Now it is Mr. Lomax who is not speaking "precisely".

To quote from Francis Weston Sears, MIT, in his excellent book
"Mechanics Heat Sound", published by  Addison-Wesley, p392, "The term
convection is applied to the transfer of heat from one place to
another by the actual motion of hot material.  The hot-air furnace and
the hot-water heating system are examples.  If the heated material is
forced to move by a blower or pump, the process is called 'forced'
convection; if the material flows due to differences in density, the
process is called 'natural' or 'free' convection.

A body above absolute zero is cooled by one of TWO processes,
'radiative' cooling to the environment or 'conductive' cooling to
another body, which in this case is AIR which is then 'convected' away
"if" there is either forced air flow by a fan or the box is not sealed
and natural convective forces are allowed to work as I previously
stated.

It is VERY important to me that the intervening air be understood to
be a working fluid that the heat is transferred TO because it's own
temperature affects how  much heat can be transferred to it.  That is
why I made to comment about Alaska versus the Sahara.  That is why hot
air does not cool as well as cooler air (and there is probably getting
to be plenty of that in this thread).

Were the working fluid a dielectric fluid (oil or fluorocarbon) in
which the board was immersed or say a heat pipe containing sodium
metal, the effect is still the same, a CONVECTIVE one involving the
CONDUCTION of heat to a intervening material between the heat emitter
and the heat absorber.  As Mr. Lomax implied but did not so state, a
vacuum offers no fluid medium and thus no convection at all.

Look, I'm not interested in a game of semantics here -I'm interested
in helping fellow PC designers understand physics in plain English; if
a lack of "preciseness" is needed to make some sense of this then I am
all for it.  Guilty as charged!

I still maintain that punching a bunch of holes in a PC board with
clad on both sides to serve as a heat sink is a worthless endeavor but
do what you will.  Until and unless I see some math to back it up, I
for one, will not use the technique.  I've done more than my share of
optical and temperature measurements (critical fluctuations of CO2 at
its triple point [where it is a gas/liquid and solid] is one such
experiment and it took THREE boxes within boxes with three temperature
control loops to do it {to hold the temp to a millidegree} but before
we did the experiments, we had an idea where we were headed and used
math to get there.

That is THE way to be "precise", in my opinion.

Bob Landman
H&L


----- Original Message -----
From: Abd ul-Rahman Lomax <[log in to unmask]>
To: <[log in to unmask]>
Sent: November 16, 1999 5:23 PM
Subject: Re: [DC] Calculating thermal dissipation of external plane
heat sink area


: At 04:19 PM 11/16/99 -0500, Bob Landman wrote:
: >Well...
: >
: >If you want to get into the nitty-gritty, there are but TWO forms
of
: >heat transfer I am aware of to either heat or cool, radiation and
: >conduction.  Convection is not a means of heat transfer.
:
: Mr. Landman is not speaking precisely:
:
: [...]
: >I should have said "natural or free convection" as none of the
: >equipment I design uses fans but instead relies upon natural
: >convection to remove heat from components to the ambient
environment
: >(which can be up to 85C).
:
: Convection moves heat. It is distinct from conduction; with
conduction the
: heat is transferred between stationary parts of a system at
different
: temperatures; with convection, the heat is transferred by the
physical
: movement of a gas or liquid.
:
: Air-cooling is both conductive and convective, but the latter is
generally
: more efficient. True, the transfer of heat between the tin/lead
surface and
: the air is conductive, but if not for convection, the overall rate
of
: transfer would be very low; it is only the constant replacement of
heated
: air with cooler air that makes air-cooling effective.
:
: >Obviously, a fan does increase the exchange rate of the air so that
: >new cooler air replaces it and thus conduction again heats the new
: >air.  Of course if this fan system is in a sealed room you
eventually
: >reach an equilibrium temperature which then depends on the
radiation
: >out of the room and that depends on the temperature of the
environment
: >outside the room.
:
: Yes, of course.
:
: [...]
:
: >Yes, of course, two surfaces that radiate (both sides of the PCB)
are
: >better than one as is twice the surface area.  And the fact is that
a
: >tin-lead plated copper plate is a lousy radiator - blacken it and
it's
: >much better.  But then what is it radiating TO?  Blacken the walls
of
: >the container (3M used to make a lampblack type paint that was ~90%
: >absorbing) and it can suck up a lot of radiated heat.  Of course
then
: >it has to radiate that heat to the outside environment.
:
: Of course. But anything which lowers the thermal resistance will
help.
:
: >I find it hard to believe the general statement that holes in a PCB
: >will cool unless the air is flowing through the board and that
depends
: >upon the position of the board doesn't it?  If you are going to all
: >that trouble, a clip-on heatsink sticking up in the breeze would be
: >far better than the PCB.
:
: First of all, the holes are almost zero trouble; perhaps they add a
penny
: to the cost of the board, if that.
:
: Secondly, the holes will increase the transfer of heat to the air
even if
: the air is not moving. But the air will move. A clip-on heatsink may
well
: be "better" but then we are talking about cost and, in one
application
: where I used a holey board, available vertical space.
:
: >As for me, I'm not a big fan of "experimental" evidence.
:
: Especially when it does not confirm the theory!!!
:
: Obviously a sound theoretical knowledge confirmed by experiment is
the way
: to go, if one can. Otherwise the theory can be worse than simple
ignorance....
:
: >I prefer
: >good theory backed by some experiments on the particular design
: >because there are too many variables in each design that influence
: >cooling.  I'd still like to see the calculations.  I've not got the
: >time to dust off my old physics or ME books but it's in there under
: >either radiation or heat transfer topics, I'm sure.
:
: We use theory and simulation and calculation in an attempt to
produce good
: design in advance of an actual physical prototype. But the proof is
in the
: prototype, in the experiment. In a real system in an atmosphere,
: conductive/convective cooling may be a larger factor, I would
expect, than
: radiative cooling. It would be odd to ignore air cooling simply
because it
: is easier to calculate radiative....
:
: Anyway, perhaps someone has the equations Mr. Landman is looking
for....
:
: [log in to unmask]
: Abdulrahman Lomax
: P.O. Box 690
: El Verano, CA 95433
: