PSI, 1445 degC was the maximum temperature reached. There was still a bit of a steam plume visible from the various unplugged ports in the shell, so there may be a little more to come once the lining is fully dry.








The flame at HT temperatures is quite yellow and very rich.




The air gap (choke) is adjusted by rotating the knurled section, which screws the whole Venturi into or out of the cast body of the injector.






1346 degC is 2455 degF.


Although welding temperatures are attainable, the CF lining is definitely not suitable for welding with flux. Even just once. Any welding would need to be dry and I'm not convinced this is a realistic proposition for a beginner. Some more testing is definitely required.


The atmosphere seemed to be reducing at all useful temperatures, which should help to minimize scale and decarb. I am not sure whether it remains reducing with the air gap fully open though. It was my original intention to jet the burner so that it would be reducing at every possible setting and I originally thought I'd managed it with both the 1” and 1/2” burners, but now I'm not so sure.


The relatively low heat input resulting from the small gas jet probably makes it slower to heat biggish bits of steel than the pros would want to live with, but as said above, I think it would give someone a reasonably decent start at hobby knifemaking.


Realistic costs are awkward to work out, mainly because I used stuff either scrounged or that I already had.


The 1/2” Amal gas injector cost a little under £40 (about $60) from Burlen Fuel Systems in Salisbury as a walk-in customer. The rest of the burner assembly is just a piece of 1/2” stainless pipe with a threaded end. Threading stainless is no fun, so I used a weld nipple to provide the thread. I had these already, along with a regulator, pressure gauge and hose.


I used somewhere in the region of 2 metres of 25mm/1” thick, 1400 degC-rated CF blanket (1400 degC is 2552 degF), maybe a litre of Rigidizer, some Harry Frazer porcelain powder and some Zirconium Silicate powder. I already had these.


The receiver that became the forge casing was free. I used one thin disc cutting the end off, part of a flapdisc tidying up the cut edge and a minimal amount of MIG wire and gas welding it back on.


I think it really needs some means of measuring the temperature to realise the benefit of the fine control provided by the Amal injector, so that is factored in. Although most of the pics show a type S thermocouple and pyrometer, a type K is good to 1370 degC/2500 degF and is a fraction of the cost of a type S. The type K thermocouple I use cost £32 (maybe $50 at current exchange rates) a year or so back and the TM902C display lists on ebay at £3.39 (about $5) delivered.


I've probably got under £120 in the whole setup (under $200), including the thermocouple and pyrometer, but it would probably be more realistic to think in terms of a couple of hundred quid for a UK-based hobby maker to duplicate it (around $300-ish).










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So this was running on high pressure natural gas? or propane?  Very cool testing thank you for posting... :-)

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I have only ever used  burners like this on a pottery kiln with a secondary air gap around the burner tube tip to induct more air and a top opening on the kiln chamber itself that could adjust the flow through the chamber by tweaking the gaps in the bricks. I always assumed (obviously wrongly) it would run very rich without it.


The kiln and burner are long gone, I dont recall ever changing the nozzle tip or what size it was, I dusted off some old notes and there is no mention of it, so it probably was a case of "it works!, dont mess with it".


Nice write up, thanks

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Extremely well written write-up Timgunn. It makes me wonder if I can find those Amal's near me.

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The Amal units are now made by Burlen. At work we got them via at least one middleman and got hit with their markup. When I bought direct, I was pleasantly surprised at the prices.


Burlen seem to specialize in carburettors and parts for classic British sports cars and I'm sure they will ship anywhere. One thing to be aware of is that the threads are BSP, rather than NP. Usually, the 60-degree 1/2" and 3/4" male US pipe threads will screw into the 55-degree BSP female threads reasonably well, as the pitch and OD are the same. I'd be a bit wary of doing this with the Amal injectors because the adjustment uses the female thread and any damage is likely to make the adjustment harder.


The larger and smaller BSP threads also have different pitches to the NP equivalents. It's not a big issue, but it's something to be aware of.


There are a number of other manufacturers/suppliers of commercial Venturi mixers. I only have experience of the Amal units, but I also know of the following:




























If any are reasonably close to you, they are probably worth a call.

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