Working backwards from bell to burner

[satamax]

Another source for old cast iron, old woodworking machinery.

Tables from a 20 inch jointer for example. But please, use only real old woodworking machinery. When you're absolutely sure it's not salvageable or useable for a woodworker, please

[Cramer]

Spoken by a woodworker I am guessing!

[satamax]

Jan 7, 2014 14:59:21 GMT -8 Cramer said:

Spoken by a woodworker I am guessing!

That's one of my jobs

[Vortex]

Cramer, I've made several cooktops and can help you avoid a lot of potential problems there. I originally thought it would be easy to remove the whole top for cleaning - it wasn't, they're heavy and difficult to remove if you have a good seal on them, (which you're breaking every time you remove it). Much better to make expansion plates on the top, like on commercially produced cookstove tops, these allow the hottest parts of the cooktop to expand and contract without causing the whole top to twist and warp, also they can be lifted off for cleaning out. If you have access to an old range top of the right size then you'll save yourself a lot of hassle.

[Cramer]

Cramer, I've made several cooktops and can help you avoid a lot of potential problems there. I originally thought it would be easy to remove the whole top for cleaning - it wasn't, they're heavy and difficult to remove if you have a good seal on them, (which you're breaking every time you remove it). Much better to make expansion plates on the top, like on commercially produced cookstove tops, these allow the hottest parts of the cooktop to expand and contract without causing the whole top to twist and warp, also they can be lifted off for cleaning out. If you have access to an old range top of the right size then you'll save yourself a lot of hassle.

Maybe something like 2 RSJ and brick over two thick steel plates and one (preferably stainless) plate for the cooktop?  Really cooking is not the main function at all so I might just brick over all three plates.  How then would I do a clean out?  Maybe a low door or two at floor level?  Cleanout needs to be as little a chore as possible so that it is not so difficult as to be one of those put it off forever things.

Hey, maybe a door just big enough to put one of those robotic vacuum cleaners in once a year!  After all, it should be only fly ash in there right?

[satamax]

Well, rsj wise, account for expansion. You either fit theses in slots or corbels. Avoid direct contact with the flame, they fail usualy between 400 and 600C°. Another idea besides the cooktop, instead of using bricks and thicker steel, i was thinking you could use thin steel like 6/10th of a milimeter, kust for making the bell airtight, and put pavers, or pizza oven slabs on top, if they rest on the rsj, it could be ok. At first, i thought you wanted to make the whole top a cooktop.

[Cramer]

At first, i thought you wanted to make the whole top a cooktop.

Thoughts are such an ever changing thing.  As long as I have enough space to place an industrial sized stew pot that would be enough cooking space (to be able to cook some burgers on the surface would be nice too).  Otherwise if the rest of the top of the bell could be brought to a simmering temperature and held there for as long as possible would be fine.  With these parameters what would be a good way to accomplish? 

[Vortex]

This'll give you an idea of the amount of thermal expansion you get on a 5mm steel top, I think if you had pavers on top it would just push them up.

[satamax]

Vortex, your plate is locked on the sides? Myself, i'd rather see a free sheet, with a joint underneath and pavers on top, but the metal has to be thin, and the structural bearing would be done another way. The metal sheet just being there to stop leaks without massonry joints. JMHO.

[patamos]

Hi All,

I did a grittle over a 4" L-feed last summer.  Searched everywhere for old cast iron stoves with removable plates.  Couldn't find them in time (for deadline), so went with a 1/4" mild steel plate roughly 18"x22" set on a ceramic gasket sitting on a rim of firebrick.  I left 1/4" of space around the edges for movement, and welded a tab 1"tall by 3" wide across the back corner of the grittle deck.  When i want to lift it up for inspection/cleaning i crimp the tab with a pair of sturdy vise-grips and carefully lift the back up with the front acting as a fulcrum.  It does take some care to not disrupt the gaskets, but when they do peel up they are easy enough to set back down again - at least so far.

The ISA wasn't anywhere close to a full bell, but the exhaust routing does drop down 3" before entering an area of horizontal flue mass with a surface for pot warming and dough rising.

It is working well so far and is nice to have the more and less concentrated areas of heat in the grittle deck...

 

[Cramer]

This'll give you an idea of the amount of thermal expansion you get on a 5mm steel top, I think if you had pavers on top it would just push them up.

Thanks for the input Vortex!  That surely would not do!  Would you think, if I can source some 1/2 inch steel plates (12.7mm) that the problem might not be so severe?  I know that would be mighty heavy but if I could make only one (of I am thinking 4) of them removable I could make that one into the cook top/radiator and brick over the others?  That would make the plates around 1ft by 2ft (.3 X .6m) but if only one would be removable (then only occasionally) that might give enough heft to not warp?  That picture IS worth a thousand words!

rather see a free sheet, with a joint underneath and pavers on top, but the metal has to be thin, and the structural bearing would be done another way.

