My compressor, 300 psi and more (..... much more :-D )

[joepage2008x2]

Im building a fully automatic bbmg that is going to run on a co2 bottle. But i do not have a co2 bottle available and im a cheap skate. so i need about 800 psi to test my gun out.
How could i get 800psi with one or two compressors without spending a lot of money, i have a lathe so dont mind maching some parts.

[john bunsenburner]

I would recomend trying to hook them up in series, that should work, or in parralel with a feeding of 100psi. Or build a pressure multiplyer?

[joepage2008x2]

I know that you can hook them up in series because my neighbour has done it to one of his compressors but i dont know if it would be too hard on the pump and kill it - 800psi is dangerous. With there being a pressure diference between the seals of 800psi in the pump they may blow, i know that i could house the pump in a pressurised cylinder but the pumps are sealed anyway.

How reliable are pressure multipliers, id rather risk a multiplier than the pump.

[Technician1002]

I would recomend trying to hook them up in series, that should work, or in parralel with a feeding of 100psi. Or build a pressure multiplyer?

DANGER
What is the maximum inlet pressure on your compressor? Unless the inlet is pressure rated, that is a good way to blow crankcases apart and in the case of fridge compressors have the shell explode. Since the type of compressor wasn't listed, I'll mention that refrigerator compressor shells (the inlet side pressure) can fail at pressures over 300 PSI. Use a pressure relief of no more than 200 PSI on the inlet of a cascaded refrigerator compressor.

[john bunsenburner]

If you build a pressure multiplier using a lathe, you can make it extremly reliable and most probably fairly cheaply. They are simple to make and you can cycle the air until you have the desired pressure for example:

Your big cylinder has a 1inch ID your smaller one a 0.5inch ID(you can just as well have 2 and 1, the results will be the same the pressure would be doubled). You start of with 200psi air. You run it through and get 400psi, you run the 400psi through and you get 800. Now if you made it very strong and relyable, cylce it again to get 1600 or even 3200, which could be regged down. This however would require you to use a proper canister rated to such pressures, I don't recomend doing it, 800psi is alright.

Of coarse you can have a far larger factor by which the air pressure is multiplied to do this you have to make the large cylinder as many times as big as you want the air pressure to be, from the smaller cylinder. This is confusing, but I couldn't find a better wording. I have examples:

If you want 800psi and start with 100psi then:

800/100=8

Smaller cylinder=0.5inch
0.5*8=4.0
Large cylinder=4.0inch

Like this you only have to cylce the air once. If you cycled your 800psi through this multiplier again you would get 6400psi!!! So be very careful, and act maturly.

[CpTn_lAw]

I found an interesting picture : hope this helps

[POLAND_SPUD]

air at 800 psi will give better performance than CO2 at 800 psi due to better efficiency of air..

I suppose you don't need 800 psi.. 600 psi should offer more less the same performance as unregged CO2 (after several shots CO2 will cool enough, which will lower its pressure)...

oh and don't forget that clouds and vortexes are quite inefficient (in a way that you'll need a some sort of detent or o-ring to let it build up the pressure before each shot

AFAIK BBMG is going to consume a lot of air so it would be a good idea to build 2 or 3 compressors and hook them up in parralel (to get more flow not pressure)

you can add a some sort of tank to the setup to store the air... the good thing is that you only need one tank and a couple of fittings (pressure gauge, quick connect fitting, ball valve for venting) and then you can hook up more compressors to the setup

[joepage2008x2]

I know how to double the pressure but doing it this way i get only one cycle and then i have to release the air at the side with the larger cylinder bore to be able to start the cycle again, i could make a 2 to 3 stage pump but then im spending money which i dont have.

My BBMG is not a cloud or a vortex because they both waste a lot of air. The cloud is one shot at high power but in an airsoft game that would be useless. A vortex fires extremely fast but low power and needs lotls of air so yet again useless.
I have a plan on how its going to work and did find a design similar to it on this site but unfortunately cannot find it to show anyone.

The gun wll be run on CO2 if in an airsoft game because about 300ft/s is the limit but will run on air for home testing for higher speeds.

