17 Nov
2003
17 Nov
'03
3:17 p.m.
On Mon, Nov 17, 2003 at 12:32:21PM -0600, Rick Oftel wrote:
Rick,
Jim, Not trying to debate science but your response
generated a question that I hope is easily answered about thermal conductivity. Possibly
an experiment over a beer would be helpful.
Trying to better understand the aluminum vs SS debate. Some folks use Rocket Engines to
heat kettles. Others use electrical elements. A few of us rely on small gas burners that
seem to outperform many of the higher BTU producers.
I agree with you here. Burners are sized by "Consumption" and
clearly not all "Burner BTU's" are created equal. The multi
jet models (e.g. Superb, 100 jets, etc.) have modest ratings (say 30-50k BTU's)
but somehow manage out-perform the "serrated disk in a cup" designs and
"single jet in a tube". The mulit jet burners are quiet, clean, efficient.
According to your statistics, aluminum is 11.25 times
more conductive than SS. Is it accurate to state that an aluminum stock pot would
transfer 11 times more heat to the liquid that the SS version using the same external
heating device? Would it be logical to conclude you would use 11 times less input heat
with aluminum than SS to achieve the same degree of "rolling boil?"
Or put another way, SS has a resistance to heat flow that is 10 times
that of aluminum. The "Delta T" needed to drive the heat across the
stainless is considerably greater (10 times if the thickness is constant)
than the "Delta T" needed to drive the same heat across the aluminun kettle.
So the heat is carried from the bottom/burner side of the aluminum kettle
to the inside/wort-side of the kettle in a more efficient manner. The
inside of the aluminum kettle is at a more uniform temp, and the
outside/bottom of the aluminum kettle is at a lower temp.
To validate this, would a "heat to boil
race" be appropriate? Two gallons in aluminum, two gallons in SS. Original water
temperature 60?F. Similar profile containers with identical burners (15 kbtu). Both
burners started at the same time. If the aluminum boils in 5 minutes, then the SS boils
in 56 minutes?
An interesting concept. Best to use DI water. (Just kidding)
I have never had trouble with scorching but use a
diffused flame source with about 100 single hole jets. I do notice more accumulations of
beer stone, hard calcium deposits (white), and general brown crud directly over the iron
supports. They must significantly help with heat transfer.
Heat transfer is driven by the temperature difference Delta T.
Heat Transfer Rates. Conduction > Convection > Radiation.
The direct contact of the kettle w/ the iron result in
hot spots in the kettle. Heat is going into the iron from the
flame (iron is a decent conductor at 50 w/cm-K.) and it takes
the "shorter path" for heat flow by conduction into the SS kettle.
The heat is coming into the SS at a faster rate (due to conduction)
than the SS is unable to distribute it.
The false bottom for boiling is usually used only when
you bitter with whole hops. The nice thing is it does not need to be in the boil for the
complete cycle but performs very well with Irish moss. If it is not yet installed, you
can scrape clean the bottom of the kettle and cut back even more on your input BTU's.
If you leave a little space between perforated plate and edge, you get a great roll in
your kettle and the perforations appear to hold the hot break quite solidly.
I find that whole/loose/fower hops work best w/ the "screen on a siphon"
that I use. Up to 50% pellets is tolerated by the screen. Advantage of
the siphon is you can always pull it out to clean it if needed.
Must be experiment time. Stay tuned - don't
touch that dial! I look forward to data about flame impingement and actual heat transfer!
What's all this about flaming on the mba? Can't we
just get along? <grin>
Thanks Jim for kicking this off again!
Symposia can be fun. (third definition.)
sym�po�si�um ( P ) Pronunciation Key (sm-pz-m)
n. pl. sym�po�si�ums or sym�po�si�a (-z-)
A meeting or conference for discussion of a topic, especially one in which the
participants form an audience and make presentations.
A collection of writings on a particular topic, as in a magazine.
A convivial meeting for drinking, music, and intellectual discussion among the ancient
Greeks.
cheers,
Jim
Rick O
Rick Oftel
--
------------------------------ *
* Dr. James Lee Ellingson, Adjunct Professor jellings(a)me.umn.edu *
* University of Minnesota, tel: 651/645-0753 fax 651 XXX XXXX *
* Great Lakes Brewing News, 1569 Laurel Ave., St. Paul, MN 55104 *
>> "Jim L. Ellingson"
<jellings(a)me.umn.edu> 11/17/03 11:13AM >>>
Greetings,
I started out w/ Keg-kettles w/ heating elements and spigots.
I direct fired the kegtle, in addition to the electircal heat.
Never did a decoction in these elemented kettles, can't
really stir w/ the element in place.
The Polarware SS Kettles are much better than converted kegs.
For a direct fired brew kettle, I find aluminum to
be superior to stainless. Isssues as I see 'em.
1. Conductivity, hot spots, scorching:
Materail Conductivity Cost
(W/m-^C) (From Matweb)
Copper 360 $1kb?
Aluminum 180 $100
400 SS 27 $100
300/18-8 SS 16 $200
I've never used copper, but in my experience, aluminum is
a superior conductor compared to SS.
Granted the Al needs to be a bit thicker, so the actual conductivity
may be only 5 times the values for the Polarware pot. The chances
of scorching are much lower w/ aluminum. Aluminum works much better for
decoction mashing. A thicker, better conductor means fewer/no hot spots.
2. Cleaning. Commercially, aluminum is not a good option since
the commercial cleaners (strong caustics) dissolve aluminum, but
few compmercial operations are direct fired. Also, commercial
brewing is a high risk occupation. (get the numbers from OSHA
if you don't believe it.) Number one hazard is burns, from the
strong caustics used in cleaning.
My question is this. What sort of chemicals are you using
on your home brew kettle? Why?
My aluminum kettles take a bit of beer stone, and keep
it until I over-do the acid/pH adjustment.
3. I'm not a fan of spigots in the brewkettle. Impossible to clean,
and easily plugged. I like the siphon/duck/whatever.
4. I'm not a fan of false bottoms. Never worked that well
in the mash tunn (Sabco), so I'm back to slotted pipes in a
cooler. In the kettle, I've had trouble w/ scorcing under
the false bottom (Sabco). Quite possibly, that issue is
reduced or eliminated w/ a "real" kettle. (e.g. Polarware).
5. Flavor: Not an issue. See Jeff Donahue's article on same.
My keg kettls are wired for heat (1100 watts at 110v) and have
spigots. I've retired them "upstairs) to HLT use, and they
work fine for that. A spigot on the HLT is very convenient.
I'm as big a fan of stainless as the next guy, but it's
not a very good conductor.
Cheers,
Jim
--
------------------------------ *
* Dr. James Lee Ellingson, Adjunct Professor jellings(a)me.umn.edu *
* University of Minnesota, tel: 651/645-0753 fax 651 XXX XXXX *
* Great Lakes Brewing News, 1569 Laurel Ave., St. Paul, MN 55104 *