It’s easy to think of air compressors as a tool that is very common however, things were not like that in the past. In fact not that long ago a compressor isn’t something you would have found in a workshop or factory.
Instead, tools used in the shop were powered by a centralized source which transferred the power in various ways depending on the tool. Usually it was through a system of wheels, belts or drive shafts. It was a large mechanical system that was way too bulky and way too expensive, and therefore only available to professionals.
Nowadays we come across air compressors pretty much everywhere. They’re common place in gas stations where we generally use them to inflate tires. They can be found in factories, workshops, even your local mechanic will make use of a tool like this.
Power tools such as sanders, grinders, staplers, nail guns, spray guns, drills, impact wrenches, and plenty of others are powered by them as well. You can also buy one for yourself online, at your local hardware store, or home depot.
Of course, the biggest advantage of compressors over a centralized power source is their small size, and the fact they don’t require a massive motor. They are also quieter, more durable, and some of them are highly portable. But, I am getting way ahead of myself because you’re here to find out how air compressors work. So, here we go!
Basic Piston Functionality
Air compressors function based on a very simple principle. When the air is compressed, its volume decreases whereas the pressure increases.
The most common way to achieve this is with the help of a reciprocating piston. There are compressors which employ rotating impellers for the purpose of creating air pressure, but I will discuss the different types in a separate article. Those which are built around a reciprocating piston are more common, and if you’re familiar with how internal combustion engines work, you will know piston compressors function in a similar manner.
Each reciprocating piston compressor has a crankshaft, connecting rod, a piston, cylinder, and a valve head. In order for the entire mechanism to work, you need power. Air compressors are usually powered by electricity or gas depending on the model. Most compressors also have a tank which is there to store compressed air for the purpose of keeping the air pressure within a set range while powering various air tools. But, let’s get back to the mechanics of it (I used an image of a liquid reciprocating piston pump to explain this (source: NVC), but with air this works basically the same)
At the top of every compressor cylinder there is a valve head that contains both the inlet and discharge valve which are basically metal flaps. These open and close and are located on top of the valve plate. When the piston moves down inside the cylinder in the space above the piston a vacuum is created.
Now, here’s the clever bit. The difference in pressure on the inside of the cylinder to the outside allows the atmospheric pressure to open the inlet valve. The air then enters the area where the vacuum used to be, and is compressed by the piston which is now going up. The inlet valve closes and the discharge valve is opened. The compressed air is stored inside the tank thereby increasing the pressure.
There are variations to this approach but the basic principle is the same. Dual-piston compressors are also very common, and they function in the same way as their single-piston counterparts do. The only real difference is there are two strokes per revolution and not one. The most common variation of a dual-piston compressor is the two-stage compressor which uses one piston to pump air into a second cylinder which creates greater pressure.
In order to keep the pressure within the desired limits, and to prevent the tank from exploding each air compressor has a switch which cuts the power to the motor when pressure inside the tank has reached the limit (which is usually around 125 PSI). In order to calibrate the pressure depending on the power tool connected to the compressor there is a regulator as well as gauges before and after the regulator.
These measure the pressure inside the tank and air-line respectively. In case the pressure switch doesn’t go off, there is a safety valve as well as an unloader valve which is there to reduce pressure inside the tank when the compressor is not being used.
Another major thing you should know about air compressors is the way they are lubricated. You will come across terms like “oil-free pump”, or “oil-lubricated”. Oil-lubricated pumps have an oil bath which lubricates the bearing and walls on the inside of the cylinder. The air and oil are kept separate with the help of the piston rings, but they are not fool-proof so there will be some mixing of air and oil which can be problematic.
On one hand, power tools need to be lubricated anyways so there is no harm in having some of the oil enter the air stream, but if you’re using air tools in woodworking or painting you will mess up the finish. Oil-free pumps have permanently lubricated bearings instead of an oil bath, and they don’t require the same kind of maintenance as oil-lubricated pumps do.
That is pretty much it. Very simple yet effective! It is downright impressive how we are able to get so much power out of relatively compact devices. If you’re that way inclined, feel free to check out some of my other articles which go into the subject of air compressors a little deeper.