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Author Topic: Water and pressure reduction  (Read 8849 times)

Gabriele

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Water and pressure reduction
« on: November 08, 2014, 03:51:24 PM »
Hello. I try to explain what i thought.

We take a box contaniner divided in 2 parts by a mobile vertical wall. In one side we put water till a determinated height,to the other side we put a spring to win the pressure applyed to vertical mobile wall. Now we introduce high pressure air to each chamber. The air compress the wall from each side with same force. Als,if we have like 20-30 bar of air pressure,the volume of water reduces by 0,1%. So the density increase but the level of water decrease like the pression on the wall. Is easy to computate. So now the wall will move and expand the spring. Now i let the air exit,the volume of water return the same and so the wall return to original position. Can it work? :-\

Gabriele

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Re: Water and pressure reduction
« Reply #1 on: November 08, 2014, 04:13:00 PM »
So.

In Yellow high pressure air

Low-Q

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Re: Water and pressure reduction
« Reply #2 on: November 08, 2014, 11:44:26 PM »
Try to do the math if you imaging the water will be compressed 50% at 2 bar pressure.
Using imaginary figures does not change the concept you're looking for. It just might make it easier to calculate.
Also have in mind that air isn't weightless. 1x1x1m of atmospheric air weights about 1.3 kg. 30 bar pressure in a 1000 litre tank is 39kg of air.
Maybe those 0.1% compression of water is accounted for by the weight of air in the spring chamber.


My wild guess is that nothing special will happen to that wall. But it is an interesting thought you brought up. It bugs me a little. I'll look closer into it.


Vidar


MarkE

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Re: Water and pressure reduction
« Reply #3 on: November 09, 2014, 01:11:43 AM »
Try to do the math if you imaging the water will be compressed 50% at 2 bar pressure.
Using imaginary figures does not change the concept you're looking for. It just might make it easier to calculate.
Also have in mind that air isn't weightless. 1x1x1m of atmospheric air weights about 1.3 kg. 30 bar pressure in a 1000 litre tank is 39kg of air.
Maybe those 0.1% compression of water is accounted for by the weight of air in the spring chamber.


My wild guess is that nothing special will happen to that wall. But it is an interesting thought you brought up. It bugs me a little. I'll look closer into it.


Vidar
It's just a balance between two pairs of forces.  The minute compressiblity of the water is so somall that for practical purposes it can be ignored.

MarkE

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Re: Water and pressure reduction
« Reply #4 on: November 09, 2014, 01:15:47 AM »
So.

In Yellow high pressure air
The wall moves when there is a force difference between the two sides.  One one side is the force that arises from the weight of the water plus air pressure and the other side is force of the spring plus air pressure.  In essence:  two opposing springs find an equilibrium position.

Gabriele

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Re: Water and pressure reduction
« Reply #5 on: November 09, 2014, 01:47:03 AM »
this way....

Gabriele

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Re: Water and pressure reduction
« Reply #6 on: November 09, 2014, 01:54:37 AM »
I tried computate an immaginary sistem formed by weights and boxes allowing the weight is on the box full of air,after a compression,to move on the first...than let the compression i gave go,and to return the weight to the ground...i got a surplus. But probably is a mistake,cause i'm not really able to do that math,really i don't know how to do that...

Green and white go on red after compression...the red compress more,i let pressure decreese,and i re-put the green white on the ground

MarkE

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Re: Water and pressure reduction
« Reply #7 on: November 09, 2014, 02:25:54 AM »
On the left the water contributes pressure based on the water height.

Assigning X left to right, Y into page, and Z as height, and taking the 0,0,0 datum at the lower left hand corner:

Weight_water = 9.8m/s2*1000kg/m3*water_volume

Pressure_bottom_left = air_pressure + weight_water/(x_wall * y)

Force_left_to_right = y * ( 0.5*pressure_bottom_left*z_water + air_pressure * z_wall )

=

Force_right_to_left = y * ( air_pressure * z_wall ) + Kspring * (box_width + x_wall - spring_uncompressed_length)   (spring K is negative value)

y * ( 0.5*(air_pressure + weight_water/(x_wall * y))*z_water ) = Kspring * (box_width + x_wall - spring_uncompressed_length) 

Solve for x_wall.

Gabriele

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Re: Water and pressure reduction
« Reply #8 on: November 09, 2014, 01:13:09 PM »
It is clear the wall moves. What for me isn't clear is,i repeat,if when i remove the compression when the level of water is rised,how much energy the water returns... 100% of elastic energy due to liquid compression or less? Thx

Low-Q

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Re: Water and pressure reduction
« Reply #9 on: November 10, 2014, 11:15:24 PM »
this way....
This shuld be "easy" to confirm if you had some mercury. And we can eleminate pressure as it is equal in both chambers anyways.


In the first experiment you use water - completely filled up. No pressure. Measure force acting on the wall.
In the next experiment, replace water with the same weight in mercury. No pressure. Measure force acting on the wall


Mercury is known to be denser than water. Well, the level of mercury would require only 7.4% of water level.
So imagine that the mercury is seriously compressed water. Water so compressed it has shrinked by 93.6% - that must require a very expensive pump, made by some very exotic materials!


Anyway. Then you can calculate the force that the mercury and water respectively is pushing on the wall. Do you find any difference?


Vidar

Low-Q

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Re: Water and pressure reduction
« Reply #10 on: November 10, 2014, 11:20:41 PM »
It is clear the wall moves. What for me isn't clear is,i repeat,if when i remove the compression when the level of water is rised,how much energy the water returns... 100% of elastic energy due to liquid compression or less? Thx
If you compress water, It will "deform". Decompressing will make the water back to what it was.
The water responds to the pressure, and nothing more. The pressure is responsible for its compression, so removing compression, you also remove the same amount of energy you put into it when compressing.
Some loss will be found due to heat when compressing anything compressable.


Less useful energy out than energy in. Less than 100% efficiency.


Vidar

MarkE

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Re: Water and pressure reduction
« Reply #11 on: November 11, 2014, 03:57:53 AM »
It is clear the wall moves. What for me isn't clear is,i repeat,if when i remove the compression when the level of water is rised,how much energy the water returns... 100% of elastic energy due to liquid compression or less? Thx
It is always less than 100%.

Gabriele

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Re: Water and pressure reduction
« Reply #12 on: November 12, 2014, 05:58:55 PM »
And by reducing temperature of the water?from 4 to 0 °C the volume increase... we extract cinetic energy of molecules and we make work...

Than we return LESS heat to the water (cause frictions the water warm up) and we generate more energy... can this be a method to break 1st thermodynamc law...? :-X