How it works

Air compressors have a wide range of uses at home and in industrial settings. Although they are common only in manufacturing plants before, they are now used in home basements and garages as a go-to device. They act as a power source for electric tools.

Air compressors are new machines. Before them, tools from shops obtain their power from a central power source with complicated systems of wheels, belts, driveshafts and other large components. Not only that these machines are enormous and heavy, but they are also costly and inaccessible to small operations [1, 2]. Nowadays, these versatile devices can already be used in powering pneumatic tools, such as impact wrenches and nail guns, in the working of air-conditioning units and even in inflating pool toys and tires. They have become less bulky and portable, and they are available in many shapes and sizes to suit different needs. This article will give you an overview of what air compressors are, their types and how they work.

What are air compressors?

Air compressors are mechanical devices that draw in air from the atmospheres, pressurise it and then release it as kinetic energy [3]. They supply air and offer pressure for electricity to all the equipment in a system [4].

Industrial air compressors feature large components, including air tanks, pumps, gauges and hoses, but there are also cordless and quick-connect hose types that are common for DIY home projects.

How do air compressors work?

Air compressors are more powerful than regular tools, and they require no bulky motors. Their portability and versatility make them essential in many factories and workshops, being used in different situations. To choose the right air compressor for a certain need or project, the way these machines work should be understood [4, 5, 6].

Air compressors work in two phases: compression and release. A key to how they work is their air end and gas engine components. The air end is the one that compresses air. It gets atmospheric air into a container called the compression chamber, where it is then forced into a compact space. The forcing of these air molecules closer together is what compresses air. 

During compression, a power source or the gas engine turns electrical energy to kinetic energy and forces the air end to move and then compresses air. This compressed air is what’s used to help a variety of tools to work.

Air compression can be done via either positive or dynamic displacement. With positive displacement, the air is forced into a chamber where the volume is decreased to compress it, and the system needs to attain its maximum pressure. Afterwards, the air from the compression chamber is released to enter into the storage tank with force.

Some air compressors with positive displacement mechanisms are used for industrial purposes, while others are for home projects. Positive displacement is also a term used to describe air compressors that compress air through positive air displacement.

Meanwhile, with dynamic displacement, a rotating blade powered by an engine is used to generate airflow. The air is then compacted to create pressure, and the kinetic energy is stored within the compressor. This is a slight variation from the other system with the use of impellers and rotating blades. Compressors of this type provide energy from the build air pressure because the blades are constantly moving. Dynamic displacement air compressors are typically used in large-scale projects, such as in chemical plants. They can’t be used for DIY activities.

What are the types of air compressors?

Air compressors can be differentiated by the type of their power source or the pressure output. However, the main difference among the types of air compressors is in their air displacement mechanisms.

Classification According to Power Source

The types of air compressors according to the power source are diesel-, gas- and electricity-powered compressors [3, 7, 8]. The way these compressors are powered largely affects their size, power and durability, thereby their performance.

Gas-powered compressors utilise gasoline to generate power, and their engines are separated from the pump systems. They are portable and better used for sites without electrical power systems. Additionally, these massive gas compressors operate continuously, which makes them ideal for any work that needs continuous pressure. Although gas compressors are cheap to buy, they may require high maintenance to keep them working well. Their fumes are also toxic, which can pollute the environment. 

Similar to gas compressors, diesel-powered ones are portable and require no electrical hook-up. Suited to heavy-duty applications, they can be more powerful than gas compressors of the same size, giving higher torque. Diesel may be more economical than gas, but diesel air compressors are more expensive than gas-powered compressors. Nevertheless, they are easier to maintain because of their simple ignition system.

Also, it is worth noting that other gas-powered air compressor models boast about hybrid internal combustion mechanisms, which allow operators to use either diesel or gas in one air compressor machine.

Electric air compressors are now the most common type when the power source is considered. They run on electric motors, and they can be used for either large-scale industrial set-ups or small home workshops because they produce no toxic exhaust fumes. Electric air compressors are also portable, but they can be limited by the length of their power cord, and they can’t be used with extension cords. These compressors are also less robust and powerful than their counterparts.

Classification According to Power Output

The classifications of compressors according to pressure output are low-, medium- and high-pressure air compressors [9, 10]. 

Low-pressure air compressors produce a maximum discharge pressure of 150 PSI. These compressors can be seen mainly in small machines and auto body repair shops. Medium-pressure air compressors release pressures of 151-1000 PSI, and they are used in different industries, such as in cement production plants. High-pressure air compressors discharge pressure greater than 1000 PSI, which is used in the operation of air pipes and clean pressure cutters, as well as in energy storage and transportation for mechanical work.

Classification According to Air Displacement

According to air displacement mechanisms, air compressors are classified as positive and dynamic displacement air compressors. Positive displacement air compressors include rotary screw, reciprocating/piston, scroll and vane types. The dynamic displacement types are centrifugal and axial compressors [2, 3, 5, 11, 12].

