On Sat, 16 Jul 2011 12:34:05 +0100, Clive wrote:

In message
,
Andy Dingley writes
The distinction in strength between Newcome or Watt's boilers and the
first high-pressure boilers (such as Trevithick''s "strong steam" of
25psi) is that the newer boilers also had a margin of safety over-
design to them. Trevithick knew that safety valves were unreliable and
often adjusted by enginemen (this wasn't even seen as a bad practice at
the time), so he not only designed for 25psi, but he designed to not
fail at 25psi, and to survive overpressure too.
Very low pressures are entirely feasible considering that the steam was
only used to raise a piston which through a beam allowed what it was
working on (Man engine etc.) To drop, the steam was the condensed by
spraying in cold water and the atmospheric pressure on the other side of
the piston did the work.

Use of vacuum as an important part of the working cycle wasn't banished by
Trevithick either: his most effective and longest-lasting engine type, the
Cornish mine engine, used high-pressure steam on one side of the piston and
vacuum on the other - made it a very efficient power source for its
day.

--
From the Model M of Andy Breen, speaking only for himself