Choosing an Air Compressor

An air compressor is an essential component in nearly all powder coating applications. It provides the air necessary for media blasting, blowing off parts, and of course, a powder coating gun. 

To those that are on the fence about whether they decide to get into powder coating, an air compressor may seem like an expensive item that takes up too much space. Don’t let that prohibit you though. Air compressors come in all shapes, sizes, and prices; and there is one available for every level of powder coater, from a complete beginner, to a large established shop. 

An air compressor is also incredibly useful to have available to you, even when you’re not powder coating. A large world of pneumatic tools becomes available to you, with the right air compressor. 

This article will cover the basic parts of an air compressor and the terminology used to describe its performance. I am hoping that this information will help you to choose the right air compressor that will assist your powder coating experience.

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Parts of an Air Compressor

In case you have no idea what an air compressor is, here is a simple explanation. A reciprocating air compressor consists of  three major parts:

-A tank

-An air pump

-A motor

The motor can be gas or electric but if you are putting it in your garage, absolutely do not use a gas-powered air compressor. A gas engine produces carbon monoxide which is a deadly, poisonous gas and is not something you want to have in an enclosed room. You can place a gas-powered compressor outside and run the lines inside but it is not as convenient as an electric compressor.  The motors job is to drive the pump.  The pump is responsible for sucking in and compressing the air.   The compressed air from the pump is then plumbed into a tank which stores the air.   There are, of course, many different configurations and sizes of these 3 components which determine the efficiency and cost of the compressor.

Understanding Air Compressor Specs

Let's discuss some of the specs that you would see when shopping for an air compressor. Hint:  The numbers you see on that big flashy sticker smack-dab on the front of the compressor are not so important when choosing one...

 

Tank size 

The tank size determines the physical size of the air compressor.  If it were not for the tank, the motor would be running constantly to supply any compressed air.  The pump will pump air into the tank until the tank reaches a specified pressure (psi).   Once the tank is filled with the specified amount of air, the motor will shut off and will not operate again until the air pressure inside the tank is reduced to a specified amount.  The tank size determines how frequently the motor runs.  Another way to word it is, the tank size determines how many air tool related tasks you can do before the motor starts to run.  A 5-gallon tank would require that the motor run for virtually every task.  For an example, removing one bolt with an impact gun will likely empty the tank enough to cause the motor to run when you have a 5-gallon tank.  A continuous air tool like a sandblaster uses so much air, it would empty the 5-gallon tank so fast that the motor would never shut off.   You can have the biggest motor and pump in the world, but if it is attached to a 3-gallon tank, the motor will run very frequently, but for a very short period of time.    If you have a very small motor/pump and a huge tank, the motor will run less frequently, but each running session will last for a very long time.  A good all-around tank size for a home garage is 60 to 80 gallons, but honestly, the bigger, the better.

Horsepower (HP)

The horsepower rating applies to the motor.  This hp rating of an air compressor is not extremely relevant to your purchasing decision.  The big "HP" number you see stickered onto the front of most air compressors, is usually an overrated peak hp number.  This peak hp number is irrelevant to the motors actual running hp which is usually nowhere to be found on the compressor.  An easy equation to figure out the actual running hp of the motor is this:

Amps x Volts = Watts

Watts x 0.00134 = Horsepower

I will fill in the equation using the specs from my air compressor motor.

15 amps x 240 volts = 3600 watts

3600 watts x 0.00134 = 4.824 HP

So theoretically, the motor on my air compressor should produce 4.824 HP, but that would only be if the motor was 100% efficient.  There are frictional losses that reduce that number by about 10% in general:

4.824 / 1.10 = 4.385 HP

The actual running horsepower of the motor is probably pretty close to 4.4 HP, yet my air compressor has a giant sticker on the front that states 7 HP.  Manufacturers tend to over-rate the HP specification for marketing purposes, and it is not just the cheap manufacturers that do this. Most of them are guilty of it.  Now you will know how to figure out the actual horsepower of your air compressor motor. But again, it is not the most important specification. The motor only serves to drive the pump and to do so without putting a bunch of strain on the motor.

Rated Pressure (PSI)

This pressure specification is another marketing number. It will be on the sticker in big bold numbers right next to the exaggerated horsepower number. All of the components used in the compressor are rated to some maximum pressure and this rated max pressure will be some number a little lower than the rating of those components. A common "Max PSI" for a garage-sized air compressor is around 150 psi. Using the 150 psi example, the motor will run, turning the pump, which pumps air into the tank, until pressure inside of the tank reaches 150 psi. Then the motor will shut off. This number is also pretty irrelevant to your purchasing decision. Along with your air compressor, you will need an air compressor regulator. An air compressor regulator takes that pressure input of 150 psi and reduces it to a pressure that you select. Nearly all air tools, and especially a powder coating gun, are rated for max pressures of much less than 150 psi. A powder coating gun operates at around 35 psi max and sandblasting is usually done at less than 100 psi. So as long as the air compressor has a max pressure of more than 100 psi, it will be plenty for most applications. Do not choose one compressor over the other simply because one states 150 psi and the other states 175 psi, as both will be sufficient.

CFM

This specification is very important and if you could only go by one specification when buying an air compressor, this would be the one to use. CFM (cubic feet of air per minute) determines how much air the pump can put out per minute. The higher the CFM, the faster it can fill the tank with air. If you are using a tool that takes more CFM than the pump is capable than putting out, once you use the reserve air in the tank and the compressor starts running, it will run until you stop using the tool and it can catch up. An easier way to explain CFM is this: Your pump is constantly trying to fill the air tank. That is its job. Any air tool that is being used is a leak in the air tank. How much CFM the tool uses is a measurement of how big the leak is. If the tank is leaking out more air than the pump can put into it, the pump will never shut off. If the tool only uses 0.01 cfm less air than the pump can produce, the pump will run 99% of the time. This only applies to continuous use tools however. If you use an impact wrench for 2 seconds, that's only a 2 second leak. A sandblaster presents a continuous leak though. As long as you are holding down the trigger on your sandblaster, it represents a leak.

