Details of my Natural Gas Heat-treating Oven...

Jeff the Millwright

Jeff the Millwright

Joined
Sep 29, 2015
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In another post I was asked by BallewBlade if I could do a more detailed post on my residential natural gas fueled heat treating oven so here it is. I think the big question on everybody’s mind most likely is how much heat does this thing generate since propane has 2.44 times the btu's of natural gas so the first thing I am going to do is post my YouTube video showing the type of flame one can expect. The video starts with just the natural gas burning and then I turn on my forced air supplied by a hairdryer hooked up to a light dimmer switch. As I add more air you can see the intensity of the flame building.




[video=youtube;AyOvqjOn04Y]https://www.youtube.com/watch?v=AyOvqjOn04Y&feature=youtu.be[/video]
 
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Because I couldn’t find any natural gas forges online as examples to work off of I pretty much had to start this design from scratch. Initially I tried using a venturi a system using the vacuum created by the pressure of the gas to suck in the air needed to get a good flame which I was hoping I could throttle by opening and closing the venture but this was a total failure.

There was some question raised about the pressure of residential natural gas in my last post so I called my gas company and was told that in Ontario where I live the maximum allowable pressure dictated by Government code is 7 inches of water for residential or one quarter of one psi. So it is not surprising that the venturi system was a total failure.

So my system uses piped in natural gas using a 3/8 inch needle valve to control the gas flow and a hairdryer hooked up to a light dimmer switch to control airflow. Don’t ask me how an AC motor can be controlled by a rheostat but it works. I also developed a much finer system to control airflow because I realized the best way to maintain a constant temperature is not by manipulating fuel flow but by finely manipulating airflow.

Before I go on anymore about operation I will show the construction which is pretty straightforward.

photo 1.jpg

Here is the shell of my furnace which is just an old piece of 8 inch pipe being cut at work. I actually in retrospect wish I would have gone with a 10 or even 12 inch piece. I only have one 1” layer of insulation and I wish I would have gone bigger so that I could have 2 inches as the outside shell can get pretty warm and it would just be that much more efficient. Also because I make mokume gane and that requires a clamping device I am a little tight for space for that. I went 16" long.


The back plate is quarter inch mild steel which I welded on but left flat on the bottom to work as legs. The front plate is held on by two bolts I welded to the shell which go through the front plate. This way I can easily replace my insulation by removing the front plate. The front door is ¼ inch stainless steel which rides on a pivot bolt and can be opened. If I am doing a knife with a handle I leave the tip of the handle sticking out the slit in the door that way I don’t have to open and close the door disturbing the temperature. But when doing folding knives I place them right in the oven and open the door to remove for quenching.

photo 3.jpg

photo 2.jpg

This next photo is after being upgraded with a 3/8 needle valve on the gas line and a 3/4" main pipe upgraded from a 1/2" line.

photo 1 (2).jpg

Here’s where I got my fire blanket, http://www.psh.ca/index.php?cat_id=225. This is the rigidizer I used all my insulation to give it a hard crust. Just pray or painted on and fire up your furnace to set it. http://www.psh.ca/index.php?item_id=RRIG2

For temperature readout I have this unit, http://www.amazon.ca/Signstek-Channel-Digital-Thermometer-Thermocouple/dp/B00IZXOLEA and for a probe I use this one HIGH TEMP FLEXIBLE CERAMIC FIBER-INSULATED PROBE Model: WD-08467-64/WD-08467-64-10 found on this website http://www.thermoworks.com/products/probe/tc_hightemp.html.

I found them very reliable and have used them many times in the last year without failure.

My original design had a half inch vertical pipe assembly for the forced air to enter and a T fitting which went to a ball valve coming off my mainline that I used to throttle the fuel. I was having trouble stabilizing my temperatures so I put a 3/8 needle valve on the fuel line which helped but I was still getting drifting and the temperature didn’t seem to correspond proportionately always to my fuel adjustments.

I began to realize that the airflow had a much more direct effect on the temperature than controlling the fuel flow. I regulated my airflow by using a dimmer switch hooked up to my hairdryer but I found the most delicate moves I could make using the dimmer switch often were too large so I realized I had to find a more precise way to control my airflow. It can be done with a hairdryer and dimmer switch but the temperature will bounce back and forth often a larger range than I wanted.

