Understanding Compression Ratios in Different Types of Compressors
The compression ratio in a centrifugal compressor is a key parameter defined as the ratio of the outlet pressure to the inlet pressure of the gas being compressed. Mathematically, it is expressed as follows:
Compression Ratio Pout / Pin
where:
Pout is the pressure at the outlet of the compressor.
Pin is the pressure at the inlet of the compressor.
Compression Ratios of Different Types of Compressors
Centrifugal Compressors
Centrifugal compressors are designed for high-flow applications. Their typical compression ratios range from 2:1 to 6:1 per stage. Multi-stage centrifugal compressors can achieve even higher ratios, sometimes exceeding 20:1. For instance, a single-stage centrifugal compressor can typically output around 2.5x, and multi-stage machines can reach up to 20:1 or more.
Axial Compressors
Axial compressors are renowned for their ability to handle higher compression ratios compared to centrifugal types. They typically have compression ratios ranging from 3:1 to 10:1 per stage. Multi-stage configurations can achieve ratios much higher, often above 30:1. Axial compressors are often grouped in series, and if we assume a conservative value of 2.3x per stage for a 10-stage machine, the total compression ratio could reach 23:1.
Positive Displacement Compressors
Positive displacement compressors, such as reciprocating compressors, have a wide range of compression ratios. Single-stage units generally have ratios from 3:1 to 10:1, while multi-stage configurations can achieve ratios up to 30:1 or even more. These compressors are ideal for low-flow applications with high pressure requirements.
Rotary Screw Compressors
Rotary screw compressors typically have compression ratios around 4:1 to 7:1, depending on the design and application. They are known for their efficient operation in various industrial settings.
Scroll Compressors
Scroll compressors generally achieve compression ratios of about 3:1 to 5:1. These compressors are used in a variety of applications, from air conditioning to industrial processes.
Considerations and Limitations
Different compressor types are designed for specific applications and operating conditions, which influence their compression ratios. Centrifugal and axial compressors are often used in high-flow applications, while positive displacement compressors are ideal for lower flow but higher pressure applications.
Centrifugal compressors can be designed in one stage or multiple stages in series. A single-stage centrifugal wheel typically outputs around 2.5x, and multi-stage machines can achieve up to 20:1 or more.
Axial compressors are always grouped in series. Assuming a conservative value of 2.3x per stage, a 10-stage machine could achieve a compression ratio of 23:1.
Positive displacement machines like Roots blowers, lobe or twin screw machines, diaphragm machines, rotary vane scroll, piston, liquid ring, and rotary lobe Wankel compressors are limited by the heat of compression. Most of these machines are limited not by power requirements, but by the heat of compression that must be managed through cooling (intercooling). This management is essential to avoid thermal growth, loss of efficiency, seizing up, or failure due to lubrication temperature limits, which generally do not exceed 175°F (80°C).
For example, while a piston compressor might be able to achieve a compression ratio as high as 15:1 or more, the practical limit for a single-stage non-intercooled machine is often around 10:1 due to the lubrication temperature limitations.
Another limitation is the length of the shaft for twin screw compressors, which affects the number of "twists" that can be designed into the shaft. This also impacts the efficiency and reliability of the machine. Additionally, some compressors need oil or another liquid for sealing the rotor at its perimeter for efficiency, while others must be "dry" compressed to meet specific process requirements.
Understanding the specific needs of your system will help in selecting the appropriate type of compressor. The choice should be based on the application, operating conditions, and the desired performance parameters.