[Gasification] Fw: Biomass heated ethanol plants

[Peter Singfield]

>Adding insulation to a cylinder increases the surface area, which 
>increases the rate of convection. If the convection rate increases 
>faster than the reduction of the rate of conduction through the 
>insulation, adding insulation can increase heat transfer.


thermal barriers Vs thermal insulation.

As the diameter of your insulation increases you delta T factor (difference
in temperature) decreases.

On the hot side it is "par" -- on the cold side -- "par"

So say hot side A = 1000 F -- cold B = 32 F 

Hot side is super critical water 15,000 psi

Cold side is ice water.

You end up with a ramp -- hot to cold going through your layer of insulation.

Delta T = (T-1 - T-2) 

The temperature difference inside and outside -- for this present example.

The less the Delta T -- the less the heat conductance can occur --  the
greater the thermal barrier to heat transfer.

The initial start up will lose heat to warming up the insulation to meet
those ramp conditions -- but after it is stabilized -- it no longer takes
in more heat energy.

The actual rate of heat flow goes down as distance between points increase
due to lowering Delta T. Total heat loss -- heat transfer depends on an
infinite number of Delta T point transfers within your insulation.

Thus though there indeed is more surface area -- on the outside -- the
energy potential to cross is also much lower .

Thus much less BTU's per second can be "passed"

Do your formulas over -- but enter in the DELTA T factor.

If the temperature did stay the same from one end of the thickness of this
insulation to the other -- your example would be True --

But in real time -- it is not that way.

Mind you -- with a low conductor of heat on the outside -- say still air --
and a much lower initial Delta T -- Say internal 200 F -- external 100 F --
it would be extremely hard to measure any difference between say -- 2
inches of good insulation -- or 20 inches.

Even in the high delta T example -- there is soon reached a point of
failing returns to.

But theoretically -- your still never going to reach that point where to
much insulation reverses -- speeds up -- the heat loss.

Your simply going to be wasting a lot of insulation for no appreciable
results.

Oh -- as your surface area is greater on the cold side your highest delta T
ramp will be closer to the center of your tube example.

It is not automatically in the center of a straight line representing the
thickness of your insulation!!

The more you increase the circumference of your tube's insulation -- the
closer to the hot side will be your highest delta T point ratios.

OK -- if your insuluating a one inch tube with twenty inches of insulation
thick from the center -- the real temperature 2 inches in from the outside
will probably be less than one degree "hotter" than the outside -- so hey
-- almost no delta T -- and alsmost no heat transfer!!

Poof -- there goes your theory!!

Peter / Belize



At 07:48 PM 8/13/2006 +0100, David Fulford wrote:
>Peter and all
>
>My lecture notes from my course on heat transfer explains the maths. I 
>have put the relevant pages on a temporary link on: 
>http://www.personal.rdg.ac.uk/~stsenerg/images/Criticalthickness.ppt
>
>Adding insulation to a cylinder increases the surface area, which 
>increases the rate of convection. If the convection rate increases 
>faster than the reduction of the rate of conduction through the 
>insulation, adding insulation can increase heat transfer.
>
>Does this give the answer you require?
>
>David Fulford
>
>