What limits the speed of an air compressor?

[strantor]

you would do better trying to figure how to get a larger volume pump, rather than trying to overspeed a small pump to gain performance.

I think we had the same idea at the same time...

 

[JimDawson]

Doing a little research it seems that most recip compressors in that HP range limit out at 1050 to 1300 RPM. Even the ones with pressure lubrication. Not sure why that is, maybe mechanical limits.

Your logic is sound, but it seems that an appropriate pump is not available. The variable speed compressors are all rotary screw, and I know that some of those will run up to 6700 RPM.

What might make sense is to run two 5HP pumps with pulleys sized to turn the pumps at 1050 RPM at maybe 120 HZ on the motor then slow it down to 525 RPM as the pressure builds. I think the net effect might be the same.

 

[strantor]

go with a screw compressor it would hold up to the higher rpm's

I've looked into it. I cannot find even a small screw compressor which is affordable. Seems $3k-$4k is the entry point for a complete unit. I can find small replacement screw units for cheaper ($1500) but then I have to design an oil recirculation system around it. But in piston compressors for example I can get a 20hp Eaton piston compressor head brand new for $1500 (that's still outside my budget, I would be looking for used).

 

[Downunder Bob]

ne thing you might want t consider is heat in the heads, which can carbonise the oil, I understand that most hobby compressors don't use much oil, but as you speed it up it will tend to use more, and running faster will increase the heat in the heads to the point of carbonising the oil, this will destroy the reed valves and clog up the intercooler and after cooler if you have one if not it will end up in your tank.

I have seen this happen one ship I worked on the compressor motors were replaced with higher speed motors. The plan was to increase the rate of compressed air. it did slightly but it quadrupled the maintenance on the compressors. In the end we had to put the original motors back on and get an extra compressor. So my suggestion is don't do it.

 

[BtoVin83]

Reciprocating things have a finite FPM plus your flywheel may not take the RPM

 

[Packard V8]

Ok reed valves, that's a gotcha that I don't have an answer for.

Don't worry about the reed valves. In kart and motorcycle engines, they function at higher RPM than a compressor head would ever see.

Also the inertia of the connecting rods

Connecting rods don't fail from their own inertia, but that of the piston, rings and pin. Inertial load goes up as a function of the square of the RPM. A compressor rod will have been designed for an RPM range. 4X is most likely beyond that envelope.

Sooo.... let's see. I suppose I could oversize the compressor head in relation to the motor and gear it down. Since I already used 5hp and 240hz as my example I'll just continue with those numbers... instead of a 5hp motor and a 5hp compressor head, I use a 5hp motor and a 20hp compressor head. I use 4:1 double-reduction between motor and compressor. At start-up, the motor is spinning 4x its nameplate 60Hz RPM and the compressor is spinning exactly at its rated RPM. At start-up the 5hp compressor is delivering the volume of a 20hp compressor (like 60 CFM vs 12CFM). As pressure builds, the motor motor slows down as before, and as pressure approaches max, the compressor has slowed to where it's delivering about what what a 5HP compressor should be delivering right before the pressure switch cuts out. How bout that?

That could be made to work. Just remember the difference between an empty startup at 0 PSI and an operating startup at 125 PSI. In the real operating range, your gain is much less than your theoretical from empty startup.

jack vines

 

[Mitch Alsup]

I think you would see the same result as running your car for extended periods at 8k rpm, i.e. early demise.

My Ferrari has an 8,500 RPM red line, and I have bounced it off the 8,750 fuel cutoff limit numerous times.

As far as running an air compressor at 4× rated flow--I wish you luck, and I hope it does not come apart in use.

 

[matthewsx]

There are plenty of good answers here but if you're really coming up on the limits of air available from piston air compressors the real answer is to go with a screw type. Surplus units can be had fairly cheap at auction but require lots of power and take up lots of space.

To answer the actual question about what limits things it's physics. It might be possible to modify an existing compressor pump as you've suggested but the fact of asking about it probably means the engineering required isn't available in-house.

I've spent lots of time modifying Briggs & Stratton utility engines to run up to 8500rpm for kart racing. It takes changing most of the internal parts and lots of time/money making sure everything is as close to perfect as possible. And this is for running only 15 or 20 minutes at a time with variable rpm, not a constant load for hours. It's unlikely any reciprocating compressor pump would last a reasonable amount of time with the type of mods required to double it's rpm.

Either modify the process that takes excessive amounts of air, or bite the bullet and buy an appropriate compressor for your needs.

JMHO,


john

 

[pontiac428]

I suspect the reason you shouldn't run your compressor as fast as Joe Walsh's Maserati is heat. Each stroke of the piston is going to generate a given amount of heat as the air is compressed. If you increase the RPM, you increase the amount of heat that the head has to dissipate. When you consider plastic pistons and splash oiling, things get complicated as RPM rises. For high RPM applications, you see vane pumps and screw pumps with intercoolers or even roof-mounted cooling stacks. I think your piston pump may have 20% overhead or less to overdrive it before it eats itself for dinner. Who wouldn't want to sell compressors that out pump the competition, if all it meant was a motor swap or a pulley change? There's probably a reason that nobody is doing it, and I would bet it is the speed limitations of the piston compressor head.

 

[strantor]

That could be made to work. Just remember the difference between an empty startup at 0 PSI and an operating startup at 125 PSI. In the real operating range, your gain is much less than your theoretical from empty startup.

jack vines

I'm planning multiple modes of operation and other goals in mind that I didn't get into at first because I didn't want the thread to get too far off the original question before I had a good understanding.

What has been discussed until now is what I would call start-up mode. Balls to the wall, as fast as the motor can go without self destruction, to attain max system pressure.

One that's happened, we enter "maintain pressure" mode. That's a PID loop where pressure is monitored by an analog sensor rather than a pressure switch with large hysteresis/headband. In this mode, if I connect a low volume air tool which causes tank pressure to drop by only say 5 PSI, the motor will come gradually to life, creeping along at just a few tens of RPM until max pressure is reacquired.

The reason for this, my other goal is noise reduction. My current oilless compressor is taking years off my life, always kicking on violently with no warning in a 90db banshee scream. With the new setup I know it's still going to be loud at startup, maybe even loaderbthan what I have now, but I can take the loud noise at startup because I'll be expecting it. I just turned the switch on, it's going to be loud; but once that's over, I don't have to worry that thing is going explode into action 90 minutes from now and make me spill coffee in my crotch.