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Date: 19 Dec 2006 20:15:37
From: Paul Monaghan
Subject: variac on popper fan - how should temp respond?
Hi folks,

Have been roasting in a poppery with PID controlling the heat and
variac for fan adjustment. I think I have read here and elsewhere a
bunch of times that when fan speed is reduced the beans will heat
faster. I have been just sort of mindlessly reducing fan speed around
first crack, when the beans really start moving.

But in the last few weeks I started paying attention to the temp
reading, and noticed that the ramp slows or even drops a degree
instantly when I reduce the fan speed. Happens every time. Not sure if
the ramp actually slows for good or if this is just a momentary thing.

TC is located in the middle of the roast chamber, maybe 2 inches off
the bottom.

Can anyone explain what is happening?

Thanks,
Paul




 
Date: 23 Dec 2006 06:42:15
From: ronnie.b
Subject: Re: variac on popper fan - how should temp respond?


On Dec 21, 9:48 am, EskWI...@spamblock.panix.com wrote:
> In alt.coffee, Paul Monaghan <monag...@shorelinelegal.com> wrote:
>
> > But in the last few weeks I started paying attention to the temp
> > reading, and noticed that the ramp slows or even drops a degree
> > instantly when I reduce the fan speed. Happens every time. Not sure if
> > the ramp actually slows for good or if this is just a momentary thing.
> > TC is located in the middle of the roast chamber, maybe 2 inches off
> > the bottom.
> > Can anyone explain what is happening?Your TC is not reading the bean temperature. Your TC is not reading the
> air temperature. Instead, it is reading some kind of average of the two.
>
> When you turn down the air speed, less hot air hits the TC. At the same
> time, relatively more relatively cool beans hit the TC. Accordingly, the
> apparent temperature drops.
>
> The only way to measure bean temperature is to turn off the air
> completely, and let the bean mass engulf the TC with no hot air
> infiltrating.
>
> --
> A nation of sheep will beget a government of wolves.
> --Edward R. Murrow

Nice catch! I missed the part about the TC being in the bean mass.
There are too many variable involved to use the PID thermocouple placed
there. Locating the TC in the inlet is much more controlable. You can
roast any amount of beans with this setup, just adjust the air velocity
to provide adequate agitation.



 
Date: 21 Dec 2006 14:48:52
From:
Subject: Re: variac on popper fan - how should temp respond?
In alt.coffee, Paul Monaghan <monaghan@shorelinelegal.com > wrote:

> But in the last few weeks I started paying attention to the temp
> reading, and noticed that the ramp slows or even drops a degree
> instantly when I reduce the fan speed. Happens every time. Not sure if
> the ramp actually slows for good or if this is just a momentary thing.

> TC is located in the middle of the roast chamber, maybe 2 inches off
> the bottom.

> Can anyone explain what is happening?

Your TC is not reading the bean temperature. Your TC is not reading the
air temperature. Instead, it is reading some kind of average of the two.

When you turn down the air speed, less hot air hits the TC. At the same
time, relatively more relatively cool beans hit the TC. Accordingly, the
apparent temperature drops.

The only way to measure bean temperature is to turn off the air
completely, and let the bean mass engulf the TC with no hot air
infiltrating.

--
A nation of sheep will beget a government of wolves.
--Edward R. Murrow


 
Date: 20 Dec 2006 19:25:57
From: ronnie.b
Subject: Re: variac on popper fan - how should temp respond?

Paul Monaghan wrote:
> Hi folks,
>
> Have been roasting in a poppery with PID controlling the heat and
> variac for fan adjustment. I think I have read here and elsewhere a
> bunch of times that when fan speed is reduced the beans will heat
> faster. I have been just sort of mindlessly reducing fan speed around
> first crack, when the beans really start moving.
>
> But in the last few weeks I started paying attention to the temp
> reading, and noticed that the ramp slows or even drops a degree
> instantly when I reduce the fan speed. Happens every time. Not sure if
> the ramp actually slows for good or if this is just a momentary thing.
>
> TC is located in the middle of the roast chamber, maybe 2 inches off
> the bottom.
>
> Can anyone explain what is happening?
>
> Thanks,
> Paul

I can't explain your setup but I'll try to explain mine. I too have a
PID'd popper, P1 to be exact, but I use a dimmer for the fan and I also
have a booster transformer for the fan that gives me 140 volts so I can
roast 235 grams with no problems. The PID has ramp and soak functions
so I set the program to my profile and leave the fan alone. I do set
the fan and the lean angle of the popper so that I have sufficient
agitation prior to turning the heat on. The beans shoot up about 2 or 3
inches over the pile. The PID runs off a thermocouple placed in the
heating area below the roast chamber and the bean temperature always
lags the heater temperature. The thermocouple in the roast chamber is
about 1 inch off the bottom. The heater temperature also lags the
profile temperature until the profile temp slows the roast above 375F.
I get first crack at about 9 minutes and 2nd occurs around 12.



 
Date: 19 Dec 2006 14:42:49
From: jim schulman
Subject: Re: variac on popper fan - how should temp respond?
On Tue, 19 Dec 2006 20:15:37 GMT, Paul Monaghan
<monaghan@shorelinelegal.com > wrote:

>But in the last few weeks I started paying attention to the temp
>reading, and noticed that the ramp slows or even drops a degree
>instantly when I reduce the fan speed. Happens every time. Not sure if
>the ramp actually slows for good or if this is just a momentary thing.
>
>TC is located in the middle of the roast chamber, maybe 2 inches off
>the bottom.
>
>Can anyone explain what is happening?

