[R-390] Tube cooling

[Tisha Hayes]

There are really three mechanisms of tube cooling; convection (airflow),
conduction (contact) and radiation (emission of heat).

The effectiveness of the tube in cooling is a function of a bunch of
things, the physical shape and size of the tube (number of square inches
either in contact with another object (conduction) or exposed to the air
where convection can do it's thing.

The old style tube shields were really more for the mechanical aspects of
keeping the tube mechanically stable in the socket with some benefit for it
not getting whacked and broken. Thermally they are pretty awful as they
prevent cooling by convection (the shield prevents airflow around the glass
bottle), conduction (only that little compression spring actually touches
the glass) and radiation (most of those shields are like wrapping the tube
in tin-foil, it reflects the heat right back at the tube).

Along came the IERC and IERC-like tube shields, by virtue of their contact
fingers with the glass bottle they increase the surface area of the tube to
allow for greater cooling due to convection and conduction. If they are
done right (flat black) they enhance cooling by radiative means (look up
"black body" and what that means in thermodynamics).

-- 
Ms. Tisha Hayes/ AA4HA

"Objective considerations of contemporary phenomena compel the conclusion
that success or failure in competitive activities exhibits no tendency to
be commensurate with innate capacity, but that a considerable element of
the unpredictable must invariably be taken into account." -- George Orwell