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Feb28-13, 11:59 AM
P: 11,911
1) Did it take energy to compress the gas into the donor tank?
2) Could we get some of that energy back by putting a pneumatic motor/generator in the whip line between the tanks and driving the generator by the pressure differential during the fill?
3) Would the recipient tank and donor tank equalize pressure regardless of whether we put a pneumatic motor/generator in the whip line?
Depends on details of the generator, but usually yes.
4) What happens to the energy we could have extracted from the gas with the pneumatic motor/generator if we don't put the pneumatic motor/generator in the whip line?
A part of it heats the low-pressure tank, another part is lost as friction in the whip line.

The pressure-drop happens in the donor tank only, in the other tank air gets compressed.

If the two gases were allowed to freely mix or exchange heat energy, the total gas would come to the same original room temperature.
This could depend on details of the expansion process.