Considerations for Selecting the Right Air Compressor

When selecting the air compressor that is right for your application, there are a number of issues which should be considered. The basic considerations will be as follows:

  1. What pressure do I need for the optimum performance of the equipment or processes that I am using the air for?
  2. What size air compressor do I require?
  3. What type of compressor should I install – reciprocating, rotary screw, rotary vane or rotary scroll?
  4. Should my compressor be lubricated or oil-less/oil-free?
  5. Will the compressor be in an area where noise may affect personnel working in the area? If so, what options do I have for either noise attenuation of the air compressor, or installing it in an area which is remote from personnel?
  6. Should the compressor be air-cooled or water-cooled?
  7. What electrical power is available to operate the compressor?
  8. How much piping, and what size and type will be required to distribute the compressed air to the points of use?
  9. Will there be a requirement for compressed air storage requiring a larger, or additional air receiver(s)?
  10. What purity levels are required for the compressed air for the equipment or processes being serviced?
  11. If the compressed air is to be dried, to what level (dew point) do I need to dry the air to for use with the equipment or processes being used?


Understanding the Economical Side of the Equipment

The answers to the technical questions above will all be in addition to the issue of capital cost of the equipment being selected, and the cost of ownership. The cost of ownership may include such items as:

  • The cost to produce compressed air (electrical energy usage) based on the mechanical efficiency of the compressor
  • The cost of installation
  • The cost for routine maintenance over the life expectancy of the equipment
  • The cost for any production losses in the event of unscheduled service requirements or breakdown.

In some cases, a user may wish to build in some redundancy by having multiple compressors as opposed to one larger compressor. This could avoid the full or partial loss of product should a compressor fail.

Understanding the Selection Process with Some Examples & Assumptions

For the purpose of this article, we are going to assume a more-or-less standard air pressure requirement of 90 PSIG for the air tools being used, and a volumetric requirement of between 10 to 50 CFM.

For sizing the compressor, most manufacturers of air-cooled reciprocating compressors will advertise an acceptable duty cycle of up to 75%. Therefore, the actual calculated volumetric requirement when adding up the tool or process requirements, should be less than 75% of a reciprocating air compressor’s output capacity at pressure. Therefore, as an example, if you have a requirement for 30 SCFM @ 90 PSIG, a reciprocating air compressor should have a capacity of greater than 40 CFM.

You would have the option of selecting other types of compressors, but we are going to assume the reciprocating compressor will be the choice compressor types. The reciprocating compressor in this case would be a 15 HP unit with a capacity of between 45 and 50 CFM. If a rotary screw or vane compressor had been selected, although the capacity may have been serviced with a 10 HP machine (as these machines are normally rated for 100% duty cycle), it is likely that the compressor would have to operate continuously, even when air demand is minimal or non-existent.

The difference between operating a 15 HP motor for part of the time, may be as cost effective as operating a 10 HP continuously with low demand. The capital cost would be higher for the rotary screw compressor, however it will likely have the advantage of having a smaller footprint, and a lower noise level.

We will not address all of the questions and considerations noted above (these should be determined by the purchaser/system designer), but it is noteworthy, that if the applications calls for an air dryer, there will be a requirement for an air after cooler on your air compressor, and that should not be overlooked. It is also noteworthy, that rotary screw compressors are designed to operate at an ideal temperature which is usually in the range of 82 ºC (180 ºF). Although smaller rotary screw compressors may have the capability of operating on auto start/stop control using a suitably sized air receiver, this operating temperature may not be reached in short operating cycles when system demand is low. The issue with not reaching design operating temperatures, is that condensate may form at the discharge of the compressor pump and mix with the oil that is circulated throughout the compression cycle. This may lead to emulsified oil which may result in premature bearing failure.