The Piston Compressor
Oil-lubricated or oil-free compression?
The cylinders, pistons, and crankshaft of a oil-lubricated compressor are lubricated with oil that circulates in the compressor. The compressed air from an oil-lubricated piston compressor contains a certain amount of residual oil, normally 10-15 mg/m³.
Most models of oil-free piston compressors have permanently lubed bearings. The pistons have oil-free piston rings, usually of teflon or carbon fibers. With this type of compressor normally a more frequent exchange of the bearings and piston rings is required than with an oil-lubricated type. However, the compressed air is free of oil residue.
Piston compressors are primarily suitable when low levels of compressed air are needed: single-stage compressors for pressure values up to approximately 800 kPa, multi-stage models can supply up to 30,000 kPa.
The single-stage compressor
A single-stage compressor has one or more cylinders which compress air from the atmospheric pressure to the operating pressure.
The multi-stage compressor
A multi-stage compressor has two or more cylinders arranged in a series which compress the air to the final pressure in multiple steps. In between the steps, the compressed air is cooled with water and air. As a result, the efficiency is improved and at the same time a much higher pressure can be achieved than with a single-stage compressor.
The Rotary Screw Compressor
The rotary screw compressor compresses the air between two rotating screws which are positioned across from each other. Together with the surrounding compressor casing, these screws create the driving screw action. A screw compressor generally works according to two principles: liquid cooling or drying. Both types are available as single-stage or multi-stage models.
Liquid-cooled rotary screw compressors
In a screw compressor with liquid cooling, the compressed air is cooled with a coolant in the compression chamber located between the screws.
The cooling agent (usually oil) circulates in a closed loop between the (1) liquid tank, (2) aftercooler and (3) compression stage mixed with air before compression. The operating temperature of the compressor is therefore held at about 80 °C, regardless of load or pressure.
Immediately after compression, the coolant is separated first in (1) liquid tank and (4) oil separator. The compressed air then flows through an (5) aftercooler and finally to the compressed air tank.
The rotary screw compressor is suitable for interval and continuous operation. The cost effectiveness is most ideal under continuous operation with a high load (up to 100 %). Using modern technology (e.g. motor speed control), the energy consumption can be lowered in the case of a minimal or fluctuating need for air.
Dry-running rotary screw compressors
The dry or oil-free rotary screw compressor compresses air without cooling the compression chamber. The operating temperature of the compressor increases as a result to about 200 °C, even at an operating pressure of 3 bar.
For a normal industrial air pressure (about 7 bar), the dry-running rotary screw compressor must therefore compress the air in two steps and cool the compressed air between the compression steps.