Compressors are essential machines used to increase the pressure of air and other gases for industrial, commercial, and process applications. Although compressors are available in many designs, their operation is generally based on two fundamental compression methods: positive displacement compression and dynamic compression.
Understanding the difference between these two principles helps engineers, maintenance teams, and plant operators select the appropriate compressor for a particular airflow, pressure, and operating requirement.
What Are the Two Basic Compression Principles?
Gas compression can generally be divided into two categories:
- Positive displacement compression
- Dynamic compression
Both methods increase gas pressure, but they achieve this in different ways.
Positive Displacement Compression
A positive displacement compressor first draws a specific quantity of air or gas into a compression chamber. The chamber is then isolated from the inlet, and its internal volume is reduced mechanically. As the available volume decreases, the pressure of the trapped gas increases.
Once the required pressure is reached, the compressed gas is released into the discharge system.
This principle is used in reciprocating piston compressors, rotary screw compressors, scroll compressors, rotary vane compressors, and rotary tooth compressors.
Dynamic Compression
Dynamic compressors work differently. Instead of trapping a fixed quantity of gas and reducing its volume, they continuously accelerate the flowing gas using a high-speed impeller or rotor.
The gas gains kinetic energy as it passes through the rotating blades. A diffuser or similar stationary component then slows the gas down and converts part of its velocity energy into static pressure.
Centrifugal and axial compressors are common examples of dynamic compressors.
Positive Displacement vs Dynamic Compressors
The two compression methods differ in their operating principles, pressure-flow characteristics, speed, construction, and typical applications.
Feature | Positive Displacement Compressor | Dynamic Compressor |
Compression method | Traps and compresses a fixed volume | Accelerates continuously flowing gas |
Flow characteristic | Relatively stable | Strongly affected by operating conditions |
Pressure capability | Suitable for low to high pressure applications | Commonly used for high-flow applications |
Operating speed | Low to moderate depending on design | Generally high speed |
Typical applications | Manufacturing, workshops, process plants | Large industrial and process installations |
Common types | Piston, screw, scroll, vane | Centrifugal, axial |
Construction | Mechanical displacement mechanism | High-speed aerodynamic design |
Flow control | Often easier to control over a broad range | Requires careful control near operating limits |
The appropriate compressor depends on the required pressure, flow rate, duty cycle, gas characteristics, energy consumption, and application.
What Is a Positive Displacement Compressor?
A positive displacement compressor works by capturing a defined quantity of gas and mechanically reducing the space occupied by that gas.
A simple bicycle pump demonstrates the basic principle. When the piston moves, air enters the cylinder. When the piston moves in the opposite direction, the available space decreases and the trapped air is compressed.
Industrial reciprocating compressors use the same fundamental concept, although they incorporate valves, cylinders, crankshafts, connecting rods, lubrication systems, cooling arrangements, and other components.
Depending on the compressor design, compression may take place on one side of the piston or on both sides.
Single-Acting Compressor
In a single-acting reciprocating compressor, compression takes place on one side of the piston during the compression cycle.
Double-Acting Compressor
A double-acting compressor uses both sides of the piston for compression, allowing compression to occur during both directions of piston movement.
The pressure ratio of a compressor is generally determined using absolute inlet and discharge pressures.
For example, if air enters at approximately 1 bar absolute and is discharged at 8 bar absolute, the pressure ratio is:
Pressure Ratio = Discharge Absolute Pressure ÷ Inlet Absolute Pressure
Pressure Ratio = 8 ÷ 1 = 8
Types of Positive Displacement Compressors
Several compressor technologies operate according to the positive displacement principle.
1. Reciprocating Compressors -: Reciprocating or piston compressors use pistons moving inside cylinders to compress air or gas.They are widely used where relatively high pressures are required and are available in single-stage and multi-stage configurations.
2. Rotary Screw Compressors -: Rotary screw compressors use rotating helical rotors to continuously compress air.Unlike piston compressors, they provide a continuous supply of compressed air with comparatively low pulsation.Rotary screw compressors can be available in lubricated and oil-free configurations depending on the required air quality.
