Can you help me understand something about Cornish beam engines?
Studying about Cornish beam engines. I was working in Cornwall this year, so the interest gripped me. As you'll be knowing, there's abandoned engine-houses everywhere there was mining in Cornwall.
I've done a lot of "steam nostalgia", but now as a welder and technologist, looking at it going forward on then the world leading edge of engine development. The mines were hundreds of metres (~ yards) deep chasing the lodes and there is no coal in Cornwall. Driving an intense competition for efficiency ("duty") - world leading edge from 1800 to 1850.
The "Cornish engine" using steam at significantly above atmospheric pressure - 45psi (3 atmospheres) was daring searing thrilling technology of its day. By 1850 advancing technology and accumulated skill had moved the baseline and 45psi was generally achievable and exceeded. Seems Cornish beam engines "hit a wall" at about 45psi to 50psi possibly60psi - apparently literally the walls of the house-built engine can't take more force. Plus is explained the cast-iron beam of the day was vulnerable to the jerking force of higher-pressure / short cut-off for greater steam expansion. Etc. So they were stuck about about 45psi.
Yet the use continued and there were even new Cornish beam engines for municipal waterworks up to 1900. The very last beam engines went out of service in the 1950's - so they couldn't have been that bad even by standards then.
Higher pressure is usually higher efficiency.
If I estimate right, about 1/3rd of the power came from condensing the steam and the vacuum it created under the piston. Given you've got a big slowly-cycling stationary engine where a condenser can be part of the overall engine concept. So you are getting that 1/3 "for free" compared to an engine exhausting to atmosphere...
My conjecture is...
Apart from the efficiency of few parts as the engine directly gave the oscillating motion working the pumps with long pump-rods direct down the shafts - that the "extra 1/3rd for free" from condensing compensated for the loss of efficiency from not being able to go above50psi?
I'm adding detail to this thought that, with this being direct condensing by water spraying into a chamber with the steam it must condense - much cheaper and simpler than "indirect condensers" used on eg. current nuclear power stations - that the mix of water, condensed steam and some lubricating oil (?) tipped back into the boiler to gain some fuel economy from heat in the "hotwell" - the oil and contamination is tolerable in a Cornish boiler at max. 50psi?
Where it would be totally intolerable to the much more sophisticated "then modern" boilers developing which gave >>50psi and much higher steam-making ("evaporative") capacity...?
Where Cornish boilers with their all-cylindrical shape - a cutting-edge technology and a huge leap forward in 1800 - had become very simple by 1850, with their single large furnace/flue tube and no "firetubes" of the later "locomotive" and "marine" boilers - no nooks and crannies for contaminants to come out doing nasty things?
I'm also seeing that "not simple harmonic motion" of the beam engine - with a passive slow pumping stoke driven by the weight of the pump rod and a rapid steam-driven return-stroke - with pumping rate being controlled by how often you "triggered" the engine to cycle - gave some efficiency advantage over a rotative engine for pumping...
Added all together...
So I'm conjecturing a status-quo where all advantages of higher pressure where negated for mine-pumping by inherent advantages retained by the Cornish engine???
Thanks for indulging me and my interest. Hoping some of you can offer knowledge, wisdom and guidance on this.
Best wishes, Rich Smith
PS - I'm hoping to estimate efficiency % for a "duty" of eg. 100million (the "magic" top figure) - work out what weight of coal and therefore the energy in Joules was in that "bushel" of coal which lifted those 100million foot-pounds of water... When I'm out of Christmas mode and have my technical head back on :-)