Compressor Sizing ..2

This is the second of two pages providing information on compressor sizing and compressor information. Page one on compressor sizing is right here.

Cylinder Air Consumption

How do you figure out air consumption of equipment?

The manufacturer of that equipment should be able to provide the demand specifications for their machine. They know how many cylinders or actuators they have including, they know the cycle rates of all air using equipment, and they likely have already figured out the total air consumption for their machine to ensure that you are providing enough air to make the machine run correctly. You just have to ask for it if that data is not included in the equipment information package they provided.

If you build that equipment yourself though, how much air will the air cylinders or air actuators on the new piece of equipment use?

This page on this site provides information about cylinder air consumption.

When you are determining the size of compressor you need, the rule of thumb is that each HP (horsepower) of electric motor size is able to generate 3- 4 CFM of air flow at about 90 PSI, (PSI = pounds per square inch). This holds true for compressors over 10 HP in size. Smaller compressors may generate less flow per HP of motor size, particularly when you get down to fractional electric motors on small air compressors.

Always make sure you ask the compressor vendor what the expected actual CFM is for a particular model of compressor. It seems that the promised compressed air delivery from air compressors is similar to the estimated mileage figures you get for a new car! They are sometimes only theoretical.

You will also need to know at what pressure that promised air flow will be, as air delivery figures at lower air pressures can be higher than the delivery of compressed air from the same air compressor at higher pressures.

Air Flow Terminology

I measure air flow from the air compressor in CFM. Others measure compressed air in SCFM, or even ACFM, which means actual cubic feet per minutes.

About Published Compressor Flow Specifications

Be careful when you are getting the air flow specifications on compressors, as some manufacturers size their compressor discharge rates as if the unit was pumping free air. As air pressure in the tank or mains air is compressed, the compressor pump has to work harder. A compressor might show a CFM rating of 8 for example, but reading the fine print will tell you that it generates 8 CFM at a much lower pressure rating than the pressure you may require.

Industrial systems (actuators / air tools) often require about 90 PSI to work to their capacity. Some will require higher pressure, and some lower of course,depending on the work each has to perform.

Use an air regulator at the point of compressed air use to dial down the pressure and run the air equipment at the lowest possible effective pressure level. You may save substantial energy costs in so doing.

Pressure Drop

Industrial plants have air compressors generating excess pressure and flow for the inevitable over-line air pressure drop. This helps to ensure that they have sufficient compressed air pressure and volume at their end application to keep their equipment running satisfactorily. The compressor you pick has to have this capability if it is an issue in your plant.

Again, some manufacturing plants will maintain a 20-30 PSI "safety margin" in extra compressed air pressure from the receiver, to help ensure that none of the plant equipment gets starved for air whether through pressure drop, leaks in the line, too much equipment being run at the same time, miscellaneous use of compressed air and so on.

Compressors & Water

Be aware that compressing air creates high humidity in the compressed air that turns into free flowing water in your compressed air tank and air lines.

Here is information about compressor water problems in case you would like to learn how to reduce and remove compressor generated water from your air system.

Ask questions and read other's answers on the forum on compressor sizing.