HomeRoast Digest

Topic: Air vs. drum roasts (was Smoky Beans) (15 msgs / 420 lines)
1) From: Ed Needham
So, 500F degree air, blowing at who knows what CFM (I won't even guess),
creates a much faster heat transfer to the beans, accounting for the faster
roast time, compared to a drum, which has similar (or lower) heat, and much
slower heat transfer to the beans.
I guess it's up to personal tastes to decide which method roasts the best
beans.  As has been mentioned here before, a faster roast tends to make a
brighter taste, whereas a slower roast emphasizes body.
I'm thinking a 'hybrid' roaster might be the best of both worlds.  Slow roast
with low air for body, or pump in an aggressive airflow to bring out the
brightness and a faster roast.  I know the commercial roasters have an
airflow adjustment, but is it enough airflow to significantly speed up a
What if the heat source was pumped into a perforated tube running through the
middle of a roasting drum, and either high flow air or low flow air was
pumped through it, maintaining the same, or similar temperature.  The beans
would be exposed to aggressive convection currents in a drum roaster, and
could be roasted, not only by time and temp, but also by air convection as a
variable.  (thinking out loud again)
Ed Needhamhttp://www.homeroaster.comed

2) From: Jim Schulman
Actually it is possible to slow down a pure 
airroaster to any speed desirable. With the Variac 
alone, that's only possible after the beans 
lighten enough to float with lower airflows (or 
unless you're prepared to do some rocking, like 
Mike), with separate airflow and heat controls, 
one can ramp the heat up slowly, while maintaining 
airflow, to lengthen the initial warmup times as 
Andy Schechter is building a Frankenpopper to go 
with his Frankensylvia. He's trying to set it up 
with a full blown ramped and cascaded PID control 
(a setpoint ramping up over time is compared to 
bean temp, that in turn computes an air temp 
setpoint, which in turn runs the airtemp PID 
loop). Problems with measuring bean temp are the 
source of delays. If it works, he'll be able to 
set it for a 3 day roast!
Of course, it may turn out that he proves the 
point he's trying to disprove. His theory is that 
only the time profile matters when roasting, not 
the method of heat transfer (convection versus 
conduction). If it turns out that the heat 
transfer method does matter, the Frankenroaster II 
may well be the hybrid you're beginning to draw up 
(although he'll doubtless add an IR bulb, and a 
few steam spouts to cover the radiation and 
humidity bases as well!)
On 15 Nov 2002 at 23:08, Ed Needham wrote:
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3) From: Ed Needham
I'll look for the thread on alt.coffee.  I must have missed it somehow.
Ed Needhamhttp://www.homeroaster.com

