Heat treatment oven from an air tank

[Ted Ewert]

I started out building a heat treating oven a few months ago. It was a total failure, other than a good learning experience. I'm going to try it again with new materials and knowledge as see if I can't make it work this time.

I ordered a dozen insulating bricks a month or two ago. They arrived and most of them were broken, so I asked for replacements or a refund. the Vendor chose replacement and it's been nothing but me nagging him to ship them ever since. They finally arrived intact today, so I have lots of bricks.

Square bricks in a round tube is not ideal, but I think it will fly. I need a good surface to mount the element on, and the bricks will serve well in that department. Here's what I got so far:

The three bricks laying out are going to be the top. I bolted them together with some 1/4-20 SS threaded rod. I have some high temp gasket material which I may put between the bricks to seal the cracks. I'm just trying to figure out the layout at this point. I trimmed the lower bricks to fit the curve of the tank, and will do the same with the top. I will then fill in the gaps with wool insulation.

The end of the tank I cutoff will become a hinged door filled with insulation. I also have to design a way to be able to replace the element should it fail. I talked to a guy who uses his oven a lot and has already gone through 3 elements. I may make the top so it slides out. I am also thinking of coating the inside with some Metricote for added heat retention.

I'll update as I get things sorted out.

 

Ted

[Steve Sells]

why bricks, Inswool is easier to work with and is not a heat sink

[Ted Ewert]

These are refractory bricks (no clay) and make very good insulators. Probably made of the same stuff inswool uses. I also need a flat surface for the heating element to attach to, as well as support for that flat surface, and the bricks provide that. 

If I had the sheet metal I'd build a large box for this oven and put a solid layer of inswool around the bricks (which would be a better way of doing things). If the tank doesn't work I'll have to do that. I could always put a layer of wool around the tank too. We'll see.

 

[Ted Ewert]

Here's an overview of today's work...

I put a hinge on the front door. Not exactly a thing of beauty but it works good.

 

I spent most of the day rewiring my control box for running 220 VAC into the element. I had to add a 40 amp mechanical relay (the black thing in back of the controller (PID)) and a switch to control it.The 220 goes through this relay and then through the SSR (solid state relay) which is controlled by the PID. The rest of the box runs off 110 which is a separate circuit.

You can kind of see the SSR, which is the gray thing right below the mechanical relay. It has a heat sink below it, which I also added a fan today to keep it cool.

 

Here's the front. The switch on the left is for powering up the box, and the one in the middle controls the 220 volt relay. It has a 110 VAC control coil which this switch activates. The red light indicates that the 220 volt circuit is active. 

This PID is specially programmed for heat treatment applications. You can ramp the heat up and down at whatever rate is indicated, and also soak the material for whatever duration you need. It's a little weird to program but not to bad. It works good once you get the hang of Chinese logic. 

 

Ted

[timgunn1962]

I think the tank might be more trouble than it's worth. Square is much easier when using IFBs.

When I built my first HT oven, I used a frame welded up from 1" x 1" x 1/8" angle and 1" x 1/8" flat. I expected to need to skin it in sheetmetal. Not knowing what temperature it would reach, I decided to run it and see, as the temperature would dictate the most cost-effective material for the skin (Plastic-coated steel, Galvanized steel, Aluminium sheet, Stainless Steel sheet, in increasing order of temperature/cost). In the event, I found it all worked fine without skinning and the IFB gave a much lower heat transfer to human skin when touched than any of the prospective sheet-metal skin materials. The result was that I seemed to have found a very safe, very cost-effective, solution wholly by accident and have used it ever since.

Whilst there is nothing technically difficult about building an HT oven, there is a lot to think about and pretty much every design decision impacts something else.  

Element life is largely dependent on element diameter: thicker wire lasts longer, all else being equal. Thicker wire has a lower resistance, so more length is needed for the same power at a given Voltage. This tends to mean that thicker wire needs a bigger oven. To reduce the power output for a given wire diameter and Voltage, the length needs to increase.

