Basic Info on Air Compressors

This is mostly my own notes from various bits of online research.

I've also found an amazingly detailed buyer's guide for compressors. I'd consider deleting my page here and linking to the guide, if this weren't doubling as my personal notepad.

First off, anything short of an (expensive) commercial compressor with a 100% duty cycle, should be considered delicate. I don't mean in the sense that you may break it if you touch it, I mean in the sense that most compressors can easily burn themselves out if they're being used past their duty cycle.

Required Definitions

• PSI: Pounds per Square Inch. This is roughly analogous to "voltage".
• SCFM: Standard Cubic Feet Per Minute. This is roughly analogous to "amperage", and it's a measurement of the speed of air flow. Higher ratings are always better. Just about any air tool you buy will be rated for a maximum safe SCFM, and a minimum required SCFM. If your compressor outputs more SCFM than your tool is rated for, you will need to use a regulator to prevent it from overworking the tool. If you're going to run a tool on a compressor, some websites recommend multiplying the tool's minimum SCFM by 1.5 in order to 'right-size' it for the air compressor.

Quantifiable Measurements of Compressor Quality

• Duty cycle: This is how long the compressor can safely run, expressed as a percentage or fraction. Generally speaking, it's an indicator of how much time the compressor needs to spend cooling down for every minute of operation.
• Horsepower: Actually, you shouldn't use horsepower as a differentiator, except perhaps in a negative sense. It's entirely possible for a less efficient compressor to just have more horsepower thrown at the problem... if a more efficient but lower horsepower engine is giving you the same amount of airflow, then you are probably going to burn less electricity AND generate less heat, with the lower horsepower engine.
• SCFM Delivery: This is a rating of how quickly the compressor can pump air into its connected storage system. You'll see "SCFM delivery at 40 PSI" a lot, and it's lower than the "SCFM delivery at 90 PSI" number. The reason for this is that what's being measured, is the flow rate of air into the storage system. When there is less pressure in the storage system, there is less resistance to overcome when attempting to force more air into it.
• Tank size: The available volume (usually expressed in gallons) to fill with air. Higher is almost always better. The only time when a small tank may be better, is when you're only going to do something small and you're faced with the prospect of having to fill up a whole tank just to do a few seconds' work. In this case, a smaller tank is better.

Cheating on Tank Size

You can hook up air tanks in series, in order to increase the overall air reservoir available to the compressor. If you're planning on performing a low-psi operation, this can be a very good thing, as the compressor will end up working far less to build up a moderate pressure in two tanks, than it would work in building up a high pressure in one tank. Bear in mind however, that if you're hooking up two tanks in series, you must take PSI into account and either use a regulator to reduce the pressure supplied, or just make sure that all of the 'downstream' gear (hoses AND tanks) are rated for the same PSI as the compressor. I'm looking at YOU, folks with a 135PSI aux tank and a 150PSI compressor!

Preventing Excess Air Use

You can use a regulator to reduce the PSI going through it. It's basically a two-way valve which clamps off when pressure on the reactive side reaches its (variable) cutoff. For certain applications, such as using a blower gun to dispose of dust, you really don't need a lot of pressure and there's no reason to overtax your compressor.

Condensation and Emptying the Tank

It seems that the humidity in the air which was kicked into the compressor, condenses into water at the bottom of the tank and can lead to rust. This is why compressors have a bleed-off valve on the bottom of the tank - so that you can blast out the water and such. I've also heard that if you have a screw-type bleed-off valve, it's advisable to replace it with a ball valve so that you can bleed off water readily without having to unscrew it all the way and empty your tank just to get the water out.

Performance Degradation in Compressors

Air leaks are probably your #1 problem when first setting up a compressor. If your compressor has leaks, then the efficiency is going to degrade. Leaks can be found anywhere the tank has an interface to the outside world. If you aren't sealing all of your connections with plumber's tape or pipe compound, then you should expect plenty of performance degradation and a general inability to maintain peak pressure without the compressor running a lot (or even constantly).

• Drain valve: The drain valve is one place where leaks are commonly encountered.
• Tank outlet: The tank's main outlet is another place where leaks are commonly encountered.
• Compressor system: The connection between the pistons and the tank can also leak.

Quick-Disconnect Fittings

QD fittings are an excellent way to prevent yourself from having to screw air tools together. I personally use QD fittings on all my blow guns, and on my hoses. I'm sure it does reduce the amount of pressure that actually gets through.

-- SeanNewton - 29 Jan 2008