Buzzkill Posted September 18, 2018 Share Posted September 18, 2018 I have built an electric heat treating furnace and just have a couple minor tweaks to finish everything up. I did a lot of calculations and for the most part I was within a fairly small margin of error in the finished product. I built it to run on 110/120v without tripping a 20 amp breaker and I have succeeded at that. I also wanted it to reach 1950 degrees F (1065 degrees C), which I have also accomplished. However, it takes over 3 hours to go from ambient temperature up to my target. Since this is my first build and I have no previous experience with electric furnaces I was curious how this compares to commercially produced ovens operating at the same voltage. If any of you have some numbers for me to use for comparison I would appreciate it. Quote Link to comment Share on other sites More sharing options...
John in Oly, WA Posted September 19, 2018 Share Posted September 19, 2018 That seems a bit slow to heat up. What's your element wattage and your interior surface area? When I built my heat treating oven I went with info I'd found on building a pottery kiln that recommended 3.9w/surface square inch. I haven't timed mine for heating up, but at full power it's much faster than that. Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted September 19, 2018 Author Share Posted September 19, 2018 When I get home I'll take a look at my notes where I have the surface area, watts per square inch, surface loading of the wire, etc. calculated. I do remember seeing the 3.9 watts per square inch and I think I was either very close to it or perhaps a little over it. It seems slow to me as well, but most of the ovens/furnaces I've seen are 220v operating around 13 amps, whereas I'm on 110v around 16 to 17 amps. Some of the reports I saw with 220v ovens claimed 75 minutes or more to reach about the same temps, but more than 3 hours still seems a bit excessive. I may do a partial disassembly and make sure I don't have any significant gaps anywhere, but I was curious about others' experiences with 110v ovens. No need to tear something apart if it's normal for its parameters. Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted September 19, 2018 Author Share Posted September 19, 2018 Ok, made it home. There are some slight differences from my original calculations because I planned on 7 ohms resistance and ended up at 7.2 ohms measured when I got it all put together. So at 110v it comes out to 3.39 W/sq. inch and at 120v it comes out to 4.04 W/sq. inch. I still haven't checked the actual voltage at the outlet I'm using. Technically I suppose even those figures aren't completely correct because I didn't compensate for the grooves in the bricks where the elements rest. Volume is 486 cubic inches. internal surface area is 495 square inches (element grooves not included). Element resistance is a total of 7.2 ohms. I have 2 elements run in parallel to get that. At 120v it comes out to 2000 watts, and at 110v it would be 1680 watts. Another number I saw to shoot for was 5000 watts per cubic foot of volume. I'm at 0.28 cubic foot, so even at the lower number I'd be at 6000 watts per cubic foot. As far as I can tell I'm within the recommended parameters for electric kilns/ovens. Most of them seem to be designed for 220v or higher voltage though. Maybe there's a good reason for that. I only have a single 220v outlet which already gets shared between the welder and the belt grinder so I was trying to avoid using it for this if possible. Quote Link to comment Share on other sites More sharing options...
John in Oly, WA Posted September 20, 2018 Share Posted September 20, 2018 It sounds like you're in a good range. I wouldn't think it would matter 120v or 240v. As long as you have the wattage for the surface area, you should be good. The voltage just means you're using more or less amps to get that wattage - but I'm no electrician or engineer, so I could be completely wrong. Of course, I'm not being much help to find a solution either. Do you have any more room in your service panel for a 240v breaker? Not hard to wire another 240v circuit. Then you could try it at 240v if your PID and element are built for it. Or just swap plugs for the different tools and use the existing 240v circuit. You're probably not welding when you're heat treating, so both don't need to be plugged in at the same time. Need some more knowledgeable people chiming in. I'm not being much help. Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted September 20, 2018 Author Share Posted September 20, 2018 I'd have to check on the regulations regarding adding more circuits. There might not be any, but I just don't know. With long heat treat cycles that are required with some stainless alloys, it is quite likely that I'd want to weld or grind during that time, so I really want to avoid swapping plugs if I can. I'll have to look at the PID specs to see if it can be run on 240v. I'd definitely have to rewire the elements though. I might even have to wind some new elements since changing from parallel to series would calculate out to about the same wattage as I have now. If I left it as parallel it would pull over 30 amps, and my wiring is only designed to handle 20 amps. It looks like one of my elements at 240v would result in about double the output, but from what I've seen the Kanthal wire gets brittle after being heated up and I don't think I could stretch it that far anyway. Just for reference sake, do you happen to know about how long it takes your oven to reach 1950 degrees F when starting from ambient temperature? Quote Link to comment Share on other sites More sharing options...
John in Oly, WA Posted September 20, 2018 Share Posted September 20, 2018 Not an accurate time at full power. Every time I've used it, I'm always ramping the temp up. But I can set it at full power, fire it up and find out. Would be good to know. I'll do that tonight and let you know. Quote Link to comment Share on other sites More sharing options...
