What Is a Gas Compressor?
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume or by imparting velocity to the gas. This can be done in numerous different types of gas compressors.
Two Broad Categories of Gas Compressors
Gas compressors fall into two major categories: Positive Displacement Compressors and Dynamic Compressors.
Positive Displacement Compressors
Positive displacement compressors are machines in which successive volumes of gas are confined within a closed space; the volume of the confined space is then decreased, thus increasing pressure.
Dynamic Compressors
Dynamic-type compressors are machines in which gas is compressed by the mechanical action of rotating vanes or impellers, imparting velocity to the gas.
Types of Positive Displacement Compressors
Within the positive displacement category, there are two primary types:
Reciprocating Compressors
The compressing element in this type is a piston that has a reciprocating motion inside a cylinder. Compressor capacity and horsepower are affected by piston displacement and cylinder clearance. Volumetric efficiency is a function of cylinder clearance, compression ratio and the properties of the gas being compressed.
Rotary Compressors
Compression is affected by the positive displacement action of rotating elements. Two very popular types are:
- Rotary screw compressors use two rotating helical screws to compress the gas. Compression takes place between the rotors and the discharge end of the compression chamber.
- Rotary sliding vane compressors consist of an external casing called a cylinder, the rotating element inside called the rotor, and sliding vanes. The rotor is eccentrically located in the cylinder. The rotor has longitudinal slots that hold the vanes. As the rotor turns the vanes slide outward, due to centrifugal force, to reach the inner walls of the cylinder. The vanes will trap a pocket of gas as they pass by the compressor inlet. As the rotor continues to rotate, the volume of that pocket of gas is decreased due to the eccentricity of the rotor. The volume continues to be compressed until the leading vane reaches the discharge port, where the compressed gas is released
Types of Dynamic Compressors
Dynamic compressors also split into two major categories:
Axial Compressors
An axial compressor is a rotating, airfoil-based machine where the gas, usually air, moves parallel to the axis of rotation. It features multiple stages, each comprising a rotor and a stator. The rotor increases the air’s kinetic energy, which is then diffused (the air velocity is reduced) in the stage, increasing the static pressure at the stage’s outlet. Axial compressors are high-flow, low-pressure machines, in contrast to the lower-flow, high-pressure centrifugal compressors. Axial compressors are more efficient than centrifugal compressors. However, centrifugals are less vulnerable and, hence, more reliable, have wider operating ranges, and are less susceptible to fouling.
Centrifugal Compressors
A centrifugal compressor imparts kinetic energy to the gas stream by increasing the air’s velocity using a rotating element called an impeller. Then it converts this kinetic energy into potential energy in the form of pressure. The gas enters the impeller at the center, and the rotating fins, or vanes, increase its velocity as it moves radially outward to the impeller’s outer diameter. Due to the continuous flow through multiple stages, centrifugal compressors are designed to thrive in higher capacities and are best suited for applications above 200 total horsepower.
Key Takeaways
- Gas compressors are broadly divided into positive displacement and dynamic types.
- Positive displacement compressors include reciprocating and rotary types.
- Dynamic compressors include axial and centrifugal types, with centrifugal ideal for higher-capacity applications.
Choose the type based on your gas properties, flow requirements, and pressure levels, and let Ro-Flo’s expertise help you make the right choice.
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