The rule of thumb for deciding how much CFM you need is to figure out what tool you will be using that requires the most CFM and add 3 CFM to be safe. An alternate rule of thumb is to simply buy the most CFM that you can possibly afford. Say your sandblasting gun will be using the most air and it uses 10 cfm at 90psi.  You should plan on getting an air compressor that can output at least 13 cfm at 90 psi.  If you plan on having someone else use the compressor at the same time, say 2 sandblasting guns being used at once, then double that number and you now need 26 cfm at 90psi.

Other options to consider when choosing an air compressor:

Belt-Driven pump vs Gear-Driven Pump 

Choose belt driven, they are much quieter.

Oiled vs Oil-less

Choose Oiled.  Oil-less compressors are very loud and the pumps usually have a shorter lifespan.

RPM

Cheaper air compressor motors will run at 3,450 rpm.  If you can afford it, try to find an air compressor that has a lower rpm rating.  They will produce less heat and are also quieter.  Most all air compressors that you will find locally that are targeted at home use will have an rpm rating of 3,450.

Single Stage vs. Two+ Stage

A 2-stage Air Compressor uses a pump with 2 pistons.  The first piston will compressor the air to a certain pressure and then send it through a cooler, and then to another piston that will compress it to a higher pressure. A 3-stage would use the same principle but with 3 pistons.  A 2-stage air compressor is capable of outputting much more CFM than a single stage. If it is in your price range, a 2-stage air compressor is ideal for the home shop to ensure you have plenty of air. It is important to mention that just because an air compressor has 2 cylinders does not mean that it is a 2-stage air compressor.  There are many single-stage twin cylinder air compressors.

Why You Need an Air Compressor for Powder Coating:

Hopefully by now, you have a better understanding of what all the specifications mean regarding an air compressor.  Let's talk about the type of compressor you would need for powder coating.  There are three important tools that will be driven by an air compressor:

Powder Coating Gun

Every powder coating gun (except for the Craftsman Powder Coating gun) relies on an air compressor as a source to propel the powder out of the gun and onto the part.  Powder coating guns do not require a very large compressor. Some hobbyist powder coaters use nothing more than a 6-gallon pancake air compressor.  I wouldn't recommend going that small, but anything that is a step up from a pancake compressor should work. A 20-gallon air compressor would be plenty for powder coating, blowing off parts, and doing other small tasks around the garage, and it will plug into your regular wall outlet.  It won't produce enough air to do any meaningful sandblasting but it is a nice budget-level consumer compressor if you have a sandblasting alternative or if you outsource your blasting.
 

 

 

Blow Gun

You will use a compressed air blow gun a lot during the powder coating process. They are great for drying wet parts, cleaning your powder coating gun, and blowing off media after sandblasting. They are also useful for lazily dusting the garage. The ideal blow gun for powder coating is one that uses a Venturi nozzle which puts out more air than a standard blow gun. Because these are not a continuous use tool when powder coating, meaning your not going to be blowing something off for 20 minutes, almost any compressor will operate it. However, a little pancake compressor will run until you let go of the trigger. 

Sandblaster

A sandblaster uses a LOT of air very quickly. If you plan to do your own blasting, you will want a larger compressor with a high-CFM output pump. The only way you would get by with a smaller one (and by smaller, I mean a 30-gallon) is if you only plan to blast very small parts and not a lot at one time. I feel the bare minimum for blasting is my own compressor which is a 60 gallon, 11 cfm @ 90psi, 4.4 HP compressor. It will keep up with my siphon sandblaster without running continuously, but I like to give it breaks because once it does kick on and start to run, it takes about 5 minutes to shut off.  Then it will start running again in another 5 minutes. The reason I give it breaks, is because consumer-level air compressors are not meant to run that frequently.  Most of the time I sandblast small to medium sized parts like engine brackets so I don't mind the breaks that much. When I occasionally blast wheels, I wish I had a larger compressor.  The compressor will have to cycle on and off about 10 times for each wheel, which equates to a lot of breaks
.

Another advantage of a large compressor when sandblasting, is the ability to use a larger sandblasting nozzle. The CFM a sandblaster uses largely depends on what size nozzle you are using. I use 1/4" ID nozzles. A small nozzle like that blasts a very small area at a time but it uses less than air than a larger nozzle. With a larger compressor, you can step up your nozzle size and blast larger areas at a time which makes the whole sandblasting task go by much faster. Industrial sandblasters use a nozzle that resembles a fire hose which they use to blasts ships and buildings, but they need monster-sized compressors to be able to supply enough air for them.
 

What this whole article comes down to is that you need a compressor big enough to sandblast the parts that you plan on powder coating. You may think you have the patience to wait for a 30-gallon compressor to catch up while you are sandblasting a set of wheels. You may have that patience for the first set of wheels, but your patience will quickly run out as you continue to sandblast more and more parts. Buy a big enough compressor from the start so your time spent in the garage or shop is efficiently used. It is worth the money. A 80-gallon single stage air compressor would allow you to get by but a 2-stage 80 gallon compressor would be ideal for a serious hobbyist or small powder coating shop.

In the next article, I will be discussing how to keep your compressed air DRY.