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Topic: Has anybody tried to control air temp by..... (7 msgs / 129 lines)
1) From: Ben Treichel
Has anybody tried to control air temp by controlling the amount of air 
(mass flow rate) that flows thru the roaster, instead of trying to 
control the input voltage?  Just curious, I can think of several 
different methods to do this, and several potential problems.
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2) From: Bob Trancho
Some of us with the Caffe Rosto use a baffle on the exhaust vents that
has the effect of slowing the air movement a bit and also redirects the
heated air back though the roast chamber.  This is specific to the Rosto
because of it's design.  I use a baffle that I can clip onto the front
of the vents as I approach first crack and then remove for a while
before replacing again to push into second crack, when needed.  Combined
with the variac, I get very good control over profiles.
Users of the variac have talked about the problem of the blower slowing
as the voltage is dropped and there has been some discussion of (and
perhaps some work on ) rewiring the blower independently of the heating
elements in different roasters.  I don't recall anyone setting up a
variable blower, just rewiring so that the blower runs at a constant
speed regardless of heater voltage.
The amount of air moving through the roaster will depend on the mass of
beans being roasted, also, so any work on this will (obviously) be very
specific to the roaster involved - a FR holds less than a Rosto, etc.
Bob Trancho
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3) From: Dan Bollinger
You can control temperature this way but there are two thing you must know.
First, if you are using a fluid bed roaster you may loose your spouting
action if you reduce airflow.  Second, if we are talking about baffles, you
can only make the air hotter, not cooler!  It's a safe bet that your fan is
unbaffled.  Adding a baffle will reduce air flow increasing temperature.
If you are talking about varying the voltage to the motor, then the only fix
is to use a variac to boost the voltage on the motor increasing airflow and
decreasing temperature.  The nice thing about using a Variac in this plan is
that you can use the cheaper and more readily available (on eBay) 5 amp
units instead of the 15-20 amp units you'd need to control the heater.  It
has some merits.
I do a variation of this on my HWP.  This unit does have a factory installed
baffle.  A 5/8" washer covering some of the roasting chamber intakes.  I
sometimes take this out or replace with a smaller washer to lower
temperature and extend roasting times.  It is probably the easiest and
cheapest roaster modification around.   Dan
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4) From: dewardh
(mass flow rate) that flows thru the roaster
Hearthware.  That's how they control (to the extent that they do control ) 
temperature in the "Precision". It is difficult to do with some small roasters 
(Caffe Rosto in particular) because air flow changes as the chaff filter clogs, 
and is more generally an issue where some minimum airflow is required to 
fluidize the bean mass, while too great a flow "levitates" the beans in a 
difficult to manage manner.  Ideally one would control both airflow and heat 
input independently . . . adjusting flow to maintain mean agitation and heat 
input to control air temperature.  "Drum" roasters (a misnomer, really, since 
the beans are still heated mainly by hot air) avoid the problem by using the 
drum as the agitation mechanism while heating the air either/both by 
circulation over an external heater or/and by conduction from the drum.
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5) From: Ben Treichel
Dan Bollinger wrote:
This was sorta my thought; cheaper & more responsive than changing the 
heater voltage. I know you 'probably' cant double the airflow without 
messing up to spouting action, but I do believe that you could get a lot 
faster temp changes with low cost.
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6) From: EskWIRED
Yep.  I do it.
I opened up the air inlets to the bean cup on my WEPP to increase airflow,
which allows for much larger batches, and I  installed a control to turn my
fan down, so I can increase the temperature and/or roast smaller batches.
Before I installed the dimmer, I used to partially block the outlet to slow
down the air and increase the temp.  You could do the same thing by closing
the air inlet to the fan.
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7) From: Gary Zimmerman
Ben Treichel wrote:
Ben, some folks on this list have fiddled with the air of their fluid-bed 
roasters in a slightly different way.  For roasting in cold garages in 
winter, they put the roaster into a box, essentially recirculating some of 
the heated air to help get the temperature up.  Not exactly what you asked, 
but another idea.  I guess it's not really "controlling" the temperature, 
just raising it when the ambient's too cold for the heaters to warm it 
-- garyZ
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