3. Scroll Compressors -: Scroll compressors use two spiral-shaped elements. One scroll remains stationary while the other moves in an orbital pattern.The movement progressively reduces the volume of the gas pockets and compresses the gas.Scroll technology is commonly used in applications requiring compact equipment and relatively smooth compression.
4. Rotary Vane Compressors –: Rotary vane compressors contain a rotor fitted with sliding vanes. As the rotor turns inside the housing, the vanes create chambers of changing volume.The reduction in chamber volume compresses the trapped air.
5. Rotary Tooth Compressors -: Rotary tooth compressors use specially shaped rotors that rotate within the compression chamber. Their design allows continuous compression and can be used in applications requiring specific airflow and pressure characteristics.
How Does a Positive Displacement Compressor Work?
The exact operating cycle depends on the compressor type, but the basic principle remains the same: capture gas, reduce its volume, and discharge it at higher pressure.
In a reciprocating compressor, for example, the piston moves through a suction stroke and a compression stroke.
During the suction stroke, the inlet valve opens and air enters the cylinder. The piston then changes direction, reducing the cylinder volume. As the pressure rises, the discharge valve opens and compressed air leaves the cylinder.
In an actual compressor, the process is more complicated than the theoretical cycle.
Factors such as:
- Valve pressure losses
- Leakage
- Clearance volume
- Gas temperature
- Heat transfer
- Valve response
- Friction
- Mechanical losses
can influence compressor performance.
Clearance Volume
A small amount of space must remain inside a reciprocating compressor cylinder when the piston reaches its end position. This is known as clearance volume.
Gas remaining in this space expands during the next suction cycle, reducing the amount of fresh air that can enter the cylinder.
Therefore, actual compressor ca pacity can differ from the theoretical displacement calculated from cylinder dimensions and piston movement.
What Is a Dynamic Compressor?
A dynamic compressor increases gas pressure by continuously transferring energy to the flowing gas.A high-speed impeller accelerates the incoming air or gas. The gas leaves the impeller with increased velocity and kinetic energy.The diffuser or other stationary components then reduce the gas velocity and convert a portion of that kinetic energy into pressure.
This process allows dynamic compressors to handle large volumes of gas at high flow rates.
Dynamic compressors are commonly divided into:
- Centrifugal compressors
- Axial compressors
Centrifugal Compressors
A centrifugal compressor uses a rotating impeller to accelerate the gas outward from the center of rotation.
The high-velocity gas then enters a diffuser or volute where its velocity is reduced and pressure increases.
Centrifugal compressors are frequently used in large installations where substantial volumes of compressed air or gas are required.
Applications may include:
- Large manufacturing plants
- Power generation
- Process industries
- Petrochemical facilities
- Large HVAC systems
- Industrial gas applications
For very large airflow requirements, centrifugal technology can offer an efficient solution when operated within its intended operating range.
Axial Compressors
In an axial compressor, gas travels approximately parallel to the shaft or axis of rotation.Multiple rotating and stationary blade rows progressively increase the pressure of the flowing gas.Axial compressors are commonly associated with applications requiring extremely high flow rates, including gas turbines and aircraft engines. Their design is considerably different from conventional positive displacement compressors and requires precise aerodynamic and mechanical engineering.
Conclusion
Positive displacement and dynamic compressors use fundamentally different methods to increase gas pressure. Positive displacement compressors trap and mechanically compress a specific quantity of gas, while dynamic compressors continuously accelerate the gas and convert its kinetic energy into pressure.
Understanding these differences is important when selecting an industrial compressor. Factors such as flow demand, pressure requirements, operating conditions, energy efficiency, maintenance needs, and application type should all be evaluated before selecting equipment.
For industrial plants, choosing the appropriate compressor technology can help maintain stable compressed-air supply and support reliable production operations. Proper maintenance, timely replacement of compressor components, and correct operating practices are also important for maintaining compressor performance throughout its service life.