Running compressor "too slowly"


I _may_ have an opportunity to pick up a compressor a bit "too large"
for my needs at reasonable cost (depends on the bidding).
GRANTED (wanna bet some people ignore this part) that doing so will
provide less air than the compressor is rated for, and GRANTED that I
don't mind that, what actual problems would be encountered by running a
5 or 10 hp compressor on a 2 or 3 hp motor AT REDUCED COMPRESSOR SPEED
(larger pulley on the compressor, or a jackshaft) such that the 2 or 3
hp motor is not being overloaded.
In engine service, lower speed is a good thing, so 3600 rpm motors wear
out much faster than 1800 rpm motors.
What, other than reduced air output that I'm willing to accept, are the
actual downsides, if any, of running a compressor slower than it's
"supposed" or rated to run?
Lack of fan action from the pulley is easily compensated by putting a
fan on it, and heat production will be lower at a lower speed anyway.
Not enough "splashing" for splash lubrication? Something less obvious I
can't think of?
While I could always harvest the large tank and try to flip the big
motor and compressor for a smaller one, the less stuff I'm trying to
swap, the better, generally.
Reply to
Ecnerwal
Loading thread data ...
Without getting into mechanical calculations, your premise sounds like it should work ok. I questioned the logic until I came to the part about "larger pulley on the compressor, or a jackshaft" in order to avoid overloading a motor of lesser hp. Others, better versed in compressor design, may have different ideas but my "knee jerk" is that you will be ok.
Bob Swinney
Reply to
Robert Swinney
The big down side is the slower motor will run forever to pump up to the same pressure the faster motor gives.
Paul
Reply to
co_farmer
At a speed below minimum, lubrication systems of compressors may not work properly. Splash lubricated compressor have little spoons that "splash" the oil around, and at low speed they would not be able to fling oil far enough. I am not sure about pressure lubed compressors, but they too have a minimum speed.
You can call the manufacturer and find out.
i
Reply to
Ignoramus20464
Should have the same starting torque with a lower HP motor turning the compressor at a slower speed.
Dan
Reply to
dcaster
I don't know if you're concerned about efficency but the larger compressor will have more friction loss than outputting the same CFM's with a smaller compressor because of larger piston contact area, larger bearing area ETC. Also there is more power loss compressing air into the inevitally larger space above the piston that must be pressurized before the outlet check opens.
Engineman
Reply to
engineman
Should have the same starting torque with a lower HP motor turning the compressor at a slower speed.
Dan *************************
And, the head will be unloaded...I assume.
Reply to
Buerste
Is it splash lubed or oil pumped? Bigger pumps usually have an oil pump. I run my Quincys way slower than max, they will last forever. I think you will not only NOT have a problem, you will LIKE the way it runs quiet and cool.
Reply to
Buerste
But that's exactly what you'd get with a smaller compressor run at a higher speed.
Reply to
Tim Wescott
Now HOW do you know they'd last any longer? A Quincy will last forever anyway.
But, I think the OP has a good idea. gear it down to the smaller motor.
Karl
Reply to
Karl Townsend
Not many motors or compressors like starting under load.
I've known of people (like me) to use refrigerator compressors as air compressors. There are a few technical things, like change the oil. A quiet running refrigerator compressor works nicely, if you start it with zero back pressure.
A compressor out of a HVAC unit might work, also.
Reply to
Stormin Mormon
It's likely that the manufacturer offered different models with the same pump on them, but with different HP motors (and outputs).
A pump that's used on a 5HP model may be the same one used on a 7.5HP model, with the 7.5HP model having a slighly larger motor pulley diameter.
A pump used on a 10HP model probably isn't the same one used on a 5HP model.
I will probably be more difficult to find a larger pump pulley, than to find a smaller motor pulley. A motor pulley that's too small could have a tendancy to slip at startup if the belt contact area with the pulley is too small.
The pump manufacturer may provide minimum and maximum pump speed recommendations. Proper lubrication is probably the only vital issue with reducing the pump speed.