4) From: Rick Farris
Jim wrote:
According to the folks at black bear (big bear?  you know, the profile
roasting people) it can't be done that way.  According to them, the bean
temperature is not a good indicator of how much heat they've absorbed.
According to them you have to keep track of the amount of heat the beans
have absorbed, not the temperature.  Why is that hard?  Because the
relationship between temperature and the amount of heat you've applied to
the beans is a non-linear equation.
So you must have a computer keeping track of the amount of heat applied, the
changing nature of the beans as the various chemical reactions (from drying
out all the way to charring) occur, *and* the temperature for finishing up.
It's that whole roast "inertia" thing.
I won't mention any exothermic reactions. :-)
-- Rick
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5) From: Jim Schulman
On 15 Nov 2002 at 23:35, Rick Farris wrote:
A cascaded PID scheme can handle a lot of non-
linearity (technically its dynamics are equivalent 
to a  6th order differential equation), and it's 
the usual control setup on highly automated coffee 
roasters. The problem is that the complex dynamics 
makes it very difficult to tune, and the tuning 
parameters could well change bean to bean. 
I personally am not a big fan of automation in any 
cooking process. Coffees, breads, cakes, roasts, 
barbecue, etc. generally seem to taste better when 
someone knowledgeable is keeping an eye on them; 
and I really dread the day when someone starts 
hawking an automated fry pan. 
But a successful control scheme lies between good 
measurements on one side, and good controls on the 
other. While I'm not big on automated schemes, I 
greatly appreciate what Andy's espresso efforts 
have added in the areas of good measurement and 
control, and I hope he comes up with similar 
goodies for home roasters.
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6) From: Dan Bollinger
Jim,  I know what you mean, around my house I'm the only one who grills
outdoors, not because I demand it, but everyone else does.  Then again,
profiled controllers on kilns is common in University art schools. Not only
does it free the artist(s) from babysitting the kiln, but some artistry,
like crystal glaze, is impossible without the controller.  Dan
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7) From: David Westebbe
Amen.  I'm not even a fan of recipes.
Coffees, breads, cakes, roasts, 
I agree totally.
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8) From: David Westebbe
t because I demand it, but everyone else does.
Then again, there are wood fired kilns where the artist stays awake for days
on end, choosing each individual hunk of wood to feed to the fire, based on
the moment-to-moment needs of the process.  Some artistry is impossible
without the judgement of the artist.
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9) From: AlChemist John
Sometime around 20:08 11/15/02, Ed Needham typed:
[also thinking out loud]
  The hybrid I was thinking about would have heat supplied by coils near 
the drum for some radiant heat, but the drum would be perforated and a fan 
would blow heated air into the drum, thus heating by convection.  Of 
course, the drum would rotate, and the air flow would be no where near 
enough the fluidized the moving bed of beans.  I wonder about the pro/cons 
of perforated drums vs not.  Could the holes be small enough to not have 
chaff fall through?  Would sufficient rpm help keep the chaff in (this 
assuming the chaff in a drum roaster is good, based on the phenol test 
[but looking for input]
John Nanci 
AlChemist at large
Roasting and Blending by Gestalt
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10) From: Dan Bollinger
John, I've been thinking along the same lines.  What I've come up with is a
rotating, solid walled drum with low velocity hot air blown through the
center of the drum heating the beans as the fall from the drum blades
located around the periphery.  The air flow would also remove chaff, too.
as a
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11) From: John Abbott
	The HotTop does just about that - but the air is pulled by an exhaust fan.
The top of the unit has a perforated grill to allow smoke out and air in to
the chamber.  The fan doesn't run through the whole cycle - but only from
first to second crack and exit. The fan pulls the heated air from the
roasting chamber - over the rotation motor and out the rear of the unit.
This may add to the extraction process of the chaff as well - but really
could be improved.
John - the retired version

12) From: Ed Needham
Is the airflow of the Hottop fairly significant?  Since I've only seen
pictures of the Hottop, I was imagining that the airflow was fairly low.  And
I am surprised that it handles heated air.  I was thinking it blew cool air
(which really doesn't make sense as I think about it).  What type of fan is
it that can handle 500F recycled air?
Ed Needhamhttp://www.homeroaster.comed

13) From: John Abbott
Ed, sorry for the delay - watching Iowa romp on Minnesota.   The fan on the
HotTop is a muffin fan - the same variety that is used on most computers. It
has a filter fabric between it and the hot air - to prevent exhausting chaff
I guess. It isn't a very forceful airflow. The fan doesn't have a huge
effect on the roast because some of the 15 prototypes were wired backward.

14) From: Rick Farris
Jim wrote:
Well, sure, but the problem isn't with the PIDs, but with the control
variable.  He's trying to control on temperature and he needs to control on
-- Rick
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15) From: Ed Needham
I guess what I am thinking of is using a heat gun to force air through a
perforated stainless tube inside the roasting drum.  I have the drum, what I
need to figure out is what to use for the perforated center tube, and It's on
my list of things to do whenever I quit my job and become independently
wealthy (or when monkeys fly out my bu**, whichever comes first).
Ed Needhamhttp://www.homeroaster.comed

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