kiln calculations

[Bad_Rockk]

Hi there,

i found a kiln build video on YouTube and it claims that his kiln has heat up times as follows:

800°C (1472°F) → 6.38 minutes

900°C (1652°F) → 10.10 minutes

1000°C (1832°F) → 15.45 minutes

1100°C (2012°F) → 25.09 minutes 

compared to other builds this heat up time ist really fast. He uses aprox. 60-80 mm /  2.3-3.15'' aerated concrete (600 kg/m3) as additional insulation around IFB 23 bricks and 4kW heating. Internal dimensions aprox. W x H x D 6''x4.5''x18''.

Is there a simplified calculation to determine heat up time to a specific temperature? I would like to understand why his design heats up much faster compared to other builds without additional insulation.

[Buzzkill]

That seems unreasonably fast to me.  However, my experience is with an electric oven I built for heat treatment, so the dimensions are significantly different than they would be for a kiln. Mine is constructed from insulating fire brick for the main body, and that is surrounded by a few inches of packed rockwool insulation, so it is well insulated.  If I take it up to the top temp you listed here it would take around 3 hours, but it would still be too hot to put your hand in 24 hours after being shut down.  I built mine to run on 110v though, so a 220v machine should heat up significantly faster.  I have a hard time believing the 25 minutes for that temp unless the chamber is really small, he's pulling something like 50 amps, and he's using heating elements that will have a short life in that kind of service.

I don't think there will be any simple formula that will help you here.  Size, shape, insulation, heating elements (and their placement), current draw, etc. will all play a part in how quickly it heats up and how well it retains that heat.

[Bad_Rockk]

The chamber is aprox. W x H x D 6''x4.5''x18'', so comparable to other DIY builds. Yes his elements won't last long because he's overloading them. 10m 1,2mm Kanthal A1, 230V and 4kW will result in a too high surface load of 10,4 W/cm^2. The recommendation I found is a rating of <3,5 W/cm^2.

I found this calculation to calculate heat transfer. I adapted  his calculations to a heat treat oven chamber of 4.7x4.7x18'' using 2.5'' thick insulating fire brick i get a heat transfer rate of 860W, when adding a 4'' aerated concrete heat transfer rate decreases by 75% to 205W.  Using 4'' rockwool sheets would decrease heat transfer rate by 91%  to 75W. So there is a big potential for saveing energy.

I know such a heat up time calculation is not exact and also no easy to do but I'll try to find an approximation.

[Frosty]

Not exact? It's not reasonable, you want heat treatment temperature to ramp up and down at a specified rate, not spike in a couple minutes. Don't you?

Frosty The Lucky.

[Bad_Rockk]

Yes Frosty, a heat treat oven should be able to perform exact temperature ramp ups and soaks. In my opinion ramp ups and soaks are mostly depending on the used controller. The oven itself determines how easy the system can be controlled and how much energy has to be used. The max ramp down rate is also determined by the oven insulation (or the possibility to have active cooling, which would make the system too complex for DIY). 

I'm talking about the maths/physics behind it. I know that this (heat up time, heat transfer rates, efficiency) can't be calculated exactly without a lot of input data and calculation power. But I rather have a rough calculation that allows me to compare different variables of such a system and try to understand it, than building one and hoping it performs as expected.

I can't evaluate the performance of that oven in the video. So for me a fast heat up time and temperature stability don't need to be contrary.

[Frosty]

I'm just curious why you'd want a heat treat oven that heats so fast is all.

Frosty The Lucky.

[Bad_Rockk]

Why waiting for 3 hours if it could heat up in 1 hour or less?

Seriously I don't have a scheduled time in the workshop every week. If there is a small time slot (0.5-2h), I'll take it and work as far as I can get. So I'll try to keep me workshop clean and ready to use. 

When buying/building tools I rather spend some extra time/effort/money to get something that suits my needs than being dissatisfied with performance/usability.

[Frosty]

DOH! Happily I need the obvious pointed out often enough I stopped being embarrassed. . . much. A good head sapping moment does that so nicely.

Frosty The Lucky.

[Buzzkill]

Just for general purposes I think you can use watts per square unit of surface area and watts per cubic unit of volume to get you in the ball park.  Again this will not be precise, but if you know the stats on a commercially produced oven and the approximate time needed to achieve a specified temperature then you can make some rough guesstimates.  More watts per surface area unit and volume should result in less time needed to achieve a given temperature if we assume that the oven will be insulated at least as well as the commercially produced product.

When I was building my oven I asked someone on here to let me know how long it took for their 220v commercially made oven to reach a specific temperature - and it was something close to the max temp you listed.  If I recall correctly it was over 2 hours.

If you are interested in my ramblings and some responses I got on this forum, you can view those here:

https://www.iforgeiron.com/topic/59936-electric-heat-treatment-furnace/'

 

[Bad_Rockk]

I already saw your topic and did further data collection. I contacted Paragon and Evenheat if they could supply heat up date of their ovens. I got fast replies and some data. I watched a lot of videos trying to find other DIY designs with fast heat up times. And there are not a lot of them. So I put it all in an excel spreadsheet to compare them:

So all the fast heating ovens have similar specs according Watts/volume and Watts/surface. And those values are aprox. double or more to the recommendations I found online. Insulation is different on the compared models. Paragon and Evenheat use fire brick only as far as I can see, Paves design uses 60-80 mm /  2.3-3.15'' aerated concrete and Loz design has a layer kaowool around his fire brick. So it seems that insulations is not as important for the first heat up as I expected. Even tough I think a good insulation will reduce total energy consumption to hold a specific temperature and will increase temperature stability. 

So I'll take those values as a guideline for my own design. 

If someone wants to share his specifications and heat up times I could update my excel and upload it here.