Max, How might you go about the structural bearing for such a design?  I am thinking RSJ again.  You said failure temperature is around 400C (approx 700f).  How much heat might be expected to accumulate within the bell?  Enough to reach this failure temperature?  Perhaps a temperature probe/gauge set in a probe well could be useful to see that a burn could be stopped before reaching the failure temperature?

went with a 1/4" mild steel plate roughly 18"x22" set on a ceramic gasket sitting on a rim of firebrick.

Patamos, no warping of the plate experienced at 1/4" thickness (6.35mm)?  That size is about the size I am talking about.  Half thick (insulating i.e. lighter) fire brick might not be too much for it to bear to cover 3 of 4 plates along the top of the bell?

 

[patamos]

No warping so far. And it has run full tilt many times, once for 8 hours at a stretch. I was quite surprised actually.
If you cover with fire bricks the metal will not be able to radiate heat as easily so temps will go up. Might be better to go with thicker metal in those layups. 3/8" or 1/2". Oxy-acetaline/propane for the cutting.

[satamax]

Cramer, i was re reading something about steel. They say here www.infosteel.be/fr/feu01_notionsdebase.php (sorry in French)

That steel is non flamable, doesn't burn etc. And that at 800C°, it has a tenth of the original strengh. At 400C° it has full strengh, but 70% of it's original rigidity. They say, that any type of fire resistance can be achieved by dimensioning the steel pieces.

Here in your case, you don't need much. Some pieces are suported on two sides, some, on only one. Let's try to work out the weight of the "slabs"

You have aproximately 167cmx84 cm external dimensions (i'll use that for safety) Let say you have 2.5in slabs (64mm)

167x84x6.4 =89779cm3or 0.08977m3 that minus a 50x50cm piece where you have exposed steel for your cooktop. 50x50x6.4 that's 0.016m3

That gives 0.0737m3

According to this chart,

www.engineeringtoolbox.com/density-solids-d_1265.html

Firebrick is between 1.8 and 2.2 metric tons per cubic meter. Let's take the higher value for safety reasons. 2.2x0.0737

That gives about 162kg. Still, keep that value, for safety reasons, but there's less than half of that which would be supported by the rsj. The rest being supported by the front, the back and the sides of the bell.

Let's aim for ten times that for a top resistance at 800C°

That's 1620kg.

My engineering booklet from the fifties, says a 100mmx50mm or 2"x4" RSJ can whistand 1864kg over a 2m span at a security coeficient of 3 So that would be plenty good enough.

Because of the 167 lengh, we are already 20% stronger than at 200cm, and that's outside dimension. which means it's even stronger At 1m it would be in the range of 3.7 tons. And i've taken the resistance at 800C°, which when cold is ten times larger, then a safety margin of 3, 3kg for 1kg of real charge. That means cold, it can whistand 30 times the weight applied before collapsing.

Anyway, if you're cautious in building so the brick corbels below the RSJ are strong enough, and you allow for expansion, you should never have problems.

Plus, i doubt that the insides will attain 800C°.

Hth.

Max.

[Vortex]

Max, yes my top is fixed in place, it has to be otherwise the front right hand corner twists up when the stove top is hot. I think you're right, as long as you're only using very thin metal with other support under it and masonry on top then it'll be fine. The problem I think is caused by the large surface area I have with a huge temperature difference across it, smaller pieces recessed into masonry or other thick metal with a gasket seal seem to be OK, as patamos has found. Cramer, the 2nd one I built has a 10mm thick top and that was worse, I had to make a big cut out above the hottest part and weld a slightly larger plate over the top of it so it would sit in the hole, that allowed it to expand and contract without twisting the whole top.

[patamos]

Hey Gang,

My apologies.  I misinformed you about the thickness of the mild steel deck on my grittle.  It is not 1/4" but 3/8".  It was a busy time of building 6 months ago and much has since come to pass...  But last night after writing i remembered that i was going to use 1/4" and had begun carving up part of the remains of an old fisher wood stove.  But a few zip discs and nose full of carbon later... i opted to farm it out to my welding friend.  He had a slab of 3/8" around, so we went with that.

Of note, i had been thinking of welding a piece to the bottom of the metal plate where the heat from the riser is concentrated, but was advised that that would increase the likelihood of warping.  Instead, i utilized 3 little 1.5"x2"x3" chunks of kiln shelf to serve as legs upon which i suspended a round 6"x 1/2" slab of cast refractory 'pancake' over top of the heat riser.  This way i was/am able to play with height of the baffle (is this the correct term?) and initial directions of gas flow in the shallow bell chamber.   The deck (bell roof) sits 4" above the heat riser and the baffle 2".  The centre still gets hotter first but is less concentrated than without a baffle.  So this this helps keep the metal stable.
I'll see if i can dig up an old picture.