John Baymore on fri 2 nov 01
First of all...... if you happen to be reading this and are not REALLY
interested in this whole subject...... do yourself a favor and just hit
DELETE now . This is a right/left wing fringe ranting by an addled
nit-picking fascist advocating one world government . =
Part of the "misunderstanding" of this concept is simply the use of
words......... many people take "heat" to mean "hot gases".
Can we agree that heat may travel omnidirectionally through a conductor,
such as a metal rod, but rises through the air?
Dai,
Hey....we can decide to "agree" on anything.....but that won't change wha=
t
is an incredibly basic physics concept........ Physics 101 stuff
_________________________
Second Law of Thermodynamics
"It is impossible for an engine unaided by external energy to transfer he=
at
from one body to another at a higher temperature."
Physics for Science and Engineering
Webber, White and Manning
McGraw Hill =
__________________________
BTW...... the "unaided by external energy" refered to above is usually
refering to a (reversible) Carnot engine....and is kinda outside the
current discussion. (A refrigerator or heat pump is the practical
application of the type of thing being talked about there.)
Without an intervening apparatus..... heat energy moves from areas of
higher concentration to areas of lower concentration. Heat energy flows
downhill. There is no inherent directional vector involved in this
transfer OTHER than that imposed by the temperature gradient. THAT is th=
e
sole driving force in the transfer amd imparts any directionality.
-----------------------------
"21-1 Heat Flow
Heat energy is always being transfered in one way or another, wherever
there is any difference in temperature. Just as water will run downhill.=
always flowing to the lowest possible level, so heat, if left to itself,
flows down the temperature hill, always warming the cold objects at the
expense of the warmer ones. The rate at which heat flows depends on the
steepness of the temperature hill as well as the properties of the
materials through which it has to flow. The difference of temperature pe=
r
unit distance is called the temperature gradient in analogy to the idea o=
f
steepness of grade, which determines the rate of flow of water."
Physics for Science and Engineering
Webber, White and Manning
McGraw Hill =
-----------------------------
We could agree now, or on the other hand, we could agree in two weeks,
after the subject has been thrashed to death----
Yeah... it could go there . CLAYART does have a way of beating certai=
n
things to death . In deference to that whole issue........ this will =
be
my last posting on the subject . I'm kinda' sorry I even brought it u=
p.
Clearly in this context.....it is not.
........or we could agree to disagree.
OK, if you want to .
*********************
I have never studied thermodynamics and have no degree in any sciences, b=
ut
I have studdied five gallon buckets full of cold liquids for the last
sixteen years. If I place a bucket on top of a heating device overnight I=
find the inside material to be nice and warm in the morning. If I place
that same heating device on the TOP of the bucket of material overnight t=
he
contents will not be heated very much at all. My conclusion: heat rises.
Klyf,
I am not a scientist and don't play one on TV . I have studied kiln
design for about 30 years.
There are very specific reasons that you have these observations...... th=
at
don't have much to do with the root concepts concerning heat energy which=
are the idea behind the "heat energy flows downhill" laws. They DO have =
a
LOT to do with gas laws and thermal conductivity . The observations
about the effects that occur in this case are accurate........ the pad
heats the stuff in the bucket...... but the conclusion you state is not
100% "dead on" true. BUT...as I said...... you are not at all alone in
this conclusion......... and in most situations having this "common
understanding" matters not a whit.
The "technical junk" behind all this does not really matter in this kind =
of
non-critical practical application....... it is sort of a case of "seat o=
f
the pants engineering" to solve a practical problem. All that matters to=
you is that what you are doing is doing the job at hand. As I already ha=
ve
said.......... the ONLY reason I mentioned it at all was IN RELATION TO
KILN DESIGN AND OPERATION ISSUES. Understanding basic heat theory IS
kinda' important in THAT application IF you really want to understand kil=
ns
and firing well.
Heat transfer in practical applications is quite complicated, and in most=
cases you cannot isolate conductive, radiant, and convective heat transfe=
r
so easily. In your "warm the bucket" case your main heat transfer method=
is likely conductive transfer..... and the effectiveness of that transfer=
is greatly affected by the intimacy of the contact between the heating pa=
d
and the heavy bucket, the emissivity of both the surface of the pad and t=
he
surface of the bucket, any space between the pad surface and the surface =
of
the bucket, the specific heat of the materials involved, the temperature
differential between the pad surface and the exterior of the bucket in
contact with the pad at any point in time, and so on. (BTW....you are
heating the floor under the pad too.) Secondarily, there is some
convective transfer occuring as the pad warms some air in the area and th=
e
air moves up the outside of the walls of the bucket, transfering heat
energy to the walls on the way. This is minimal, but it is occuring. If=
the content in the bucket is a liquid.... the warmer liguid is also boyan=
t
and circulates upward in the bucket moving heat energy upward. Then ther=
e
is the effect of the hot air around the bucket decreasing the possible he=
at
losses from the walls of the warm bucket because the differential in
temperature is reduced.
