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Trompe pressure

 
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I wanted to ask, forgive me if I am being naive, but in using a trompe system the height is needed to generate pressure. But couldn't the same be done by forcing the water down the pipe at speed, like a large weight on a board or something similar to force the water through the pipe and reduce the height needed? I have seen very old copies of Compressed Air magazine from the 19th century and cities like Paris were almost entirely run on compressed air. Cars, buses, trams, underground trains, machinery, refrigeration, everything. Does anyone else know about this. ?

Also, the Romans were great engineers, right. But why would they stick water in aquaducts 100ft plus up in the air unless it was to drop the water down again? Otherwise they would have settled for a few feet above the ground or as much as was needed to create flow.And we know they built massive structures, like the civilizations before them. It doesn't make sense to me to go through all that work with their architectural engineering if the returns are minimal such as water in housing etc.
 
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From my understanding, I think in the trompe system a chamber is made underground where the Earth can withhold the high pressure.  Air is forced down into the chamber via downward moving water where the air increases in pressure until the pressure is useful.  The chamber has a vertical outlet pipe where there is a valve and is then used to drive things in a pneumatic fashion.  It is a very intriguing concept.  I don't know enough about it really to brainstorm adding efficiency by pushing the water with weights.  Certainly gaining elevation would help in increasing water pressure, but I figure it would take some doing to create this pressure with weights in a fashion that would work simply.  ,

I don't recall any indication that the Romans built their viaducts with this in mind, though they did have a very strong engineering culture.  They had all of the necessary engineering modalities, functioning in different systems, for instance, to build a locomotive, but they did not put all the pieces together to build any locomotives.  Imagine if the Romans had trains!  Yikes.  They might have taken over the whole world.  I do not know why the Romans built their viaducts so high.  You may be able to find out that information by studying Rome a bit more by finding forums that are looking into Roman engineering.  Sorry I couldn't be more helpful.  
 
pollinator
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Robert Bedwell wrote:I wanted to ask, forgive me if I am being naive, but in using a trompe system the height is needed to generate pressure. But couldn't the same be done by forcing the water down the pipe at speed, like a large weight on a board or something similar to force the water through the pipe and reduce the height needed?.



Wouldn't work.  Aircraft carriers are pretty heavy, an yet they have no effect on the water pressure below them.

Just having weight on water won't increase the speed, not unless the weight was "pumping" up and down, and the energy to pump the weight would far exceed any energy you got back from the compressed air.

The reason the big Trompe systems aren't more widely used is because there are very few places where the conditions are right to make one, and even then they are horrendously expensive to build.  

As for the smaller Trompe systems, like the ones used on forges, etc. unless you already have a convenient source of elevated water, it's cheaper and more efficient to use a regular motorized air compressor/fan/etc.

Note: its not the "speed" of the water that compresses the air, it's the water pressure caused by the "head", i.e. how high the water column is.  The air pressure is equal to the water pressure at the bottom of the Trompe caused by the "head".  The speed of the water just serves to pull the air in near the top of the Trompe (it causes a partial vacuum).
 
Roberto pokachinni
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One way I thought of with doing a trompe type system would be to take a couple large propane type tanks, and have a windmill.   Everytime the wind blows it compresses air into one of the large tanks.  One can be used for running pneumatic tools, and the other is getting pressurized.  
 
Robert Bedwell
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Ok, thanks for enlightening me. Has anyone tried the height of a house to drop water through a trompe, kind of like a drainpipe system with a water tank in the attic?
 
pollinator
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As I understand it, the Romans built their aqueducts as high as they did so that they could bring it as far as necessary. If the source of the aqueduct was a mountain lake, and a minimum slope was required for gravity to move the water down the aqueduct (open-topped pipes overflow instead of getting pressurised), then having the water descend the mountain would decrease the distance it could travel.

I don't see there being much energy-storage potential to this idea. First, the water needs to get up to attic-level. There is a limit to how large a water tank you could place in an attic not originally designed to take that kind of weight, and a practical limit to how much money a reasonable individual will spend on retrofitting their house to serve as a weird sort of water tower.

Then the water needs to be moved into the upper storage tank. That takes energy. What was the point of this excercise again? Oh yeah.

Also, any system small enough to be able to run on stored greywater wouldn't have much capacity at all. Better to hook up a couple of air tanks to a compressor powered by solar panels or a wind turbine.

-CK
 
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Well......
Yes I realize I'm late to the discussion....
Trompe's compress air via air entrainment and depend on the weight of the water to actively trap, and transport, air from the top of the water column to an entrapment vessel.
While velocity may help keep air entrained for a short distance anything other than a vertical drop contributes to air dropping out of suspension and and the stream "delaminating" into two separate streams (one of water headed down and another at the top of the pipe of air escaping ( going upward because of displacement rather than pressure)).
Beyond that restriction, air pressure is more a function of  water pressure at the entrapment vessel which is a function of fall (I.E. every 2.6 feet of fall generates one psi) that is to say if you have a 30 foot high column of water, you will generate 11.5 psi of pressure, simple physics demands if you have 12 psi of air pressure at the entrapment point fed by a 30 foot column, you will reverse the entire column of water. If you feed the water column at higher pressure and/or velocity then you are spending energy to create energy and entering a losing battle of transitional losses. If at any time you slow the column you reduce efficiency as the entrained air has more time to assert its natural buoyancy.
Air can be compressed as a function of velocity via a "ram pump" at a small scale, (depending once again on entrainment) and the bunyip pump could be easily modified to pump air as easily as it pumps water. a simple displacement compressor can be made with a float switch and an electric valve, Google patents has dozens of them, all with micro differences working on the same principle.
If you have enough air volume to spend it profligately, the the simple compounding of pressure per square inch can be used to create ever higher pressures up to the point of liquidization (liquid oxygen)
 
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