So my solution was I bought a three amp computer case fan and converted a computer power source to drive my computer fan and to control the fan I bought a PWM (pulse-width-modulated) speed controller. This is the site I got mine from but they don’t seem to have the 15 amp model I bought anymore. Http://www.canakit.com/electronic-kits/motor-controllers

My PWM came with a one turn potentiometer that I replaced with this one that I ordered online http://canada.newark.com/vishay-spectrol/534-11104/wirewound-potentiometer-100kohm/dp/20M1956 which gave me 10 turns of adjustment which was beautiful because this is what allowed me to have very precise flow of air and after this I could stabilize my temperatures within about a 10°F range.

The computer case fan that I used was only a three amp one but there is a four amp and I would definitely go for the extra boost. http://www.amazon.com/Delta-Electronics-PFB1212UHE-F00-120x120x38mm-connector/dp/B004Y15ALK This is the one I would get in hindsight.

Here is my computer fan with the and PWM with high resolution potentiometer and converted power supply.

photo 2 (2).jpg

So with a half inch air feed pipe and my 3/8 line with needle valve for my fuel supply I was able to get temperatures in the 1950°F range to do my stainless steels and with my PWM speed controller I was able to hold my temperatures within a 10°F range with a little vigilance. And once the temperature stabilizes it’s pretty easy to hold the temperature like that.

Then along comes my urge to do some s90v which requires a temperature of 2150°F and using my system I couldn’t get much over 2000°F. So I increased the diameter of my vertical pipe feeding my air to three quarters of an inch and unfortunately I still couldn’t quite reach 2150°F using my computer case fan method. Perhaps if I had the four amp model instead of the three amp I could’ve made it but if I want to go that high I have to go back to the hairdryer method. But luckily because I am at such a high range temperatures it seem to stabilize there pretty good even using my dimmer switch method so I have that covered.

I think I am definitely limiting myself in heat and temperature by having a 3/8 inch needle valve for my gas feed. The mainline going to my garage is 1 inch so I definitely am restricting my volume of gas. Apparently doubling the size of a line increases the flow four times so if my gas feed was a three-quarter inch line I would be getting four times as much gas into my furnace.

My setup suits my needs but my point is that anyone wanting to use natural gas for a larger furnace I think could get a lot more heat than I have simply by using a larger diameter gas supply pipe that the 3/8 I am using which of course would need proportionately more forced air of course.

And I have read that natural gas is used in industry to heat treat and actually surface carburize metal so when I heat treat without foil I make sure to run a very rich flame to keep the oxygen in the furnace to a minimum to avoid decarbonization. https://en.wikipedia.org/wiki/Carburizing. I may be wrong about this but I have never had a problem other than with my s90v with micro-chipping but I believe that is due to using too low of a tempering temperature. And I can tell my flame is very rich because I cut back the air or add gas till I know I have incomplete combustion because I can see flames coming out my front vent in my door.

I find there are a lot of advantages to using residential natural gas opposed to bottled propane. Firstly it is an endless supply and you never have to worry about a bottle running low in the middle of a run. Secondly apparently it costs about one third as much as propane for the equivalent btu's. And thirdly it is less dangerous because unlike propane which is heavier than air and settles in low spots being more vulnerable to explosion natural gas is lighter than air and rises and dissipates.

Anyway I hope someone out there found my presentation interesting and if you have any questions please free to ask.
 
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So do you think if I went with a 10in pipe and a 1/2in needle valve I could get adequate heat control with just a blow dryer? I would like to avoid the computer fan. Where is a good place to get needle valves?
 
Needle valves are available almost anywhere. Try eBay.

The fan should be a better type than a hair dryer or computer fan. A squirrel cage fan would work better.

Your straight in the top middle setup has a few issues. The most important is to make sure the cords and gas lines stay well away from the forge shell. The burner is normally brought in from the side at the tangent, at a 15-20° forward angle, and about 4" up from the rear.
 
I actually think BallewBlade that you would be okay with just a 3/8 needle valve with a 10 inch pipe. The limiting factor in my own oven using a 3/8 needle valve is not the gas flow but the airflow. I seem to have more than enough gas for my needs using a 3/8 inch needle valve.

