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Kiln dynamics


#1

I’ve just made myself a little burnout kiln from Shinagawa
insulating fire brick. My heat source is a 15kw Sievert burner bottom
centre, firing upwards. I could do with some help with venting it. I
was thinking perhaps I need a variable vent-hole.

The kiln capacity is 7 litres (1.8 US gal). 150mm (6in) above the
burner is a kiln shelf with an inch space on two sides. A device
under the shelf splits the heat to help it flow around the shelf.
From the shelf to the kiln top it’s 135mm (5in) and near the top I
have an analogue probe. The top lid is two fire bricks, and the kiln
works well with a approx 10mm (1/2in) vent gap - just a slot -
between the bricks.

It’ll get to 650C in about 10min. About the vent, I tried with a
30mm vent hole at top centre, but it needs to be much bigger when the
burner is working on full. I worry about wasting energy once the
kiln’s up to heat and the burner’s just maintaining 650C.

The vent as I see it needs to be big enough when operating at fast
heat-up at 200kPa, and only as big to allow the burner to run
efficiently when turned down for maintaining a soak temp.

Would a vent hole be better at both these jobs than a vent slot?

Anyone have a variable vent-hole idea that’s also low-tech?

Details. The Sievert 2954 burner is mounted on a Sievert 3025
support, the one with a cast iron base and valve at the end of a long
brass pipe. It’s on a regulator at 200kPa (28 psi). This burner I
already use under a tripod with st/steel mesh to anneal sil, gold and
copper (hence this post runs on from the previous one on annealling
Cu). The mesh is useful as a means to hold the workpiece, as it
doesn’t get too hot and I can raise/lower to control the heat.
Sievert make other big burners, according to this LPG Torches
pamphlet I have, and the one I prefer is 42mm dia, 15kW, with a short
150mm flame length. The weed-killer burner they have is 50mm dia,
86kW, and a 2 foot flame.

Kiln photos are at

Auckland
New Zealand
www.adam.co.nz


#2

Hi Brian,

That is a pretty aggressive approach to burnout! I prefer a more
gentle approach myself.

I made something similar for metal melting out of a small oil drum
and ceramic fibre. about a foot high and wide. It will melt up to 6Kg
of bronze in about 40 mins. A Sievert 2942 burner. Burner inlet side
bottom and outlet centre top. I made the holes the same size about 2"
diameter, and I can adjust the burner distance from the hole to give
me not quite complete combustion to give a slightly reducing
atmosphere inside. I made a bigger better one for another foundry and
put a swivelling 1/8" steel plate on the top but they found that once
it was set up just adjusting the burner distance was all they needed
to do.

In answer to your question I would think a hole would be better I
don’t know if you can get ceramic fibre blanket there but it is easy
to deal with, and can be rigidised, (pottery suppliers have it in the
UK), or backed up with a bit of stainless steel weldmesh. You
obviously want the airflow through to get rid of the carbon. How long
does it take to give you a clean mould and what size flasks?

Another thing that works well and is very cheap is white insulating
building block. Celcon make them in this country. Don’t use the grey
ones they are concrete but the white ones are very heat resistant to
metal melting temp, and a couple of pounds for 9"x4"x18" they need to
be treated gently.

It is surpring what will stand the heat, I am still using a furnace
I made as an emergency measure intending to ditch it after a couple
of uses, I is still going eight years later!

keep up the experiments,
regards
Tim Blades.