HPA Compressor

[billybob_81067]

I'm thinking of building my own HPA compressor and I've got a pretty big hydraulic cylinder that came off of some piece of construction equipment. The ram is approximately 2.5-3 inches in diameter and the whole thing is probably 8-10 inches in diameter, so it's a pretty good sized cylinder. What I was wondering is if anyone has ever done this before. I'm planning on using this 2-way cylinder as a HPA compressor... Just fill the ram side up with shop compressor air and then hydraulically cram it to high psi. I'd just have valves on the inlet and outlet so that you would open the inlet to let new air in and then close it before you compress and open the outlet to let the compressed air into the HPA tank.

Sorry if I'm not making a whole lot of sense, but it's 2 in the morning and I'm a bit sleepy

I'll clarify some more later

Oh yeah... basically I just wondered if anyone has done it before and if there were any problems. Basically I just don't want to make a huge pipe bomb and have it go off in my face

[billybob_81067]

Pic

Here's a picture to clarify my ramblings from last night.

[billybob_81067]

anyone?

???

[nerobro]

your biggest problem would be that you'd need to get the hydraulic fluid to the pressure you will need to get the air to. And then..... the seals that handle 500-1000psi won't be able to handle air at 500-1000psi.

without replacin all the hoses involved, and having a hydraulic compressor that could handle making 3000psi... you're not going to get to far.

Now.. ther'es another issue. Hydraulic fluid by being non-compressable is much safer to work with at high pressures. not to say it's SAFE by any means. you're still dealing with 3000psi. And then you'll be dealing with a fairly large volume of 3000psi air on the output. if anything let loose there it would all try to expand...

I dunno if I would reccomend this. I'm fairly sure that the rams aren't designed for this kind of pressure......

anyone who knows more about hydrualics have anyhting to add?

[Cristobal]

I would be worried about keeping my air output clean from grease and oil... compressed air and oil don't mix :O When you compress the air in the cylinder its going to get hot, which could ignite the oil -- and well that's just not good. There's a warpig article about the danger of people getting oil in their hpa tanks when filling; same basic deal.

[billybob_81067]

New Design Idea

Ok, so I got to thinking about what you two were saying and came up with another design. Basically just use the cylinder to compress the air in it's own seperate chamber rather than having it within the cylinder itself. That would get rid of the grease/oil issue and also would get rid of a couple seals. I figure this cylinder has got to be good for at least 5-10 tons of lifting/compressing force...

Also the air chamber could be made smaller in diameter than the cylinder therefore giving a better power ratio to the cylinder over the air...

Just something to think about

Here's another pic:

[Redkey]

We have loads of hydraulic equipement where i work. It's all 3,000 psi stuff. Our cylinders range anywhere from 1,000 to 50,000 lb capacity.

If I were you I would run the calculations to see if this is worth doing. Things to consider... max pressure of pump, flow of pump, size of hydraulic ram, size of compression chamber and input/output pressure.

Looking at your last diagram I would suggest to run this in both directions. Otherwise the return stroke is doing very little... consider how a booster pump works.

let's see....

if you had a 3" diameter hydraulic ram with 12 inches of stroke coupled to a 2" diameter 12" long ram to compress the air... the following would be more or less true with a 3,000 psi hydraulic system and 100 psi input air pressure.

hydraulic max force = ram area * pressure = 21,200 lbs
hydraulic volume = ram area * stoke = 85 cu inches = 0.37 gallons.

Max pneumatic pressure = ram area * force = 6,750 psi
Max pneumatic volume = ram area * length = 38 cu-in or 38/1728 = 0.022 cu-ft

pressure at 0" stroke = 100 psi (input pressure)
6" stroke / 6" air = 200 psi (reducing volume by half doubles the pressure**)
9" stroke / 3" air = 400 psi (half again)
11" stroke / 1" air= 1,200 psi (one third this time)
11.5" stroke / 0.5" air= 2,400 psi (half)
11.6" stroke / 0.375" air= 3,200 psi and requires about 10,000 lbs of force. Volume at 3,200 psi = Area * length = 1.1 cu inches.

**Pv=NRT however you have to take heat loss into consideration since compressing a gas makes it hot. To simplify the equation i left out the heat loss part.

so... according to my calculations (not sure they are 100% correct) you'll need about 60 strokes of this system to pressurize a 3000/68. Not sure what the compression efficiency is on a compressor... let's just guess that heat loss represents a 25% reduction in efficiency = 75 strokes.
Which doesn't seem like very many strokes when you look at how fast a HPA compressor runs... on my LMF compressor the third stage cylinder diameter is about 1/2 in diameter... your system has 16 times more surface area.

How fast can you... errr... stroke? Hydraulics are not superfast. I'll guess that you could cycle this system 4 times a minute, 15 seconds per cycle... so, it would take something like 18 minutes to pressurize a 3000/68.

Another thing to consider here is that the high pressure cylinder of a HPA compressor is not air tight. The piston runs fast reducing the amount of time the pressurized air has to slip past the rings. Because your system would be running much slower the air would have more time to leak, meaning you would have to have a very good seal. My LMF compressor has 10 rings on the high pressure cylinder.

Also... at 4 strokes a minute you are putting out 0.087 cu-ft of 100 psi air or 0.58 cu-ft of unpressurized air per minute. Not very fast.

