Building my own low-tech HPA compressor using hydraulic rams

[quickenberger]

I have recently gotten into paintball and am looking for a more permanent solution than going to the local dive shop to get my HPA bottle refilled because they are on the verge of shutting down so there will be no HPA sources for about 50 miles. I have done quite a bit with hydraulics and wondered about looking for a series of hydraulic cylinders to make my own multi-stage air compressor. Running the general F=PA calculations I can get about 3,000 PSI with 3 stages which is what I want. For cooling I would use oil on the rod side of the cylinders where check valves force flow back to the reservoir. Air compression would be done through check valves with intercoolers between each stage and the extension, retraction of the cylinders would be done with a continuous cycle reciprocation circuit. All cylinders, hoses, fittings...etc would be rated for over 3000 PSI so that shouldn't be a concern. Basically three smaller cylinders would be forced to extend/retract by one larger cylinder and limit switches would cause a solenoid operated flow control valve to shift from retract to extend. I have most of the stuff (pump, drive engine, working rated cylinders, some hoses and lots of AN fittings) so it wouldn't be a huge financial investment. It does not have to operate quickly either. Flow control valves could time things so heat buildup is not as much as well.

My question is if this is something that someone would recommend not doing for some unforeseen reason?

Here is a site that is doing something like this for methane compression.

http://www.biomass2methanol.org/biomass_compressor.htm

They don't have a working prototype that I can find. Most high pressure air compressors are three of four stage reciprocating piston type with forced air cooling and large heat syncs. They are quite expensive even used, around $2500 to $4000. I am assuming that I can accomplish HPA for less than $1000 if you include the stuff I already have.

I'd love to hear your thoughts

 

[zdas04]

I worked on a commercial product that sounds much like what you are describing. That project had a really crappy business model and never came to anything, but technically it was actually pretty cool.

Some of the things we found were:
[ul]
[li]At 3-4 strokes/minute the process is not adiabatic and you lose most of the heat of compression to atmosphere (the cylinders get pretty hot so you need to make sure your elastomers can take temperatures above the boiling point of water), and we could get away without interstage and after coolers.[/li]
[li]We got significantly improved performance by double acting the cylinders (i.e., one end of the gadget is just hydraulics and the other is just compression). We tried a simple spring loaded cylinder and got really fast wear. The compression cylinder with the rod running through is is markedly smaller than the other end, we made that cylinder our second stage.[/li]
[li]You are doing 200 compression ratios, I would make it a 4 stage with two "compressors". You'd want to do the math to get the cylinder and rod sizes right, but I'd expect it to work out to something like (1) first stage: 2 inch ID; (2) second stage: 2 inch ID with a 1.25 inch OD rod; (3) third stage: 1.75 inch ID; (4) fourth stage: 1.75 inch ID with a 1-inch rod. Check your ID's and rod ID's for rod load and volumes.[/li]
[li]You'll probably find that the hydraulic ends of the two compresors are the same size if you get the ID's and rod sizes right.[/li]
[/ul]

This is a reasonable project for someone who already has many of the hydraulic elements.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[quickenberger]

Thanks for the reply, If I used two cylinders and daisy chained them together to essentially make a four stage pump, would there be major problems due to lack of lubrication? There could be some residual lubrication I guess but that wouldn't last long. Also I have thought about using pilot operated direction control valves so that it would be completely hydro-mechanical. Any thoughts on pilot operated control valves?

 

[zdas04]

The one I worked on used teflon ring and shaft seals, no problems with lubrication.

It also had mechanical pilots. As I recall (it has been a few years) at a max hydraulic pressure the pilot would reverse to send pressure to the other side of the hydraulic cylinder, but I can't recall any details on the valve. I do recall that we never even considered using solenoid valves

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat