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I've been looking at solutions to compress gases and getting stunned by the cost and maintenance requirements of existing systems to fill cylinders (whether for SCBA or chemical storage) to 2000-4000psi: The cost of entry is over 4 figures, with commercial systems like those used in fire stations and SCUBA shops running into 5 figures. And, as far as I can tell, they are all direct mechanically driven multi-stage pneumatic pistons.

Meanwhile, sitting in my garage I have a manually-driven hydraulic bottle cylinder that can lift 20 tons, and typical hydraulic systems circulate hydraulic fluid at 5000-10000psi.

So I have in mind a very simple gas compressor: a single piston sealed on both ends. On one side it is driven by a single hydraulic pump line. On the other are two gas valves: An intake and an output. Operation is simple:

  1. Hydraulic pressure shut off.
  2. Gas intake valve open.
  3. Hydraulic flow reversed until piston has drawn in the maximum capacity of gas.
  4. Intake valve closed.
  5. Hydraulic pressure run until piston has reached maximum compression
  6. Output valve opened (if not one-way and hence automatic), then closed once pressure equalized with output cylinder.

If the piston lacks the compression ratio to reach the desired psi, then the charged cylinder is switched to the input line and another cylinder to the output.

Am I missing a reason this won't work? Or does it already exist in some form close to this?

If not, where and under what name might I find a long two-sided piston with pressure fittings on each end?

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    $\begingroup$ How is that different from the mechanically-driven pump, except that it is several orders of magnitude slower? I think you're forgetting how small the volume of compressed gas is on each stroke of the pump. $\endgroup$
    – Dave Tweed
    Mar 15, 2016 at 12:15
  • $\begingroup$ @DaveTweed: The mechanically-driven pumps are extremely slow too: It can take 12-24 hours to compress a cubic meter to 3000psi. And at least the cheaper ones are notoriously fragile: they have many valves and rings that need frequent replacement. My question is essentially: Assuming I have a hydraulic pump, isn't a piston like this essentially a single off-the-shelf part? If so it should be relatively cheap and durable. $\endgroup$
    – feetwet
    Mar 15, 2016 at 15:14
  • $\begingroup$ It's the same piston!! All you're changing is how it is moved -- from mechanically driven at several hundred strokes/min to hydraulically driven at one stroke per tens of seconds. Unless you're talking about a piston with significantly higher displacement, it still takes the same number of strokes to get the job done. And how would a hydraulic pump be any less complicated? Wouldn't you have the same electric motor driving it? $\endgroup$
    – Dave Tweed
    Mar 15, 2016 at 15:34
  • $\begingroup$ I was imagining using a larger hydraulic piston. But the key difference, I assumed, is that a mechanically-driven piston can't compress anywhere near as much as a hydraulic piston. (If that's incorrect, an answer explaining why would be appreciated.) Now, perhaps hydraulics does not have a theoretical advantage but the technology is so mature that it is significantly cheaper and more reliable? $\endgroup$
    – feetwet
    Mar 15, 2016 at 16:46
  • $\begingroup$ @DaveTweed - Here's an image of the cheapest mechanical compressor on the market. It has only two stages, and requires an 85psi input. If hydraulics offer no advantage then perhaps I'm just confused as to why this costs $1000, requires a rebuild roughly every 100 hours, and compresses only .04CFM, when I imagine buying or salvaging a single large piston and a 10kpsi hydraulic pump for half the price that could compress an order of magnitude faster. $\endgroup$
    – feetwet
    Mar 15, 2016 at 16:54

2 Answers 2

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There are two aspects to this. First is that the main advantage of hydraulics is that is provides a compact means of greatly multiplying force but in compressing gasses you have to a huge amount of work on the gas itself so the constraint is the power input so you are always limited by the capacity of the motor driving the compressor.

Consider that the gas in a cylinder with a volume of 1 cubic metre at 350 bar would occupy 350 cubic meters at atmospheric pressure. So if you have a cylinder with an intake volume of 1 litre that's 350 000 cycles to fill the cylinder.

The other very important factor is that high pressure compressed gasses whether for SCBA or industrial gasses is that they need to be clean and dry.

Conventional compressors and hydraulic cylinders are lubricated with oil and designed so that piston seals are slightly leaky to lubricate the seals.

Providing oil free gas compression is a much greater technical challenge and requires high pressure dry sliding seals which is why it requires frequent rebuilds as the seals have a high wear rate.

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    $\begingroup$ The clean requirement is critical. Air with oil in it compressed to 2,000-4,000 PSI gets warm and may well explode. This is why, for example hydraulic accumulators are always filled with inert gasses, not shop air. $\endgroup$
    – Ethan48
    Mar 15, 2016 at 22:51
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The elephant in the room is that while gasses are compressible, hydraulic oil is only negligibly so.

This difference has two major effects:

Firstly, the output volume flow is limited by the intake volumetric flow divided by the compression ratio. Let's say we have a hypothetical compressor that pulls in 10 L of air per second at STP, and raises the pressure to 300 bar. Once the output gasses have cooled down from temperatures best described as "red hot", the output volume will be 1/30th of a liter per second. All that work, and we only have 30 mL of compressed gas.

The second issue is that as you compress a gas it heats up. This is an effect that is inherent to the physics of compression. The heat (motion) already present in gasses doesn't go away when they are compressed. When our example 10 L (10,000 mL) of air is smashed into a much smaller volume of 33 mL, all the heat energy the air already contained remains, but in a much smaller volume. The air has been compressed to occupy a space that is 0.3% of its original volume - a size reduction of 99.7%. All of that heat has to be absorbed and dissipated by the compressor mechanism somehow before we will end up with cool high pressure gasses.

Keep in mind that air at 300 bar has an O2 partial pressure of around 63 Bar. High pressure O2 is extremely dangerous around oils or anything else flammable. For example, the oils like those used in and are essential to seal hydraulic systems.

Your energy source doesn't matter, whether hydraulic, electric, or steam. They are just how you apply the energy to the system, and make very little difference to the complexity of the compressor mechanism itself.

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