Let's Air out a few things about Compressors

There are several categories that compressors fall into, but when we talk about compressing air to use for tools in your typical shop or garage 99.9% of the compressors that are used are piston type compressors, so that is what we'll focus on here. Compressors range in size from your tiny portables used for small airbrushing jobs to your massive 10 to 25 hp compressors typically used at the larger production shops.

Now to the nuts and bolts, within the piston or reciprocating type compressors as they're called in fancy terms, there are two major differences 1 stage and 2 stage. This has nothing to do with how many pistons are in the compressor, so let's explain the difference. When we talk about a 1 stage compressor we're referring to the way it compresses the air into the storage tank or receiver. A 1 stage compressor accomplishes this in a single stroke of the piston, on the down stroke it sucks in the air controlled through a set of valves and on the upstroke it compresses the air into the tank. 

There are upsides and downsides no pun intended, to a 1 stage compressor the upside mainly being the price because it's a simple system and a lot less expensive to produce. These are for your small application type uses and they range from low HP to around the 5 HP range. When shopping for these type of compressors don't be fooled by tank size or some of the hyped numbers that manufactures put on the labels they can be easily inflated by changing pulley sizes of the motor and increasing the revolutions on the compressor.

Which now leads us to the down side of a 1 stage compressor, and that's in the way they compress the air in a single stroke into the tank. They generate a lot more heat, and a by product of that is condensation which leads to water in the storage tank and lines.
As pressure builds in the tank these compressors have to work harder to continue to fill the tank and are not as efficient at doing so. Don't misunderstand 1 stage compressors are fine, just know that you get what you pay for and they are only suited for the correct applications. If you over work them you'll have to contend with a lot of moisture in your lines and a compressor that won't last very long.

                                                

On to 2 stage compressors, you have very few manufactures producing 2 stage compressors in the low HP range because people looking in that range are typically looking at price not so much the quality. Your typical 2 stage compressor will start at 5 hp on up. What makes a 2 stage compressor a more efficient system is in how it compresses the air into the tank, the first larger piston compresses the air to a mid range level and then passes it through an inter-cooler system. This inter-cooler can be as simple as a piece of copper tubing to a sophisticated series of pipes with fins to help cool the air before it enters into the next smaller piston. Then the second phase of the compression hence 2 stage, is the further compression within that second piston to a higher compression level and into the tank.

Since the air is compressed in 2 stages it is done so without generating as much heat therefore enters the tank cooler which causes less moisture. 

Also the revolutions that they typically run are much lower which causes less strain on the components which gives you a longer lasting compressor.