This list could go on and on and on....... but it is really not worth
getting too far into this. We are just keeping a bucket from freezing, f=
or
god's sake!
If you place the pad on top of the bucket, first of all, the mass of
materials in the bottom of the bucket likely is somewhat isolated from th=
e
heating pad by some air...... a poor conductor of heat energy......your
main method of transfer here. So conductive heating will be pretty
ineffective. Second, any convective heating of the air over the pad tend=
s
to move upward due to the bouyancy of the hot gases. Again..... no heat
into the materials in the bucket. Radiant transfer is not all that
effective in this case because the source is not a good emitter(low temp)=
and the surface of the pad is not an effective radiator. Plus the effect=
of radiant heat transfer is related to the square of the distance from th=
e
emitter to the receptor...... so the further from the surface of the
materials in the bucket... the less heat is transfered. =
Again...... this could go on and on. Net result....... pad on top......
bucket doesn't get as hot. Simple.
By the way...... you'll get more heat to the bucket if you place a sheet =
of
aluminum foil shiny side up under the heating pad under the bucket. Will=
impact where some heat energy is going...... acts kinda' like you can
envision light would act. "Heat Mirror" . However in practical
application...... this practice COULD also cause he pad to catch on fire =
(not what it was designed for). With the foil, less heat energy will go=
into the floor below the bucket and more will be directed upward (radiant=
issues) into the materials you want to heat.
I use a small propane space heater in my shop in the winter (yes Mel, I a=
m
a total cold wimp). When I want to warm my hands I hold them a ways over
the top of the heater and use the rising heated air. I have held my hands=
the same distance under the heater and I feel no heat at all. My
conclusion: Heat rises.
Again accurate observation........ less accurate conclusion if you are
looking at it from a scientific standpoint. As I mentioned above, some o=
f
the distinction comes from the use of the word "heat". I read "heat" as
"heat energy"....... I think you are looking at it as "hot gases". Their=
behaviors are not ALWAYS identical. So........ yes.... in most practical=
applications hot gases tend to rise. You are 100% correct in this
observation. Your practical observation coincides exactly with the usual=
gas laws invloved.
A great portion of the heat energy from the propane heater has gone to
warming the byproducts of combustion and the HOT AIR being discharged by
the heater. That is it's job....to heat air....so that it can be used as=
a
convective heat transfer method. So the heat energy is being transfered=
to the air....and the hot air is being moved to an area above the heater.=
=
The air is hotter there.... your hands held in that air get hot. So
yes...... the heat energy from the heater has been moved to an area above=
the heater. Little to none of the hot gases have gone down below the
heater. So no convective heat transfer is occuring in that lower area. =
Only radiation and some limited conduction. Doesn't get very warm.
The more accurate conclusion is that hot gases rise when placed in an are=
a
of cooler gases of the same basic composition. In this case the heat
energy "rises"....... but ONLY as the byproduct of the hot gases rising.
As nit picky as it seems............ and I am sure it does ..........
this basic concept IS important in an intimate understanding of certain
kiln related issues.
I am curious about this John, if heat is omnidirrectional, how can I stan=
d
under the stack of a kiln at cone 10 and not get fried in all that fallin=
g
heat?
The vast preponderance of the the energy contained in those flue gases is=
moving upward. Again not because "heat energy rises" but because the gas=
es
are buoyant. (One of the basic components affecting what we call "draft"=
in a kiln) The hot gasses are emitting heat energy in radiant form
too........ heat is moving in all directions away from them...... but
because the gases are composed of very dispersed matter (not much thermal=
mass there in a unit of space) they don't act as good radiators of heat
energy to the surrounding space. (If you've ever stood around a campfire=
on a cold night you have experienced the radiant transfer issue....... ho=
t
on one side cold on another . Acts kinda' like light.) As to
conduction..... in this case the cooler air around them acts as a poor he=
at
transfer medium.
That's why you don't get "fried".
And it is actually quite dis-similar to what is going on in the bucket
scenario. In this case it's mainly convective transfer of hot gases...an=
d
in the bucket case....porbably mostly conductive transfer.
Best,
..............................john
"Who IS the perpetrator of an ultra right/left-wing plot involving a
one-world government and a devious banking conspiracy "
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop 2002 Dates TBA"=
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