Main Question or Discussion Point
Air weighs about 800 times less than water, 1 litre of water weighs 1000 gms approx. while 1 litre of air weighs 1.25 gms approx. So theoretically it should need less energy ( in fact a lot less) to create a rough vacuum in a given volume than it would need to remove a liquid (water) from the same volume. If you remember Otto Von Guericke in the 17th. Century, (more than 400 years ago) performed his famous sphere experiment in Magdeburg by removing the air from two hemispheres of bronze that were joined together creating a vacuum in the resulting sphere by using a simple piston vacuum pump. 18 horses could not pull the spheres apart, (when he was in Berlin he repeated the experiment with 24 horses.). The vacuum that Guericke created probably had a value of about 1 Torr. (1/760 atmospheric pressure) . Looking once more at Guericke’s piston pump, one wonders if today one could make a reciprocating vacuum piston pump out of graphite and Teflon, which would be almost friction free, lightweight and heat resistant and could be run at high rpm. The pump could be a multi-cylinder (say 4 cylinder pump) with a box fitted to lengthen the stroke of the piston giving each cylinder a capacity of about 6 litres. Using such a pump it should be possible to empty a 7 cubic metre container to a pressure of 1 Torr in about 38 seconds. Thinking about this air at atmospheric pressure contains about 2.69 x 1025 molecules per cubic meter while air at 1 Torr contains about 3.5 x 10 22 molecules per cubic meter. Again the distance (mean free path) between air molecules at atmospheric pressure is about 6.7 x 10 - 6 cms and at 1 Torr it is 5 x 10-3 cms . Therefore there will be enough molecules to pump out even when the pressure reaches 1 Torr. , since air expands to fill available space and will exert an equal pressure on all points within the container. Consider that at each stroke of the pump it has to shift a maximum of only 7.5 gms of weight (i.e. weight of 6 litres of air) atmospheric pressure is sealed off so it doesn’t count. Such a pump should need to lift only the weight of the air and its own weight (weight of piston connecting rod etc.) My question is how much power will it take to run. A minimum of about 250 Watts I am guessing. Any comments ?