Air compressor duty cycle

[Ike Carlson]

What exactly is it that determines an air compressor's duty cycle?

I have a 5hp Emax 2 stage with a duty cycle of 60% according to the manual.
I called and they said it is because of the rings/valves. Is this a bunch of BS or is there some truth to it. It seems the rings should be fine and the valves would be at operating temp in under a minute.

I am currently rebuilding the compressor because the previous owner left it outside all winter and the bottom end rusted up. I upgraded to a Viton crank seal to have a better temp range and safety margin for heat.i also plan on cleaning up any flash in the head.

It has a cast iron "cooler" between the two stages, but I think this can be improved greatly.

It seems to me that cooling between stages would reduce temps by a significant amount and should increase the duty cycle.

Am I being unreasonable or does this sound right?

I've also played with the idea of making it full pressure lube.....

 

[TRWham]

It's your compressor. If you think you can do better than the people who built it, no one will stop you. You might consider that those valves are attached to a valve plate that is, in turn, attached to a cylinder head so the thermal mass is a little greater than that of a few pieces of sheet metal. Plus the motor might have something to say about it as well.

 

[Ike Carlson]

It's your compressor. If you think you can do better than the people who built it, no one will stop you. You might consider that those valves are attached to a valve plate that is, in turn, attached to a cylinder head so the thermal mass is a little greater than that of a few pieces of sheet metal. Plus the motor might have something to say about it as well.

The motor I'm getting is continuous duty, so that's no problem.

It's a fine compressor, I just think it could be a bit better, that's all.

 

[engineer2]

i believe it has to do with overheating whether the motor or the pump. Carbon buildup can be an issue. If the motor is rated for 100% duty cycle, you are good to go. Pump cooling can be helped with a large computer fan or similar.

 

[Ike Carlson]

i believe it has to do with overheating wither the motor or the pump. Carbon buildup can be an issue. If the motor is rated for 100% duty cycle, you are good to go. Pump cooling can be helped with a large computer fan or similar.

There isn't any carbon that I have seen yet, and the only thing still together is the head/disc valves. The flywheel has a 16" fan built in and I had thought of making a shroud to direct the air right at the compressor.

 

[pstemari]

I'm confused. How would you get carbon buildup in a compressor pump? Overheating and burning oil?

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[Ike Carlson]

I'm confused. How would you get carbon buildup in a compressor pump? Overheating and burning oil?

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That would be my guess, but I've never seen it. Good rings and synthetic oil should be all you need. A compressor would have to get pretty dang hot to make carbon. I would lean more toward dust buildup from a bad air filter.

 

[Ike Carlson]

Ok, quick idea before falling asleep...

If I put a power steering/hydraulic/oil pump on the compressor, I could make it a full pressure lube system by drilling the crank/rods and then feeding oil to the end of the crank away from the flywheel. I could "gear down" the pump to provide the proper flow and call it a day. Heck, I could even have an oil filter.

 

[cvairwerks]

You are overlooking what are the material composition of the rings and valves.....Do you know what they are, and what the thermal characteristics of them are? How about what the cylinder composition and what it's machined specifications are? How do you know that by running the compressor at 100% duty cycle, you are not overheating the rings and increasing the expansion of the rings and thereby increasing the ring pressure on the cylinder wall and either increasing the friction load on the piston/rod assembly or the cylinder wall to the point of overloading or accelerating the wear?

As to making it pressure lubed....get real...go buy a pressure lubed unit instead.

BTW...at 60% duty cycle, the compressor should be running about 36 minutes out of every hour. If you are using enough air so that it's consistantly starting more often than a minute after shut down, you probably need a bigger compressor.

 

[matt_i]

Ok, quick idea before falling asleep...

If I put a power steering/hydraulic/oil pump on the compressor, I could make it a full pressure lube system by drilling the crank/rods and then feeding oil to the end of the crank away from the flywheel. I could "gear down" the pump to provide the proper flow and call it a day. Heck, I could even have an oil filter.

