Thoughts on Gas Forge Design and modification

Questions about gas forges come up often here and in other places. Here is an unfinished article I wrote on the subject:
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How to make a Gas Forge Better
by Pete Stanaitis
I have seen many emails on various newsgroups and heard many folks discuss the subject of making a gas forge, but wanting to change from some existing design for some reason. Often, the person has some sort of general improvement in mind, but may not know how to evaluate that desire against any possible side effects that might occur. Also, they may not have to capability to actually design and implement that improvement. I am no expert on the subject, but I have worked with heat and heat treating on and off throughout my career. I have learned that there is a lot more than meets the eye to these designs. For instance: I worked for a company that designed and manufactured heat treating furnaces and when they needed new burners designed, THEY even went to the experts.
So, here below, I am attempting to provide Food for Thought for those interested in buying, building or modifying gas forge designs:
First, here is a list of attributes that are more or less common to all gas forges. Although I have done my best to cover ever aspect of design, I'm sure this is not a complete list. If you are considering buying, making, designing or modifying a gas forge, you should consider every one of these attributes as they apply to your needs and goals. Whole books probably have been or could be devoted to each of these attributes. This list is just intended to get you thinking. -Size of overall unit -Size and shape of chamber (Capacity)---(will it hold the kind of work that I do?) -Weight (Can I lift and carry it if need be?) -Fuel efficiency (Fuel usage rate) -Noise level -Electrical requirements (Does it need a blower or is it naturally aspirated). If it does need a blower, does it run on the power that I have available?) -Ease of operation -Convenience of use -Speed of getting to temperature -Maximum temperature obtainable (Can I forge weld reliably in it?) -Material Cost to build -Labor time to build -Complexity to build (Can I build it myself or how much of it do I have to farm out?) -Reliability -Cost to Buy (if it is a commercially available unit) -Ease of Ignition -Type of Ignition system -Does it stay lit or do you have to play with it? -Safety -Toxicity of materials -Ventilation requirements -Fire precautions for use -Longevity of insulation materials -Cost and time to replace insulating materials -Atmosphere in forge (potential for scaling/a measure of burner efficiency) -Controls available -Accessories available -Portability, does it travel well without damage? -Control of exiting heat and gases (Does it have or need doors?)
Wanting to make things better immediately begs the question better than what? So, once you have determined what you like and what you don't like, you need to determine how you will know that you are indeed getting the desired result. This is pretty easy when you are talking about capacity, overall size, etc.. But if you want to compare fuel usage, ultimate maximum temperature, etc., you need Measuring Devices.
Minimum Measuring Devices (instrumentation): -Pyrometer with appropriate probes (usually Chromal vs. Alumel thermocouple) -Low pressure gage on gas regulator (about 0-30 psi) -Accurate scale to measure propane usage ( GOOD bathroom scale) -Rulers to measure openings, overall dimensions (or the largest and smallest things you will want to heat in it) -Stop watch to measure heat-up time and fuel usage rate
Optional instrumentation Exhaust gas analyzer Sound meter
Attributes that don't take to direct measurement(For these attributes you need to be able to describe what the baseline is for that attribute, so you can detect a variance) -Appearance -Ease of use (unless the usage process is broken down into individual parts) -Ease of setup (Portability)
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Thank you Spaco for sharing that.
I do a prototyping approach myself based on past experiments. Failures and successes all help when designing a gas forge.
If you keep the fire box (or heating chamber) small compared to the burner you will definitely achieve high temperatures.
How high? High enough to melt bronze in 5 minutes and that's naturally aspirated. If you add an air source the temperature goes up with the added cost of extra oxidisation, but if you want to melt iron then you need these temperatures. Also at the really high temperatures the refractories start to degrade, and you have to replace them sooner.
Being able to melt bronze is more than enough heat for the average blacksmith, so forced air isn't really necessary for a hobby application. Also if forced air is left out of the equation then you can be a little more mobile.
"Better than what?" was an excellent question. This can only be based on experience. Better than the one I made first, better than the one I bought for a bucket load of cash. Better than something dreamed up isn't really quantifiable.
If you plan to make a forge, and have no experience, then you have to listen to those that have made them. Don't be afraid to ask "stupid" questions.
Regards Charles
spaco wrote:

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Chilla wrote:

The forge/ metal melting furnace that I built got blessed with a forced air burner made of 2" plumbing parts.
Nothing against the naturally aspirated burners, as I have friends melting and pouring iron with them, but I had the blower...
Forced air burners are about the most tolerant designs to use if things are not just right. More or less fuel, more or less air, dead easy to tune. Want lots of heat add lots of fuel and adjust as required.
Having the power go out has never been a problem. :-)
I'll second the notion that a small firebox gets hotter faster. I made mine too large. Good refractory is a help too, stay away from hard firebrick except maybe some half splits or kiln shelves for lining the bottom of the firebox. Insulating castable refractory works good, ceramic wool or foam, better, and the reflective coatings like ITC 100 add that much more to the efficiency.
Cheers Trevor Jones
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