Another Oven Build

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  • It's a while since I've built an HT oven. It seemed like time to get off my backside and build another one, or two.

    The last time I built one, I stepped the length up from 18" to 22.5", increasing the length by an extra brick. It was a bit slower to reach temperature than the 18" oven, but not excessively so. It seemed like there was a fair chance that an oven would still reach temperature if I built it with yet another brick for a 27" length. The only way to be sure was to build one and it seemed like a lot of effort with the risk that the resulting oven would not do the job. I tend to regard 1206 degC, 2200 degF as an acceptable design temperature, being a little over the maximum temperature requirement for any of the readily available blade steels I am aware of.

    At Owen's hammerin earlier this year, I was chatting to a guy who had one of my earlier 18" ovens and wanted something longer. He already has the control box, which represents about half the cost and effort of a build. He also only works with Carbon steels. Carbon steels need lower temperatures than stainless steels, not usually more than 1000 degC, so there is minimal risk a 27" oven will not meet his needs, even if it cannot manage stainless temperatures.

    The plan is to build a 27" oven and test it, for which I'll need a control box, then build either another 27" oven if it will reach 1200 degC, or build a 22.5" oven if it won't, to go with the new control box.

    I've made a start on the oven.

    My earlier ovens had fully-welded frames. These made element replacement a job for an angle grinder with a thin cutting disc and a welder. It also made them awkward to modify. This one has welded sub-frames which bolt together. A discussion with a well-known smith some time ago had me rethink the element connections. My early ovens had the element connections on the back: dead easy to do. Making the back removable entailed taking the elements out of the side and is much more hassle. The idea behind it is that it should be possible to bolt 2 ovens together and HT swords. I have already built one 42" sword oven which uses a modified control box to power 6 kW of elements from two 13A sockets, so I know there's no difficulty electrically.

    The bits of box section between the oven and the hinges are there to space off the hinges and help to even up the pressure on the 1" blanket I use as a door seal.

    I think the odd bits of faffing about with the elements coming out of the side add at least 50% to the build time, possibly more. Progress to date has certainly been slower than I'd like.

  • I just get them as tight as I can.

    I tried cementing them once and just don't have the knack. I found the bricks just sucked all the water out of the cement and it woudn't squidge once it was that dry. Short of really wetting the bricks out first, it just wasn't going to happen for me.

    I live in Lancashire and work in a shed. Drying things out properly after wetting the bricks seemed likely to be weeks of a job and I'm not that patient. The cement stuff is still in a tub under my bench.

    The best bricks for ovens (JM23 IFBs: very light and very good insulators) seem to be the worst for soaking up the water. They have a chalky texture.

    I've tended to make the top and bottom frames as accurately as I can, erring a little on the tight side, then use an off-cut of a coarser brick as a file to adjust the bricks to fit. I then use a ratchet strap to pull things up tight and weld in the verticals.

    An old file or sanding block will work if you've not got a bit of coarser brick.

    For this one, I've welded the uprights to the bottom frame and bolted the top frame on.

  • The stuff I’ve actually got done on the oven has been mostly the fiddly bits that don’t look much but take a lot of time.

    There’s still a fair bit to do, but by Thursday evening I’d got to the point where the oven was more-or-less ready to test.

    I still had no control box, so decided to just power it up and see how hot it would get in an hour. The temperature readout used was a handheld unit and the thermocouple used was the one that I intend to use in the oven.

    I was hoping to reach 1100 degC within the hour and 1177 degC (2150 degF) within an hour and a half to give a reasonable compromise between capacity and speed: 2150 degF still seems to be enough for any of the stainless steels likely to be encountered by anyone using one of my ovens.

    In the event, 800 degC was reached in 22 ½ minutes, 1100 degC took 54 ½ minutes, the temperature at an hour was 1125 degC and 1177 degC (2150 degF) took 71 minutes.

    I am reasonably happy the oven ticks all the boxes in terms of temperature.

