Re: Question about drum roaster design

On Sat, 17 Sep 2005 13:59:46 GMT, "Dan Bollinger"
<danNObollinger@xxxxxxxxxxxxxxxxx> wrote:

>Let's look at the modes:
>
>First, the slower the drum is turning, the longer (in percent) the bean is in
>contact with the drum and heat is being transferred by conductance. ...
>

The tenor of the chaper is that virtually all heat transfer is by
convection even in conventional drum roasters.

>Second, the faster the air is moving past individual beans the higher the heat
>transfer rate is by convection. The reason being that heat transfer is at a
>greater rate when the temperature differential is higher. Pushing cooler air
>away from the bean means the differential is maintained at a higher level.
>
>If the drum and air are both heated by the same heat source and their
>temperature is nearly identical -- as is the case with a perforated drum in a
>closed (non-ventilated) roaster -- increasing the drum speed will increase heat
>transfer (and speeding the roast).
>

This was my impression. On my PIDed airroaster, if I hold the inflow
air temperature constant, and blow faster, the roast speeds up. I
thought this was due only to increasing the available heat. Now this
convection bit comes up and I'm scratching my head. The ideal way to
tell is a completely sealed drum with constant heat input (or more
properly, held a constant temperature) -- if it's slow whirl, slow
roast, fast whirl, fast roast, tha would nail it.

>
>What's next, a PID'd drum speed roaster? ;)

That would be more elegant than some of the current automation
systems.


--
jim schulman
<jim_schulman@xxxxxxxxxxxxx>
.

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