[Homeroast] On Topic: Popper-Roasting

Martin Maney maney at two14.net
Thu Jul 11 14:04:40 CDT 2013


On Thu, Jul 11, 2013 at 11:04:11AM -0500, Brian Kamnetz wrote:
> I roasted with a stock West Bend Poppery 2 for several years. First in New
> Mexico, where it worked fine with 1/2 cup of greens. Then I moved to
> Columbia SC and could hardly get the beans to roast, in two different
> buildings. The roast would get close to done, but then could barely be
> coaxed over the top. (I removed the bimetal thermostat.) I finally settled
> on an untested hypothesis that the increased level of moisture in the SC
> air took more oomph to heat; my analogy, maybe bogus but the best I had,
> was that if I were to run pure water through the popper it would never get
> close to roasting temps, so as the level of moisture increased (and there
> was a LOT of moisture in the air in SC compared to NM), it seemed it would
> take more oomph to heat the air. Another possibility, one that I could not
> test, was that the power level in the outlets was lower in SC than in NM.

I've often heard, and have repeated, the "moister air takes more heat"
(for a given temperature rise), and it's true, but it's always been
handwavy - no real idea how big a difference it makes.  So I had a few
minutes and went a-googlin' in search of answers, and if the numbers at
www.engineeringtoolbox.com aren't all wrong, the difference it makes
is...  pretty small.  For saturated air at 25°C, the water vapor adds
about 4%.  The energy per degree of both change somewhat with
temperature, but ignoring that, let's calculate a bit...

Assume a heater power and volume of airflow with a 200°C rise between
the inlet and outlet.  If we change that to air with 100% relative
humidity (which is about 2% water by weight), the outlet temperature
will be reduced by 7.2°C if I've got this right - call it 13°F, so it's
the same as if the ambient temperature changed from an afternoon high
of 85 to an evening roast-time reading of 72.  Or in yet other terms,
it's as though the heater power were reduced about 4%, equivalent to a
2½ volt drop from a nominal 120V.

martin "never met a problem that didn't go better with numbers" maney

-- 
Here's my message to the record industry and its allies:
I'm not a thief.  I'm a customer.  When you treat me like a
thief, I won't be your customer.  -- Dan Gillmor




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