For a 230V, 3000W, oven, about 18" is the shortest chamber length I have been able to comfortably build using 16AWG Kanthal A1. I built 4 ovens with 16AWG elements and they went to knifemakers. There were 2 element failures, which I felt could be at least partly attributed to the element wire diameter being small. I built the next using 1.6mm/14AWG elements and increased the length to 22 1/2" to give a longer groove to put them in. I tested the oven and found it would easily achieve 1300 degC/2372 degF, the upper range limit of the type N thermocouple I use (type N has better high-temperature stability than type K and is MUCH cheaper than types R or S).

The next 2 ovens I built used the same 16AWG elements and were each 27" long. Again, they would easily reach 1300 degC. They were designed/built to join together to make one long oven for swords.

The thermal image was taken just after pressing my bare hand hard against the door (2" Ceramic Fiber board) for 10 seconds. I'd previously used IFB for the door and was checking to see whether the CF board also seemed safe.

Joined together for long stuff:

 

Apologies: this may have become something of a thread hijack and that was not my intention.

If you are looking to keep it small, I would recommend going with 110V. You already have a ramp/soak controller, which is very helpful for controlling overshoot when you have powerful elements relative to the thermal mass of the oven.  It does mean that you need to use the ramp function and that you need to set the ramps having mapped the heating (and cooling?) curves for the oven, since most of the industrial ramp/soak controllers I have encountered do not have the guaranteed soak function that is built in to many of the kiln controllers.

Do you have a link to the controller manual?

 

 

 

[Ted Ewert]

Thanks Tim, I appreciate all your input and pictures. I built my first oven the same way you did, with only angle iron, but I used clay bricks and it failed. I also prefer square for this type of oven and I'm going to take your advice and abandon the tank. I never had a good feeling about the tank as an oven anyway. 

I'm using 18 gauge for the element and we'll see how it goes. I've got about 10 ohms through the wire cold which should give me between 20-25 amps, somewhere around 4500 watts. That should warm things up pretty good. I'm making the top removable and will use thicker gauge wire as warranted.

This PID doesn't need any precalibrating for ramp up. It automatically calculates the rate of heating and turns the element off if its heating too fast. I only have a paper copy of the manual, ebay link removed

Ted

 

[Irondragon Forge ClayWorks]

We just use one of our three ceramic kilns (1 propane & 2 electric). Much easier than building one.

[Buzzkill]

I'm in the planning, pricing, and calculating phase of making one of these furnaces, and I have a few questions for anyone with experience building or using them.  Did you coat the floor with anything or put any kind of protective insert on the floor to protect it?  Did you coat the fire bricks in the chamber area with anything like ITC, Plistix, or Matrikote, and if so was there any noticeable difference before and after?

I assume that the thermocouple should be centrally located and since it's more likely to be damaged if installed in the floor, the ceiling is the best option, but if there's a good reason to put it somewhere else I'd like to know where and why.

I haven't found (yet anyway) a "standard" for watts per cubic foot or something comparable.  Is there a target to shoot for there?  Right now my calculations would give me about 1500 watts for about 0.27 cubic feet of volume.

Any good info in these areas is appreciated.

[stevomiller]

Nice work gentlemen!

As an equipment engineer and fabricator I utilized SCR/thyristors in many of my resistive heater products (I build other stuff now).

If you can find one on eBay (for cheap since new from distributors they aren’t) that meets your needs and will play with your temp controller they are great. You will get rock solid temperature control regardless of line fluctuations, etc, and they will allow your heating elements to last 3x compared to using just an SSR with normal on/off function.

The SCR will clip the AC sine and can limit voltage and current, kind of like PWM does for DC, only different haha. Anyways, just throwing it out there for those of you that want to build to true industrial/mfg quality and durability.

BTW Eurotherm is a good provider of 120/208 single phase units.

