Recommendation? DIY Heat Treat Ovens - Specific Design Questions

[Cushing H.]

I will let bricks to protruding slightly so kaowool on door can seal it tight

yes.

Hardly seems very portable, does it?? (I will need to use in in my garage - storing off to the side, then moving in to place when I need it. Maybe I should plan on a rolling table too???

 

[Natlek]

so - I finally had a chance to go through the dimensions and material count.

For overall out side dimensions I get 20 inches wide, 22 inches high, and 22 inches deep. Is that an unreasonable size for a 6"x5 1/2" chamber?? (natlek - yours seems to be about that - my apologies for the non-metric dimensions.....)

Don t worry about that hanging here I became half American

 

[Cushing H.]

after going through the design equations, it just seems like a far cry from the DIY's posted on the web ... like this:


most of which have something like a 4 by 4 chamber, and only that 2 1/2 inch brick between the 2000F inside and the outer surface you can (not!) touch.....

 

[Hubert S.]

It does seem very big. I have brick and an inch of blanket on the sides and it does not get too hot to touch. I will pay more attention next time and measure actual temperatures. It will only be O1, so not crazy hot. I think neglecting the convection in the calculation might be the reason.

Without blanket or board, the temperature does go above whatever temperature the binder in the board cooks off at. When I first fired mine up, there was a bit of smoke and an unpleasant odor. I took one of the boards off, and you could see that the board had turned dark along the seams in the brick.

A cart is convenient, I have mine on a cart with a plate quench vise on the right and the tempering oven below the heat treat oven.

 

[Cushing H.]

I did do all the calculations assuming 2000 on the inside, that might be part of it. Also,the added insulation helps a lot with reducing the amperage needed to get to temp.... i could take a stab at the calculation assuming some level of convection on the outside. Working on taxes now though ... not a lot of time

 

[Natlek]

that is actually pretty close to what I was thinking - except you seem to have less framing on the inside than I was thinking (which is 1 inch, because I can easily buy 1 inch angle iron....

Find some shop which have tool to bend sheet steel .You don t need any angle iron , not for door or for box.... I will use 1.2-1.5 mm thick sheet metal it s enough rigid to make door and cabinet without angle iron ...

Pause this video on 0.50 and there is your door And on 38 min. you can see how easy they make it ........

 

[John mc c]

I had all my sheet steel laid out and marked ready to cut and fab outer box when I saw I had a job box toolbox sitting unused in the corner,it was just a matter of cutting it down bit smaller and cutting and hinging end for door,30 min job
Something like that would save you a lot of time and work if fabbing isn't your thing
I also have 2" of wool all around my bricks,top bottom sides,bricks are stable and outer shell cool in use but all in all it's a big heavy oven and not very moveable.mobile table would sort that out tho

 

[Cushing H.]

I had time this morning to play with those calculations. Just using convection loss at the surface gives something like 500-600F with 1-2 inches of insulation - which is much too high base on experience of all. For those that care, it looks like BOTH radiation and convection are important - and I no longer have the computer programs needed to solve that mixed problem (nonlinear simultaneous equations) - it is kind of a crap shoot anyway as to whether you get the parameters right. My main interest anyway was the total heat loss (not so much outside temp), as I am limited to 13 amps or so to drive the coils. looks like in the range of chamber size I am looking at I will be ok. I think now I will go with 2 inches of wool around the blocks and reduce the size of the oven a little. I am concerned that if I go with 1 inch I will be pushing limits on ability to hit 2000 F.

FWIW - there is definitely a strong trade-off between outside temperature and total power needed, with cooler outside temp driving significantly lower power needs.....

 

[Natlek]

I had time this morning to play with those calculations. Just using convection loss at the surface gives something like 500-600F with 1-2 inches of insulation - which is much too high base on experience of all. For those that care, it looks like BOTH radiation and convection are important - and I no longer have the computer programs needed to solve that mixed problem (nonlinear simultaneous equations) - it is kind of a crap shoot anyway as to whether you get the parameters right. My main interest anyway was the total heat loss (not so much outside temp), as I am limited to 13 amps or so to drive the coils. looks like in the range of chamber size I am looking at I will be ok. I think now I will go with 2 inches of wool around the blocks and reduce the size of the oven a little. I am concerned that if I go with 1 inch I will be pushing limits on ability to hit 2000 F.

FWIW - there is definitely a strong trade-off between outside temperature and total power needed, with cooler outside temp driving significantly lower power needs.....

Somewhere in my previous posts was link for some PDF which I can t find it now. Short , there was text ......more insulation improve heat uniformity in oven .And that thing which no one mention is probably more important then anything else in oven . Make sense to me .....less difference in temperature between hot and cold face of bricks /since the bricks are additionally insulated with ceramic blanket/ must improve temperature inside ?
My first tube furnace ......ceramic tube I use in that oven weight about 1.3 kg .That tube literally is in middle of ceramic blanket from all sides so when furnace is ON complete tube get to same temperature , heat uniformity is 100% all around inside tube .Low thermal mass of tube have some advantage so that oven get very fast to 800 Celsius .....5min. with 1800w .Now I have 1200W and take little more time .
In your case , I would use 1.5 inch brick and rest 4 inch kaowool. With 1.5 inch bricks it will get faster to temperature and have better heat uniformity inside then with 2 inch bricks .And of course less total power ? I don t see any reason to use 2 inch bricks /which have poorer thermal characteristics then ceramic blanket and they have more thermal mass weight / if you have 4 inch ceramic insulation over bricks ?

