gumpfer
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Let's compare two ways of compressing air. Typically it is squeezed mechanically into a smaller volume. Or there is pressure equalization, filling a volume from a higher pressure volume. (A third way to compress air is by directly heating it.) Any way you do it, for a given weight of air at a given set of conditions, to get a certain pressure there is the same amount of heat involved.
But heat and temperature aren't the same thing. Scuba tanks are filled by pressure equalization. Filling them too fast is not safe because a lot of heat develops quickly and there's no time for it to dissipate. Over-pressuring the tank is the result.
My design goal is to turn this phenomena to my advantage. The potential for a normal amount of air to develop more pressure than expected because of very fast delivery is easily explained.
In mechanical compression the compression and resulting heating takes place in the compressor, not the plumbing or tank. Air is delivered fairly slowly to the tank after substantial cooling has already taken place. In pressure equalization, the compression and same resulting heating all takes place in the tank.
Textbooks warn about keeping oil out of the plumbing. A few drops of oil in a closed pipe that is suddenly filled with high pressure air might explode and burst the pipe. This class of phenomena can be advantageous but oil is not needed.
Imagine a normal piston compressor feeding a tank that is pre-filled to 200 psig. This tank (not the compressor) is the source for compression by equalization of a smaller volume. For the sake of fast equalization the smaller volume to be filled from this source is placed inside the tank. This is the equalizer.
The intake pipe extends into the tank and is closed by a series of two check valves with a space between them. The intake pipe has to be large enough to accomadate the amount of atmosphere that will enter the equalizer at slightly above atmospheric pressure.
In the intake pipe between the two check valves is a large port that is open and shut by a fast acting valve that can be controlled by some convenient means.
When the equalizer is full of atmosphere the big valve on its side opens and if not for compression heat the resulting pressure would be about 199 psig in both tank and equalizer. But thermal equilibrium is not reached as quickly as pressure equalization. If the equalizer is well insulated there won't be time for the "no work" final result of free expansion. The rush of tank air into the equalizer compresses the air in the equalizer and the compression heat is trapped in the equalizer by the closure of the valve. If the valve were left open for a long time, thermal equilibrium would be reached and the resulting pressure in both tank and equalizer would be slightly less than 200 psi. But this is a way of overshooting equilibrium and the whole tank has contributed a little of its heat to the contents of the equalizer which blast into the tank en masse.
The sudden departure of the equalizer contents creates a momentary depression or relatively low pressure zone at the equalizer's intake check valve and the next charge of atmosphere enters under its own impetus with the help of the compressor which is only acting as a supercharger. Not resisting tank pressure.
The air compressor as we know it is an outdated machine. Simplistic thinking about compressed air has people assuming that pressure is just pressure, never mind the details. But the pressure in any container can change according to the motion of the air inside. Still air gives the maximum reading on the pressure gauge, that is static pressure. If the air moves in the tank, the pressure goes down till the air stops moving. There are any number of ways to get air into a tank. As the skeptics inform us, trying to design a more efficient air engine is a waste of time. A cheap way of compressing air is what is needed.
The equations for pressure equalization are simple but hard to find. They are implied in a few air brake manuals but spelled out in only a very few.
The condition of a given batch of air is constant as defined by the combined gas laws pv/t = Constant.
Full equalization of two conditions (waiting for thermal equalization too) results in the sum of the two original constants. pv/t of the compressing source + pv/t of the compressed destination = a new pv/t.
For the process I'm talking about we don't wait for thermal equalization so the sum of the two original pv/t conditions = the sum of the two new pv/t conditions.
Thanks for the forum.
Gumpfer
But heat and temperature aren't the same thing. Scuba tanks are filled by pressure equalization. Filling them too fast is not safe because a lot of heat develops quickly and there's no time for it to dissipate. Over-pressuring the tank is the result.
My design goal is to turn this phenomena to my advantage. The potential for a normal amount of air to develop more pressure than expected because of very fast delivery is easily explained.
In mechanical compression the compression and resulting heating takes place in the compressor, not the plumbing or tank. Air is delivered fairly slowly to the tank after substantial cooling has already taken place. In pressure equalization, the compression and same resulting heating all takes place in the tank.
Textbooks warn about keeping oil out of the plumbing. A few drops of oil in a closed pipe that is suddenly filled with high pressure air might explode and burst the pipe. This class of phenomena can be advantageous but oil is not needed.
Imagine a normal piston compressor feeding a tank that is pre-filled to 200 psig. This tank (not the compressor) is the source for compression by equalization of a smaller volume. For the sake of fast equalization the smaller volume to be filled from this source is placed inside the tank. This is the equalizer.
The intake pipe extends into the tank and is closed by a series of two check valves with a space between them. The intake pipe has to be large enough to accomadate the amount of atmosphere that will enter the equalizer at slightly above atmospheric pressure.
In the intake pipe between the two check valves is a large port that is open and shut by a fast acting valve that can be controlled by some convenient means.
When the equalizer is full of atmosphere the big valve on its side opens and if not for compression heat the resulting pressure would be about 199 psig in both tank and equalizer. But thermal equilibrium is not reached as quickly as pressure equalization. If the equalizer is well insulated there won't be time for the "no work" final result of free expansion. The rush of tank air into the equalizer compresses the air in the equalizer and the compression heat is trapped in the equalizer by the closure of the valve. If the valve were left open for a long time, thermal equilibrium would be reached and the resulting pressure in both tank and equalizer would be slightly less than 200 psi. But this is a way of overshooting equilibrium and the whole tank has contributed a little of its heat to the contents of the equalizer which blast into the tank en masse.
The sudden departure of the equalizer contents creates a momentary depression or relatively low pressure zone at the equalizer's intake check valve and the next charge of atmosphere enters under its own impetus with the help of the compressor which is only acting as a supercharger. Not resisting tank pressure.
The air compressor as we know it is an outdated machine. Simplistic thinking about compressed air has people assuming that pressure is just pressure, never mind the details. But the pressure in any container can change according to the motion of the air inside. Still air gives the maximum reading on the pressure gauge, that is static pressure. If the air moves in the tank, the pressure goes down till the air stops moving. There are any number of ways to get air into a tank. As the skeptics inform us, trying to design a more efficient air engine is a waste of time. A cheap way of compressing air is what is needed.
The equations for pressure equalization are simple but hard to find. They are implied in a few air brake manuals but spelled out in only a very few.
The condition of a given batch of air is constant as defined by the combined gas laws pv/t = Constant.
Full equalization of two conditions (waiting for thermal equalization too) results in the sum of the two original constants. pv/t of the compressing source + pv/t of the compressed destination = a new pv/t.
For the process I'm talking about we don't wait for thermal equalization so the sum of the two original pv/t conditions = the sum of the two new pv/t conditions.
Thanks for the forum.
Gumpfer