Rotary Screw Compressors for Intermittent Use?

Rotary screw compressors are best for high volume continuous operation. What about situations with more stopping and starting?October 13, 2012

Question
I've come across a small rotary screw compressor with 5000 hours on it for what seems to be a fair price. It has a 40 gallon tank and doesn't have any drier or aftercooler.

I was looking to upgrade to a 10hp, two stage piston compressor for the (almost sole) purpose of running two Dynabrade sanders. We also need air for non-continuous small air tools, finish work, and our wide belt, but the guys doing this stuff would also be running the air sanders so not everything will be happening at once. Also, we'd probably be looking at just running the sanders for 3-4 hours a day and only 2-3 days. This is all initially, as six-nine months from now we'd plan on running them pretty much all day, every day.

How harmful would it be to start/stop the rotary screw for our limited operation in the beginning? Would it be better to switch to a smaller tank so it would be forced to run more often? Is there anything else to consider in switching to a rotary screw?

Forum Responses
(Cabinetmaking Forum)
From contributor J:
Just keep in mind, rotary screw maintenance is very expensive. 900 for a fluid and filter change. Other than that, great. Aftercooler and drier is a big deal. Can't speak to the frequent cycling on/off issue.


From contributor L:
Screw compressors work best for continuous air use. They typically don't pressurize the tank above 125 pounds, so they may cycle more often. The kind of controls are important and most likely on a very small screw they will have the cheapest scheme. If it has the next level up in controls, it will go to "unload" when pressure is reached, then after an adjustable timer set point is reached, the motor will shut off. If the air pressure drops before shut off, the unload valve will close and the tank will be supplied again. If the motor shuts off you can have the high amperage draw of motor starting and the heat buildup in the motor caused by too much cycling. The best schemes employ some sort of variable speed drive (expensive).

Starting/stopping sequence needs to be followed or you will get oil carry over. On larger compressors the screws end up using less electricity. They will run a long time between overhauls (for the well made ones.) Synthetic compressor oil is expensive. So are separator elements. Then there are oil filters and air filters.

The complete system includes an air to air cooler, refrigerated drier, coalescing filter, condensate filter and regulator.

If you have a good recip compressor now, you might consider adding a second one and an alternator so they trade load.


From the original questioner:
Can anybody comment on the starting and stopping frequency? This is my primary concern as we just don't need air running all of the time and I want to make sure this type of compressor will service our needs (without our non-continuous needs damaging the machine).

The deal is $1000 for a 20 year old machine. Only known problem is it tends to shut down after getting overheated (about an hour's worth of work). This seems to be a new problem as the owner has only ever used it as a backup to their main compressor and this was only discovered as the main compressor was being serviced and offline. The owner seemed to think it was a thermostat that could be replaced but his boss is only interested in selling it. I plan to call Sullair about it on Monday to see what they say about the overheating.

I have an air dryer that needs to be repaired and will handle the increased CFM. I'll need to purchase a filter. How often does the oil need to be changed? Owner said this was done about a year ago and the machine has only been run as a backup.


From contributor L:
Oil change intervals vary by manufacturer of compressor and by the type of oil used. There are several different oil bases and some of them are not compatible. 4000 hours is probably a safe time. Top of the line compressors with full synthetic oils can sometimes go to 12,000 hours. So this compressor, for your use, probably won't need an oil change for quite a while. Note the hour meter reading when you service it and ask for the reading when the current owner serviced it and if the separator was changed out. Sullair should be a good compressor. They should be able to help determine the overheating problem easily. I don't think you will hurt it with intermittent use.

From contributor Y:
I have a two year old 7.5 hp Sullair Shoptek. Great little machine. It unloads and runs for about 12 minutes when the 80 gallon tank is full (150#). I have control of this setpoint as well as the tank pressure (I regulate my manifold pressure and have a pretty big swing in the tank pressure which minimizes machine cycles on light air days), but a 20 year old machine... I doubt you have as much control. The cycle frequency is about 5-6 starts an hour. Motor overheating is the concern.

I would get the overheating thing figured out before you buy it or else you are likely just to have an expensive project. Sullair recommends the oil changed about every year or so, no matter what (if I recall correctly). Can't be beat (rotary screw) for regular air usage. It runs non-stop when we are machining parts on our CNC. The rest of the time it cycles occasionally for finishing, staple guns, etc. I wanted the quietness this unit offered as well as the integral dryer. It is expensive air for low volume (very sporadic) use; high volume (continuous, and closer to its max output the better), I think it is hard to beat.

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