Re: starting a steam loco

Stephen Furley wrote:
On 8 Oct, 12:21, a...@xxxxxxxxxx (Andrew Robert Breen) wrote:

In atmospheric engines, iirc, the weight of the pump rods descending was
used to start the engine - that pulled the piston up, steam was let in
behind it - and then the water sprays were put on to condense the steam
and create vacuum.

Yes, in the atmospheric engine the cylinder was constructed above the
boiler, and directly connected to it. The piston was normally at the
top of the cylinder, held there as you say by the weight of the pump
rods on the other end of the beam. Steam was allowed to rise from the
boiler into the cylinder, a valve was then closed, and water was
sprayed directly into the cylinder, thus condensing the steam, and
creating a vacuum below the piston, which was then pushed down by the
atmospheric pressure above it; there was, of course, no separate
condenser at that time. The water was then allowed out of the
cylinder, and air and steam let back in, so that the weight of the
pump rods pulled the piston back to the top of the cylinder, and the
cycle was repeated.

I am familiar with the mode of operation of this (Newcomen) type of engine, but I would query the statement, "Steam was allowed to rise from the boiler into the cylinder". I would have thought some sort of positive induction would be needed to get the steam to displace the air that would inevitably leak into the cylinder when the engine sits idle for some time. I can see one option being to lift the weight[1] (or halt the engine with the weight in the raised position and have a catch to hold it there in order to start the engine next time)

The Watt engine improved upon this in two ways: it provided the separate condenser, and instead of air pressure acting on the top of the piston, the steam from the boiler, which was at atmospheric pressure (Watt was very much opposed to the use of "strong" steam, ie steam at above atmospheric pressure) provided the pressure on top of the piston, against condenser vacuum below. Both of the engines in these two pumping stations have been converted to use "strong" steam, but otherwise appear as built. What I am curious about is how, as built, such an engine would be started.

In the engines as they are now, steam is admitted to the chamber above the piston, with air (presumably) below it. The steam pressure is enough to lift the weight, and fill the top of the cylinder with steam, emptying the lower side of air, into the condenser. The steam valve is closed and the equalising valve opened, the weight drops, and the steam ends up in the lower chamber. The equalising valve is closed, the condenser and inlet valves opened, and once more the positive steam pressure pushes the cylinder down, forcing the exhaust steam into the condenser, which condenses it, begins to form the vacuum, and with this greater pressure difference, the machine runs faster, and after a number of strokes the vacuum is at the appropriate level, and the machine runs itself (looking at the vacuum gauge, which at Crofton is an ex railway vacuum brake gauge) it takes a good 10 or so strokes for the condenser to reach its working pressure.

What I don't know is how you would start a Watt beam engine as built. For the above process to work, there need to be two working strokes with the positive steam pressure acting against atmosphere to get steam, first of all into the top chamber, and second of all into the condenser, to get a vacuum started. All I can suppose is that there would be some starting valve to admit live steam to the condenser directly, to build a condenser vacuum to power the first working stroke.

[1] for mine type pumping engines there is no weight as such, the weight of the connecting rod to the water pumps at the bottom of a deep shaft is enough. On the pumping engines at Kew and Crofton, there is no long shaft, so there is a whopping great weight on the other end of the shaft.

Robin
.

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