If the heat runs flat out (i.e. no control), lowering the airflow
raises the airtemperature and usually speeds up the roast. If the PID
control is on the bean temp, lowering the fan will raise the in
blowing air, but just enough to maintain the programmed ramp. The
roast ime is unchanged. If the PID is on the supply temperature, then
changing the airflow doesn't change the air temperature, and lowering
the airflow slows down the roast.

Your setup's sensor seems to be responding mostly to bean temp, but a
little to the inflowing air.


  
Date: 19 Dec 2006 23:04:19
From: Paul Monaghan
Subject: Re: variac on popper fan - how should temp respond?
On Tue, 19 Dec 2006 14:42:49 -0600, jim schulman
<jim_schulman@ameritech.net > wrote:

> If the PID
>control is on the bean temp, lowering the fan will raise the in
>blowing air, but just enough to maintain the programmed ramp. The
>roast ime is unchanged.

Thanks Jim.

To clarify, I'm not trying to change the profile, but rather make sure
the heating capacity of the popper is sufficient to allow the PID to
keep the profile. I roast in my garage. In the summer the popper
always keeps up so I just leave it alone.

But in the colder months the popper starts to struggle to maintain the
profile near the end. I know this because my PID controller has a
screen that displays output as a percent of total. I've learned that
as I get into the 90% range it is time to do something to increase the
heat output of the popper, before I hit 100% controller output and
start to lag the profile.

One thing I do is to remove the scr from the heating circuit by
pulsing a switch connected across the scr output, to get rid of the
scr voltage drop. This helps quite a bit.

The other thing I have been doing is reducing fan speed with the
variac. But not only does this seem to cause a "pause" - at least - in
the profile, my output immediately starts shooting up, which is the
opposite of what I would expect.



   
Date: 19 Dec 2006 19:06:08
From: jim schulman
Subject: Re: variac on popper fan - how should temp respond?
On Tue, 19 Dec 2006 23:04:19 GMT, Paul Monaghan
<monaghan@shorelinelegal.com > wrote:

>The other thing I have been doing is reducing fan speed with the
>variac. But not only does this seem to cause a "pause" - at least - in
>the profile, my output immediately starts shooting up, which is the
>opposite of what I would expect.

Interesting. The output of the heater is a good measure of the energy
you are putting into the roaster.

Higher airspeed improves the heat transfer per degree difference
(between bean and air), but lowers the air temperature. These
counteravailing effects typically result in a convex curve, where the
roster's efficiency is maximized, and energy input minimized, at some
perfect airspeed.

My direct evidence for this, however, is pretty scant. If I lower the
fan so the beans barely move, the heater time increases, if I raise
the airspeed so the beans barely stop the flow, the heater goes to max
as the airtemperature plummets. There is no reason to assume the
fuinction between these extremes isn;t smooth.

Unfortunately, since the beans' tendency to loft changes through the
roast, the curve will probably shift, and finding the sweet spot could
be a pain. I basically try to keep the beans bubbling at a roughly
even rate through the roast. But I've had some mishaps when I haven't
paid atention: chaffy beans can create stalls at the end of the roast,
no chaff beans can become too light and kill the heat.

My conclusion: Convection (airflow) is a great way to heat beans,
permitting very fine control; but it is a very unreliable and finicky
way to move the beans.


    
Date: 20 Dec 2006 15:29:32
From: I->Ian
Subject: Re: variac on popper fan - how should temp respond?
On Tue, 19 Dec 2006 19:06:08 -0600, jim schulman
<jim_schulman@ameritech.net > wrote:

>On Tue, 19 Dec 2006 23:04:19 GMT, Paul Monaghan
><monaghan@shorelinelegal.com> wrote:
>
>>The other thing I have been doing is reducing fan speed with the
>>variac. But not only does this seem to cause a "pause" - at least - in
>>the profile, my output immediately starts shooting up, which is the
>>opposite of what I would expect.
>
>Interesting. The output of the heater is a good measure of the energy
>you are putting into the roaster.
>
>Higher airspeed improves the heat transfer per degree difference
>(between bean and air), but lowers the air temperature. These
>counteravailing effects typically result in a convex curve, where the
>roster's efficiency is maximized, and energy input minimized, at some
>perfect airspeed.
>
>My direct evidence for this, however, is pretty scant. If I lower the
>fan so the beans barely move, the heater time increases, if I raise
>the airspeed so the beans barely stop the flow, the heater goes to max
>as the airtemperature plummets. There is no reason to assume the
>fuinction between these extremes isn;t smooth.
>
>Unfortunately, since the beans' tendency to loft changes through the
>roast, the curve will probably shift, and finding the sweet spot could
>be a pain. I basically try to keep the beans bubbling at a roughly
>even rate through the roast. But I've had some mishaps when I haven't
>paid atention: chaffy beans can create stalls at the end of the roast,
>no chaff beans can become too light and kill the heat.
>
>My conclusion: Convection (airflow) is a great way to heat beans,
>permitting very fine control; but it is a very unreliable and finicky
>way to move the beans.

When futzing with my FR to make sample roaster, I settled on keeping
the air speed constant and increasing the element power to increase
the temperature as the roast progresses.

I settled on a 13.8VDC regulated supply, which is about 2V higher than
the diode bridge / dropping resitor in the stock FR, to get enough
airflow when the beans are cold.

As the beans get lighter, more air passes over the element, cooling it
more.

Try fixing the fan speed, cranking the variac to max and use it on the
element.

--
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