Water power for your machines

A bit worrying that among the butchers, carpenters and general mechanics, dentists would also find invaluable, an off-hand grinder with no tool-rest and supported apparently only by the turbine's attachment to the tap....

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The Bray patent application is interesting but to succeed he would have needed prove novelty of design, not of "improvement".

His is a simple Pelton turbine but what would appear to be his claim is fundamentally flawed.

The true Pelton "bucket" (yes, it is called that) is a twin component like a pair of spoons side-by side. The central wall is sharp, and exactly on the centre-line of the single nozzle. There may be more than one nozzle per turbine, but only one per bucket.

The result is that the water divides into two equal streams flowing down their own sides of the wall to impinge on the bottom of the bucket - impulse action.

The spoon shape of the bucket then reverses each stream by somewhere around 150º or so, making it leave the bucket somewhat to the side of the wheel, and possibly giving a reaction component.

This smooth reversal extracts the bulk of the jet's potential energy.

The bucket usually also has a semicircular notch in its rim. I take it that this eases its movement into the jet at the best angle.

Bray's provision of two nozzles means the water will hit the bucket close to the bottom of the "spoon", making the reversal less efficient. Further, some of the water would rebound up the central wall, so the two jets would interfere with each other.

He would have been better, perhaps, using two single nozzles on opposite sides, but I doubt he could have patented that. Many large industrial turbines were built with multiple nozzles, each with its own valve, for speed control.

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I have seen a small grain-mill driven by two very simple reaction-turbines. Located in the Pyrennean foothills, it was clearly preserved and open to visitors, but closed when we saw it. Even so, I managed to paddle into the large, arched tail-race to inspect the two turbines, of open construction.

Bright shiny stainless-steel bolts contrasted with the dark cast and wrought irons, showing recent repairs.

The runner resembled a simple extractor-fan. .An iron ring maybe 150 mm deep (this is France!) by 500mm diameter, having four, rigid, "spoke" vanes inclined at 45º to the plane of the wheel. The wheels are on vertical shafts rising through the floor above, perhaps driving the stones directly rather than through gears.

The water simply falls onto the turbines from curved elbows protruding from the wall, with their sluices or valves presumably in the mill, built basically on the dam. The head (from memory) would be about 3 or 4 metres.

I spotted a large salamander in the shallow tail-race water.

According to the interweb there should be a pressure regulator in every house just after the stop tap. I've never seen on in any of the houses I've lived in, but if the mains pressure is below about 3 bar you wouldn't need one. Even without a reducing valve, you'd only see the full head if no-one was running a tap, so no pressure drop in the pipes.

Nigel Graham 2 and MichaelG:

Thanks for pouring cold water on my Peltier! And for those unusually fine drawings.

This business of ‘head’ is interesting …and I think the missing factor is probably the inevitable frictional loss in the very long pipes.

As an aside … I once had to do an immersion test on a piece of military hardware to [if I recall correctly] eight foot depth. As an innovative cost-saving, we put a good strong lid on a domestic [grey plastic] cold-water tank and added a vertical ‘stack’ of plastic drain-pipe. … the bulging sides of the tank were an adequate demonstration that the head was effective [because the aspect-ratio [bore/length] was sufficient to make the friction trivial].

MichaelG.

Ega -

Sorry - I wasn't trying to pour cold water on it. Just crediting the right inventor!

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Michael -

Friction? What you describe reads as a static rig, with no flow. So once the elasticity of the tank and pipe has equalised itself with the pressure, there is no further flow so no friction. The head will be 8 ft irrespective of pipe diameter.

Where friction may matter is in long flows, such as given by Duncan's example.

Turn all the taps off in every home connected to that reservoir and the pressure at the first regulator would indeed be that 10Bar (I Bar represents a depth of 10m, for practical purposes.)

I suppose the classic pressure / flow exchange example here is the Hydraulic Ram, driven by the stream whose water it is extracting.

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isn't the Pascal such a daft unit? Mathematically tidy enough for any ISO bureaucrat with an ology in Dimensional Analysis of Physicists, but at 1 / 100 000 Bar far too tiddly for real-life engineering. Yet far too big for Acoustics, which works in µPa, or, 1 X 10^(-11) Bar !

(For airborne sound, 0dB is set at 20µPa, the faintest sound pressure level discernible by the fully healthy human ear.)

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My work-place had a big hydrostatic test tank for external pressure, for testing marine equipment containers. Some idea of the loads involved were given by the nature of the lid, like a giant steel bowler-hat weighing 7 tons; and held down for the full pressure-range by 36-off 3-inch BSF bolts with specially-made nuts. These were probably twice the length hence mass they needed be, too, to make lifting them to chest-height and threading them onto the dangling bolts, more fun. The seal was just an O-ring of about 3/8" dia material on the lower, turned joint-face.

One day its operator was given a test-piece, an empty equipment pod, and the test specification....

It imploded well before maximum. Spectacularly too - he showed me the video from a camera inside the tank.

He duly handed the wreckage back with a report, something like:

Test pressure required XXX Bar, reached by steps of --- Bar of ~~~ minutes.

I followed this, but the casing collapsed at only ZZ Bar.

He told me his manager asked, "Is that all?"

Dave remarked that was he was not the item's designer or builder, he'd obeyed the test instructions, it broke below maximum and he was not expected to analyse the failure.

"What else could I write?"

Evidently a manager used to managerial word-smithing!

I have a vague recollection that you extract maximum power from an electrical device if it's resistance is the same as the internal resistance of the supply, so perhaps the max power from a water device happens when the flow is sufficient to drop the water pressure to half what it was with no flow. Of course this could be rubbish, no doubt someone will elucidate

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Sorry … I evidently expressed myself poorly : my reference to friction in long runs of narrow pipe was relating to Dave’s original puzzle.

Yes I am [and was] fully aware that until we get down to capillary sizes, the pipe diameter is irrelevant … that was the point of me mentioning our test rig at all.

MichaelG.