cascading/stacking air compressors

[knighty]

could do with some higher pressure air at work, it's not for important stuff, blowing up tyres etc..

current hydrovane runs to 120psi

some tyres get filled to 100psi (hgv stuff)


seams a shame to replace the good hydrovane, and it fills a 1500litre air tank that I wouldn't want to put any more pressure into

I do have a 200litre composite gas tank, made for holding compressed natural gas at 3600psi

I've been thinking I could feed a small compressor with 120psi (dry) air and have it fill the high pressure tank

but can't find a decent/cheap small compressor rated to higher pressure... anyone just run a nromal compressor with pressure on the input ?

180psi out would be enough


I could bodge a fridge compressor to do it, and it would take the pressure no problem... but it's a bit of a bodge (oil return etc.)

 

[Wendelspanswick]

When I bought my SIP 3HP compressor sbout 20 years ago it was rated and produced 150PSI (and it still does) but I saw that the later versions of the same model are only rated to 120PSI!

 

[roofman]

could do with some higher pressure air at work, it's not for important stuff, blowing up tyres etc..

current hydrovane runs to 120psi

some tyres get filled to 100psi (hgv stuff)


seams a shame to replace the good hydrovane, and it fills a 1500litre air tank that I wouldn't want to put any more pressure into

I do have a 200litre composite gas tank, made for holding compressed natural gas at 3600psi

I've been thinking I could feed a small compressor with 120psi (dry) air and have it fill the high pressure tank

but can't find a decent/cheap small compressor rated to higher pressure... anyone just run a nromal compressor with pressure on the input ?

180psi out would be enough


I could bodge a fridge compressor to do it, and it would take the pressure no problem... but it's a bit of a bodge (oil return etc.)

Trying to put 180 psi into a tank will really get the lights phasing...do you mean piggy backing the pumps? surely the end output will still only equate to the best of the 2 so whats the point or am i missing the plot?.

 

[knighty]

I mean feeding 120psi into the air inlet of a small compressor and then turning it up to 180psi

(100 to 120 inlet really, drops to 100 before big compressor kicks in and takes it back to 120)


won't use the air receiver of the small compressor, I'd plumb it straight to my higher pressure tank

pressure 'across' the small compressor will only be 60 to 80psi

 

[addjunkie]

Compressors work in ratios from what i remember. And you cool between stages to improve efficiency. One machine discharging into another is quite common in oil and gas plants.

You will need to make sure you have xs air from the first machine to keep the second machine happy. Id take air from your reservoir into next machine, but to get the higher pressure you will need so ething more robust with better safety valves etc for the higher pressure.

Looking at your hydrovane does it have an adjustable spill off pressure if no air is been used or does it stop and start, can his not be adjusted?

 

[Crater Creator]

If you're meaning a normal single-piston compressor as your second, smaller one, I don't think it will work - or at least would be very difficult to make it work.
The second compressor will see a piston force on it's upstroke proportional to the rising discharge pressure (it would be compressing 100 up to 180 in your example). No matter how much pressure you have squeezed in there on the downstroke, the piston is trying to overcome 180psi on each upstroke.
If you've got a 2-piston setup that helps as the inlet pressure in one cylinder's downstroke will assist the opposite piston's upstroke.
Alternatively a great big flywheel would let the inlet pressure assist rotation.
The second compressor's swept volume can be much smaller than the first - about 7 times smaller for a 100psi (7 bar) inlet.

It's not as straightforward as it seems, industrial multi-stage recips are quite cleverly designed with balanced forces on the crank or pressurised crankcase, cylinder areas etc.
On recips, the compression ratio is usually imposed on the machine by external controls, it's not the same as a car engine compressoin ratio with a certain "unswept volume". The clearance area at the top of the piston on a compressor would allow for an extremely high compression ratio if the driving motor was big enough and there was no ancillary pressure switch or PSV to stop the machine.
You'll also be needing an intercooler between the stages or things will get very hot.

OK I'll button it now - enough waffling.

 

[Mee]

When I bought my SIP 3HP compressor sbout 20 years ago it was rated and produced 150PSI (and it still does) but I saw that the later versions of the same model are only rated to 120PSI!

Some are now 140 or 145 psi.

 

[Johnnybravo]

There are two valves on my Hydrovane that control the pressure coming out of the rotor chamber. On the lesser powered versions these open at 7 bar. On the higher powered version these open at 10bar. Only difference that I could see was the spring pressure. I am currently rebuilding a Hydrovane to run my plasma cutter. I have swooped these springs out for 2 that open at 5 bar as I really don't see the point of reducing high pressure down to the level needed for the plasma. Just a waste of energy and introduces more water into the system that is necessary.

 

[knighty]

thanks for the replys, I (stupidly) didn't think about the atmospheric pressure on the back of the piston, bit of a brain fart their

guess it would work with another hydrovave feeding higher pressure air in... but at the cost of another hydrovane I might as well just get a little high pressure compressor

guess I'll get an old fridge compressor and use that, doesn't need to be fast and it'll be fine with the pressure :-)


Johnnybravo, I'd be worried about turning the pressure down on the pressure release valve on the hydrovane head - they use back pressure to work properly and for the oil return - hat's why that pressure valve is there, so the hydrovane part gets up to pressure quickly instead of waiting for your air tank to fill up to pressure :-o