homemade radial compressor?

Well, a good piston compressor will deliver about 3.2 CFM per HP at 90 PSI, so it should deliver 6.4 CFM at 45 PSI. I doubt you will do a lot better than that with a multi-multi stage centrifugal blower.

Windjammer type blowers (often used in old computer tape drives) would produce something like one PSI at 3500 RPM, with 5 - 7 stages. Internally, they didn't have really smooth airflow, so you could probably do better. I had a 2-stage centrifugal 3450 RPM blower off a pipe organ. It had rotors about 24" diameter, and produced 80 in of water near stall. 80 In of water is about 3 PSI differential.

With 8" diameter rotors, your pressure differential per stage is going to be WAY less than that. Your choice is to go to much larger rotors or much higher RPM. Turboprop engines using centrifugal compressors that size usually run around 80 thousand to 100 thousand RPM. They probably do develop 3:1 compression per stage at those speeds.

A great book I picked up on eBay is "Gas Turbine Theory" by Cohen, Rogers and Saravanamuttoo. It has a bunch of VERY simple procedures, at least for a theoretical book, for figuring these things out. But, their numbers do assume some aerospace mettallurgy. I think you can plug in the right factors for the materials at your disposal, though.

Jon

Nope. CFM means cubic feet of free air per minute. It does not mean the volume of compressed air. CFM output only improves marginally as delivery pressure is lowered.

With a well designed rotor you can get a maximum of 3:1

Our compressor LP is 7 stage and delivers 120 psi ( at 4500 RPM) into a 3 stage HP (10,000 RPM) that outputs 312 psi at 40#/sec. The motor is ratings are on a similar scale 7000V 690A 11,250HP. Pat

You might pick up some tips from books about making model gas turbines - they typically have a fairly simple centrifugal compressor stage. Mind you, they are going at 100000 rpm . Some are made from wood and carbon fibre !

Dave

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When you say "radial", I take it to mean that you want a piston compressor with a number of cylinders radial to the crankshaft. The rest of the group appear to think "centrifugal". You can probably build a "radial " machine, but a fractional horsepower centrifugal compressor is not a practical machine. Commercially made centrifugals tend to be 500 HP and above. Screw compressors are now becoming available in smaller sizes,but, again, there are few under 10 or 20 HP. You can probably buy a Chinese made compressor for far less than you can build it for even if you already own the motor.

[ ... ]

Not all of us. I was thinking of a radial graphite vane pump, such as those made by Gast. However, I'm not at all sure that even those could make the 45 PSI minimum which he wants.

A quick visit to their web page (they call them "rotary vane pumps") shows the highest pressure available for any form of that is 25 PSI, so we can drop that idea. :-)

Enjoy, DoN.

They get up to 15:1 per stage out of turbine engine centrifugal compressors, with a max of two stages, but you won't get 15 times 15 with a two-stage. Diffuser pressures downstream of the compressor are typically around 350 psi.

Dan

CFM performance relies on really close tolerances between the head and piston, with an absolute minimum of volume when the piston is at TDC. Any volume not swept by the piston (the volume in the discharge port between the valve and piston, for example) will store compressed air which expands when the piston starts downward again, and so intake of new air is delayed and the CFM drops as tank pressure rises. It's for that reason that old gas engines make poor compressors: the head volume spoils it.

Dan

I'm actually picturing a Nash type water ring compressor.

Shawn

I doubt it. We were using them (Cooper rather than Gast, but same design) for an emergency backup vacuum/pressure system, and could get 5 inches of mercury but not 10. An inch of mercury is almost exactly 1/2 PSI. They're great for flow, but not for pressure.

There ya go, agrees with what makes sense. Always nice when that happens.

My only experience of those is as steam turbine condenser air extraction pumps, running at inlet pressures of 150mbar and outlet pressures of 1bar. I would expect that they could be designed for the OP's 40psi without any major problems. As for CFM, the ones I dealt with had 600hp motors on them... go figure :-)

Mark Rand RTFM

Look at the HP models. The smallest will do 80 psi, 205 CFM at 100HP. Granted, this is way more than the OPs 3/4 horse could do, but he did ask for ideas.

Shawn

Thanks for all the replies (I got knocked off the internet for a while, but now I am back on, if only temporarily, and I am overwhelmed at the responses). I was thinking of centrifugal, I guess, instead of radial. I meant to distinguish between and axial compressor (like in the modern multistage jets) and one that just sucks air in near the center and slings it outward with vanes in one step. I had read somewhere years ago that this design was called radial and that the highest compression ratio to be achieved this way was 3:1 (something to do with the compressibility of air). It would probably look like a squirrel cage blower (found on window unit air conditioners), but with much closer tolerences and better shaped vanes. I realize now from the responses that I am a babe in the woods here and I need a whole lot more research before I even begin to think about tackling such a project. I was just trying to make use of this good electric motor and I thought I could make a high air flow compressor with medium psi (even as low as 25 psi would be a good deal for me). I didn't envision making pistons nor cyliners, but just vanes and a close fitting shroud that would hold the pressure when the vanes are spinning and a jet openning for the output

- something very simple - so I could check out my machining capabilities and just see how difficult it is to go from idea to something practical. Back to the library, for now, I guess. thanks to all, littleberry

I have some theoretical info on vane type compressors. Unfortunately it is very theoretical, but not that detailed, otherwise I would gladly do a few calc's.

It does say that these compressors are used in applications with free air deliveries up to 150m3/min, and pressure ratio's up to 8.5 to 1, and in special applications up to 20 to 1. (That is somewhat bigger than what you are thinking of!!!) Lubrication of the vanes is important, unless using carbon vanes.

I will be doing some more research, if you are still interested, let me know.

modern

compression

As I said earlier, they can generate pressures as high as 15:1, but that's at very high RPM. They aren't a squirrel cage at all, but a wheel with fins on it just like a turbocharger wheel. Go here:

A couple of pictures of centrifugal compressor wheels.

Squirrel cages are fine for moving vast quantities of air at low pressures and without much noise, but as compressors they're no good. They can't take the enormous centrifugal forces at the high RPM needed for higher pressures.

Dan

A 15:1 compression ratio is pretty high for a single stage centrifugal compressor. Ingersoll Rand Centac that I worked with had a 150 psi discharge pressure, but got there in 4 stages. The final stage impeller was about

6"(150mm) in diameter, spun at 22,000 rpm and cost a bundle of money.There was an intercooler between stages,and a refrigerated dryer on the discharge to knock out any moisture. The Compressor was driven by a 3.3 KV 50 Hertz 500 HP motor. The air dryer had a 60 HP motor driving a screw compressor. It could handle to output from 3 Centac's

Tom

A Pratt & Whitney PT6 turboprop engine has a hybrid compressor, with four axial stages giving about 1.25:1 each, and one centrifugal stage of about 10" in diameter. The whole assembly spins at around

60,000 RPM, depending on model. Diffuser pressures are on the order of 350 psi. There are smaller turboshaft engine with centrifugal impellers of five or six inches, but they spin at speeds of about 100,000 RPM. like a turbocharger. The compression stage of a turbine engine requires enormous power to drive it; for example, the Allison C250 engine used in the Bell Jetranger helicopter has a *net* output of 420 HP, but the turbine section is producing over 1600 hp. Three-quarters of that is required to drive the compressor to keep the whole thing going. Higher compressions are possible, but the machine has to be able to take it and you need a lot of power to achieve it, just like any piston-pounder.

Dan