Compressor Size

Are you in the hunt for a new compressor? If so, you do want to take compressor size into the equation when you are deciding what air compressor to get.

It will help you to know that you can generate around 4 CFM of compressed air at 90 PSI for each horsepower of compressor motor size. Knowing that is one thing, knowing why that is important is another. An explanation follows.

Also, this link takes you to a page about helping you understand how air compressors work. This will be good background information, and a help when your air compressor doesn’t work!

About Compressor Size and air flow

Compressor size a small 12 VDC tire pump compressor

If you acquire an air compressor that has a fractional horsepower motor, that’s a motor that is less than 1 HP in size, your new air compressor really won’t be generating much in the way of compressed air.

This photo is of a 12 VDC fractional HP, tankless, air compressor.  It’s purpose is to be plugged into a vehicle’s auxiliary power supply and pump out compressed air to fill a car or truck tire.

If you have used one, you know that they work fine, but they are very slow. It takes minutes for them pump up a tire. Make no mistake, this is a true air compressor, some brands of which can output air up to 250 PSI, but again, very, very slowly.

Would you pick this air compressor to run your blow gun? Would you pick this size of air compressor to run your air drill? Of course not. It’s easy to see that this puny air compressor does not have sufficient compressed air output to run any air tool!

But the air compressor I bought looks big enough!

It is easy to understand that the 12 VDC air compressor pictured above would be too small for running air tools being that in many cases it would be smaller than the air tool.

It becomes more difficult to understand the compressor size issue when many of the big box stores are selling smaller air compressors that come bundled with a selection of air tools, and some of those air tools can only be run for short periods of time with the compressed air output of the air compressor they are bundled with.

You might expect that if the store is selling a compressor bundled with an air tool that the compressor should be big enough to run that air tool the way you want that tool to run. That would be a mistake.

Bundled air compressors and air tools

Air compressor bundled with air toolsThe Porter Cable pancake air compressor shown is currently available as a combo-kit from the Home Depot. In this case, the combination of this air compressor with these two air nailers will work.

The air tools that are bundled with this air compressor are “single shot” type air tools. These tools will use a shot of air to drive the brad or the nail into the work piece, and then they will not use any  more air until the trigger is pulled and then they cycle once more.

After a few nails have been driven, the air pressure drops, and the compressor starts to rebuild the air pressure. Since the work being performed is nailing, there is usually no problem to wait for a minute or two for the tank to rebuild before doing some more nailing.

The intermittent use air tool is the ideal tool to use with small air compressors.

If you get a fractional horsepower air compressor, and it comes bundled with air tools such as air wrenches, air drills, air sanders continuous demand tools like these, I’m afraid that you will be disappointed with the results. Some air tools like these consume 10 CF of compressed air per minute and many of them use even more than that.

If we look at our rule of thumb again – you can generate around 4 CFM of compressed air at 90 PSI for each horsepower of compressor motor size. Let’s say, for example, you purchase a new compressor with a 3/4 HP electric motor. The most that 3/4 HP compressor can generate will be 3 CFM of compressed air at 90 PSI. If your air tool requires 10 CFM of air at 90 PSI to run, you will only be able to use that air tool for a few seconds before the compressor tank is empty, the compressor is running, yet you run completely out of air and the air tool comes to a stop. That air compressor is just too small to run that air tool.

Air Compressor Size is critical!

Once again, before you acquire that new compressor, give some thought to how big it needs to be to run the air tools you want to use.

Go one step further.

Figure out what air tools you want to use, and visit the air tool store and find those air tools, before you buy the air compressor. Don’t necessarily buy the tools, but do look at the tool manual or specifications which should tell you what that air tool needs in terms of compressed air flow and pressure to run properly. Find the air tool that consumes the most air, and pick the compressor that will supply that air tool satisfactorily, and you will then, logically, have enough air available with that new air compressor to run all the other tools you will have that consume less air than that one.

Compressor one-foot-itis!

Folks that have owned boats in their life will have a pretty good idea about what I mean here.

If you have a 20′ run about boat, you are convinced that a boat just one foot longer would resolve all the space issues in the 20′ boat. If your boat is a 24 footer, then you’ll have the same thinking about getting a 25 footer and so on.

Same with air compressors. If you have a 1 HP with a 5 gallon tank, we expect you will soon yearn for a 1 1/2 HP air compressor with a 10 gallon tank, an air compressor that’s just a little bigger than the one you have now – that will solve all your compressed air supply problems.

Nope, it likely won’t. At least it won’t if you don’t take the time to think about what you want to use the compressed air for, and ensure that you pick the air compressor that is able to supply both the air pressure and compressed air flow needed for all your air tools, before you lay down your cash for a new compressor.

Comments and questions about air compressor size are welcome. Please use the comment box below.

 

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Comments

  1. David Burnsed says:

    I have an older 80 gallon upright tank not sure of brand . The electric motor is a Dayton model number 5K481G 1750 rpm and it’s a 1 1/2 hp . My question is what size pump do I need ?

    • David, just found this query. So sorry for the delay in responding.

      Yours is a difficult question to answer. The logic suggests that if you get 4 CFM at 90 PSI or so from each HP of motor, then the pump you would want would be one that would produce 6 CFM of air running with a 1 1/2 HP motor.

      Thing is, the older air compressors were much better built than are the new, were more heavy duty, and as such the original 1 1/2 HP motor might have handled a bigger pump.

      I don’t know the motor, but I assume it’s a 120 VAC? If so, I wouldn’t get a pump much bigger than I’ve noted above, if this were my compressor.

      Also, a 1 1/2 HP motor on an 80 gallon tank, that is a pretty small motor for a large tank… suggesting that maybe the motor might have been 240 VAC, and as such, could have handled a much bigger pump.

      I hope I haven’t muddled the water too much. Good luck.

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