[POLAND_SPUD]

my current setup has one compressor and a 8-9 liter tank and works quite good... but for a fullauto you might need 2 depending on air usage of your gun

well I just realized that you are talking about a BB sized gun
my semiauto 16mm marble gun has a 120cc chamber...

what's chamber volume of your gun? for a 6mm gun you probably don't need more than 20 - 30cc at 500 psi for each shot.. with a good and efficient design you could even use 10cc without losing too much power

[spudtyrrant]

during airsoft you should run the bbmg 200-250fps that is plenty depending on what task your using it for if your clearing rooms even less than that

[Gunner]

POLAND_SPUD: Are yours compressors made by Danfoss?

[Technician1002]

If you want 800psi and start with 100psi then:

800/100=8

Smaller cylinder=0.5inch
0.5*8=4.0
Large cylinder=4.0inch

Like this you only have to cylce the air once. If you cycled your 800psi through this multiplier again you would get 6400psi!!! So be very careful, and act maturly.

Energy transfer and hydraulic math.

John, I hope I can pick on you with no hard feelings.. But the above is true in a slow moving hydraulic system. In a pneumatic system which can achieve high kinetic energy transfers the expected results may change drastically.

First lets look at the accelleration curve of the piston assy with a good fast high flow valve. We won't look at the full stroke travel at first , but instead just the first 1/2 stroke, just to the mid point and look at what is happening.

On the acceleration side.. Large cylinder 100 PSI 4 square inch 400 lbs push (disregard shock heating, cooling, valve COF, etc for this example)
This will be a powerful launch.. against what is slowing it down.. Lets look.

On the deceleration side of the first 1/2 stroke.. Small cylinder starting at 1 atm pressure ( 0 BAR) and at mid stroke at 1/2 volume or 2 atm ( 1 bar) 15 PSI pressure. The force on .5 square inches is 7.5 lbs.

What is the force balance? 400 lbs against 7.5. At half stroke, its still picking up speed in a big way.

At 3/4 travel, still not much better. Add the kinetic energy near the end of the stroke to the 400 lb push of your primary air. The outlet pressure can be quite a bit higher than 800 PSI. Without high outlet pressure the kinetic energy has to go somewhere, possibly out the end of the case of the pump unless a proper bumper is installed.

John, sorry to pick on you man.. Your start was wonderful on the ratios and such for hydraulics.. but stopped short of energy transfer with the kinetic energy component of pneumatics. The Airow Gun uses this kinetic energy transfer to achieve high pellet velocities through kinetic to gas to kinetic energy transfer.
http://airowgun.com/

Edit, In consideration of the kinetic energy transfer and such, a 1:1 piston volume may very well do the pressure multiplication. The first 9/10th of the stroke is pretty much transferring energy to the piston. The deceleration distance is approximately 1.10th the stroke for the sudden change of kinetic energy to compressed gas, at about a 10:1 ratio. The airowgun doesn't even do 1:1. The compression piston is larger than the 22 pellet it fires. Straight hydraulic pressure won't do that with a bow. It's kinetic energy transfer.

[POLAND_SPUD]

POLAND_SPUD: Are yours compressors made by Danfoss?

no the ones I had and have now... but yeah danfoss does make them

the one I use now was made in Soviet Union about 30 years ago I suppose (seriously)

there was almost no rust and it works fine... what is surprising is that AFAIK my grandparents thrown it away because the compressor broke down

upon closer inspection I noticed it wasn't the compressor itself but the starting circut so I replaced it and voilà!! it works


lol never trust the maintenance guy

[john bunsenburner]

Tech, this was considered by me, though nto mentioned, none the less your example is still true, to a degree.

I would fill the tank and the small cylinder with 100psi air too, to avoid extra pumping and wasting of time. Therefore:

1/2 stroke: 200psi
3/4: 400
full stroke is then not quiet as drastic, as the system was help up by 50% of its force at 3/4 of a stroke, haha, im not half dumb, i didn't mention it though. But thanks a bunch for pointing it out, i have that done to me all the time(teachers tell me to make mistakes so they can explain) because people know i don't care, thats for education my self and the others!

[absinthe]

on the fridge compresser front i found that mine will cut out at more than like 30 psi of forward pressure (output at almost 450psi though but the higher the output the higher input pressure it will cope with.. at 100psi output it will stall at 20psi input)