Re: emergency egress

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

At 12:17 AM 9/17/01 -0400, Ludwik Kowalski improvised:

> > What about an escape "pipes" made from strong canvas to be
> > available at every floor? Pipes attached to escape ports would
> > roll down and be pulled away by fire fighters on the ground
> > (creating an acceptable slope). Sliding down into a receiving
> > platform (able to handle frequent arrivals)

JohnD replied:

> Slight physics problem:  What happens to the energy when a
> person descends 500 meters in such a slide?  I don't like the
> picture that is forming in my
> mind: frequent arrivals of people with 3rd degree friction
> burns....
> -- You can't dump the energy into the people (they would get too hot).
> -- You can't dump the energy into the slide (it would get too hot).
> -- You can't make the slide conduct heat to the outside, since part of
> its job is to carry people past a region of flames.....

I AM NOT SUGGESTING THAT THIS WAS A PRACTICAL
PROPOSAL. BUT …

Let me do some number crunching. The term "thermal energy"
(THENA) will be used instead of "heat". Why? Because some
may still think that heat is a substance. (The letter A was added
to avoid a conflict with the word "then." Think of THENA is a
Martian word for thermal energy, if you wish.)

An object who’s mass is 100 kg and who’s elevation is 100 m
has 10^5 J of potential energy with respect to the street level.
For the obvious reason the kinetic energy of that object must
not be excessive at the bottom. Therefore roughly 2*10^4
calories of THENA must appear in the process. Where does
it appear? Partly in the sliding object, partly in the canvas and
a tiny fraction in the air.

To address the issue of "the 3rd degree burns" let me assume
(very unrealistically) that 100% of THENA appears in the sliding
body and that it is equally distributed in it (thermal equilibrium).
In that case the body temperature would change by only 0.2 C. This
is for a water bag; for a block of aluminum dT would be about 1 C.

But thermal equilibrium is not going to be established. During
the slide down most of the THENA would still be in a thin layer
of the material. Why? Because the rate at which THENA "flows"
(toward the inner parts of the body) is small in comparison with
the rate at which THENA is generated in the sliding down process.
The so-called "local temperatures", near the skin, could indeed
reach several hundred degrees. Burns of the 3rd degree are
possible. But they can perhaps be prevented if properly designed
bags were used for sliding. This reminds me of materials used
to protect a space capsule entering the atmosphere.

Protecting the canvas is another matter. That material must
accommodate as many as 100 sliding bags, or more. The first
thing which come to my mind is to impregnate the material with
water; this would facilitate the removal of THENA through the
process of evaporation, perhaps accompanied by the intense
ventilation of the pipe. The main point is that an escape pipe
is feasible. I am sure that more practical solutions can be
invented.

May I suggest that this "proposal" be presented to students as
a problem on calorimetry. It does not have to be linked with
what happened at the WTC on September 11.

In teaching calorimetry I always emphasize that scientists
learned to quantify thermal phenomena long before the concept
of energy was formulated. They invented a model of limited
validity (heat imagined as a fluid) and used it successfully
before realizing that a much better model can be constructed.
Ludwik Kowalski