There are two generic principles for compression of air or gas — positive displacement compression and dynamic displacement compression.
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:
The core operating principle of a positive displacement compressor is straightforward: air is trapped in a confined space, and that space is mechanically reduced to increase the pressure of the air inside. Unlike dynamic compressors, which rely on high-speed rotating impellers to add velocity to the air, positive displacement compressors physically squeeze a fixed volume of air into a smaller space with each compression cycle.
This mechanical approach to compression gives positive displacement compressors a key characteristic: they deliver a consistent flow of compressed air regardless of the outlet pressure. As long as the compressor is running, it continues to trap and compress air in fixed volumes, making it a reliable and predictable source of compressed air for a wide range of industrial applications.
The way this compression is achieved varies depending on the type of positive displacement compressor. In a piston compressor, a reciprocating piston moves back and forth inside a cylinder to compress the air. In a rotary screw compressor, two interlocking helical rotors trap and compress air as they rotate. In a scroll compressor, one spiral element orbits around a fixed spiral to progressively compress the air toward the center. And in a tooth compressor, two synchronized rotors with tooth profiles compress air as they rotate in opposite directions. While each design is mechanically distinct, they all share the same fundamental positive displacement principle.
Positive displacement compressors offer several characteristics that make them the preferred choice for the majority of industrial compressed air applications:
Consistent output pressure: Because positive displacement compressors compress a fixed volume of air with each cycle, they are able to maintain a stable output pressure even as demand fluctuates. This makes them well suited for applications that require a reliable, steady supply of compressed air.
Wide pressure range: Positive displacement compressors are capable of generating a wide range of discharge pressures, from low pressure applications all the way up to very high pressure requirements. This versatility makes them suitable for everything from general manufacturing to specialized high pressure processes.
Suitability for variable demand: Positive displacement compressors can be paired with variable speed drive technology to match their output to the actual air demand of an operation, delivering significant energy savings in applications where demand fluctuates throughout the day.
Self-priming capability: Positive displacement compressors are inherently self-priming, meaning they can start and operate without requiring the compression chamber to be pre-filled. This simplifies installation and improves operational reliability.
Broad application range: From small workshop piston compressors to large industrial rotary screw compressors, positive displacement compressors are available in a wide variety of sizes and configurations to suit virtually any application or industry.
Positive displacement compressors are found across virtually every sector of industry, powering a broad range of processes and applications. Some of the most common include:
In manufacturing, positive displacement compressors provide the compressed air needed to power pneumatic tools, actuators, clamping systems, and conveying equipment. In the automotive industry, they deliver the air that drives assembly line tools and robotic systems. In food and beverage production, oil-free positive displacement compressors supply clean, uncontaminated air for packaging, conveying, and process applications. In construction, portable positive displacement piston compressors power drills, jackhammers, and spray equipment on job sites. And in the oil and gas industry, large-scale positive displacement compressors are used for gas gathering, compression, and transportation.
The common thread across all of these applications is the need for a reliable, consistent supply of compressed air at a predictable pressure, which is precisely what positive displacement compressors are designed to deliver.
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.
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