Air Compressor Questions

I am looking at replacing my old shop air compressor and have been looking at the rotary screw type compressors because they are quiet.
They are a lot more money than a piston type compressor. The smallest ones I have seen are 5HP which would be about the right size for what I want.
Does anybody whether they output clean air or is there a lot of oil to be removed?
Would I need to run a big filter and refrigerated dryer to paint or use a plasma cutter?
Thanks, BobH
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That's what we run at work, and there is no oil in the air. Course, these are commercial units (3x 400 HP + 1x 250HP units). As for the dryer, any compressor needs one to get the moisture out for painting, etc.
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Anthony

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Anthony wrote:

I dunno , we run a rotary screw type at the cabinet shop and I get a good bit of oil . And water , but that's another issue entirely . OTOH , I run a piston type at home , and have no problem at all with oil . I'm not runnin' anywhere near the same volume though . And in fairness , the screw comp at work was purchased used , and I kinda suspect we got took ...
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The 250 runs our gauge and measuring equipment supply air, which is super- clean air. Not sure what type of conditioning it goes through after the compressor though. I haven't been in the compressor room very often, it's in a separate building that I have little reason, or time to visit.
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Anthony

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BobH writes:

From the compressor, compressed air is saturated with water, and the only way to get rid of it is with a refrigerated dryer. Undried air for painting is risky at best.
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Richard J Kinch wrote:

Well, not the only way, but probably the most economical way on this relativley small scale...
Paul
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Paul writes:

What other way? Dessicants aren't feasible for volume air.
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Richard J Kinch wrote:

Well, actually that's a common method for really large volumes in industrial settings. Achieving <ppm moisture levels is routine.
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Paul writes:

After a refrigerated dryer, right?
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Richard J Kinch wrote:

No refrigeration at all. This method employes dryer beds in parrallel, one in service and one regenerating, connected via three way valves. After some time interval, less than the time of moisture breakthrough for the bed, the three way valves switch, lining up the in service bed to regen and the newly regened bed to in service. The regening bed is then vented to atmosphere and a relatively smaller flow of dried air is purged through the bed. The reduced pressure releases the trapped moisture in the dessicant and to vent (PSA or pressure swing absorption). This is all done with timers or PLC's, with moisture measurement instruments to monitor the air dew points.
The primary driers (one on each compressor) give plant air with -40 to -60 degree F or better dew points. For instrument air some of the dried air goes to another set of drier beds giving dew points of -150 deg F or lower. If I remember correctly -164 deg F is no moisture and its not uncommon to see dew points just about that low. I've got a table at work relating ppm H2O to dew points that would be more precise.
Now these are pretty big compressors, 1000 to 1500 HP, and corresponding large volumes of air, but I'll bet that this process would scale down with no problems.
Just where the economics curves for refrigeration vs desiccant dryers cross is unknown to me.
Regards Paul
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You will need both oil filter as well as air dryer, for your application. It is not a very big deal to get that.
i
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Unless your going to turn it on and run it for hours straight you will not be happy with one. They are made to run, most are 7X24 type machines. I was going to get one before I looked into one. For a plant or high volume user they are GREAT. For a home shop they are not the ideal setup. They don't normally cycle on an off they run and unload so they stay running the whole time you need air and then some. The GD model I was looking at was a super nice unit but it's MIN run time was 15 min. ( so if it turned on 4 times under light use it would run for a hour) and it was really designed to be turned on that the start of the shift and off at the end..
William....
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wrote:

I agree with some of the previous posters. Rotary units are typically made for continuous operation, with a loading/unloading control loop based on the pressure settings. I don't know if your situation involves a separate enclosed room where the compressor is located, but if you have that situation, the air dryer should not be in an environment where the temperature is really elevated. They don't work very well when the ambient temp is up in the 100+ F range. As a note to Snag -- If you are getting oil and water in the distribution lines, there MAY be an issue with one of the control/valve components on the compressor. I had that experience a couple of times with a 25HP Quincy, where the valve assembly got stuck, and proceeded to allow significant oil into the air lines. Not pretty, especially in a cleanroom environment.... Otherwise, they work very well, and the air is pretty clean. There is no substitute for point-of-use coalescing filtration, though, and in-line adsorbers that change color when they start getting contaminated.
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BobH wrote:

Thanks for the information Everybody!
The main reason I was looking at the screw type compressor was for the quiet operation. The compressor lives in the shop with me and I want something as quiet as reasonably possible. Putting it outside or in a separate room are not really options for me. Currently, I use an old Speedair that is so noisy that I almost never use it. The oil-less ones are worse. I need something to keep up with a small bead blast cabinet, but if I had a compressor that did not chase me out of the shop, I would use compressed air a lot more.
Are low RPM compressors available as small as 5 HP?
Thanks, BobH
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For quiet operation, get a better, slower compressor. The 10 HP Quincy that I resold recently, was unbelievably quiet.
i
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BobH wrote: Thanks for the information Everybody!

I've never heard a rotary screw that was truly "quiet". They make a totally different sound, more like a hum than a chug-chug, but it really isn't that quiet. They do put sound-absorbing huts around them to reduce the hum. I have a small Quincy, and it was pretty quiet with a 1 Hp motor on it at 450 RPM, but when I upped the motor to the design rating of 2 Hp, and changed the pulley for 915 RPM, it got surprisingly QUIETER! I think the intake valves were rattling at the lower speed. This is right in the shop with me, and with no acoustic cover or intake muffler. I think you could cut the noise quite a bit with a very simple muffler on the intake, just like a coffee can with some holes in it or something, but I haven't bothered.
I do wear an ear protector when I will be in the shop with it, but I usually wear that anyway when working with the machines. If I put a sound damping "house" over it and an intake muffler, I think it would so quiet I wouldn't need the protectors.
Jon
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I wonder if a screw compressor is a good choice? My understanding is a screw compressor runs constantly, and "unloads" when the air is not needed. Unless you use allot of air continually is a screw compressor a good choice? I think Iggy may have a better choice, a good quality, 5-10 HP slow turning, cast iron, two stage compressor may be a better choice. Greg
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I do not have anything currently, but that's what I would buy. A slow reciprocal compressor. The Quincy that I had, was so quiet that it felt a little unreal.
i
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Remoting the inlet filters will also help dampen the noise. We did that with a 25Hp Quincy and it really helped reduce the noise level, even when the unloaders kicked in. At that point it was easy to hold a conversation standing right next to it.
Craig C.
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