But you may want to try just using a half-inch gas globe valve to start with because that may actually be all you need. I went to a needle valve before I really understood that the easiest way to control the temperature is by manipulating the airflow much more than the gas flow. Besides a 3/8 stainless steel needle valve is probably in the neighborhood a hundred dollars so you may want to try the globe valve first to avoid the cost. Being a millwright I just picked one up out of the scrap metal bin at work so I got mine for free but I don’t think it is really necessary.

Much more important than the gas flow is being able to finely control the airflow. So yes if I were you I would get a half-inch globe valve to work as my main shutoff valve and use that to throttle your gas and then use a hair dryer hooked up to a 120 V dimmer switch and that should work. I did many heat treatments that way so you shouldn’t have a problem it’s just easier and takes less fidgeting the more accurately you can control your airflow and to a lesser degree your gas flow. You can always upgrade later to get more control but that should get you with a workable oven.

And yes Stacy E. Apelt is certainly correct that a squirrel cage fan would produce more airflow but a hairdryer produces plenty; the problem is with 120 V supply I couldn’t find a solution that gave me a very high resolution for control of the current.

My understanding is that DC motors control RPM with voltage and that AC motors which would run off household current are controlled only by controlling the frequency and that’s why people use variable frequency drives for their grinder motors and such but even if this was used it doesn’t give a high resolution of control. That’s why I switched to a 12 V DC computer case fan motor.

And I’m talking about a fan motor in the 3 amp range giving about 190 ft.³ per minute so these are very powerful little fans and I am upgrading to a 4 amp 252 ft.³ per minute 5500 rpm which I hope will get me to 2150° for s90v. Your average computer case fan is only about half an amp. I would have much rather used some sort of 120 V AC motor such as a squirrel cage but I just couldn’t find an accurate rheostat at that voltage. Because with my pulse width modulator and 10 turn potentiometer I think I can get about 1/10 of 1% increments in RPM and I don’t see a 120 V solution coming anywhere near that. I’m sure there are solutions but they would be much more expensive that would make my home built little furnace impractical. I spent a lot of time looking for a very accurate 120v rheostat and could not find anything.

And yes Stacy I am sure you are completely right about the positioning of the burner and I think anyone should take your advice but I did find a solution to give me a very even temperature in my furnace despite my ill-conceived burner placement. If you look at my video you can see I have a stainless steel baffle plate under the burner flow which dissipates the heat giving me quite even temperatures in the area where I place my blades. I did move my probe around checking my temperatures in this area and it only varies a few degrees. But I think anybody building a furnace would be wise to take your advice.
 
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I saw the baffle diverting the flames and wondered about that.

The large 12VDC fans are excellent for blowers. Speed control and thus air volume control is really simple.
 
Thanks for the input Stacy, and Jeff thank you for this post! I work at a welding shop so i can get most of these materials for free or really cheap. I think I will stick with an 8in pipe and locate the burner like how Stacy said and try out the blowdryer with a dimmer switch for now, see how that works.
 
Stop and think about what you are saying.
You are putting some serious effort and design work into building a forge that will make many knives.
You will spend maybe $100 and a couple dozen hours making the forge body.
It will run hundreds of hours a year, for ten years or more.
During that time it will burn hundreds of dollars of fuel, and forge hundreds of dollars of steel.
This will make thousands of dollars worth of knives.

But, you want to save $50-100 by using a hair dryer to power the burner????




If that doesn't seem clear, I will use this example to make it clearer:

You build a race car from the ground up - weld up the frame, build the front end, roll cage, rear end, etc.

You decide that there is no reason to spend a lot of money building a supercharged engine when a junk yard Honda engine is cheaper.

You put in the Honda engine and take it to the track ... And it doesn't perform like everyone else's car???

You blame the transmission, the tire size, etc.. The truth is it was your fault for trying to scrimp on the most important part of a race car ... the engine.

The blower is the major and most critical part of a blown forge. You won't get the best results with saving a few bucks on an inferior part. The forge shell can be most any found item. The refractory can be of several types. The burner can be made from piping odds and ends, in anything from galvanized to stainless ( galvanized is not recommended, but will work the same as stainless).

The blower is what makes it all work or not work. Spending the needed funds to get one that is the right size and controllable is well worth the cost.



A basic squirrel cage blower and a fan control will work OK. A proper 12VDC forge blower with a rheostat control will be super. Between these are several choices that are far better than a hair dryer.

