I would like to make a small homemade radial compressor in my amertuer
machine shop. I have a 3/4 hp electric motor (12 amps, 110v, 3600
rpm) with a 5/8" shaft that I would like to use to drive it. I don't
know where to start - can I expect a 3:1 compression ratis? I wonder
what kind of cfm at 45-50 psi I could expect.
I also wonder what kind of profile I should use for the fins, and how
many fins I should configure. Is there public data as to various fin
profiles verses efficiency and compression ratios? I am gueessing
that the compressor should probably wind up being about 6-8 inches in
diameter and maybe 4-6 inches in lenght. If there were plans (already
checked out and proven) I would certainly prefer to just buy the plans
instead of having to work all this out on my own. Any ideas would be
very much appreciated. littleberry

Littleberry wrote:
I would like to make a small homemade radial compressor in my amertuer
machine shop. I have a 3/4 hp electric motor (12 amps, 110v, 3600
rpm) with a 5/8" shaft that I would like to use to drive it. I don't
know where to start - can I expect a 3:1 compression ratis? I wonder
what kind of cfm at 45-50 psi I could expect.
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.
I also wonder what kind of profile I should use for the fins, and how
many fins I should configure. Is there public data as to various fin
profiles verses efficiency and compression ratios? I am gueessing
that the compressor should probably wind up being about 6-8 inches in
diameter and maybe 4-6 inches in lenght.
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

Jon Elson writes:

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.

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.

"Jon Elson" wrote in message
ervers.com...

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.

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


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


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

"Littleberry" wrote in message
om...
I would like to make a small homemade radial compressor in my amertuer
machine shop. I have a 3/4 hp electric motor (12 amps, 110v, 3600
rpm) with a 5/8" shaft that I would like to use to drive it. I don't
know where to start - can I expect a 3:1 compression ratis? I wonder
what kind of cfm at 45-50 psi I could expect.
I also wonder what kind of profile I should use for the fins, and how
many fins I should configure. Is there public data as to various fin
profiles verses efficiency and compression ratios? I am gueessing
that the compressor should probably wind up being about 6-8 inches in
diameter and maybe 4-6 inches in lenght. If there were plans (already
checked out and proven) I would certainly prefer to just buy the plans
instead of having to work all this out on my own. Any ideas would be
very much appreciated. littleberry

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.

"Littleberry" wrote in message
om...
I would like to make a small homemade radial compressor in my amertuer
machine shop. I have a 3/4 hp electric motor (12 amps, 110v, 3600
rpm) with a 5/8" shaft that I would like to use to drive it. I don't
know where to start - can I expect a 3:1 compression ratis? I wonder
what kind of cfm at 45-50 psi I could expect.
I also wonder what kind of profile I should use for the fins, and how
many fins I should configure. Is there public data as to various fin
profiles verses efficiency and compression ratios? I am gueessing
that the compressor should probably wind up being about 6-8 inches in
diameter and maybe 4-6 inches in lenght. If there were plans (already
checked out and proven) I would certainly prefer to just buy the plans
instead of having to work all this out on my own. Any ideas would be
very much appreciated. littleberry

In article ,
Tom Miller wrote:

"Littleberry" wrote in message
. com...

I would like to make a small homemade radial compressor in my amertuer
machine shop. I have a 3/4 hp electric motor (12 amps, 110v, 3600
rpm) with a 5/8" shaft that I would like to use to drive it. I don't
know where to start - can I expect a 3:1 compression ratis? I wonder
what kind of cfm at 45-50 psi I could expect.

[ ... ]

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".

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.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

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.

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

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.

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

"Tom Miller" wrote in message
...
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".

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

Shawn

On 14 Apr 2005 18:05:39 -0400, DoN. Nichols wrote:
In article ,
Tom Miller wrote:

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".

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.

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.

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. :-)

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

On Fri, 15 Apr 2005 00:15:50 -0400, "Shawn" shawn_75ATcomcastDOTnet wrote:


"Tom Miller" wrote in message
...
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".

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

Shawn


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

"Mark Rand" wrote in message
...
On Fri, 15 Apr 2005 00:15:50 -0400, "Shawn" shawn_75ATcomcastDOTnet
wrote:


"Tom Miller" wrote in message
...
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".

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

Shawn


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

http://www.nash-elmo.com/english/pdf...L_500_E_Rz.pdf

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.

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.

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 he
http://www.grc.nasa.gov/WWW/K-12/airplane/centrf.html

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


wrote in message
oups.com...
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.

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 he
http://www.grc.nasa.gov/WWW/K-12/airplane/centrf.html

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.

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