Reducing the upper pressure limit cut-out is another option, in conjunction with a slower pump speed. The upper end of the cut-out limit is where the highest power demand is involved. Reducing an original upper limit of 175psi to 150psi, for example.
Reply to
Wild_Bill
One more issue is that if the pump does not get hot sometimes, then there could be condensation in oil (water in oil, milky oil).
i
Reply to
Ignoramus20464
I certainly wouldn't want to run a splash lube compressor too slow. I pressure lube one shouldn't have a problem at any speed.
Reply to
Pete C.
I'm running a compressor underpowered but at specified speed.
I bought an old Atlas Copco 3-stage, 400PSI compressor with a 10 HP 3450 RPM motor. I put a 3 HP 3450 RPM motor (previously on a seawater pump) on it. Connected to an 80 gal. 600 PSI-rated Halon tank, I get 150 PSI of air okay. Sluggish starting.
Upgrade: Replaced the 3HP with a 5HP 3450 RPM motor from a large fan. Somewhat better starting.
PROBLEM: Starting torque is okay with either motor in warm weather.
For either motor, at winter temps (say, 20F to 0F or lower) I have to turn a propane salamander on the compressor for maybe 15 minutes in order to start it. Once started, and with medium air use, its own heat keeps it warm enough to restart in the cold.
Bonus question: Anybody have a manual, exploded parts diagram or parts list for an Atlas Copco model "KT 630 A1"? (The company doesn't admit to ever having made this old (50 yrs?) model.) Email me if you do.
Reply to
Mike Spencer
Does it have unloaders? If not, perhaps electrically unloading it would help to start it. Then again, gearing it down might also help - particularly if it already has unloaders.
Sketch of an electric unloader:
Compressor out======+======checkvalve====tank===piping to shop==== | | | | Electrically operated valve to air
Set up where the valve is operated by a time delay relay such that the valve is opened for the first 10-15 seconds each time the motor starts.
Reply to
Ecnerwal
Yes. The unloader works fine. It's just a very clunky old compressor. Three pistons and crank with cold oil. Probably *should* have a special motor designed for prolonged starting load.
I use it so little in very cold weather that it's not worth the cost of the pulley or the trouble or the reduced pump-up time in warm weather.
The object of the post was to offer a possibly useful data point for the OP who was thinking of gearing down in a similar situation, i.e. that I get by without reducing speed except for extra bother in cold weather.
Reply to
Mike Spencer
I am the OP, so that's fine and appreciated. Of course, you're also using a 400 psi compressor at considerably lower pressure, so that probably helps to keep your load down from the original design load. I won't really know until I get to inspect things exactly what I'm looking at, but I would guess they are typical shop air compressors aimed at 150-175 psi, which I probably can turn down to 110-120 for my purposes just fine and gain some that way, but I suspect I'll still need to slow whichever one I get (if any - never can tell with auctions, and I'm not going into a bidding frenzy) down some to run it on a motor I can actually run without ticking off my electric company (rural line, single phase, motor HP limited).
I used to have to throw a blanket over my lathe to keep the heat in and put a lightbulb under it to get it to start in the winter, since it was in an unheated shed. I wonder if there's synthetic compressor oil, and if so, if that would help any.
Reply to
Ecnerwal
On Feb 7, 6:01=A0am, Ecnerwal I wonder if there's synthetic compressor oil, and
There is synthetic compressor oil. Should help. WW Grainger lists some in their catalog.
Dan
Reply to
dcaster
In 35 years, I have had to replace 2 wrist pins, 1 HP piston, numerous valve bodies, a bunch of valve disks, 3 sets of rings, various oil pump rebuilds and a boatload of gaskets. But, that's on four pumps and mostly due to the "Run em till they die" maintenance program. Now, we actually keep them clean so they cool better, run them slower, change the oil using synthetic and keep up on the leaks.
Don't forget I need a ship-to address if you want some 9mm cast.
Reply to
Buerste

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.