Heat treatment oven overpowered?

T

Tehemton

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Sep 6, 2016
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Hi, I'm in the process of making my own HT oven. There's a lot of good information material online and I think I've got everything sorted.
It's a 6x6x15 inch oven at 3100W (240v@13A)with a 14 gage kanthal A1 wire coiled up to 12mm diameter. The power density is roughly at 1.13/cm2 and surface load on the element coming up to 2.3W/cm2.
I just figured out that my line can supply up to 20A at 230-240V and I would like to increase the power as that means heating up is much faster. But I read somewhere that if your oven is too powerful, the PID controller has a hard time at stabilizing and will overshoot.
So my question is, if I have the scope to increase the power by using a thicker element and changing other parameters ofcourse; is it worth it? Am I missing something here?
I plan on using the Rex c-100 controller as it is easily available and cheap. Would probably replace it down the line.

Also I've seen in some places that they add two SSRs. Why would that be? To me it seems like potentially an additional point of failure. But maybe I'm missing something here.
Any advice would be appreciated. Thanks.
 
I don't think the PID will have any problem controlling your oven at 3100 watts, or 4100 watts. It will just have a different learning curve, and once the PID has learned the control parameters it should do just fine. As you said, it will just heat up quicker. Remember, the PID will require this learning process to prevent major overshoot and undershoot on first heat up. It will still need time to stabilize even after learning, but all ovens require this stabilizing time.

Remember, it's not the "thickness" of the element that determines amount of heat, but the current draw (wattage) of that element. The higher wattage might be thicker, but that's not the way to rate the element.

Ken H>
 
For your size oven, 3100W is fine.

If you want to make some changes, I would consider a 6X6X18 oven ... or 6X8X24.
 
The Rex C100 seems to be a very basic controller. It has no ramp/soak capability. This should not be a problem when trying to control to Austenitizing temperatures, but if you are likely to use it for tempering, it may tend to cause the workpiece temperature to overshoot unless you take additional precautions. Tempering with the blade in a tray of dry sand is about the easiest way of dealing with it, so it's unlikely to be a dealbreaker.

The Rex seems to have much of its configuration preset at the factory. It's not entirely clear whether these preset parameters are changeable from the front panel. Whether this is a good thing or a bad thing depends on how familiar you are with controllers in general and the Rex C100 in particular. It does mean that you need to get it right when you place the order.

Using a ramp/soak controller allows you to set the rate-of-change of temperature and often greatly improves the control. Most of the affordable ramp/soak controllers are aimed at industrial users and are not exactly user-friendly when it comes to programming patterns. I tend to use the AutomationDirect Solo 4848VR or the very similar (identical bar the badge?) Omega CN7823. I buy whichever works out cheapest at the time. I have not used the Auber Instruments SYL 2352P personally, but it seems to be generally liked West of the big pond and is probably worth considering if you are in North America.

If you use the Rex C100, allowing for future upgrade to a 1/16th DIN ramp/soak controller only really needs a couple of inches extra to be left on all the wires so that they'll still reach if the new controller has a different terminal layout.
 
Thanks guys!
I'm taking Stacy's advice and going with a 6x6x18 considering that the ss foil takes up about an inch or so. And increasing the wattage a little.
Finally going with 14 gauge kanthal a1, 24.3 meters long coiled up with an inner diameter of 12mm. It should be at 3320W. The numbers seem fine so far. Let's see how it performs.
I'm from Mumbai, India and the dealers are selling good controllers at ridiculously high prices. So I'm thinking of ordering one from China. The c-100 is cheap enough that it wouldn't hurt to chuck it out a couple of months down the line.
I'm still not clear about the two SSRs though. One seems enough. And the second one isn't redundancy either. Would there be a specific purpose behind adding two of them? Like I said before, it seems like another possible point of failure down the line and it would probably produce a lot more heat too in the control box as compared to adding just one SSR.
Thanks again.
 
Being in Mumbai is relevant.

The USA uses an odd power supply system: basically I gather it comprises a 2 phase 110V supply where the phases are 180 degrees out of phase with each other to give 220V phase-to-phase. With two live feeds to get their 220V, they use 2 SSRs. Everywhere else, including Mumbai as far as I can tell, seems to use a single phase, 220V-ish supply and only needs the one SSR.
 
timgunn1962 said:
Being in Mumbai is relevant.

The USA uses an odd power supply system: basically I gather it comprises a 2 phase 110V supply where the phases are 180 degrees out of phase with each other to give 220V phase-to-phase. With two live feeds to get their 220V, they use 2 SSRs. Everywhere else, including Mumbai as far as I can tell, seems to use a single phase, 220V-ish supply and only needs the one SSR.
Click to expand...
That clears it up. Thanks man.
 
If you have single leg 220 ( hot leg and neutral/ground), then one SSR will work. If it is two leg ( two hot legs), you need two.
 
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