Screw? Recip? Vane? Size?
If the compressor is yours and it works well and is sized right, putting a 3 phase motor on it would be quick and cheap.
If you wan't something bigger in a recip and have the money, buy a Quincy QR25 pressure lube on the biggest tank you can get. Other good brands are Champion, Curtis Masterline, Saylor Beall and maybe a Kellogg. Bigger IR units are still pretty decent. Anythign above 25-30hp in a recip is starting to get a bit obnoxiously heavy and vibratory for most shops.
If your are looking at a screw, Palatek makes a decent, simple unit. I believe Saylor Beall and Curtis use the same design. Kaeser's aren't bad either. Quincy is usually a safe bet, but i think a lot of their smaller screws are belt drive. Not bad, but extra maintenance. My preference is a direct drive screw. Make sure you team up any screw with a big air tank to minimize the inlet valve cycling. Atlas Copco screws are high quality but parts and service are expensive. Avoid anything made in Italy, India, or China.
A vane is a simple compressor with a small footprint. They may have a cost advantage over a similarly sized screw and are easy to overhaul. Maintenance is simple and service can usually be done by a trained staffer. Hydrovane and Mattei are the biggest companies, although there is also Davey. Mattei is Italian but have used Hydrovane designs.
If you are looking at a used screw, check the hours. They will usually need a bearing job between 35,000 and 50,000 hours. Vanes will not, since they run plain bearings in the compressor. Figure a separator change out every 8000hrs or so. All will carry over some oil vapour, so invest in some good filtration and change it one a year.
