5.1: What are Compressors?

Compressors are components in pneumatic systems that convert mechanical energy from a prime mover (such as an electric motor or internal combustion engine) into the potential energy of compressed air. They supply air at a sufficient pressure and volume to perform useful work within a system.

There are two main categories of air compressors:

1. Positive Displacement Compressors – Increase pressure by reducing air volume in a confined space.
2. Dynamic Compressors – Increase pressure by adding kinetic energy to moving air and then converting it to pressure energy.

Each type has various subcategories that cater to different industrial needs.

Positive Displacement Compressors

A positive displacement compressor delivers compressed air into a receiver tank using a moveable component inside a housing. These compressors are generally not affected by changes in working pressure, aside from minor losses due to leakage and volumetric inefficiency.

Piston Type Compressors

The most common type of positive displacement compressor is the reciprocating piston compressor. It consists of a piston inside a bore, driven by a crankshaft connected to a prime mover. The compressor operates through a series of suction and compression strokes:

How a Reciprocating Piston Compressor Works

1. Intake Stroke: The crankshaft pulls the piston downward, creating an increasing volume in the bore, which lowers pressure. When the pressure differential is sufficient, the inlet valve opens, allowing atmospheric air to enter.
2. Compression Stroke: The piston moves upward, compressing the air. As the pressure rises, the outlet valve opens, allowing compressed air to flow into a storage tank.

Single-stage piston compressors operate up to 100 psig, but industrial applications often require higher pressures, leading to the use of two-stage compressors, which compress air in multiple stages to increase efficiency and reduce heat buildup.

Vane Compressors

A vane compressor consists of a housing, a rotor, and multiple floating vanes. As the rotor turns, centrifugal force pushes the vanes outward, forming air pockets. These pockets decrease in volume as the rotor turns, compressing the air.

Advantages of Vane Compressors

• Lower initial cost for small applications.
• Minimal starting torque requirements.
• Suitable for pressures up to 150 psig.

Helical (Screw) Compressors

Helical compressors, also called screw compressors, use two meshing rotors to compress air. Air is drawn in at one end and compressed as it moves toward the discharge end.

Types of Screw Compressors

1. Dry Helical Compressors: Use timing gears to maintain clearance between rotors, eliminating the need for lubrication.
2. Oil-Flooded Compressors: Use oil for lubrication and sealing, improving efficiency but requiring oil separation downstream.

A variation of this type is the single screw compressor, which uses a single rotor and offers low noise and vibration.

Lobed-Rotor Compressors

Lobed-rotor compressors, similar to dry helical compressors, employ a timing mechanism to prevent rotor contact. While efficient at maximum speed, they experience slip losses at higher pressures. These compressors can reach pressures up to 250 psi with power ranges from 7 to 3000 hp.

Dynamic Compressors

Unlike displacement compressors, dynamic compressors increase pressure by accelerating air and then converting velocity energy into pressure energy.

Centrifugal Compressors

Centrifugal compressors use an impeller to generate centrifugal force, which pushes air into a diffuser where its velocity decreases and pressure increases.

Key Features

• Ideal for high-volume, low-pressure applications.
• Sensitive to fluctuations in demand; running below rated flow can cause a surge, damaging components.

Radial Compressors

Radial compressors operate at very high speeds (20,000–100,000 rpm) and are commonly used in aerospace and industrial applications.

Axial Compressors

Axial compressors move air parallel to the rotating assembly. They provide constant airflow at moderate pressures (around 90 psi) and are typically used in high-volume applications such as blast furnaces and aircraft engines.

Multi-Stage Compressors

For higher efficiency, multi-stage compressors compress air in multiple stages. The two-stage piston compressor is a common example.

How a Two-Stage Piston Compressor Works

1. First Stage: A large piston compresses air and sends it to an intercooler, where it is cooled before entering the second stage.
2. Second Stage: A smaller piston further compresses the air before final storage.

By cooling air between stages, two-stage compressors reduce the energy required for compression and prevent excessive heat buildup.

Compressor Output Control

To maintain system efficiency and safety, compressor output must be regulated. Several methods exist:

Unloading Methods

1. Bypass Control: Releases compressed air into the atmosphere, keeping the compressor running at full load (inefficient).
2. Start-Stop Control: Turns the motor on and off based on system pressure (may lead to excessive motor wear).
3. Inlet Valve Regulation: Keeps the inlet valve open when demand is low, reducing compression.
4. Inlet Throttling: Restricts the inlet port to control airflow, though it maintains high power consumption.
5. Total Inlet Closure: Shuts the inlet valve completely, but this method is rarely used due to inefficiency.

System Efficiency and Losses

Even the best-designed pneumatic systems experience efficiency losses due to heat dissipation, friction, and airflow restrictions.

Heat of Compression

Compression increases air temperature, but much of this heat dissipates before reaching the point of work, reducing system efficiency. Intercoolers and aftercoolers help remove excess heat.

Frictional Losses

Efficient system design minimizes frictional losses by using properly sized pipes, reducing bends and elbows, and selecting the correct compressor type and size.