Choosing an Air Compressor




How to Choose an Air Compressor





In this article, I will explain how to choose an air compressor that would be suitable for powder coating. Regardless of powder coating, an air compressor is one of the best tools to have in the shop.  By itself, all an air compressor does it make noise and take up space, but the tools that it can power are great.  Air tools are often cheaper than their electric counterpart, and most of them are better.  If you are thinking that you don't want to get into powder coating because you have to go out and buy a big expensive air compressor, just think about how much an air compressor can help you out in the garage.  I don't even include my air compressor in my powder coating budget because of how many other things I use it for.  It is another one of those things, that when you own one, you will never want to be without one again.



Parts of an Air Compressor

In case you have no idea what an air compressor is, here is a little explanation.   A reciprocating air compressor consists of basically 3 major parts:
-A tank
-An air pump
-A motor



The motor can be gas or electric but if you 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 which 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 still not as convenient as an electric compressor.  The motors job is to turn the pump.  The pump is responsible for actually 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

Lets 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 the air pressure inside the tank is reduced to a specified amount.  The tank size determines how frequently the 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 .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 .00134 = 4.824hp

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

4.824 / 1.10 = 4.385hp

The actual running hp of the motor is probably pretty close to 4.4hp, yet my air compressor has a giant sticker on the front that states 7hp.  Manufacturers like to over-rate the hp figure for whatever reason and it is not just the cheap manufacturers.  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 all that important.  The motors requirement is simply to be able to turn the pump and to do so without putting a bunch of strain on the motor.

PSI

This number is another marketing number.  It will be on the sticker in big bold numbers right next to the fake hp number.  The psi is a measure of the pressure of the air inside of the tank.  A common "Max PSI" for a garage-sized air compressor is around 150psi.  Using the 150psi example, the motor will run, turning the pump, which pumps air into the tank, until pressure inside of the tank reaches 150psi.  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 input of 150psi, and reduces it to a psi that you select.  I have never found a need for more than 120psi.  A powder coating gun operates at around 35psi max and sandblasting is usually done at less than 100psi.  So as long as the air compressor has a max pressure of more than 100psi, it will work fine.  Do not buy one compressor over the other because one states 150psi and the other states 175psi.

CFM

This number is very important, 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 .01cfm 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 a impact wrench for 2 seconds, thats 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.  Say your sandblasting gun will be using the most air and it uses 10cfm at 90psi.  You should plan on getting an air compressor that can output at least 13cfm at 90psi.  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 26cfm at 90psi.

Other things to look at when choosing a 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 also be quieter.  Again, I say, only if you can afford it.  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 psi and then send it through a cooler and then to another piston that will compress it to a higher psi.  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 you can afford it, a 2-stage compressor will provide you with all the compressed air you ever need in a home shop.


Hopefully by now, you have a better understanding of what all the numbers mean on an air compressor and hopefully I wrote that well enough that you could actually understand it :)  Lets talk about the type of compressor you would need for powder coating.  There are basically 3 tools that an air compressor will operate that you will need to powder coat:

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 big compressor at all.  I have even heard of people using pancake compressors with their powder coating guns.  Now, I wouldn't recommend going that small, but anything that is a step up from a pancake compressor should work.

Blow Gun:

You will use the a blow gun a lot during the powder coating process.  They are good for drying wet parts, cleaning your powder coating gun, and blowing off media after sandblasting.  They are also great for lazily dusting the garage.  There are some blow guns that use a Venturi nozzle to blow out more air than the standard blow guns.  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 do.  However, a little pancake compressor will run until you let go of the trigger. 

Sandblaster:

A sandblaster uses A LOT of air.  If you plan to do your own blasting, you will want a larger compressor.  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, 11cfm@90psi, 4.4HP compressor. It will keep up with my siphon sandblaster and is actually able to shut off while I am still blasting, 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 it was not meant to run that much.  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" nozzles.  A small nozzle like that blasts a very small area at a a time but it uses less than air than a larger nozzle.  With a bigger 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 sandblaster 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. 

So what this whole article comes down to is that you need a compressor big enough to sandblast the parts you plan on blasting.  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 parts.  Buy a big enough compressor from the start so your time spent in the garage isn't miserable.  It is worth the money.  A 2-stage 80 gallon compressor would be ideal.

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