Compressor questions. Specifics on the screw

[updatedsecurity]

Hello.
I recently got a new machine that requires about 18cfm. I would really like the idea of a screw compressor because its quiet and has a dryer in it. My questions are these; Apparently the screw compressor needs to have a 100 percent duty cycle. The machine when sitting does not really use the full 18 cfm. What does the compressor do when the machine sits idle? Does it just keep cycling? Where does it put the air that is not being used? What happens if the machine is rated for 18cfm and the compressor for 24 is that bad for the compressor?. At the moment I am making do with an old piston that runs about 30 percent of the time. Its loud and the air coming out of it is hotIt has a bunch of dryers and filters on it but the noise and the constant on off cycle is annoying.

I got quotes for an atlas copco for about 9k and one called kaishan for about the same. I am also looking at the KairFree from kaiser which is their compressor leasing thing where you just pay 2.2$ for each hour the compressor produces air and they do all the maintenance and quarantee no down time.

Thank you

 

[EmGo]

I would really like the idea of a screw compressor because its quiet and has a dryer in it.

You're going to get a million replies but I get to be first -- early bird catches the worm, yay

Screw compressors are not necessarily quiet, and they don't as a class have driers built in. The air coming out of any compressor is hot and should be post-processed since machines like dry air.

Other than that, we had a 20 hp sullair, cost $3,000, ran at least 18 hours a day seven days a week in a nasty environment (for a compressor, really humid hot air), and was trouble-free. The oil is expensive and you can't ... no, shouldn't just use whatever you can grab down at walmart, but other than that, was great. Ours lived in a separate little hutch which is a good idea for any compressor, almost all of them are annoying, sound-wise.

You can spend the next month going all anal on this but in the end, I don't think it matters. A recip is probably cheapest, just put it in a separate room. I had an antique Ingersoll-Rand that was water-cooled, that was pretty nice and very simple but they don't grow on trees.

 

[Clive603]

For 18 cfm I'd take a Hydrovane HV04 vane type over a screw every time.

Probably quieter, more tolerant of stop-start duty cycles if need be and known to last for a very long time given regular oil changes. Just remember to warm it up first thing. Min is an open one with a drier / cooler radiator thing on the front which works OK with a decent filter unit on the output. Not a super duper drier but it's an excellent first stage to ease the load on a high end one if you need really dry air.

Built in a box ones are clean installation and quieter than the open ones but reputed to be harder to work on should attention be needed.

Pity they don't do a regulated speed version that small but I imagine a hooking up a VFD with some intelligence wouldn't be hard. Minimum speed is maybe 800 or 900 rpm so could get it down to 10 cfm if less air would do for a while.

Clive

 

[john.k]

Yes ,compressor builders want you to use frankensense and myrrh in their machines .............however ,in 4 x 500hp screws of three good brands ,all we ever used was a R&O 68 steam turbine grade oil.............the problem with compressor oils is oxidation .....the oil forms a yello/brown scale called shellac.(I dont know if it is).........but this is at 100% duty cycle and 110C operating temperature ...........and yes ,compressors do dischatge a lot of water ...........I used to drain several hundred gallons every morning from the big airtank (20ft highx 10ft dia. ).............for some reason auto drains and steam traps dont work ,they just block up and stop discharging.......and next thing you know the paint shop foreman is on your hammer with air hoses spraying water .

 

[john.k]

Incidentally,the new owners replaced the good compressors with two Indian made machines that were output modulated via motor speed controll...........complete disaster ,both hopelessly unreliable ........and the new owners were the agents for the Indian made compressors ,and wouldnt accept reality in any form.