Compressors

Table of Contents

What are Compressors?

Compressors are mechanical devices that are used to increase the pressure of compressible liquids or gases by decreasing the volume of it. The two mains components are a power source and a compressing mechanism. The temperature of the compressed substance is also increased as a result of this. Air compressors are the most common type of compressors. In this case atmospheric air is generally taken in. Some factors which affect the performance of the compressors are speed of rotation, pressure at suction, pressure at discharge and type of refrigerant being used. Real life applications include phase shifting refrigerants for refrigeration cycles, powering air tools and propelling gas through pipelines. They come in various sizes as some can fit in a glove box and others are found in huge plants [1]. The two main types of compressors are positive displacement and dynamic compressors. 

Types of Compressors

The two main types of compressors are positive displacement and dynamic compressors. These are then further divided into more types as seen in the diagram below.

Diagram showing different types of compressors

Positive Displacement Compressors

A Positive displacement compressor takes in a specific volume of air into a chamber and then reduces the volume of the chamber to compress the air. A mechanical linkage is responsible for the reduction of the chamber's volume. Positive displacement compressors can be broken down into two main categories known as reciprocating and rotary compressors.

Reciprocating Compressors

In reciprocating compressors, gases are drawn into a chamber by the inward stroke of the piston via a suction valve. The outward stroke of the piston then compresses this gas and it is positively displaced via the discharge valve. A cooling mechanism is put in place as heat is generated during the cycle. Single Acting reciprocating compressors have one inlet and one discharge valve resulting in one compression cycle for each time the crankshaft turns. In double acting reciprocating compressors, there are inlet and discharge valves on either end of the cylinder, This results in two compression cycles being completed from on crankshaft turn.

A diaphragm compressor compresses air via the up and down motion of a flexible diaphragm or membrane. This diaphragm is connected   a connecting rod and crankshaft which drives the up and down motion. When the diaphragm is pulled downwards air enters and when it it pushed up, air is discharged.

Diagram Showing Diaphragm Reciprocating Compressor

Rotary Compressors

In rotary compressors, the air is compressed by the rotation of a component inside. The rotating component determines which type of the rotary compressor it is. 

Type of Rotary compressor	Diagram	Description
Screw		Air enters the compressor through an inlet valve and becomes trapped between two asymmetrical rotating rotors with very little clearance. This rotation causes the pressure of the air to increase by reducing its volume.
Lobe		As the lobe rotates, the size of the cavity decreases which causes compression of the substance within.
Liquid ring		A liquid is fed into the compressor. The rotor produces a centrifugal force which creates a moving liquid ring to be formed along the inner walls. Multiple seals are created as a result of the ring. These seals accumulate between the vanes of the machine to form compression chambers. Air that enters the machine gets trapped inside the compression chambers. As the blades rotate, the liquid ring applies pressure to the gas within the chamber causing its volume to decrease and pressure to increase.
Scroll		There is a fixed scroll and a moving scroll. The air enters the scroll and moves towards the center. As the moving scroll rotates, the chambers become smaller and the air gets compressed due to the decrease in volume. The air is then discharged at the center of the scroll.
Vane		As the shaft rotates, the vane housing also rotate due to the sliding mechanism of the vanes, Air enters the largest opening and exits at the smallest opening. This reduction in volume causes the air to become compressed.

Comparison of Reciprocating and Rotary Compressors

Dynamic Compressors

In dynamic compressors, the inertial forces created by the blades of the machine drives the compression process. 

Key Differences Between Positive Displacement and Dynamic Compressors

References