Watching your guys’ great creations and ingenuity,

Steve

 

[stevomiller]

Ted I just noticed your SSR isn’t mounted to a heat sink. You absolutely need a heat sink, preferably with thermal past betwixt the SSR and it. If you choose not to, your SSRwill be de-rated by 50% , and I very likely to fail. Sometimes catastrophically so.

Please take what I am saying to heart, I don’t post things as fact if they aren’t. 

[Ted Ewert]

Thanks for your concern Steve, and you're right, a heat sink is necessary for these SSRs. However, I do have a heat sink (with heat sink compound) although is a little difficult to see in the pictures. It is mounted directly below the SSR, and underneath the enclosure. I also have a fan mounted to cool it off as it tends to get quite warm. 

I keep meaning to get back and finish this project, but I'm having so much fun at the anvil I haven't got around to it. I'm also running out of room in my garage and don't have a good spot to put it. I need to do some remodeling!

Thanks,

Ted

On 8/17/2018 at 9:13 PM, stevomiller said:

As an equipment engineer and fabricator I utilized SCR/thyristors in many of my resistive heater products

How is the SCR controlled? The controller I have is a strictly On / Off type. Wouldn't you need a variable voltage for these things?

[Ted Ewert]

Buzzkill... all good questions. You don't need any type of coating if you're using high temp insulating bricks. I like to put my thermocouple low as to more accurately represent the temperature close to work They are not particularly delicate devices and easily avoided when putting the piece in to be treated. I put mine in the back.

The power in calculation pretty much depends on what type of insulation you have and how long do you want to wait for it to come up to temp. I'm using around 4 Kw, but I haven't tried it yet so I can't give you any direct feedback. My first oven wasn't insulated well, I used 1.5 Kw, and it didn't get close to the heat I needed. I talked to a guy on another forum who was using basically the same setup I'm building right now and he was very happy with it.

I'm no expert, so take this info for my opinion.

Ted

[Buzzkill]

Thanks for the response.  A follow up question comes to mind though:  How far into the chamber does your thermocouple protrude?  I was thinking about getting one of the ceramic sheaths to protect it a bit, but since I have zero experience here I don't know if that's a good idea or serious overkill.

I'm still waffling a bit on the wire diameter for the heating elements, which of course also affects the resistance, coiled length and/or coil diameter, etc. so I'll do a few more calculations before ordering parts.  If it's reasonably possible I want to run on a 110v circuit and pull no more than 18 amps but still be able to hit the temps needed for heat treating some of the stainless blade alloys.  It might be a tall order, but I'm going to give it a shot.

[Ted Ewert]

You only need the tip in the chamber. It's the reaction of the dissimilar metals connected together at the tip which generates a voltage. Make sure you don't run any power wires close to the thermocouple wires, and keep them as short as possible (they're susceptible to interference).

Be careful if you're ordering a "1500 watt" element from ebay. They will indeed produce 1500 watts when run in a 230 VAC circuit.  Make sure they specify what voltage the power spec is derived from.

I wound my own element, which is simple enough. If you have a multi meter you can measure the resistance of the wire and calculate the current draw (and power) based on the voltage you're using. If you want about 18 amps at 115 VAC, you'll need a wire which measures 6.4 ohms from end to end theoretically. That will give you around 2 KW. You will get some loss across your SSR, and the resistance of the wire will increase slightly with temperature, so you could probably get away with a little less resistance. Test it before installation. If it trips your breaker you need a longer element. 

I would go with as thick a wire as you can, they hold up longer. Also, remember to stretch out the coil before you install it. There has to be some distance between the windings to avoid overheating. Your connections to the element are going to get real hot too. I use a steel crimp connector with no insulation on the end of the element, then I connect that to a threaded rod which travels through the insulation to a convenient point for the external connection. The threaded rod should be made of stainless steel (as well as the nuts and washers). 

You should figure that the element is going to fail at some point and you'll have to replace it, so design accordingly. I'm securing my top (containing the element) with nuts and bolts so I can remove the whole thing.

