Types of Air Compressors: Working Principles, Classification, Applications

• All compressors fall into 2 major categories i.e. Positive-Displacement and Dynamic categories.
• Positive displacement compressors work by trapping a fixed volume of air and mechanically reducing the space it occupies (e.g., pistons, screws).
• Dynamic compression imparts velocity energy to air via high-speed impellers, which is then converted into pressure.
• Reciprocating air compressors are ideal for high-pressure requirements but produce pulsating flow.
• Rotary Screw compressors are preferred for continuous duty and compact installation with pulsation-free discharge.
• Centrifugal compressors are best suited for very high-volume, oil-free applications.

1. Positive-Displacement Compressors

In these compressors, a specific quantity of air is trapped in a chamber. The volume of that chamber is then reduced, causing the pressure to rise before the air is discharged. A key characteristic is that at a constant speed, the airflow remains relatively constant regardless of discharge pressure.

A. Reciprocating Compressors

The most widely used type for both air and refrigeration, reciprocating compressors work similarly to a bicycle pump.

• Mechanics: A piston moves within a cylinder, driven by a crankshaft and connecting rod.
• Configurations:
  • Single-Acting: Compression happens on only one side of the piston.
  • Double-Acting: Both sides of the piston are utilized for compression.
  • Multistage: To avoid excessive heat, air is compressed in stages with cooling (intercoolers) in between.
• Best For: High-pressure applications and facilities with intermittent demand.

B. Rotary Compressors

Instead of pistons, rotary compressors use rotating elements to compress air. They are known for continuous, pulsation-free discharge and high speeds.

• Rotary Screw (Helical-Lobe): Features two intermeshing rotors (male and female) that trap and compress air as they rotate in opposite directions.
• Rotary Vane: Uses a rotor with sliding vanes that create compression cells.
• Scroll: Employs two spiral elements—one stationary and one orbiting—to compress air.
• Best For: Continuous duty, compact installations, and sizes ranging from 30 to 335 hp.

2. Dynamic Compressors

Dynamic compressors impart kinetic energy (velocity) to a continuous flow of air using high-speed impellers. This velocity is then converted into pressure energy through diffusers or volutes.

A. Centrifugal Compressors

These are the most common dynamic type of compressor used in industrial settings.

• Working Principle: Air enters the center of a rotating impeller and is thrown outward at high speed. A stationary diffuser slows the air down, converting speed into pressure.
• Oil-Free by Design: Since the lubricated gears are separated from the air chamber by atmospheric vents and seals, they are inherently oil-free.
• Best For: Very large volume requirements and applications where oil-free air is mandatory.

Comparison Chart: Types of Compressors

Selection Criteria: Key Questions to Answer

When choosing a compressor, engineers should consider the following:

• Duty Cycle: Is the demand constant or intermittent?
• Pressure Requirement: What is the minimum pressure needed at the furthest tool?
• Air Quality: Is oil-free air required for the process?
• Life Cycle Cost: Does the higher efficiency of one type justify its higher capital cost?

Selecting the right compressor is a balance of performance, cost, and application needs. While reciprocating compressors excel at high pressures, rotary screws offer installation flexibility and continuous flow. For massive, oil-free requirements, centrifugal compressors remain the gold standard.