I cover several of these 2 stage, oil free compressors and am wondering what shaft speeds are typical and what range the gas pulse frequencies will occur.

I don't have any nameplate data, so I can't be any more specific other than to say they are 2 stage with about 150 hp motors.

Thanks,

Danny

Danny, are they screw type compressors?

Hey Pete,

Yes, they are 2 stage oil free screw compressors.

Danny

Without the nameplate data you have difficulties, but the following method may help. First identify the largest amplitude harmonics in your velocity spectra. Chances are that these will be multiples of your lobe pass (or meshing) frequency. All the compressors that I've worked on have 4 lobes on the male rotor and six on the female rotor. So divide the lobe pass frequency by these numbers to get the shaft speeds. If the calculated frequencies match actual peaks in your spectra, then you're probably on to a winner. This method works on the compressors that I check and has been verified by calculating out the known data from the manufacturer. Of course if your compressors have a different design, it may not work. I've even been able to identify the gearmesh frequency on some compressors by looking for a high frequency peak with known sidebands.

Al,

Thanks for the input.

I regularly use the technique you describe on single stage compressors, (it also works well with gearboxes) but I think the oil free 2-stage must operate at significantly higher speeds than the more common single stage like the IR SSR or the Atlas Copco GA serires.

I have identified two frequencies as being likely candidates. They are at 69,600 cpm and 104,400 cpm. That ratio would seem to imply a 4-6 lobe combination but at 17,400 rpm it is much higher than what I normally see. Maybe it is a harmonic, but I havn't been able to find anything that I could consider running speed in the spectrum.

If I don't get the nameplate data from my client soon, I'll get it next month when I go to the site again. In the maen time, I'm going to ask this question to the folks at IR in Davidson, NC and see if they can give me some general idea.

Thanks again,

Danny