FIRST: Determine what you need by talking to your supplier of air operated equipment. Observe how stuff works in their factory and talk to your operators Request CFM and PSI required. Consider the lowest possible air pressure to save operating electrical expense. Research the compressor equipment suppliers – Do they have local service? Is it 24 hr/day? What is their hourly rate? Where is it manufactured? Will parts be available in ten years? Do they have references? Is the local representative knowledgeable? Is this company licensed to install and work on air compressors?
SECOND: Determine what electric voltage and amperage you have available. Common electrical services are single phase 230 volt systems. Three phase systems can be 208 volt, 230 volt or 460 volt. Changing or increasing the electrical service can be very expensive. Is an oil free compressor required? Can a food grade lubricant be used?
THIRD: Commit to a written specification. Contact prospective suppliers for equipment quotes, installation, and preventative maintenance service.
Lately, the growth of internet shops have sprung up offering locally assembled air compressors or machines from manufactures that do not have any facilities in the US. These outlets and parts could disappear. It is also possible to source American products from outlets with no local presence. I strongly object to this as air compressors need local hands on service. This is especially true when it stops. Remember, the first cost of an air compressor is a small part of its operational expense over its life.