Dividing the compression cycle into four stages results in less heat generated per cylinder, that is if all other conditions are approximately equal. That is theory. However, these other conditions such as the end pressure, interstage cooling, ambient temperature and type of oil can make a crucial difference. Generally, three or four stage compressors can be used for pumping NITROX unless the manufacturer has a valid objection, not including insurance woes, warrantee fudge factor, or liability concerns. As with air compression, it is not a good idea to operate the compressor for extended periods in 100F ambient temps, and particularly not at sustained pressures above 3500 psi. Nitrox fills should be planned for the coolest part of the day. The oil should have a high flash point, 490F or higher. Try using a food grade oil such as NUVAIR 455. Food grade (PAO) oils probably hold up better in EAN service as suggested by lab tests. Try not to exceed 36% EAN. Under no circumstances exceed 40%. Given this, either type of compressor, in good condition, sold by a reputable manufacturer, should give satisfactory service.
