Piston or rotary compressor – which is better?

The crankshaft is driven by prime-mover that is either an electric motor or a fuel-driven engine. The machine will come with an option to have an air tank to hold the amount of compressed air. The crankshaft operates and forces the piston to move up and down inside the machine. There is a valve at each end of the cylinder. When the piston moves one-way air enters the cylinder. Then, when it moves the other way the air inside the cylinder is compressed. As the volume decreases, pressure increases. The full compression cycle is the process by which the air comes into the cylinder and then gets compressed to the required pressure. Depending on the pressure requirement, the number of stages is decided by the manufacturer. In a single-stage piston compressor, the piston will compress the air in one stroke. One stroke means the piston has a full-motion with one rotation of the crankshaft. In layman's terms, a single-stage piston will travel forward & backward directions with one rotation of the crankshaft. A single-stage unit can operate with lower RPMs and noise levels. A two-stage unit uses two pistons. One crank uses one piston to bring the air into a cylinder. It's then transferred and a second piston for the further air compression and in turn for pressure elevation. In the case of an oil-lubricated piston compressor, the piston and crankshaft need lubrication to operate effectively. When the piston moves in the cylinder the lubrication oil gets mixed with compressed air. This means that the compressed air coming out of the compressor will have some oil in it. In the case of an oil-free piston compressor, the crankshaft needs lubrication to operate effectively whereas the piston won’t be lubricated with oil. This means that the compressed air coming out of the compressor will not have any oil in it. However, it is very essential to understand that, in piston-type compressor the compressed air output shall be pulsation type (not constant) and unbalanced vertical forces cause cylinder ovality, acting along with the piston weight and also an unbalanced couple necessitating heavy foundation. They are also very bulky in size and weight.