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?
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.
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.
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.
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.