BTW, not every light dimmer works well. A fan control is far better and the same or less cost. Most run $10-20. Look under hydroponic stuff and/or search fan speed control. A new blower and control can run only $100.

I have used this $6 speed control many times to run a simple Dayton shaded pole squirrel cage fan:
http://www.ebay.com/itm/Variable-So...C-DC-Motors-/141226907354?hash=item20e1c796da

Blowers under $50:
http://www.ebay.com/itm/Dayton-Blow...B-WITH-CORD-/121773105897?hash=item1c5a3e22e9
http://www.ebay.com/itm/Dayton-Blower-Fan-1C180-No-7021-200-/141788804457?hash=item2103457569
 
I have been on this site reading for about a year despite only recently posting so I know Stacy that you are a very knowledgeable person and I have learned a great deal from your posts so I hope you understand I’m not arguing with you I just think we have a misunderstanding.

I do appreciate the reasoning behind your recommendation for this unit if it were a forge but I think that is why we seem to be kind of at cross purposes because it is actually not meant to function as a forge but instead as a heat treating furnace.

I think the confusion lies in the fact that me and BalewBlade began this discussion in a previous forum where we discussed the fact that we both do only stock removal so this is not meant to be a forge which purpose would be to quickly heat metal for forging but instead as a heat treating oven to maintain a very constant specific temperature. Perhaps the fact in my video I left the door open so the viewers could see the flame made it appear as a forge when in fact it is operated with the door closed as an oven with just a small vent hole open for gas exhausting.

So a more fitting analogy would be this is not a race car this is actually a sedan driven by a fastidious senior citizen obsessed with driving exactly the speed limit. So in this case the driver wouldn’t care if they had a supercharged V-8 or a Honda motor from the wrecker because both would get them to the speed limit instead what they would be concerned with would be the cruise control that maintained a constant speed.

That is why I am not concerned with what kind of blower is used because a hairdryer would supply more than enough air to reach the temperatures I need for the heat treating so the blower is not the major part as it would be in a forge but instead the critical part is the temperature control system which is the fan controller because what is needed is not lots of heat but a constant temperature. And I actually think a squirrel cage blower would be overkill for the small amounts of natural gas one could obtain from 1/4 psi residential service.

So to use another analogy being a millwright we often have to align motors to pumps or turbines to generators so that the coupling faces are within tolerances of a few thousands of an inch. If the machine is not too big we will put a dial indicator on one side and slowly jack it over using a fine threaded jacking bolt to slowly creep it over until we get it to exactly where we need it in one move. An ironworker would grab a 20 pound sledgehammer and bash the thing back and forth not having a clue how far he was moving it hoping it would land in the right spot eventually.

So to control temperature using either a hairdryer with a dimmer switch or a squirrel cage fan with a one turn speed control is like using a sledgehammer and using my 10 turn potentiometer with a high output computer case fan is like using a jacking bolt and a dial indicator. Because as I said the most important and critical part of this unit is the control system not the blower because it is a heat treating oven needing a constant temperature and not a forge needing lots of heat.
 
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Fwiw.
A typical hairdryer uses a universal motor, that is will run equally well on AC or DC. (actually a touch better on DC)
Most AC motors with brushes are universal.
But variable speed powertools, the motor is universal but the speed control trigger requires requires AC..

Anyway a dimmer switch electronically cuts off portions of AC sinewave effectivly lowering its "on time". and works much like the prior mentioned PWM speed control. Whats similar but on a DC circuit. Its a very fast switch turning the motor on & off so rapidly is does not get up to its normal speed. Or in the case of a lightbulb, does not let the bulb come to full brightness.
A Rheostat is a variable resistor what reduce its output voltage by wasting unwanted power as heat. Simple in operation but much less efficient. However an old style sewing machine pedal can be used to foot control a coalforge blower nicely.

We use hairdryers as gasforge blowers,(my heattreat ovens are electric) cheap to buy at Goodwill, put out plenty of air & even in the dirty forging shop last hundreds of hours.
Cheap to keep a couple spares on hand, When one eventually expires, just plug in a new one & keep going.
 
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I understand that in your situation, a hair dryer works.

I just wanted to make the comments on design choice because there are hundreds, maybe thousands, of people who will read this thread and say, "Oh, a hair dryer is a great blower for a forge."
 
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