What limits the speed of an air compressor?

[brino]

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.

An admirable goal........well, at least for my crotch.....

This is a very interesting thread and I will be following along closely.

Thanks @strantor for the original question and everyone for their posts!

-brino

 

[JimDawson]

One thing that has not been discussed here is scroll type air compressors. They are typically much quieter than either recip or rotary screw. Might also be adaptable to variable speed, not sure about that. Eastwood is one manufacturer. https://www.eastwood.com/eastwood-elite-qst-30-60-scroll-air-compressor.html I have wondered if it possible to salvage the scroll compressor out of a heat pump unit and make an air compressor out of that. I think most of them are in the 10HP range for an average size home unit. Some of the newer ones are variable speed.

One thing that would help your situation immensely would be adding extra storage capacity. I have about 260 gallons of air storage (160 gal before the air drier and 100 gal after) and after the initial start up, mine only kicks on occasionally even with the differential set at 5 PSI. I have the max pressure set to 125 PSI even though the system is rated at 175 PSI. Normally the compressor runs about 3 minutes or so to top up the system even with moderately heavy air usage. I am running my 5 HP compressor with a VFD, and I use a 10 second ramp up to speed with coast to stop set. I have thought about adding analog controls, just been too lazy to do it.

 

[strantor]

One thing that has not been discussed here is scroll type air compressors. They are typically much quieter than either recip or rotary screw. Might also be adaptable to variable speed, not sure about that. Eastwood is one manufacturer. https://www.eastwood.com/eastwood-elite-qst-30-60-scroll-air-compressor.html I have wondered if it possible to salvage the scroll compressor out of a heat pump unit and make an air compressor out of that. I think most of them are in the 10HP range for an average size home unit. Some of the newer ones are variable speed.

One thing that would help your situation immensely would be adding extra storage capacity. I have about 260 gallons of air storage (160 gal before the air drier and 100 gal after) and after the initial start up, mine only kicks on occasionally even with the differential set at 5 PSI. I have the max pressure set to 125 PSI even though the system is rated at 175 PSI. Normally the compressor runs about 3 minutes or so to top up the system even with moderately heavy air usage. I am running my 5 HP compressor with a VFD, and I use a 10 second ramp up to speed with coast to stop set. I have thought about adding analog controls, just been too lazy to do it.

I have thought about scroll compressors. I've long lusted after that beautiful beast Eastwood sells. I recently had my entire home HVAC system replaced and I told them to leave me the 2.5ton condenser. I had every intention to make that my new shop compressor. It's a quiet unit with a huge cooler to boot. But the more I researched the more I was turned off to the idea. Refrigeration systems circulate oil in addition to freon. The compressor needs oil going through it just like a screw compressor does. In order to do it properly I would need to design an oil separator and oil cooling/recirculation system. You can find example of guys just taking the scroll units out and slapping them directly onto an air tank, and it works, but I suspect not for long. I could find absolutely no data on how long they last when used in this manner.

 

[Buffalo21]

We sell and service industrial boiler room equipment, we are constantly bombarded by power utilities to install VFDs on every 3 phase electric motor possible, sometimes its practical, sometimes it is not. We install VFDs on the boiler blower motors, that maybe save $500/yr in electricity, but the approved control system to run the VFDs, can cost anywhere between $35,000 to $85,000, depending on complexity and other control issues.

They constantly ask about the air compressor modules, that are sourced from either Quincy, I-R and Atlas-Capco, all of them say the same thing. The operating speeds of the modules, is optimized to maximum performance, required air flow rates and long term reliability, they say its a balancing act to get the best of all 3 requirements at the same time. Altering the motor speed gains you nothing. We install them on the modules all the time, but they are locked in at 60 Hz, all they really gain, with the installation, is a soft start for the motor.

 

[strantor]

We sell and service industrial boiler room equipment, we are constantly bombarded by power utilities to install VFDs on every 3 phase electric motor possible, sometimes its practical, sometimes it is not. We install VFDs on the boiler blower motors, that maybe save $500/yr in electricity, but the approved control system to run the VFDs, can cost anywhere between $35,000 to $85,000, depending on complexity and other control issues.

They constantly ask about the air compressor modules, that are sourced from either Quincy, I-R and Atlas-Capco, all of them say the same thing. The operating speeds of the modules, is optimized to maximum performance, required air flow rates and long term reliability, they say its a balancing act to get the best of all 3 requirements at the same time. Altering the motor speed gains you nothing. We install them on the modules all the time, but they are locked in at 60 Hz, all they really gain, with the installation, is a soft start for the motor.

I'm not out to copy Quincy, Atlas, or anyone else. What I've described here is, as far as I know, not done by anyone. I'm not locking anything in at 60Hz. This not a soft start, and power factor correction is not among the design goals.

 

[Superburban]

One should also be cautious with lowering the speed of the compressor. Splash lubricated compressors may not get enough lubrication. Also, the pulleys with a fan built in may not flow enough air at slower speeds.

Otherwise, I like the idea of a bigger head unit, then slowing it down as the pressure increases. The air flow could easily be fixed by a separate fan. If you are looking at heavy usage, you could run a separate external oil pump, with jets to cool the bottom of the pistons, and even an external cooling system for the oil. Both with temps monitored as part of the main motor control system.

Is it overkill for a air compressor system? is that not our way of doing things?

 

[strantor]

Ok, allow me to steer this out of theory zone and into action. Disregard all previous talk of 240Hz and overspeeding the compressor head by 4x its rated RPM.

