Another Oven Build

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  • I'd intended an update towards the end of November, but Photobucket was acting the goat at the time and there seemed little point without pictures.

    I got the second one wired up and ran it for its initial warmup and tuning runs in late November.

    At the time, it was without a door switch, needed a couple of cables securing and needed a bracket for the thermocouple, but it worked.

    I’d missed out a wire in the control panel, which lost me a good couple of hours, then I couldn’t find a door switch. I was sure I’d got one somewhere, but finding it in the disaster area that is my shop was going to be be a major undertaking. It seemed best to just order a new one.

    I always give my ovens a run up to maximum temperature before running the autotune. The rationale is that this gets an Oxide layer on the thermocouple and the tuning run is then carried out under “real” conditions.

    Until now, I’ve tended to regard 2150 degF, 1177 degC, as being maximum temperature, because that is the maximum recommended temperature for S90V, which was the most exotic steel I could reasonably foresee anyone wanting to treat in a homebuilt oven back when I built the first one back in 2009.

    Things seem to have moved on somewhat since then and there has been some discussion lately about needing higher temperatures for things like M35 HSS, which needs up to around 1240 degC.

    The specs for the Evenheat KO-series ovens show they have a maximum temperature of 2400 degF, 1316 degC, and are suitable for use up to 2350 degF, 1288 degC. The KO-series is Evenheats high-temperature range of HT ovens. Their KF-series is the range more folk are likely to be familiar with. The KF-series has a maximum temperature of 2200 degF, 1205 degC.

    I therefore decided to give this one a run up to 1288 degC on the grounds that I’ll have to do it sometime, if only out of curiosity, and if it breaks it getting that hot, I’d rather it happened early on before I put any more work into it.

    It reached 1250-odd degrees without too much difficulty with the controller still applying full power. With the factory default PID terms, it then started reducing power (this was the pre-tuning run, so it had not been tuned to the process at that stage).

    The temperature was still rising, but at a greatly reduced rate. Patience is allegedly a virtue and I’ve never been considered virtuous. I put the controller into manual, applied full power and took it up to 1300 degC.

    I wasn’t sure what would happen at 1300 degC, which is the upper limit of the type N thermocouple range, but I was expecting the controller to show a fault. Interestingly, the display dimmed as it does when the setpoint is changed and the controller is waiting for the enter button to be pressed to accept the new value, the controller cut the output until the temperature dropped below 1300 degC and then the display brightened and it seemed to carry on from where it left off. Of course, it was still running in manual and I don’t know what it would have done in Auto. No doubt I’ll find out at some point, though I’m in no hurry to push the limits of an oven that already has someone’s name on it. It is quite encouraging though: It does not appear to just shut down and lose everything.

    Since then I've faffed about a bit.

    The control panel is complete.

    The black thing to the left of the controller is a USB port for programming the controller and logging the temperature.

    There's a USB to RS485 converter on the inside of the panel. The software only works with Windows and the USB to RS485 converter needs Windows7 or later, but it's the best I can do at the moment.

    The untrimmed tyraps are the ones I changed when I fitted the wire I'd originally missed. They will get trimmed.

    The controller has been Auto-Tuned.

    The oven itself is pretty boring, but there are a couple of things which are unusual.

    The out-through-the-side element connections can be seen in this pic.

    And enclosed for safety

    The reason for this is that I wanted to make the oven relatively easy to extend in a way that could easily be reversed.

    It's not pretty, but adding 2 layers of 2" Ceramic Fibre board to the door gives another 4" of chamber length.

    And on test:

    I ran out of the Ceramic Fibre board with just 2 layers in the door. With the back of the oven having a similar construction, there's no reason why both the front and back cannot both be extended by 4".

    The basic oven has 27" of IFBs and a seal at each end made from 1" CF blanket. This compresses in use, but is still around 1/2" when compressed, giving a chamber length of 28".

    With both ends extended by 4", that should give a 36" chamber.

    The temperature seemed slower to rise with the extension and it certainly won't do 1300 degC at 36" long. I don't really see this as a problem though. 36" seems like Carbon steel territory to me.

    I have a "slave" contactor, SSR, etc. set up to run the second oven bolted to the first. I couldn't try it today because both ovens need to start from the same temperature. I'll have to try to find time to run it between Christmas and the New Year.

  • At "normal" length, it'll do anything we're ever likely to want, including HSS.

    Once the extensions are on, there's more surface area for heat loss and the heat input is unlikely to be able to keep up with the losses at higher temperatures. I think it'll probably still manage "normal" stainless temperatures no problem.

  • More playing.

    I took some pictures with a thermal imaging camera because I wanted to understand where the heat was going. Thermal Imaging can be quite difficult to interpret for a number of tediously technical reasons, but it told me what I needed to know and I was pretty happy with the results.

    I had run the oven up to 1100 degC (2012 degF) and held it for an hour. I was concerned about the surface temperature and safety, given that the surface temperature was showing around 300 degF (about 150 degC) on the door. I felt it would probably be reasonably safe because the Ceramic Fibre board is pretty poor at transferring heat.

    I pressed my hand against the door for 5 seconds

    Then took it away and photographed the door

    The poor heat transfer between the board and my skin meant that I suffered no damage and no pain. I'm reasonably happy that the risk of burning oneself through accidental contact with the outside of the oven is acceptably low.

    I've now stuck the 2 ovens together.

    I'd taken reasonable pains to make sure both ovens were close to identical in terms of their construction, hoping that they could be controlled from a single control box, albeit with a second SSR slaved to the first to feed the second oven from a separate 230V, 13A supply.

    I lashed up the slave supply and fed it off the control box.

    I attached a handheld readout to the thermocouple in the slave oven, set the temperature to 800 degC (1472 degF) and started it.

    The indicated temperature of the slave oven seemed to rise faster than that of the master, showing a difference of around 60 degC at 300 degC and 240 degC respectively. The gap then closed until both readings were within a couple of degC when the master reached 800 degC. Leaving it at 800 degC for another half hour, the gap opened up somewhat until, after half an hour of "soak", the slave was showing 795 degC with the master at the 800 degC setpoint.

    Having gone to the trouble of building the ovens, I would want to see better control than this. Running off a single controller was an attempt at keeping the cost down and I don't feel the cost saving justifies the reduction in performance. In future, any joining of ovens will be done with each of them controlled independently by its own control box. This will give the tight control expected of an HT oven.