  • Piston/Reciprocating Air Compressors

These are the most common type of displacement compressors that work through a piston inside a cylinder. Piston air compressors pump air into their chambers, with inlet and outlet valves guiding it. They use piston rods that make downward and upward strokes. Air flows into the chamber with downward strokes and out to the storage tank with upward strokes. They are commonly used in small work worksites, not for continuous use.

Piston air compressors can also be single or two-stage type. Both of these types complete full compression cycles or strokes. 

In single-stage compressors, a rotor spins, which forces the piston to move up and down. The valve opens and the air is pulled into the chamber when the piston moves downward. When it moves up, the air is forced into the output chamber and through the output. Most of these models run quietly, and they are relatively cheap compared with others.

When more power is needed, two-stage compressors are used. They feature two compression chambers on either side of the piston. The rotor spins to operate these two pistons at the same time. The large piston gets air in and pushes it out to the intercooler, which uses water to cool the air. The small piston takes in a large volume of air to pressurise it and then forces the pressurised air into the output. Two-stage compressors are common in large factories, not for small projects.

  • Rotary screw compressors

Typically used in large industrial machines, rotary screw compressors are easy to operate and maintain. They have two helical screws that guide the air into the chamber. These two screws rotate in opposite directions, which traps and compresses the air between them. When the screws turn, their motion creates a vacuum, and trapped air is compressed as it goes through the screw. Compressed air is forced through the output.

  • Scroll compressors

Scroll compressors make use of orbiting and stationary spirals. When the orbiting spirals trace the path of the fixed spirals, the volume of space between them is reduced. Air is taken in at the outer edge of the scrolls, and the compressed air is discharged near the centre. These compressors need no lubricating oil because the scrolls make no contact. However, in the absence of oil to remove the heat of compression, the capacity of this type is limited. It’s often used in home air-conditioning compressors.

  • Rotary vane compressors

Vane compressors release specific volumes of air at high pressures. Their built-in rotors have variable arrangements of blades, which guide the air into the chamber before compression. The working principle of this type is similar to that of rotary screw compressors. Instead of screws, vanes are on the rotor and being rotated.

In these compressors, the spinning rotor or motor is at the off-centre of a rounded cavity. This rotor has blades that automatically adjust their arms to create various sizes of cavities. When the arms approach the air input, they become longer, which results in large cavities. As the motor spins and the air moves with it, the arms go near the output and become smaller. This produces a smaller space between the vanes and the casing, thereby compressing the air.

  • Centrifugal compressors

Centrifugal or radial compressors are dynamic displacement compressors. They usually use electric high-speed motors and are applied in heating, ventilation and air-conditioning units.

Centrifugal compressors bring air into the centre through a high-speed rotating fan. A centrifugal or outward force expels the air that hits the fan radially outward. The air travels around the casing, and it slows down and increases in pressure when it passes through a diffuser. This generates more kinetic energy. The pressurised air is then released through the outlet pipe.

These compressor types are used for higher-capacity commercial and industrial applications because of their continuous flow through. They can produce oil-free air for some types, and their maintenance is lesser than that of reciprocating or rotary screw compressors.

  • Axial compressors

Axial compressors have large fans that pull air at high velocity. What guides the air to the next fan is the stationary fan blades, which slow air pressure and increase pressure. The air becomes pressurised due to its high kinetic energy when the space between fans becomes smaller. Again, the pressurised air is forced through the outlet.

These types of air compressors are limited in their functions, which makes them uncommonly used. Axial compressors can be seen mainly in large air separation plants, as well as in aircraft engines.

How do you find the right compressor?

Compressed air is now used in everyday life. It’s needed in many air-powered tools and devices that make life comfortable and efficient. With such uses and the many kinds of compressors available, how do you determine the right compressor for your needs?

Although the type of compressor one should buy will be mainly dictated by the cost and the needs or applications for which the compressor will be used, other factors can also be considered [5].

  • Air quality

Many compressor types need oil to lubricate their mechanisms. The use of oil can create environmental problems because of the toxic fumes that can be released. This risk can be reduced by using oil-free compressors. The problem with such a compressor is that they are more expensive. Most facilities that need a clean manufacturing environment use oil-free compressors despite the price.

  • Energy use

The most energy-efficient compressors are the best types, especially for long construction projects. Energy efficiency can be determined by the compressors’ speed variability and use of natural gas. Variable speed-driven compressors increase or decrease output on demand, which saves money and energy. By contrast, a fixed-speed compressor needs constant movement at the same rate, which wastes energy during the cooling period. This is because the compressor still runs without producing power.

With regard to the use of gas, natural gas compressors work more efficiently and provide better heat recovery capacities than those powered by electricity or diesel.

  • Portability

If the application requires transferring from one location to another, getting a portable compressor unit is the best option. These small, lightweight compressors may not produce as much power as the large ones, but they can still provide energy that can power small construction projects.