 

[Cushing H.]

Just a couple thoughts on that. Higher thermal mass will get up to temperature faster, BUT when you open the door, higher thermal mass will allow for faster recovery when the door is closed. My guess though is that in this case, if you are pretty quick in getting the door back closed, the difference between 1.5 and 2.5 inch brick would be pretty small.

On the other hand, 2.5 brick will be more stable if you are just stacking without using mortar. In my case it kind of does not matter, as my supplier does not carry 1.5” brick, and I really do not want to have to manually saw down all of the bricks...

It was kind of a fun and instructive exercise, but - as you would expect- it still comes back to the same ballpark of total insulation others have been reporting

 

[Natlek]

C Cushing H. , you need low thermal mass for higher energy efficiency.............
Thicker bricks will absorb and store more heat energy.....

Low thermal mass furnaces reduce overall heat treatment costs by increasing production and improving product quality. Their special linings require less total heat input to reach operating temperatures quicker than old-style refractory lined furnaces. Therefore, the lower thermal conductivity of the linings delivers much lower heat loss from the furnace, achieving impressive energy savings.

 

[Cushing H.]

Natlek - I think whether low thermal mass is "good" depends on your objectives - and also I think the quote you have above is mixing two different things. the ability to get to temperature fast is a result of low thermal mass - BUT the uniformity of heating and ability to recover from placing something cold inside in an oven like this is dependent on the ability of the lining to absorb energy, then re-radiate it.

A high thermal mass lining will therefor have a more uniform temperature, and the resulting radiation field will likewise be more uniform. It will also be able to more quickly bring the oven back up to temp when you, say, put a cold knife blank in to it (the energy to heat that blank is already stored in the higher heat capacity lining instead of needing to wait for the energy to be delivered into the chamber by the electric coils). So ... bricks on the inside make sense to me.

But it is also true that higher insulation will reduce loss to the outside (likely also help with uniformity of temps in the oven), BUT a really high insulation value adjacent to the cavity will interfere with that absorb-re-radiate thing, and really interfere with the ability of the oven to recover from an open door or something cold being put inside.

so, from what we have talked about so far, I agree that the "best" design would be something like an inner lining of 1 inch bricks (thick enough to store that energy and give that thermal recovery), surrounded with blanket of something like 4-5 inches ... which would give you both the good insulation and effective local heat storage right at the cavity. I just cant buy 1 inch bricks, and dont want to cut down thicker ones.... :-(

 

[weo]

I just cant buy 1 inch bricks, and dont want to cut down thicker ones....

I thought you could buy just about anything on the 'net. They shouldn't be that expensive to order online.

 

[Cushing H.]

I thought you could buy just about anything on the 'net. They shouldn't be that expensive to order online.

Probably. But the guy i worked with at the local supplier (a highly experienced bricklayer) was just so much ... fun... to work with (longer story) that i want to give them my business. And they stock the 2.5” blocks.

 

[Jesse Latham]

If the purpose of the fiber insulation is to reduce the temp of the outside shell instead of maintaining temp in the oven. Consider bricks inside a tin shell with a second tin shell with an air gap and fan for cooling.

 

[Cushing H.]

If the purpose of the fiber insulation is to reduce the temp of the outside shell instead of maintaining temp in the oven. Consider bricks inside a tin shell with a second tin shell with an air gap and fan for cooling.

Good thinking - but not quite so simple i think. I think it better to say exactly the same thing - but “backwards”: “the purpose of the fiber insulation is to keep the heat inside the oven” (though a nice side effect is to keep the outside cooler). By keeping the heat inside the oven, you avoid the “waste” of heating going to heating the room outside of the oven, thus reducing the amount of heat you need to put into the oven via the elements (more “efficient” energy wise). An extreme case would be so little insulation that you could never get the oven up to temperature...

(example, for anyone interested - in the calcs i ran yesterday, with one inch of insulation the estimate was 250 watts needed to maintain temperature, whereas with 2 inches of insulation the estimate was 190 watts needed to maintain temperature (again, these values are off - real numbers are higher ... but hopefull this helps make the issue clearer?)

 

[timgunn1962]

It's probably worth looking at Calcium Silicate board for the outer insulation layer. It has good insulation characteristics and is rigid enough to make the build relatively easy. It is available (at least over here) with a temperature rating of 1000 degC (1832 degF) and is less of a hazard to health than most other materials of interest to "us". I used 2 1/2" of Calcium Silicate board, backing up 1" of 2300 degF ceramic fibre board, for the roof and door of a quick-and-dirty sword-length HT oven with some success. The only reason I have not used it on my more recent ovens is that my local supplier went out of business. I expect to use it again in the future.

 

[Cushing H.]

It's probably worth looking at Calcium Silicate board for the outer insulation layer.

Agreed. though I did look at this, and my local supplier advised against it given the cost versus cheaper wool. It also occurs to me that steel outside, when painted black, actually helps keep the outside temperature down better (black radiates heat better than all other colors.....)