There are several types of oil-less air compressors available on the market, and the selection of a compressor will usually be determined by the application’s volume and pressure requirements, and in some cases, how and where the air will be used. The industry has changed over the past 50 years, and going back to times prior to the 1970’s, most oil-less air applications for standard pressure (up to 125 PSIG) were serviced by reciprocating air compressors. For large industrial applications, these compressors were available in the single stage (for pressures to approximately 100 PSIG) and 2-stage machines (pressures above 100 PSIG). For high volume applications, these machines were usually massive, with cast iron frames, and were water cooled by way of cooling jackets in the compression cylinders and heads. Most of these machines were double-acting (compressed air on both the upstroke and down-stroke of the piston) using a cross-head design. Also available for much larger volumes (approximately 1500 CFM and up), were multi-stage centrifugal compressors. These compressors were usually 3 stages or more using centrifugal impellers, with the final stage turning at extremely high speeds (up to 60,000 RPM). This required sophisticated precision bearings, and the machines were usually monitored for vibration to determine bearing integrity.
In the healthcare sector (hospital breathing air) during this time period, liquid ring compressors were the standard for smaller air volumes. These machines were available in the single stage and 2-stage versions and were mechanically inefficient by comparison to reciprocating machines. Healthcare regulatory standards and installation and maintenance requirements did not permit the use of reciprocating compressors for hospital applications throughout that period. As rotary screw compressors evolved late in the century, dry versions of the machines provided a more compact and cost effective method for producing oil-less compressed air in larger volumes. The sophisticated screw profiles and close tolerances used in these compressors made them cost effective for the most part in mid-size to larger machines only. Today’s oil-less screw compressors may have dry screw compression chambers or water-injected versions which enhance the sealing and cooling in the air end.
In most of the methods noted above, water-cooling is still a requirement. As water has become a valuable and limited resource in recent years, many municipalities place restrictions on water consumption/usage. Many industrial applications for larger volumes of compressed air, therefore, require additional equipment for closed loop coolant systems which may include cooling towers or refrigerant coolers. In many of these larger industrial applications, that is simply the cost of doing business.
For many applications requiring smaller volumes of air (under 100 SCFM), the older standard of reciprocating air compressors has been improved to include air-cooled machines with advanced piston ring materials and technologies and re-designed cylinders and heads for better heat dissipation and higher duty cycles. Also, the more recent development of air-cooled rotary scroll compressors serves as an alternative to reciprocating compressors. Rotary scroll compressors offer advantages of compact size and usually lower noise levels. Disadvantages include lower mechanical efficiency, and although there is little maintenance required (air filters, bearing greasing, vee belt maintenance) over the life of the machine, they tend to be a commodity item, as final failure is usually catastrophic, requiring replacement of the complete air end. In the case of both reciprocating and rotary scroll compressors, these machines are usually available in multiple pump systems to increase volumetric capacity and provide backup redundancy. Multiple machines operating with start/stop controls also offer an advantage of greater efficiency, as the number of compressors operating will match system air demand. Both types of machines are usually available in sound attenuating enclosures for noise sensitive applications. In choosing either a reciprocating or rotary scroll compressor for your application, the following factors will likely determine your selection:
- Space requirements
- Routine maintenance requirements
- Noise level considerations
- Mechanical efficiency
- Initial/capital Cost
- Cost of ownership over a specified period of time
Although some users may consider newer technology or aesthetic appearance to be beneficial, these preferences may come at considerable cost.