Working principle of compressor

Definition

A compressor is a machine that takes in a gas at low pressure, compresses it to a higher pressure, and delivers it at the required pressure for useful application. In simple words, it is a device that reduces the volume of a gas and thereby increases its pressure and temperature.

In thermodynamic terms, compression is a process in which work is done on a gas to raise its pressure, usually with a corresponding rise in temperature. The compressor is designed to perform this work efficiently and continuously.

Main Content

1. Compression of Gas by Volume Reduction

• The fundamental principle behind a compressor is the reduction of gas volume. When gas is confined in a smaller space, its molecules collide more frequently with the walls of the container, causing pressure to increase.
• This principle can be explained using Boyle’s law, which states that for a fixed mass of gas at constant temperature, pressure is inversely proportional to volume. As volume decreases, pressure increases.

In practical compressors, this volume reduction is achieved either mechanically or dynamically. For example, in a reciprocating compressor, a piston moves inside a cylinder and squeezes the gas into a smaller space. In a rotary compressor, rotating elements trap gas and compress it gradually. In a centrifugal compressor, the gas gains velocity and pressure through high-speed impeller action.

This concept is the core of the compressor’s functioning because every compressor must create a pressure difference between the inlet and outlet. The inlet side draws in low-pressure gas, and the outlet side discharges high-pressure gas after compression.

2. Work Input and Energy Conversion

• A compressor does not create pressure from nothing; it requires external energy to do work on the gas. This energy is usually supplied by an electric motor, engine, or turbine.
• The mechanical energy supplied to the compressor is converted into pressure energy and sometimes also into heat due to compression.

When the gas is compressed, the work done on it increases its internal energy. As a result, the temperature of the gas usually rises during compression. This is why the air coming out of an air compressor is often hot. In ideal systems, engineers try to minimize energy losses by making compression as efficient as possible.

Different compressors use different methods of energy conversion. In a reciprocating compressor, the motor drives a crankshaft and piston arrangement. In a screw compressor, rotating screws trap and compress the gas. In a centrifugal compressor, mechanical energy from the impeller is transferred to the gas as kinetic energy, which is then converted into pressure energy.

The efficiency of a compressor depends on how effectively the supplied work is used for compression rather than being lost as heat, friction, vibration, or leakage.

3. Compression Cycle and Pressure Delivery

• A complete compressor working cycle consists of suction, compression, and delivery. First, low-pressure gas enters the compressor. Then it is compressed to a higher pressure. Finally, the compressed gas is discharged to the required system.
• Valves, rotors, impellers, or vanes control the movement of gas during this cycle and ensure that the pressure is built up properly.

In a reciprocating compressor, the suction valve opens when the piston moves downward, allowing gas to enter the cylinder. When the piston moves upward, the suction valve closes and compression begins. Once the gas pressure becomes higher than the discharge pressure, the delivery valve opens and the gas leaves the cylinder.

In rotary and dynamic compressors, the same cycle occurs continuously, but without the same piston-type motion. Gas enters through the inlet, is compressed by rotating parts, and exits at higher pressure. The delivery side is usually connected to a receiver, storage tank, refrigerant line, or industrial pipeline.

This continuous pressure delivery is what makes compressors essential in systems that require a steady supply of compressed gas, such as pneumatic tools, refrigeration circuits, spray painting systems, and process plants.

Working / Process

1. The compressor first draws in low-pressure gas through the suction port or inlet when the pressure inside the compression chamber becomes lower than the atmospheric or supply pressure.
2. The gas is then trapped and compressed by the moving mechanical element, such as a piston, screw, vane, or impeller, which reduces the available volume and raises the pressure and temperature of the gas.
3. After the gas reaches the desired pressure, it is discharged through the delivery valve or outlet into the required system, where it is stored or used for practical applications.

Advantages / Applications

• Compressors provide high-pressure gas for many industrial and domestic applications, making them extremely versatile and useful in engineering systems.
• They are essential in refrigeration and air-conditioning systems, where they circulate refrigerant and maintain the pressure difference needed for heat transfer.
• They are widely used in pneumatic tools, air brakes, spray painting, gas pipelines, manufacturing plants, medical equipment, and chemical processing industries.

Summary

• A compressor works by reducing the volume of gas and increasing its pressure.
• It requires external work, which is converted into pressure energy and heat during compression.
• Its operation generally involves suction, compression, and delivery stages.
• Important terms to remember
• Compression
• Suction
• Delivery
• Pressure rise
• Volume reduction