[Homeroast] Drum rotation > Jimma > PID iRoast

miKe mcKoffee mcKona at comcast.net
Thu Sep 30 11:20:45 CDT 2010

First I'll say I'm not a roaster designer or manufacturer so can't say
exactly how specific drum rotation speeds are chosen for different roasters.
Before his passing Ron did a fair amount of testing for various drums and
drum diameters to determine how fast was too fast for his Q drums. IE at
what speed would the beans not loft but instead have centrifugal force push
them against the wall. IRC the HotTop drum rotation is ~30rpm (checked it
but that was almost 3 years ago so memory fades), USRC 3k ~65rpm, Behmor
very slow ~6rpm. Observation of other commercial drum roasters and their
observed bean loft means using similar speeds to measured (counted) USRC
drum speed. 

A common mistake is thinking the drum itself is the primary heat source for
drum roasted beans. Modern drum roasters convection accounts for about 80%
of the roast energy, air roasters even more. While the specific pre-heat of
the drum and roaster IS VERY important at the beginning of the roast, it's
importance is it's stored energy which is applied primarily via convection
and radiation not conduction. With a bit of understanding of the types of
heat in the roast process and observation of various roasters it becomes
clear constant loft of the beans by the drum is the goal. Convection is the
primary heat transfer in all modern roasters.  (See a good article by Terry
Davis, originally written for Roast Magazine IIRC
http://www.ambexroasters.com/information/read/heat_transfer.html )

If the beans aren't vigorously tumbling convection can't occur for the
majority of the beans stuck in a barely moving mass. This leaves conduction
and radiation as the more predominant roast energies, which is the problem
with the stock Behmor. Radiant energy primarily from the heater and
pre-heated roast chamber primarily affecting the beans on the outside of the
mass and conduction later in the roast bean to bean. At the same time I
split out the heater for variac control Alchemist John put in a faster drum
motor, with marked improved results. (We both had/have first production run
Behmors from Joe) I never got a roast I was satisfied with just by directly
controlling radiant heat, ie the heater. Some "ok" roasts to be sure, but
nothing as good as Caffe Rosto profiled roasts or later CCR HotTop roasts.
Convection or lack thereof was the apparent difference.

IF a faster drum rotation causes problems with the Behmor Alchemist can
chime in since fairly certain he's been using IIRC ~45rpm drum motors for
close to 3 years.

It's easy to turn beans brown, it's a lifetime Journey learning how to make
beans sing. IMO (and that of many others) a huge part of the process is
learning how to effectively control your particular roaster.

Slave to the Bean  miKe mcKoffee
URL to Rosto mods, FrankenFormer, some recipes etc:

Ultimately the quest for Koffee Nirvana is a solitary path. To know I must
first not know. And in knowing know I know not. Each Personal enlightenment
found exploring the many divergent foot steps of Those who have gone before.

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> -----Original Message-----
> From: homeroast-bounces at lists.sweetmariascoffee.com 
> [mailto:homeroast-bounces at lists.sweetmariascoffee.com] On 
> Behalf Of Jim Couch
> Sent: Thursday, September 30, 2010 7:49 AM
> Believe I saw some comments from Tom cautioning about the "dangers" of
> spinning a Behmor drum too fast, something about it causing 
> chaff collection
> problems and messing with the ability of the beans to 
> transfer heat to each
> other perhaps problems would be a better word than dangers..........
> On Thu, Sep 30, 2010 at 8:36 AM, Robert Yoder 
> <robotyonder at hotmail.com>wrote:
> >
> > Hi MiKe,
> >
> >
> > Thanks for your posts!  So much to learn!
> >
> > >
> >   Even directly controlling the heater via variac
> > > didn't help enough because drum rotation was so slow yielding poor
> > > convection. If I hadn't acquired a CCR HotTop so soon 
> after the Behmor
> > > likely would have mod'd it further with faster drum and bean temp
> > probemaking it a decent roaster.
> >
> >
> >
> > Can you clarify the drum-rotation/poor convection comment? 
> Is there a
> > typical shop-roaster drum-rotation speed?  How is that chosen?
> >
> > Happy Roasting,
> >
> > robert yoder

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