Compressor Sizing

It seems many folks buy an air compressor without considering compressor sizing. What they intend to use the air compressor for, or how big an air compressor they are going to need to provide adequate compressed air to run their air tools and compressed air using equipment?

For compressor sizing it is necessary to figure out how much air you're going to need. That is... the air requirement for all the air tools, all the blow guns, all the air cylinders, all the air operated diaphragm pumps, all the everything that is in your plant or workshop that uses (or has the potential to use) compressed air and whether or not that compressed air demand is going to be constant, or change as your tool use changes, production using air equipment increases, and so on.

Compressor Size Matters

There is lots of additional information about compressor sizing on this page. As well, the comment form at the bottom of the page invites you to ask a question about compressor sizing, or make a suggestion to help others.

To Help Compressor Sizing Here Are
        Simpler Blocks Of Information

When first venturing into the air compressor-sizing arena, folks tend to be more than a bit intimidated. There's such a vast array of compressors, of types, of capacities, with a veritable feast of accessories; it's easy to become confused!

And, though there are many manufacturers of compressors world-wide, all are focused on selling their own products, which may impart a certain bias to the information that's available from them about their compressor-sizing, you think?

Consider What's The Compressed Air For?

Have you determined what you'll be using compressed air for? In order to correctly size your compressor, you really must do some homework towards figuring out the volume of compressed air you will require. Here you will find a list of compressed air uses that may affect your overall compressed air requirements.

The above link you may have just visited took you to a short list of some in-plant compressed air applications, this to help you determine which of those you have or are planning to have, equipment that will be consuming your compressed air and needs to be accounted for when compressor sizing.

Is Air Use Quantity A Total Or Periodic Demand?

If it is necessary in your plans to run all of those compressed air using appliances at the same time, and for long periods of time, then you are going to require a much bigger compressor than if they are used intermittently and sporadically.

If your compressor is for continuous air use then start making a list of everything in your plant that uses air, and leave a space beside each to note the expected CFM demand for each. Better yet, use a spreadsheet so that the air consumption numbers can be set up to give you a running total of the air you'll need.

Know that compressed air consumption of tools varies depending on the size of the air tool, it's speed, and the efficiency of that air tool. You need to get the specific compressed air demand for that air tool from the manual or the manufacturer of the tool for accurate assessment of the overall compressed air demands of your shop.

You may find the chart below handy in approximating the CFM requirements of an assortment of standard air tools. This is a guide only. Your actual consumption of compressed air will vary, depending on your tool size and design.

Air Consumption Chart

Air Tool Consumption

Air tool consumption changes based on variables. Grinders will have changes in consumption due to grinding wheel size, and rotary sanders will have their consumption change due to the pad size. Even the same tool from two manufacturers will have different air consumption rates.

Try to get the air consumption figures for a specific air tool from that tools booklet / manual, a label on the tool, or from the manufacturer! If contacting the manufacturer, have the model number and serial number of the air tool at hand.

Consider "factoring in" the highest air consumption figures you find for the tools you expect to use. You may oversize your air compressor capacity by doing so, but under sizing the compressor you purchase may bring greater long term costs to your plant as you grapple with air tools or air cylinders that do not run as expected due to a lack of compressed air supply.

Other Compressed Air Use

Air tools are only one of many applications in a plant that consume air. It's necessary to give consideration to all of the ways that you will consume air in your plant and applications.

Some folks don't consider an air-driven blow gun as an air tool, yet these can sometimes use more compressed air than expected.

Air driven automated production machinery can consume far more air than some air tools as air operated production equipment with valves, air cylinders and other air actuators is often run for 2 or 3 shifts per day, with uptime measured in weeks. Each piece of equipment may have dozens of air actuators or air cylinders operating at high speed.

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 and Water Generaton

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.

Forum On Compressor Sizing

Here is a form on which you can post questions only about compressor sizing. Should you have a different question, please use the ASK page or find and use the link from the list of other forums found on the sitemap page.

Please do review the postings of others found below the form, and add your comments or advice about compressor sizing there too.



Need a little help sizing your air compressor?

Need a little help sizing your air compressor? Here is where you can ask a question about it.

Please note that I work on responses to information and technical requests daily so please allow up to 24 hours for a response. Thanks. Bill

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