Ted

[Buzzkill]

I've been planning to wind my own coils.   That has put me knee deep in calculations for the different diameters and lengths of wire to get the resistance needed. After that it's figuring out the length per coil using different diameter mandrels, how many coils per inch for the various wire gauges when tightly wound, how many inches of tightly wound coils are needed for the overall length in order to produce the stretched length needed for the channels in the chamber.  The problem I'm having as I go up in wire diameter is that the length needs to increase significantly to achieve the desired resistance.  That means a larger a larger coil diameter to keep the same coiled and stretched lengths of wire.  Of course as you go up in wire diameter and length, the cost of the elements goes up as well.  I'm looking at 13 to 17 ohms resistance on two elements run in parallel at this point.  It just depends on how closely I want to run to the upper current limit and risk tripping breakers.

I plan to double or triple the wire back on itself and twist pigtails on the ends of the elements and run those through the bricks to ceramic terminal blocks to deal with the anticipated heat issues and avoid any pinch points inside the chamber where the temperature is high.  I've seen in a few places that it's not uncommon for the element wire to fail/burn where stainless bolts were used for connections inside the heating chamber.

[stevomiller]

If the wire is attached to a screw where the wire and screw get very hot the thermal cycling can cause the fastener  to grow due to expansion and reduce contact between the screw and wire. Reduced contact area can overload the contact point causing it to fail faster, as well as reduce the load carrying capacity at that juncture 

[Ted Ewert]

I use "high temperature" crimp lugs with a nominal 1/4" O ring which attaches to 1/4" stainless threaded rod between 2 nuts. If anyone has a better solution, I'm all ears. 

[Ted Ewert]

Finally got back to this project. I got the box built and part of the frame done. The element is hooked up to the thru bolts.

[Buzzkill]

On 8/23/2018 at 3:13 PM, Ted Ewert said:

I use "high temperature" crimp lugs with a nominal 1/4" O ring which attaches to 1/4" stainless threaded rod between 2 nuts. If anyone has a better solution, I'm all ears. 

I'm not sure if it's better or worse, but I can share how I dealt with that.   At each end of the coils for the elements I left about 3 feet of uncoiled wire.   I doubled the wire back on itself and then did it again, giving me 4 strands of element wire.  Then I clamped the coil end of the wires in a vise and chucked up the other end in a drill.  Using relatively slow speed on the drill I twisted the 4 wires together.  This has the effect of creating a significantly stiffer wire while reducing the resistance in the wire to a quarter of what a single wire would be.   After that I "drilled" a hole through the firebrick using some 1/8 stock and pushed the twisted ends through the brick.  To avoid any sharp bends I bent the wires around my finger to roughly 90 degrees to keep the wire from pulling back inside the bricks.   Using some high temperature wire and some ceramic wire nuts I ran the wires from the SSR to the elements.  During a couple test runs lasting several hours each I periodically checked to see if the connections were heating up by hovering my hand close to the wire nuts and touching the outer insulation of the high temp wire.  I never detected anything hot to the touch.   At most it was only ever slightly warmer than the air temperature around the oven even after several hours of use.

[Ted Ewert]

Contacts will stay good as long as air doesn't get between the surfaces and cause oxidation. I'll give this a try and see how it goes. I made the thing so I can take it apart if I have problems. 

I got the door built and mounted today. Should be done by this weekend. 

[Buzzkill]

There's always the possibility of galvanic corrosion when 2 different metals are in contact with each other.  However, it should be minimal in this case without any real electrolyte to facilitate the process. I'm not sure whether the temperature attained in one of these ovens affects this process much.

[Ted Ewert]

I'm almost finished with the oven. I'm experimenting with a latch, roughly based on the window latch principal. It works, but then it occurred to me that I should probably make it operable from the front. I'm thinking of wrapping the whole thing in a layer of wool since there are some small air gaps between the bricks. The latch arm might get in the way in the present configuration.

I built the door out of 2" angle iron and inserted a piece of oversized wool to provide a nice seal since the brick face is not perfectly flat. 

[Ted Ewert]

Fixed it