Pamela Regentin on wed 21 mar 07
Ivor and Olive Lewis wrote:
>>>"Heat only flows one way, downhill, to places where things are cooler. To make it go the other way takes a lot of energy and a heat pump."<<<
I don't understand this statement since it is paradoxical to "hot air rises" and that cold air is heavier and sinks. Are you speaking of heat but not air? Also, heat generates its own energy. Thus, I am confused.
Pam
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Ivor and Olive Lewis on wed 21 mar 07
Mel tells us <<`most potters give away about 50 percent of the kilns =
energy up the stack.`>>
That is why it is most important to understand how burners function and =
exploit the controls we have; the primary air shutters, the secondary =
air orifice and the damper.
Keeping gas velocity in the stack to a minimum allows time for heat of =
combustion to soak into our ware and the structure through Conduction. =
Excess primary air cools the atmosphere in a kiln by dilution. Combusted =
gas must be kept as hot as possible.
Heat only flows one way, downhill, to places where things are cooler. To =
make it go the other way takes a lot of energy and a heat pump.
There was a wonderful article In Ceramics Monthly several years ago =
about Recuperators and heat recovery. All incoming air was heated by =
exhausting flue gases. Apparently the system was so effective the final =
effluent was not hot enough to singe paper.
Though I forget her name, twenty years ago there was a wonderful potter =
down the Yorke Peninsula, a graduate of what is now University of South =
Australia. Her husband was an Engineer with BHP. She was funded to build =
a kiln. Hubby did the design work and incorporated a recuperator to =
conserve energy. Just about halved the fuel bill.
This sort of project is for professionals. A lot to study about high =
temperature plumbing and instrumentation.
It was pleasing to read that article. I thought it was a breakthrough =
for CM to print such a technical article. Nothing like it since. Just =
art stuff.
If you go back a few years through the volumes of Interceram there has =
been some interesting work done on the design of ceramic surface =
burners. and about industrial firing times.
Good wishes to all,
Ivor
claystevslat on thu 22 mar 07
Pam -- In this case 'downhill' is metaphorical -- what he's saying
(at least this is as I understand it) is that a concentrated area
(a 'high') of heat surrounded by an area of lesser temperature
will 'go downhill' until within a contained system the heat is
of a consistent extent.
Heat can be used to generate particular kinds of energy, but
it is iteself a measure of the total thermal energy present
in a body. (Temperature, by contrast, is a measure of the
average energy of the molecules in a body.)
The language of science can be terribly obscure and precise or
metaphorical, apprehensible, and susceptible of misapprehension.
The 'heat goes downhill' metaphor is a great and reliable tool
once you're comfortable with it.
Best wishes -- Steve Slatin
--- In clayart@yahoogroups.com, Pamela Regentin
wrote:
>
> Ivor and Olive Lewis wrote:
>
> >>>"Heat only flows one way, downhill, to places where things are
cooler. To make it go the other way takes a lot of energy and a heat
pump."<<<
>
>
> I don't understand this statement since it is paradoxical to "hot
air rises" and that cold air is heavier and sinks. Are you speaking
of heat but not air? Also, heat generates its own energy. Thus, I am
confused.
John Baymore on thu 22 mar 07
Let me see if I can help with this .......
"Heat flows downhill" is one rather poetic way of stating one of the most
basic of thermal engiineering concepts. Another way to look at is is the
way that I have been presenting it in my college kiln design classes since
the 70's:
Heat energy always moves from areas of higher concentration to areas of
lower concentration.
This concept is one of the reasons why stalling an uneven kiln for a long
time tends to even it out. If the heat input to the overall chamber in
general is set in equilibrium (as mucgh heat energy is being input as is
being lost from all sources), the hot areas get cooler and the cold areas
get hotter. Heat is "flowing downhill" across the temperature gradient.
It is also the basis for why the hot flames and gases, as well as hot
electric kiln elements, transfer their heat energy to the (relatively)
colder pieces in the kiln.
Also keep in mind that heat energy and temperature are different. Heat is
a form of energy, while temperature is the result of the application of
heat energy to some physical substance. Substances with no heat energy
applied to them are at Absolute Zero Kelvin (a different temperature unit
scale than F or C).
If you apply one unit of heat energy to some substance you will raise the
temperature of that substance some very exact amount. That is refered to
as the Specific Heat of the substance. Different substances have
different Specific Heat values. You can apply one unit of heat energy to
one weight unit of some substance and get a resultant temperature increase
of, say, 5 temperature units. But you can apply that SAME one unit of
heat energy to one weight unit of a DIFFERENT substance and get maybe a 10
temperature unit change .....or maybe a 1 temperature unit change.
In your experiental understanding of "heat" from daily life, like many,
many people you are confusing what happens to hot gases when suspended in
cooler more dense gases with how heat ENERGY behaves. That old "tried and
true" saying "heat rises" is incorrect. Heat energy's movenments know no
relationship to gravitational direction (up, down, sideways).
This prevalent misconception has lead to generations of potters building
kilns with well insulated arch / roof areas.... and poorly insulated
floors. Then they wonder why the floor tends to fire cold . If the
hot face (inside) of the arch brick is at 1250 C and the hot face of the
floor brick is at 1250 C, and both areas are constructed of the exact same
substance, they will both pass equal units of heat energy through them in
a unit of time. (There are some slight engineering variances on this
fact .... like what is happening on the cold face..... but they are small
enough to be somewhat inconsequential in the case of this example.)
The above heat flow concept is extrapolated from the First, Second and
Third Laws of Thermodynamics (Google it) and also sort of relates to a
concept you'll find mixed in ther called "Entropy".
Kiln design and operation is based pretty heavily on that nasty science
stuff . I've been incorporating the "why" component into my kiln
classes for many years.
best,
....................john
John Baymore
River Bend Pottery
Wilton, NH
JBaymore@compuserve.com
http://www.JohnBaymore.com
Ivor and Olive Lewis on sat 24 mar 07
Dear Pamela Regentin,=20
Sorry to have confused you. I suppose to put it that way is =
metaphorical. Things that have high temperatures pass heat energy to =
places where there is a lack of heat energy, as indicated as by low =
temperature readings. In doing so it can accomplish what is called =
"Work"
To take heat from a place that has a low temperature requires work be =
done. The motor and compressor do this in a Freezer. Then, Heat is being =
pushed uphill !=20
Thanks to other readers who have helped to clarify this.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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