I like your concept but I think its going to be a ton of work. Also by drilling the crankshaft you weaken it, it wasn't designed nor validated for the lost cross-section plus the possible stress risers in the journal. But if you have the tools and the time, you'd have an awesome compressor!

There was a Quincy 325 sitting at HGR last week for ~$900 negotiable that seemed just fine.

 

[sberry]

Some are listed for 100%with synthetic oil. Just run it. Fixxi g it better likely leads to problems, they worked a long time before you came along.

 

[Showkey]

Duty cycle on compressor is one of the things that nobody really follows in daily use.

Also when you look at some specifications some better oil less models are actually 100% duty cycle.

 

[cstmg8]

What am I missing here? Duty cycle is the amount of air usage a compressor can keep up with if the tank is depleted, correct? If you're spray gun needs 13 scfm at 90 PSI, and the tank is used up, a rated compressor can run and keep it going at pressure/flow. This is the whole reason I upgraded to a big enough compressor, with special attention to duty cycle rating.


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[Ike Carlson]

I think I got a bit "tinker happy" before bed last night. I'm not going to drill the crank, but I could put a small pump and cooler on it to cool the oil if running temps are high.

 

[rlitman]

What am I missing here? Duty cycle is the amount of air usage a compressor can keep up with if the tank is depleted, correct? ...

No, that is CFM.

Duty Cycle is the percentage of time that your pump is designed to be running. At 100% duty cycle, the pump can run continuously. Less than that, and it is designed to expect some off time to cool down each time it runs.

 

[BarryWells]

i believe it has to do with overheating wither the motor or the pump. Carbon buildup can be an issue. If the motor is rated for 100% duty cycle, you are good to go. Pump cooling can be helped with a large computer fan or similar.

What carbon ? There's no combustion here ?

 

[BarryWells]

I have a big Puma and an Atlas and both are 100% duty cycle. The Puma I had to push the reset button a couple times on real hot days so I hung an 18" Blaster fan beside them. No more problems.They're shoved together in a corner so the ventilation *****. Keep your rig where it gets good flow or make it happen somehow.

 

[engineer2]

What carbon ? There's no combustion here ?

Paraphrased from an SAE paper on the subject:
Passing oil into the air compressor head is a normal byproduct of the piston, ring and bore lubrication process. The oil in the discharge air is oxidized to form carbon deposits if there is sufficient heat.

Oil vapor + 150psi air + 225°F (max) = gradual carbon deposits

I had an old 1970's Craftsman compressor that I used to sandblast an automobile frame. I took the head off afterwards to clean all the carbon deposits out from around the reed valves. Nowadays synthetic oil would probably minimize the issue.

 

[seber]

If you need 100% duty cycle, your compressor is too small. If you are using air at a steady rate then you should buy a scroll compressor instead.

 

[Marctrees]

I may be wrong... but when I occasionally blast extended hours w my 5 hp I have a big *** fan blowing directly on both the motor and comp and don't worry about DC.

That fan makes a huge effect removing heat.

If I was doing that everyday, then would get suitably sized comp.

Marc

 

[American Locomotive]

What carbon ? There's no combustion here ?

All piston compressors push some oil past the rings. Compressors also run hot, so some of the oil that gets into the cylinder head area will carbonize.

 

[finn]

Duty Cycle is meaningless without a design target run time. We used to qualify engines on, among other tests, a 62% load factor (duty cycle) for 2000 hours on the Dino. Load fact of 62% meant that the engine cycle was designed to burn 62% of full load, rated speed fuel rate. In other words, if the engine burned 100 lb/hr of diesel fuel at rated speed and load, it would burn fuel at a rate of 62 lb/ hr over a 20 minute test cycle.

That cycle was the equivalent of a B10 life of 500000 miles in a medium duty truck application. (90% of the trucks would run 500k miles without pulling the head or pan, as long as application guidelines were met). Typical in service fuel consumption was probably less than 20 lb / hr, from memory.

A compressor would probably have similar design criteria. The highest valve, piston, ring, and bearing wear will occur at or near the pressure cutout setting, where temperatures and pressures are highest, but total run time, ie design life, is part of the equation too. % run time / hour in service is a convenient way to translate life to the consumer.