[TheForge] Chip forge

[Steve Smith]

Demon Buddha wrote:
> 
> 
> Steve Smith wrote:
> 
>> I could be completely off base here, but why does the surface area 
>> matter in radiative transfer? If the chips are all pretty much at the 
>> same temperature, isn't the steel already getting its max radiation, 
>> regardless of the chip surface area? Isn't the air (whatever) itself 
>> between chips radiating at the same temp as the chips? I think the 
>> steel is already getting the max IR since it is completely surrounded 
>> by a constant temperature environment.
> 
> 
>     Area has a direct bearing on the rate of thermal dissipation, all 
> else equal.  Let's use an absurd example to illustrate.  suppose you 
> have two equal masses of steel (same alloy and all that so that the 
> volumes are precisely equal).  one is in the form of a cube (let's say 1 
> cu in.) and the other rolled into foil 1" wide x .001" thick x 1000 
> inches long. Surface area of the cube will be 6 sq in, and the foil will 
> be 2002.002 sq. in.

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. The chip 
is in a constant temperature environment and cannot lose any heat--until 
it is pointed at the cold steel. Put a hot chip next to the steel. I'm 
saying that it makes no difference whether the chip surface is smooth as 
the proverbial baby, or rough and corrugated as all get out. What does 
count is the cross sectional area of the chip as seen from the steel, 
not the surface finish.

So I think your fine example applies to the steel, but not to the chips 
while in the forge. It would also apply to a chip removed from the forge.

Say you have two hot forges with different types of ceramic media in 
them. The first forge has 1/2" spheres, the second has 1/2" x 6" rods. 
The rods have round ends. In the second forge, the rods are arranged so 
that the round ends all point directly at the cold piece of steel. Both 
forges will heat at the same rate. The length of rod increases surface 
area, but doesn't affect heat transfer to the steel as the steel can't 
see it.

Steve