Think of it this way... scuba tanks are rated for their uncompressed volume... 80 cu-ft. HPA compressors are rated the same way. For example, my small Mako scuba compressure will do 3200 psi at 2.4 cfm using a 1 HP motor (i think). it's probably running at 300-400 strokes per minute. Takes about 40 minutes to pressurize a empty 80 cu-ft scuba tank. The hydraulically driven system described above would take about 140 minutes.

I could go on and check my math. But others will probably do that for me. Anyhow, I think you would be better off buying a small scuba compressor...

[billybob_81067]

Ok... I'm a bit stoopid when it comes to calculations...

I wish I could afford a nitro compressor and maybe I can someday, but I'm going to try and pursue this first! LOL.

Say I've got a cylinder with a 3 foot stroke and I want my air chamber to have a six inch diameter. How many strokes would it take then to fill a 68 cu in tank to 3000 psi? And how much pressure would it take?

Also my shop air source is at 175 lbs so that would make a difference too...

I suck at calculations

Thanks!

[Redkey]

first the cylinder diameters.

3,000 psi on a 3 inch ram will = pressure * area = 21,200 lbs of force.

21,200 lbs of force on a 6 inch ram = force / area = 750 psi.

To get high pressure from lower pressure your cylinder ratios need to go the other direction.

now for the higher pressure, longer ram...

2 inch diameter ram, 36 inch stroke, 175 psi starting pressure.

24" stroke (12" air) = 525 psi with 38 cu-in volume
30" stroke (6" air)= 1,050 psi with 18.8 cu-in
34" stroke (2" air)= 3,150 psi with 6.3 cu-in

which seems about right... you tripled your starting volume and nearly doubled your starting pressure. Looks to be about 10 strokes to fill a 3000/68. much better.

Although i'm not sure how well you could get something like this to seal. Nerobro is right about the hydraulic fluid being easier to seal when compared to air. An intesting project if you have everything you need laying around in the shop. Keep us posted.

And... be careful.

[billmi]

Here's my thoughts....

If your knowledge level is to the point where you need to ask about it in a forum, you are not an expert in the field.

High pressure gasses can be deadly. Just this last year a man was killed while filling a tank because some of the components in his system were not up to spec. Unless you are an expert at high pressure gas engineering, you are most definitely running the risk of building a device that can fail catestrophically taking the life of yourself and possibly someone else.

See you on the field,
-Bill Mills

[akscubainst]

Goto http://www.airspeedpress.com They have a book you can buy that shows how to build a booster. Which is what you're wanting to build.

I agree with Billmi that you'd better do some serious research before you mess around with HPA.

I own a Scuba shop and have years of experiance with HPA and am still recovering from a simple mistake I made last month. I got in a hurry and didn't back off a regulator before charging it. I put 2000psi in a 25' 200psi hard plastc hose. The hose exploded right about chest level. I had shrapnel wounds all up and down my arms, across my chest and in my face. It's been a month now and my ears are still ringing. The largest piece of hose I've found was split in half and about 2-1/2" long. I was extremely lucky and very fortunate not to have been killed or messed up my eyes.

Moral of the story is HPA isn't something to mess around with.

The book I've mentioned is for divers who use mixed gases which can't easily be run through a compressor. For instance to use trimix you fill a Scuba tank with a few hundred pounds of helium then top it off with air. Anyone who has ever filled a HPA painball cylinder from a Scuba tank knows that the pressure in the Scuba tank drops fast. Same Idea with the helium cylinder except that the helium costs about $200 per fill so you want to be able to use every last bit of it.

One of the projects in the book is a booster concept almost exactly like the idea you describe except they've already considered things you haven't like where to put check valves and which cylinders will work and which won't.

Anyway do you're research and don't get hurt.

DM - akscubainst - www.thescubatank.com

[billybob_81067]

Thanks for all the replies!

Redkey, Yeah I thought that that would make a big difference! It will probably be quite awhile before I actually get to this project (I've got a lot of other projects lined up as well ), but I'll keep you all posted when I do start.

Billmi, Thanks for your concern, but we all learn by asking questions do we not? And before I actually do anything I will do much much more research... I was just tossing around the idea and seeing if it was even feasible.

Akscubainst, thanks for the webiste! I'll check it out and see what other designs there are. Sorry to hear about your accident, get well soon!

[nerobro]

This ties in well with another thread. the thread about truely constant air... thanks for the lead, I need to order a manual about making moving high pressure seals ;-)

[Big'n slo]

Its already half built for you

Just motorize THIS

[dfs]

http://www.princedirect.com/pages/closeout_list.cfm?category_id=2

its rated @ 3kpsi working and 6kpsi shock load (bursting pressure is usually 1.5-2 x the shock load pressure). there are 5kpsi cylinders out there but they are too $$$. this one should give me a 1.25:1 pressure boost over hydraulic pressure and ill need > 25:1 compression ratio @ 150 psi shop air input. the volume between the output check valves and the ram itself will have to be kept to a minimum to keep efficiency up and keep the comp ratio high. i should be able to get about 4.5 cu. in. @ 3.8kpsi per stroke if my dead volume at the end of the stroke is =<1 cu in.

for the pump, i was also going to use a barnes/haldex or parker/oildyne 500 series 1 hp power unit with a built-in pressure solenoid.

http://www.parker.com/parkersql/bulk...2%3A11%3A35+PM

http://www.haldexbarnes.com/products/hps.htm