HomeRoast Digest

Topic: More thoughts on .. now insulation (13 msgs / 255 lines)
1) From: petzul
Thanks for the input.
It has been years since I worked with this stuff, but I just got an idea...
I have my SS bowl encased with a chamber made with Aluminum flashing and 
other pre formed parts. It looks pretty good, but all I did was stuff it 
with fiberglass insulation that used to be on the outside of an old 
dishwasher... ( It was handy).
Any ideas if it would gain me much to take the bowl/ case off, turn it 
upside down, remove the fiberglass insulation and pour it full of that 
castable ceramic insulation?
That sure would fill in the gaps, and would not slip or compress.
Hmm, I could almost just leave it all together and turn it upside down.. 
fill it right up to the motor!
Would definitely be wearing a mask.
Daydreaming again, here in LHC.
Jim Karavias wrote:

2) From: Jim Karavias
Hmmm.  Good question.   I'm not sure.   I know that some kilns with <  
4" of ceramic fiber insulation are running upwards of 2500deg and are 
just warm to the touch on the oustside.  I used a cone of ceramic fiber 
batting to direct heat to the base of my hand cranked popper when I was 
using my grill as a heat source. It could get up to 700deg under the 
batting  and I could pick it up without any discomfort.   I suspect the 
fiberglass you're using was just for sound insulation and doesn't 
provide anywhere near the insulation the ceramic batting would.  One of 
the complaints about fiber kilns is that they're difficult to 'soak'.  
SInce they don't have much thermal mass they dont radiate a lot of heat 
after the heat source is cranked back.  If you want some thermal mass to 
stabilize temps then fiber isn't going ot help.  Castable would be 
better in that area.    I like having some thermal mass to smooth out 
the temp. ramps.
I think you can get 2 lineal feet of batting from ceramic supply houses 
for about 10 bucks wich doesn't seem like too big a loss for the 
experiment.  You can make your own castable pretty easily with 
vermiculite, pearlite and cement.  There are recipes online and you 
wouldn't need to spend much on it.  Actually, was thinking of using a 
mailbox as a form for a castable coffee roaster body with one or two 
electrice bbq starters as heat.  Just haven't had the time...
Blah blah blah.....:-)
petzul wrote:

3) From: petzul
Good point on the thermal mass.
To me it would be more important that the insulation did not 'soak'. 
Fiber would be better because then I could allow the beans to stabilize 
the temps.
Using the theory that beans become exothermic sometime between first 
crack and second crack, it is necessary to cut back the heat a bit to 
avoid rushing the end. Therefore the less thermal mass in the oven the 
better, so you can put on the brakes easier.
I thought about the BBQ starters as heater elements also, they are 
inexpensive enough and readily available. The problem I saw in my 
application was protecting the handle from the environment it would be in.
Your suggestion of casting it into a form now brings a whole new world 
of possibilities :)
It may be possible to form fit it to the outside of the roasting bowl, 
and cast it in place with a minimum of ceramic/cement.
IYO would I need to use screws for support also?
Maybe build little ceramic support blocks?
Will now look for a few lineal feet of batting, here in LHC.
Jim Karavias wrote:

4) From: Dan Bollinger
Ahem,  Ain't no theory, Peter, 'tis fact that beans are exothermic around
I agree, a low thermal mass roaster is better for profiling a roast.
I used ceramic grommets turned on the sides and pass the coiled nichrome
through them.
I get my nichrome and ceramic blanket and paper from:http://www.infraredheaters.com/Dan

5) From: Oaxaca Charlie
 Jim wrote: 
  You can make your own castable pretty easily
 That's exactly what I did to begin insulating my brick
oven, outside of the firebox. 6" of the home made castable.
It crumbles easilly, so carefull ... There is also a layer
of aluminum foil in the middle (for heat "bounce back").
Outside of all that went 24" of rockwool insulation, then
more foil and a metal cover. 
                                         Oaxaca dreamin'
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6) From: Dan Bollinger
Charlie, That's called hypertufa. It's not very strong. People add
reinforcement mesh or fibers sometimes.
Sorry, it doesn't work like that. For infrared to reflect off a metallic
surface you need a minimum of 5/8" air gap in front of the reflector.

7) From: Ken Mary
Hmmm, 5/8 inch?, minumum?. I would have thought a couple wavelengths would
do, a few hundredths of an inch. Could you explain your calculations? Sorry
but a statement such as yours naturally invites a request for proof.

8) From: Dan Bollinger
Ken, I didn't make any calculations. I've heard this twice, both from mfgrs.
of two different types of aluminum house insulation. Both said to install
the aluminum faced insulation in wall cavities with the aluminum facing
outward, and with at least a 5/8" air gap.
hope this helps, Dan

9) From: Oaxaca Charlie
--- Dan Bollinger  wrote:
 You may be right about this, but some brick oven designers
and builders(Alan Scott, for one) don't seem to know that
aluminum won't bounce heat without that 5/8"air gap. There
are even specific instructions on which direction the shiny
side should face etc. 
                                         Oaxaca dreamin'
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10) From: Ken Mary
I understand it now, thanks. The building industry must have some valid
reasoning behind this number. It may not be totally in regard to radiation,
but other factors likely enter the equation. From the pure physics point of
view, the surface does the reflecting. But other objects within a wavelength
or so will interfere. The 5/8 inch number was off the map so it confused me.

11) From: Dan Bollinger
Ken, I'm not sure how it works, but offhand I'd agree that a wavelength
would be sufficient. Do you agree that a shiny surface buried in some other
material, say concrete, would have no infrared reflective qualities?  Dan

12) From: Gene Smith
I'm "Talking through my hat," as my Grandma Smith would have said, but it 
seems to me that the infrared energy would have been absorbed before 
reaching the shiny stuff, effectively negating the point of having it 
Gene Smith
riding the wild learning curve, in Houston

13) From: Ken Mary
In thinking about this in depth, there will always be some reflection as
long as the refractive index of the 2 mediums in contact (concrete and shiny
metal) are different. There is constructive or destructive interference when
the distance between surfaces is near a multiple of a half wavelength.
Surfaces in contact will transmit heat by conduction. Intimate contact as
when the metal and fresh wet concrete are joined will transfer most of the
heat by conduction. But covering dry concrete with a layer of metal foil
will create a thin air film which acts as a very effective insulator because
air is a poor conductor of heat.
The heat emitted by a radiating surface is proportional to the 4th power of
the temperature. At room temperature, the heat transmitted by radiation
between surfaces is small enough to be ignored.

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