I've chosen a 3hp motor from among my cache and just picked up a 5hp compressor head from HF. As you can see on the motor nameplate, it's a 1760RPM/60Hz motor motor with a stated maximum safe speed of 3600RPM. So I should safely be able to run it up to 123Hz. But I'm getting a strong "I live life 10% over the edge" vibe from this motor so let's say 135Hz/3,960RPM is my max.

My compressor head is 1050RPM max, 650RPM min (for cooling I guess?), 17.3 CFM @ 40 PSI, 15.2 CFM @ 90 PSI, and has a 14.5" flywheel. Since the compressor overspeed condition only happens at the beginning of charging an empty tank, I feel comfortable overpseeding it by ... uh.. (arbitrarily) 25%. Unlike the motor which stands a real chance of centrifugal forces pulling itself apart, I'm not too worried about the compressor failing with an occasional 25% overspeed condition. So call it 1313RPM max.

So... (1313RPM × 14.5")/3960RPM = 4.8" motor pulley diameter. They didn't have a pulley that size at Tractor Supply so I got a 4.5" pulley. Looks like with a 4.5" motor pulley at 3960RPM I'll only be able to overspeed the compressor by 17% (1229RPM). Shame. Anyway, still looking good.

In the beginning before any pressure builds, I think the volume is a direct function of cylinder displacement. So with a 3.14" bore, 2.36" stroke, and two cylinders (36.52C.I.D.), at 1229 RPM, I should be moving 44,883 cubic inches/min (26CFM) from zero PSI. By the time 40PSI is reached, I estimate we should still be below the 3HP threshold and therefore still running 17% excess compressor RPM and therefore the rated 17.3CFM becomes 20.2CFM.

Somewhere between 40PSI and 90PSI, I estimate, is where we max out the 3HP motor's power and RPM starts to drop off. Wherever that point is (I haven't done the math on that yet, I think I'll just discover it empirically) this turns into a 3HP compressor. But it's still a 3HP compressor spinning way above 1760 RPM and moving a lot more air, a lot sooner in the process, than a normal 3HP compressor. Right at the point before cut-off, is when the speed finally is pulled down to the 1760 nameplate RPM, and when it finally becomes a 3HP compressor, moving the same amount of air as a standard 3HP compressor; it's just a 3HP compressor geared down to turn a 5HP head. And then it shuts off. So it only spends a few seconds being the good little boy that the nameplate advertised. All the rest of time it's being a very, very bad boy.

All that sound more reasonable than the wacky numbers I spouted in the OP? Anyone wanna make a prediction about operation? Be careful now; if you opt to say nay, realize that we are no longer in theory mode and you might be eating crow in short order.

 

[projectnut]

Not all compressors are created equal so to speak. Kellogg for instance at one time made 6 different models. However they came in 70 different cfm and pressure ratings.

As an example the popular model 321 dual stage series compressors came in 16 different sizes and used motors ranging in size from 1 hp to 2 hp. They had pressure ranges from 100 psi to 250 psi, compressor speeds from 300 to 823 rpm, and piston displacements from 4.32 cu ins to 11.9 cu ins. The basic rule of thumb is that the lower the pressure rating, the lower the piston displacement, and the higher the speed. Conversely the higher the pressure rating the higher the piston displacement, and the lower the speed.

 

[JimDawson]

Just a thought here, you may be able to use the current limit on the VFD to control the speed at least in the initial phase for pump up. Typically as you exceed the motor base speed (1760 in this case) the current goes down as the RPM increases. Below the base speed the current goes up as the motor slows down. This may only happen when operating in sensorless vector mode, because the VFD is trying to produce constant torque below the base speed, and constant HP above the base speed.

Not sure is this would be useful and I'm not sure if current limiting would actually slow the motor down or just trip out the VFD on overcurrent. It might depend on the available settings on the VFD. The theory is, as the load on the motor increases as the pressure comes up, the VFD starts backing down the speed to keep the output current at the max set point.

 

[strantor]

One should also be cautious with lowering the speed of the compressor. Splash lubricated compressors may not get enough lubrication. Also, the pulleys with a fan built in may not flow enough air at slower speeds.

Otherwise, I like the idea of a bigger head unit, then slowing it down as the pressure increases. The air flow could easily be fixed by a separate fan. If you are looking at heavy usage, you could run a separate external oil pump, with jets to cool the bottom of the pistons, and even an external cooling system for the oil. Both with temps monitored as part of the main motor control system.

Is it overkill for a air compressor system? is that not our way of doing things?

My thoughts exactly. If it's worth doing, it's worth overdoing. And in some cases it's worth overdoing even if it isn't worth doing at all. But I think this is worth doing.

I will keep the lube system mods in mind but (keep in mind I have no idea what I'm talking about) at only 25% intermittent/rare overspeed I don't think they will be necessary.

As for separate cooling fans and monitoring temps, absolutely. A yet unmentioned (maybe obvious) piece of this puzzle is a PLC. PLC will establish the operating modes and run PID. PLC will monitor temps, pressure, maybe flow, I don't know what all else yet. I don't want to get too deep into the weeds until I at least get it moving air. But with quiet in mind, I think I will house this in a sound dampening cabinet with muffled cooling fans drawing in outside cooling air which should allow me to run below the 650 minimum rated compressor RPM once in "maintain pressure" mode. ...but wait, you implied the oil might be the issue at low RPMs. Why is that? Can you expound? Does "splash lubricated" mean that the crank goes down and slaps the surface of the oil bath, splashing it back up onto the pistons/cylinders, and that's why the low RPM might be a problem? If so, then I see where you are coming from. I will give this more thought, pending clarification. Thanks for the input.