Understanding Air Compression: Positive Displacement vs. Dynamic Compressors

There are two generic principles for compression of air or gas — positive displacement compression and dynamic displacement compression.

Positive displacement compressors

Positive displacement compressors work with a constant flow regardless of outlet pressure. In positive displacement compression, atmospheric air is drawn into one or more of its compression chambers, which are then closed from the inlet. As the volume of each chamber decreases, the air is compressed internally until the pressure reaches the designed build-in pressure ratio. Then, the valve opens and the air is discharged into the outlet system.

Positive displacement compressors are the most common type of compressor grouping seen in industrial settings and include:

• Piston compressors
• Screw compressors
• Tooth compressors
• Scroll compressors

Dynamic compressors

Unlike positive displacement compressors, dynamic compressors work at a constant pressure and are categorized based on their axial or radial design. The performance of dynamic compressors is affected by external conditions, such as changes in inlet temperatures.

During dynamic compression, air is drawn between the blades on a rapidly rotating compression impeller as it accelerates to a high velocity. Subsequently, the gas is discharged through a diffuser, where the kinetic energy is transformed into static pressure. Depending on the main direction of the gas flow, these compressors are called radial or axial compressors, all of which are designed for large volume flow rates. 

Dynamic compressors are often referred to as turbo compressors for its ability to produce significant horsepower.

Understanding the key differences between positive displacement and dynamic compression will help you choose the right compressor for your application needs. To learn more about different types of compression and compressor technology, download a copy of Atlas Copco’s Compressed Air Manual.