[TheForge] Chip forge

[xlch58 at swbell.net]

Demon Buddha wrote:

>
>
> Steve Smith wrote:
>
>> Andy,
>> I agree--the more surface area the steel has, the faster it heats 
>> when in the forge and the faster it cools when out of it. The large 
>> surface area makes for faster heat transfer *due to the temperature 
>> differential* between the steel and its environment.
>>
>> The chip surface area doesn't affect how fast the steel heats.
>
>
>     Agreed, the steel's surface characteristics will determine the 
> rate at which it will absorb heat.  The chip configuration may, 
> however, have an effect on the heat available to the steel.  I would 
> imagine that changes in the chips would have a marginal effect on 
> this, but it may not be so.  Interesting item to muse upon.

The heat transfer rate  by convection, radiation or conduction is 
directly proportional to the area of the source and sink at 90 degrees 
to the direction of travel and inversely proportional to  the distance 
it must travel for conduction and radiation.   It is also directly 
proportional to the difference in temperature between source and sink 
for convection and conduction.   A chip forge would use all three 
modes.   The heat from the gas to the chips would be primarily by 
convection with some radiation.   Conduction does not play a role in 
heat transfer from gases in a forced convection situation.  The heat 
from the side of a chip exposed to the gas to the other side would be by 
conduction and the heat transfer from chip to chip would be by 
conduction, and radiation, the proportion dependent upon the area in 
direct contact.   I also expect that the chips would give heat bas to 
the gas at some point.  An irregular surface on the chips would not only 
provide more surface for convection, but promote more turbulence which 
would also speed heat transfer and generally be a boon to convection, so 
it would be a plus in picking heat up from the burning gases.  The 
irregular surface  would probably be a minus for conduction of heat from 
chip to chip since it would probably reduce the area in direct contact.  
All of the chips are constantly picking up heat and passing it on.  Any 
heat not picked up by a chip, the iron or the lining is wasted to the 
atmosphere.   Of interesting note, most people know that black items 
will pick up heat by radiation more readily than other colors.  Many 
people do not realize that black will also more effectively radiate 
heat.   If radiation was the major player in a chip forge, I would 
expect to see black chips.  I suspect that convection is still the 
dominate transfer mechanism.    The other thing tha would come to play 
here is the conductivity of the materials in question.  Some materials 
will preferentially store heat rather than conduct it.  Others will 
conduct/radiate it a varying rates, i.e. aluminum vs iron.   I am not 
sure which properties are the goal for a chip forge.  I have spent the 
last five years  doing a design exercise on an aircooled aviation 
engine.  Heat transfer is the haireist part of the exercise, as most 
aircooled engines are thermally limited.   One thing that I have leaned 
is that many mechanics and even "engineers" are pretty clueless about 
heat transfer.  I have often noted that years of time spent in front of 
a fire watching bits of metal heat up teaches you a lot.  Like the 
effect distance has on heat transfer rates for conduction.  It is the 
thing that lets us hold the end of an iron two foot bar a half inch in 
diameter with one end orange, and yet not burn our hand.   It is also 
the reason why it is uncomfortable to try that with aluminum, or even a 
two inch diameter iron bar.   In the first case the conductivity went up 
due to materials, in the second due to the area at a 90 degree angle to 
the direction of travel.     It also effects what it takes to bring the 
things up to heat, the iron bar with the bigger conduction path can 
diffuse the heat faster than the smaller bar.  It is not just the mass.  
Put a neck in the iron bar and it will choke the heat transfer.....


Charles