Positive Displacement Compressors have a cavity that allows a volume of gas into the engine at atmospheric pressure. This space then becomes smaller, reducing the volume and at the same time, increasing the gas pressure. On the other hand, Piston compressors can combine several stages of compression to achieve the desired pressure which makes them particularly suitable for high-pressure applications. Both oil-lubricated and oil-free engines are available with special designs and are capable of compressing gases other than air. Additionally, if you also need a compressor for your garage, we highly recommend you check out Garage Master Blog.
However, in a double rotor compressor (Screw Compressor or Scroll Compressor), the air is trapped and sealed (usually with oil) between the two dual rotors. When the rotor rotates and unites the air, the air is pushed along the rotor into the smaller space until the pressure increases which then allows the given volume of air to enter the cavities in the compression chamber.
Furthermore, you might also need to know that there are several types of single rotor compressors – Vane, Liquid Ring, and Scroll. In a scroll compressor, the rotor is a single spiral oscillating against the same fixed spiral and, as these spirals move against each other, the air spaces trapped between them become smaller. This decrease in volume forces a fixed volume of intake air to increase the pressure.
Displacement compressors are also referred to as Constant Flow Compressors because the compressor will produce the same flow at a certain motor RPM, independent of the existing outlet pressure.
Finally, you may also be interested in understanding Dynamic Compressors. It’s a little fancier, think of Turbos and Jet! In the compressor industry, you are unlikely to find too many ejectors or axial compressors, as these are usually used for aviation. In a dynamic compressor, increased pressure is achieved by accelerating the gas using an impeller then slowing down the fast-moving air in the diffuser and volute to transfer the kinetic energy into the pressure.