    There was a certain amount of condensation formed on the frame as the 50mm ceramic fibre board used for the door and back dried out. I’m sure this will have had some effect on the heating rate, so it seemed possible the second run might heat up slightly faster through the lower temperature range. It seemed unlikely to make any difference at all to the rate-of-rise at the higher temperatures though.

    The surface temperature of the door seemed to be higher than I’d like, so it might need an extra layer of board. I didn’t measure the temperature or try touching it. It’ll be something that needs to be checked when it gets a more of a proper test.

    The Ceramic Fibre board was pretty unpleasant to be around while it was drying out and getting up to temperature. It gave off a fair bit of smoke that got to the eyes, nose and throat. I had picked up the board (relatively) cheap and don’t have the spec. so I’m hoping it was just the binder burning off and that the second run is smoke-free. If not, a bit of a rethink will be in order.

    I couldn’t really see well enough to post last night as a result of my eyes and nose streaming.

    I gave it a second run on Friday evening. There was no smoke this time, so I assume it was just the binder burning off on the first run up to temperature as I’d hoped.

    I gave it an hour and checked the temperature as it went through the 60-minute mark; 1116 degC this time, so slightly slower than last night, though not enough to worry about. I then left it for another half hour.

    Temperature after an hour and a half at full power was 1253 degC, 2287 degF and still rising, albeit pretty slowly. By the time I’d switched on the camera and taken a couple of photos, it was 1255 degC, 2291 degF. That’s getting uncomfortably close to the 2300 degF rating of the bricks, particularly since the element temperature will have been somewhat hotter than the chamber temperature. I powered it down and left it to cool.

    The door was definitely hotter than I'd like, but is still touchable (briefly; I tried, didn't stick and didn't blister) and I think it'll be OK as it is.

    I’ll find out on Saturday morning whether the bricks have suffered any damage to the element grooves.

  • Progress continues to be painfully slow, but some has been made. I have some time off this week and hope to get back on track.

    I now have 2 ovens more-or-less built and I got the control panel together today.][/URL]

    The 2 ovens are about ready for wiring up. I’m expecting the cable to arrive tomorrow. Once wired, I’ll need to get covers over the electrical connections and do a bit of tidying up, then give them a thorough test.

    The control panel is to go with one of the ovens. The other one is due to go to someone who has one of my earlier (18”) ovens but needs something bigger. He already has the control box, so it’s just a case of swapping 3 plugs.][/URL]][/URL]][/URL]

    Once I’ve got each oven tested on the control box, the plan is to lash up a slave SSR with a second 13A supply in parallel with the one in the control box, bolt the 2 ovens together and see if they’ll give decent control working as a single oven. If so, that’ll give a working length of 58” and I’m hopeful I’ve built in the facility to extend it a little further still. I’m aiming to get over 62”.

  • It depends on the layout of the elements, Stew, but in most cases it would be counter-productive.

    If there is a section that is beyond the elements, it'll work. However, in most HT ovens the idea is to have a consistent heating regime throughout and the elements therefore extend the full length. In that case, adding anything would be replacing air, which has very little thermal mass, with something else that has considerably more thermal mass. It would just increase the time taken to get it up to temperature.

    At least, that's how I understand it.

  • They'll all do about the same: the upper limit is effectively set by the materials used, right up to the point where the oven gets too big for the heat input (which is limited by the UK 13A, 230V domestic plug/socket) and will not reach that temperature.

    Most of the big-name knifemaking ovens seem to be advertised as being good to 2200 degF, 1205 degC, and the 27" one has already exceeded that. There are a few which are supposedly good to 2400 degF, 1315 degC, and I gather these carry a significant premium.

    To build one would probably require higher-temperature Insulated Fire Bricks and probably the PM version of the Kanthal A1 used for the elements: relatively spendy stuff.

    I've not seen any HT info on any available blade steels that would need much over 2150 degF yet remain within the sort of budget that would seem consistent with regular use of a homebuilt HT Oven. As a result, I've not really looked into it too closely.