This question, in short, is: Can a gas be compressed with high pressure liquid?

As a scenario, imagine a tank full of atmospheric gas at STP. Could something like a pressure washer pump (in this case water, but generally any) liquid into the tank thereby compressing the gas inside the tank? Pressure washers often reach pressures that are quite high (easily 2000+ psi depending.) This seems like a cheap (if inefficient) way to get high pressure gas.

I haven't been able to find any material on this idea, and I find it highly unlikely that I am the first person to think this. What this indicates to me is that this is NOT a good idea but the reason is not obvious to me.

If the reason happens to do with the choice of gas/liquid, would this be solved with different materials? For instance, if the water is a problem for some reason could something like ln2 be used to compress nitrogen gas in some sort of specialized pump that could handle those type of temperatures?

  • $\begingroup$ These are used already, well, pressure vessels to control water pressure in plumbing systems and water is added or removed to control the pressure. Anyone who has a solar thermal system will have added or removed water from one side to control the pressure. $\endgroup$
    – Solar Mike
    Sep 5, 2019 at 5:37

2 Answers 2


This is common in any farm water system. A pressure tank is on a side branch of the line from the well to the farm water system. At the cut-in pressure the well pump starts, and pushes water into the tank until the system reaches the cut out pressure. Typically the two pressures are about 20 psi apart. There is some adjustment available.

Water would absorb air under pressure, so some mechanism is in place to regulate the amount of air in the tank. This was usually some combination of a float valve that released excess air, and a bleeder valve in the top section of well pipe that led some air back in the pipe so when the pump started, a slug of air was included. I'm sure there are other ways.

Failure of either of these would result in a waterlogged tank. The well pump would cut it, almost immediately pressureize the system, cut off, and repeat as often as several times a second. Hard on pump.

More commonly now there is either a bladder or a diaphragm in the tank to separate the water and the air. These require no additional components. Usually the tank is pressurized to about half the max pressure of the system.

I have two in my farm system, one at the house and one near the far end of the irrigation line 1500 feet away. A third of a mile of moving water has considerable momentum. My solenoid operated valves were taking a beating when turning the water off. I was popping connections from the pressure surge. Adding a 25 gallon pressure tank meant that the surge would just overpressurize the tank somewhat.

Water compressing air is used in hydraulic rams. The momentum of a pipe full of water is halted with a 'clacker' valve. The moving water loses it's momentum rushing into a air containing side chamber. That air pressure then is used to move a small amount of the water to a higher elevation. A well tuned system (Air volume, large in stream pipe, and small diameter discharge pipe have approximately equal volumes) can pump water to surprising elevations. More info here: https://en.wikipedia.org/wiki/Hydraulic_ram

water pumped air can also be used as a way to generate energy. Consider a vessel with an open bottom and two one-way valves on top. When a wave passes it compresses the air in the vessel. A valve opens and that compressed air goes to a storage tank. The wave falls, that valve shuts, and another one opens admitting air into the top of the tank.

The compressed air can then be used to operate a turbine. The efficiencies of this are low. 6 foot waves would only generate about 3 psi. There are better ways to extract energy from waves.


Yes indeed, air can be compressed by pressurized liquids directly as you suggest. However, there are problems that accompany this sort of scheme.

First, if the gas is significantly soluble in the liquid, then the liquid will pick up gas while being pressurized which will then come forcibly foaming out of solution when the pressure is released.

Second, you have to be careful to prevent any liquid from finding its way into the piping that carries the compressed gas and vice versa.

For these reasons, it is common to place a thin rubber membrane between the gas and the liquid inside the tank where the compression takes place.

  • $\begingroup$ The gas is being compressed by the liquid, which why the OP mentions a pressure washer... $\endgroup$
    – Solar Mike
    Sep 5, 2019 at 5:35
  • $\begingroup$ thanks, error is corrected in edit. -NN $\endgroup$ Sep 5, 2019 at 6:22

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