What size of compressor pump do I need?

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It’s been a while now since David sent in the question “about what size of compressor pump do I need?”

He has the original 80 gallon upright tank, but he’s not sure of the brand of compressor, or tank. The electric motor originally (he thinks) on the compressor is a Dayton, 1 1/2 HP, 1750 RPM and the model number of the motor is 5K481G. I’ve added an image of the motor just below to give you some idea of the type of compressor motor we’re talking about.

Dayton 1.5 HP 1750 RPM Electric motor
Dayton 1.5 HP 1750 RPM Electric motor Photo source: grainger.com

With that tank and that motor, what size of compressor pump?

Let’s step back a bit and look at this another way. Rather than focusing on the pump that will suit the tank and motor, wouldn’t it make some sense to figure out how much of an air compressor pump will be required for the jobs he wants compressed air to do for him? In other words, what air tools does he plan on using?

Not knowing that may mean that he can get a pump that will work just fine with the tank and motor he has but it may not provide enough air for the work he wants doing. Here’s a page that addresses this issue more fully, if you care to have a read.

This page about the best compressor for a small shop might help too.

So, what size compressor pump do I need?

Now back to David’s original question. This is a difficult question to answer specifically.

The logic suggests that  you if you typically would get 4 CFM at 90 PSI or so from each HP of electric compressor motor, then the pump you would want would be one that would produce 6 CFM of air since you could power it with the 1 1/2 HP motor.

Yet an 80 gallon compressed air tank is fairly large. That would suggest that the original setup had a larger pump in order for the compressor not to have to run to long when it cycled on at low pressure cut in. I wonder then if the original electric motor (if not the one David says it was) might have been larger? More likely a 5 HP motor based on the tank size. That being the case, the original pump could have been fairly large, might have had a couple of cylinders and might also have been two-stage.

All that being said, for the motor presently on the compressor you would be best with a pump designed to deliver 6 CFM at 90 PSI, one similar to that shown in the photo below.

compressor pump 6 CFM Schulz brand
Compressor pump 6 CFM Schulz brand
Photo: eBay.com

These pumps are readily available and cost in the $175-$200 range on line.

Given that the existing motor has a 1750 RPM, when a new pump is acquired, make sure it’s sheaved correctly to give the pump the rotation it needs. That may mean a larger or small motor sheave.

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