On Wed, 2 Jul 2008, Tom Barry wrote:

Tom Anderson wrote:
On Tue, 1 Jul 2008, Tom Barry wrote:
...

This seems like an extraordinary proposition to me. Could you refer me to
any documents giving more details?

Yup, someone took notes at a TfL talk in March, which are online he

http://www.freewebs.com/going_underg...ogspot.com.doc

Excellent document, thanks!

Relevant section:

Ah, now the thing that i found extraordinary was the idea that heat was
building up in the deep tubes over the course of decades - that the reason
things are hotter today is that heat generated in the 70s or whatever has
yet to escape. I just can't imagine the time constant for diffusion of
heat from the tubes to the surface being that long. I have no basis for
this in geophysical knowledge or engineering experience, but i feel it in
my gut.

And what our corresponded writes on this subject is:

"However, in the closed environment of the deep lines, the movement of
rolling stock, working equipment and people using the lines emitted heat
over time. This dissipated into the walls and then into the subsoil,
raising the ambient temperature and drying out the clay and reducing its
ability to conduct heat away. Thus they noticed after 20-30 years of
operating the early deep level lines that the ambient temperature had
slowly but steadily increased over that time..."

That is, it's not a matter of heat buildup, but of the heat from the
tunnels changing the nature of the surrounding ground. That i can believe
- although i would never have thought of it myself!

He also mentions what i suspect is the main reason for the increased
temperatu more services. We have a train every two minutes on many
lines, which i would guess is more than a hundred years ago, and i would
hazard a guess that they accelerate harder and go faster, and so use more
power per train, too (perhaps except the latest stock). Again, this is
purely my gut talking!

"Most heat is generated from the equipment, brakes etc underneath the
train. Plans are being made to draw this heat out from under the trains
when they stop at platforms and then vent this heat out of the stations.
Obviously this would only work in those stations that have adequate
ventilation to street level..."

Interesting stuff, particularly since the TfL tube cooling press release
quoted the Mayor thus:

?It always perplexed me that boffins could produce mobile phones the size of
a credit card yet passengers would emerge dripping with sweat from Tube
trains that lacked air conditioning.'

Oh christ, don't get me started. I'd never presume to imagine that our
glorious leader's grasp of physics would leave him up to speed on this
issue, but you might hope that he'd have someone explain it to him.

Someone like this:

http://www.boriswatch.co.uk/2008/06/...s-a-new-voice/

The 'boffins' do at least appear to be on the case, here (already ruling
out air-conditioned Tube trains straight off, for obvious reasons),
although whether or not any of the current crop of ideas reaches
operational status is open to question. This effect was known about
when the Jubilee was extended, so it's hardly new stuff.

What's probably true is that it's one of those engineering problems
where there's no magic bullet big ticket way of fixing it (other than
total replacement), but a few judicious improvements could add up to a
considerable improvement.

No magic bullet, no. I do have high hopes for the impact of regenerative
braking, though - my own physics is pretty wafty, but if the 30% reduction
in power consumption (which ISTR is claimed - and why isn't it more? boo!)
translates to a 30% reduction in heat output, that should mean a 30%
reduction in the temperature differential between the tubes and the
atmosphere. That should be a good few degrees.

On top of that, i think our best bet might be heat pipes. I'm really not
an expert on these, but the point is that they're thing which conduct heat
incredibly well, far better than a metal rod. Run some of those along the
tunnels, and install surface-linked air conditioning at substations and
stations to pump heat out of them, and you have a way of cooling the
tunnels. The two questions are whether these can move enough heat to be
useful, and how much they'd cost, and i have no idea about answers to
either.

Anyway, some more info here (along with a repeat of the 'ground warming
up' theory):

http://en.wikipedia.org/wiki/London_Underground_cooling

And some splendidly technical knowledge from Profs Maidment and Missenden,
who, refreshingly, really do know their physics:

http://www.cibse.org/pdfs/Cooling.pdf

tom

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