January 29, 2020 Air Compressors

What is an air compressor? A look at the types and working principles.

An air compressor is a power tool that is used to convert power into energy, usually stored as pressurized air. Air compressors several methods to force more air into a cylinder/cylinders, thereby increasing the pressure. The compressor shuts off when the pressure reaches its upper limit, and it rests in the tank until a need arises.

The compressed air can be used for a variety of applications by using kinetic energy as the tank depressurizes. When the compressed air reaches its lower limit, the air compressor turns on again to re-pressurize.

The main difference between an air compressor and a pump is that an air compressor works on gas/air while the latter works on liquids. Air compressors are designed to use several sources of power, including electric motors, gasoline/ diesel engines, vehicle engines, or power-take-off ports.

Air compressors are primarily categorized according to the amount of pressure discharged i.e.

• • Low-pressure air compressors, which deliver a pressure of 150 psi and below,
  • Medium-pressure compressors, which provide a pressure of 151 – 1000 psi,
  • High-pressure air compressors, which deliver a pressure above 1,000 psi.

The process of air compression can be divided into two methods, including positive displacement and dynamic displacement.

Positive displacement

Positive-displacement involves forcing air in a chamber, which is followed by decreasing the volume to compress the air. When the maximum pressure is achieved, the compressed air is forced out of the compression chamber into the outlet system.

Dynamic displacement

Dynamic displacement compressors utilize a rotating component to impart kinetic energy to the air, which is converted to compressed air. The spinning part accelerates and decelerated the air, which in turn becomes pressurized.

Types of compressors and their working principle

1. Reciprocating Air Compressors

A reciprocating air compressor is a positive displacement type of compressor that utilizes a piston to compress and displace air within the compression chamber. These compressors have one or more cylinders, crankshaft, valve head, and they can be powered by a gas engine or electric motor, which allows for ease of mobility and remote use.

They also have more moving parts than other types of compressors, and you’ll need to lubricate them often. The pressurized air is then released into a storage tank through a valve where it is held until called for use.

Reciprocating compressors come in single or two-stage variations, and this affects the amount of pressure delivered.

i Single-stage compressor

In a single-stage compressor, the air is compressed only once. It involves the following steps;

• • • Air is drawn into the compression cylinder.
    • The air is then compressed with a piston in a single stroke.
    • The air is finally discharged to a storage tank.

ii Two-stage compressor

A two-stage compressor follows the same process as a single-stage compressor, but the compressed air is sent for a second stroke before it is drawn to a storage tank. The double-pressurized air is then stored, and it is used to power high-powered equipment.

Reciprocating compressors can be used in various applications, including natural gas processing, oil refineries, and chemical plants. Also, they play a significant role in refrigeration technology, automotive workshops, and agriculture. In addition, reciprocating compressors are more energy-efficient, but they require more maintenance than other compressors in the same range.

2. Rotary-screw compressors

Rotary-screw compressors, as the name suggests, use two helical screws that rotate to force air into a chamber. As the screws turn, the air gets trapped, and thus, pressure builds up within the chamber. And as the air moves through the rotors, there is a decrease in space between the chamber and the rotors, and this is what leads to compression of the. The pressurized air is then displaced to the outlet.

These compressors are the easiest type of compressors to maintain as they come with an internal cooling system to help reduce excess heat. The operation of a rotary-screw compressor differs from that of a reciprocating machine in that the former generates power energy in a continuous sweeping motion and with very minimal pulsation.

Also, rotary-screw compressors come pre-lubricated to offer smooth operation, but some are oil-free. However, you’ll still need to lubricate the bearings. Typically these compressors are large and are designed for continuous use at large construction sites and applications with a power range of 5 horsepower up to 350 horsepower.

3. Rotary vane compressors

Rotary vane compressors, on the other hand, utilize a slotted rotor with blades placed all around the rotor, which guide and compress air within the chamber. The blades move in and out of the rotor, thus forming some compression pockets. As the rotor turn, the volume of air is reduced gradually, and this raises the pressure. This technology is extremely efficient and provided extended years of service.

Rotary vane compressors provide superior energy across different levels of air demand from agriculture and pharmaceutical applications to automotive. Also, they provide continuous energy power while consuming less energy than their rotary-screw counterparts.

4. Rolling piston compressors

These compressors use a rolling piston to force/compress air against a stationary vane. The piston acts as a partition between the rotor and the vane, and it causes compression of atmospheric gases by reducing the volume of the chamber. Ideally, rolling air compressors are designed to work at higher speeds than the ordinary reciprocating pistons, and they ensure a continuous flow of compressed air. These compressors are mainly used in cooling applications that not exceed 15 kW.

5. Ionic liquid piston compressors

Ionic liquid piston compressors use an ionic liquid piston instead of the ordinary metal piston to pressurize the air. This liquid piston generally reduces the need for bearings and seals, which helps to improve the efficiency of the compressor. These compressors are particularly useful in applications involving highly volatile gases such as in hydrogen filling stations and other hydrogen-based applications.

6. Centrifugal compressors

Centrifugal compressors are rotary kind of compressors that use a very high-speed spinning disk to drive gases into the cylinder where they are converted into pressure energy. Also, they have a diffuser (divergent duct), which is responsible for converting velocity energy to pressurized air.

These compressors are primarily used for continuous commercial applications such as industrial refrigeration, petrochemical, and natural gas applications. Also, they capable of high pressures of above 1000 psi, and their application can be thousands of horsepower. These compressors also have wide applications in snowmaking operations, and also they are used in combustion engines and medium-sized gas turbines.

7. Scroll compressors

Scroll compressors feature two interleaved spiral-shaped vanes to compress air or liquids. Generally, one of the vanes is fixed, while the other one orbits eccentrically, enabling them to trap and compress air in between before it is finally discharged at the outlet port.

Also, these compressors tend to have high volumetric efficiency due to the reduced volume clearance between the fixed and the orbiting vane and, also the absence of suction valves that lead to pressure losses.

Engineers and manufacturers find these compressors useful for several reasons. One, these compressors have few moving parts, which means less maintenance and higher reliability. Also, the design runs quietly, and the gas flow is continuous, which reduces any chances of gas pulsation that can cause noise.

8. Diagonal compressors

Diagonal compressors are also referred to as mixed-flow compressors and have a similar working principle as centrifugal compressors. However, diagonal compressors have an axial and radial high-velocity disk at the exit from the rotor. Also, the diffuser is used to produce a diagonal flow from an axial direction instead of a radial one. When compared to a conventional centrifugal compressor with a similar stage pressure ratio, diagonal compressors may have 1.5 times more speed, and they are widely used in aeronautics.

9. Axial compressors

Axial compressors use an array of fan-like airfoils to compress air progressively. The airfoils are designed in rows, usually in pairs, one stationary and one rotating. The rotating foils also referred to as blades or rotors, accelerate the air. The stationary foils, on the other hand, decelerate the air and redirect it to rotor blades for the next stage of compression.

Most axial compressors are multi-staged, and they are preferred where high airflow or compact construction is needed. This makes them perfect for medium to large gas turbines, chemical plants, and pumping stations. However, they are relatively expensive and require high-quality materials.

Conclusion

In selecting an air compressor, the choice will generally come down to energy output levels. There are several factors you’ll need to consider when choosing the right type of air compressor. These include energy efficiency, portability limitations, and the need for additional features such as thermal overload protection and belt-drive systems. The different types of compressors come with their unique capabilities and downsides, and ultimately, you will want to ensure the compressor you choose will be compatible with your tools.

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