John in Oly, WA Posted September 21, 2018 Share Posted September 21, 2018 I fired up the oven last night and was surprised by the results. The oven's interior is 6.25" x 8.5" x 18". I set the control to heat it at max power to 1950f. It hit 1000f in 25 minutes. So I thought, gee, another 20-25 minutes it'll be at 1950. Nope! It took another hour and 50 minutes to get the other 950 degrees. Total time 2 hours and 15 minutes. Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted September 21, 2018 Author Share Posted September 21, 2018 Wow! Maybe I'm not as far off the mark as I thought. My interior is 4" x 4.5" X 27" so the difference in shape might play some part in it. My PID only displays Celsius temps, but I've noticed that at first I'm getting about 20 degrees a minute increase, but that gradually drops off to only about 3 degrees a minute. The last 150 degrees or so takes a lot longer than I expected. Do you know the resistance of your elements or the watts rating for them? Quote Link to comment Share on other sites More sharing options...
John in Oly, WA Posted September 21, 2018 Share Posted September 21, 2018 After my surprising (to me) results it doesn't look like you're far off at all. If I remember correctly - I have a bad habit of doing all the research, learning what I need to know to build something, then once it's built and running properly, promptly forgetting all those details - I bought the 3000w, kanthal A-1, coiled wire elements from Duralite. Quote Link to comment Share on other sites More sharing options...
Latticino Posted September 21, 2018 Share Posted September 21, 2018 For the one I built I actually had to put a limit on the coil energizing time in the control loop (I have an older West Controller that is a bit more configurable than some of the modern controls) to avoid having the elements burn out from overheating. I never run mine up above 1650 deg. F. I think you have to be careful with standard nichrome elements above a certain range as they will start to droop, get brittle, burn out... over a certain temperature. Hopefully your brick grooves will help with this. My first set of coils was just recovered from a decommissioned electric duct heater, and were only pinned into the fiberboard walls with some additional nichrome wire. Worked well up to around 1200 deg, but when I started using it for 1650 and tried to increase the duty cycle they burned out pretty quickly. I now am using some coils reclaimed from an old ceramic kiln, and the ceramic coil channel holders work much better (though they supply more thermal mass as well). My heat treat oven is lined with two layers of fiberboard (1.5" of high temperature board and 2" of low temperature board insulation), so the heating characteristics will be very different than yours. However the rapid initial heating and slower heating as you approach temperature can have two sources. Of course the heating speed is directly proportional to the temperature difference between the coils and the interior of your oven. As the temperature of the oven approaches more closely the energized coil temperature the physics of the situation indicate that heat transfer will slow. The other potential source can be the PID loop feedback algorithm for your controller. Most of these are configured to avoid overshoot, so they reduce the output of the coils (either by cycling or with SCR) as the feedback signal indicates the temperature is closer to the setpoint. Other than to your electric bill, and for the circuit wiring, it shouldn't matter whether you are running at higher or lower voltage. I wouldn't be surprised if the coil wire thickness has some relationship as well (as it will be carrying more amperage). Quote Link to comment Share on other sites More sharing options...
John in Oly, WA Posted September 21, 2018 Share Posted September 21, 2018 Your PID would be slowing down the heating as it nears the target temp too, trying to ease in to the target temp and not overshoot. That might play a small part in the timing. Overlapped with Latticino - what he said. Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted September 21, 2018 Author Share Posted September 21, 2018 No doubt that the further from ambient temperature you get the slower it will heat up. If I remember correctly the resistance of these wires also increases with temperature which would have the effect of lowering the wattage. My PID keeps the elements on full power until it's within a few degrees of the target. It has an indicator light which shows when the circuit for the elements is energized so I can see that easily. Wire coil thickness may play a part though. I'm using 15 gauge Kanthal A1 wire for my elements. I'm using 2300 degree rated 2.5" insulating fire brick for the main structure, but I've surrounded the entire thing with 2 inches of mineral wool insulation to help cut down on heat loss. I'm going to try to tweak a couple things this weekend and then do another timed test run. Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted September 24, 2018 Author Share Posted September 24, 2018 I made a few minor changes, but it didn't seem to affect the amount of time the oven took to reach 1065 degrees C. I guess I'll have to accept the amount of time it takes or do some major modifications. I am curious about whether a smaller diameter wire would affect the heat up time, but I probably won't spend the time and money to find out until one of the elements fails. On an only slightly related note, I decided to check my propane forge temperature since I have the means now. I drilled a hole for the thermocouple in a piece of IFB and used that to block off my pass through at the rear of the forge, so it was out of the flame path completely. It took a little over 20 minutes, but it went from cold forge to 1300 degrees C and was still climbing when I stopped. The thermocouple is only rated to 1300 C and I didn't want to ruin it. I was a little surprised that the forge interior still appeared to be at the high orange/low yellow range at that temperature though. I also found that inserting a piece of cold steel dropped the forge temperature by more than 40 degrees C before it started to climb again.. I'm not sure how all the physics in a forge work together, but I also found that during the initial heating I could maintain a steady 1065 C at about 4.5 psi. However after running at max pressure and over 1300 C, it only took a little over 3 psi to maintain 1065 C once it cooled back down to that level. I'm going to play around with it some more and take some notes. I wish I hadn't already coated the forge interior with Matrikote. I would have liked to have gotten a before and after comparison for that. Quote Link to comment Share on other sites More sharing options...
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