Author: Bob Pickard
You have your Blasting unit, you’ve selected your cabinet and now you are ready for your air supply unit. So it’s down to the hardware store, building center, or air supply specialty store. You didn’t realize how many businesses sold air compressors until you started shopping around for one.
When you walk into the store, you see rows of air compressors. They range from shoe-box size to what looks like a Saturn rocket ready to be launched into space.
I hope this information on air compressors will help if you find yourself in the situation of trying to select an air supply for sandblasting.
There is a misconception about how much air you can obtain from a compressor unit and the horsepower rating on the unit. The amount of air a compressor can deliver is measured in CFM (cubit feet per minute) at a specified PSI (pounds per square inch). It is not measured in HP (horsepower). The HP is how much electrical power the motor will consume or how much your electrical bill will increase when you use it.
I have seen some compressors rated at 5 HP that put out 4.0 CFM at 90 PSI and another rated at a 3.5 HP that delivers 4.2 CFM at 90 PSI. The 3.5 HP compressor will cost less money and works just as well as the 5 HP compressor. When you are shopping for a compressor, it is more important to compare the CFM than it is to compare the horsepower.
The basic compressor unit consists of a motor and an air pump. The first is an oil-type that requires oil to keep the internal parts lubricated. The second is oil-free and requires no lubrication. The oil-free air pump is usually connected directly to the motor shaft and is built as one unit. It is usually a single piston or single stage and the maximum air pressure is 110 PSI. The oil-type air pump is connected to the motor with a belt. It can either be a one- or two-stage pump and the maximum pressure can be 200 PSI. A two stage air pump will require at least a two HP motor to run it.
The next item on a compressor unit is the air tank, which stores the excess air. When a tank is added to a basic compressor unit, a pressure switch is also added. This switch is used to turn off the power to the motor when the air pressure reaches a preset air pressure. It turns the power back on when the air pressure drops below another preset level.
One of my compressor units turns on when the air pressure drops to 90 PSI and turns off when the air pressure reaches 120 PSI. I have another compressor that turns on at 150 PSI and turns off at 175 PSI. When you buy a compressor unit with a tank, the pressure switch will be preset at the factory. I have found that most pressure switches are adjustable and the settings can be changed.
The tank will also help cool the air, allowing the water to collect in the air tank, not in the abrasive tank. This water should be drained off periodically. The cooler the air is when it reaches the blasting unit, the less moisture you will have. On blasting systems where a large volume of air is used, a condenser coil is placed in the air-line to cool the air and collect the moisture. If you use galvanized pipes in your air supply lines instead of plastic pipes, the metal will also help cool the air.
Most compressor units will come with an air regulator, but there are some compressor units that do not have an air regulator. The regulator should be large enough to carry the CFM you will need for your blasting unit
Another consideration in choosing a compressor unit is power (as in electrical power). Do you have adequate wiring for the type o compressor you are going to buy? If you get a compressor unit that requires 220 volts to operate and you don’t have it, your electrical contractor will cost you more than the compressor unit. Usually, compressor units over 2 HP require 220 volts to operate
SELECTING THE RIGHT SIZE AIR COMPRESSOR
If you are using a pressurized blasting unit to airbrush, and do small area blasting, a ¾ to 1.5 HP with 2 to 3 CFM at 90 PSI will work just fine. These small compressor units are 110 volts and will plug into any ordinary electrical outlet. If you get one with a small tank, it will be easy to take with you to do small projects outside the shop.
The next size air compressor is the range from 2 to 4 HP equipped with a 20 gallon tank producing 4 to 8 CM at 90 PSI. You can get this size with wheels so it can be moved easily and can be taken to on-site jobs when electrical power is available. This is the portable size most generally found in shops and requires 220 volts. It will handle most projects that are 9 square feet or less.
When you get into doing room dividers, plate glass windows, and 1/2 or 3/4 inch plate glass with multi-stage blasting, you will need something that will deliver more CFM. These electrical air compressors are 5 to 10 HP with 12 to 50 CFM at 200 PSI equipped with tanks of 80 gallons or larger. The noise generated by these units is very loud and they produce a large amount of heat. Some shops build a room to enclose the compressor unit or place the unit in a building outside to reduce the noise in the shop.
Another air compressor unit you need to know about if you are going to do on-site blasting is the gasoline-engine-powered air-compressor unit. These units are powered by a small gasoline engine, from 5 to 8 HP, weigh 140 to 220 pounds and deliver 9 to 13 CFM at 125 PSI. They are ideal when you are blasting on-site for building contractors.
An air-compressor unit is very low maintenance, but here are some things that you need to know. If you have an oil-type air-pump, the oil level of the pump must be checked and changed regularly. Your service manual should specify the type of oil to be used and how often it should be changed. The air filter should also be checked and changed when it gets dirty. How often you change it will depend on how much dust is produced where your compressor unit is located.
Do not run your air compressor unit without an air filter, because it will greatly reduce the life of the compressor. You should also drain the water from the tank and moisture traps on a regular basis. The last thing I should mention is to check the pulley belt for excessive wear and replace it when necessary. The first of each month, I check each of these items.
The size of air compressor unit you will need, will depend on two things: the type of blasting unit (siphon or pressurized) and the opening size of the tip you are using. This chart will help you in selecting the air-compressor unit you will require.
This chart is based on a 60% air and a 40% abrasive ratio mixture.
Unit Selection by CFI
Tip Size 40psi 70psi
.4 .6 1.0