Sunday, 25 May 2008

Thinking about thermodynamics in Wales

I recently spent two nights in Wales with a friend who collects railways, crematoria and hats.

We didn't find any new hats, but did visit two railways and a crematorium.

We stayed in the Trefeddian Hotel, Aberdovey, a delightful establishment where the amenities included an 'adults only' lounge. We were pampered with strawberries, and of course lots more too, for breakfast, and delicious five course dinners in the evening. If one doesn't finish a bottle of wine, they keep it till the next day, so we ordered both red and white on the first night and finished them off on the second.

On the Talyllyn Railway I had a chat with a friendly fireman while the engine was getting up steam. He told me that all the steam is vented to the outside (extremely inefficient) and when I peeped inside the driver's cab I noticed that the maximum safe pressure was 150 pounds per square inch. That provided food for thought during, because it was the first time I'd seen any figures for a steam engine, and didn't know the conversion factor from pounds per square inch into anything useful

Although the scenery was very pretty, with wooded hills and waterfalls, I should have enjoyed the journey a lot less had I not been busy converting pounds per square inch into pascals - the answer is about 6900, so the engine operated at about a million pascals, or 10 atmospheres.

Once I got home I looked up the vapour pressure of water and discovered that that pressure corresponds to about 180 degrees C, and by making a rough estimates of work done by the expanding steam in the pistons, and the heat put in, I estimated an upper limit of 6% for the efficiency.That makes steam trains seem much less fun.

It is not because I'm an ardent environmentalist, but because I find waste aesthetically unappealing. Even though I can remember how unpleasant railway travel could be when steam was almost universal, I'm not immune to the superficial charm of the surviving steam railways. The technology is very old and yet it works, thus do we condescend to previous generations. Steam engines make a cheery chuff chuff chuff and whistle frequently, and those that survive often potter along through pretty countryside, but using 94% of the fuel to produce the steam and smoke, and only 6% to make the thing move seems an unreasonable preference for the superficial.

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