The Two Main Types of Compressors to Know
Compressors are universal and are used in a wide variety of industries and businesses. These are the two main types of compressors, their differences, similarities, and how to tell which works best for you.
Have you ever wondered how a machine achieves what it does with the help of a compressor?
Then you’re not alone!
Their variety ensures that each is specifically tailored to serve a particular purpose and to the best of its ability.
That being the case, our review of main types of compressors and their applications will give you a good knowledge of the best out there whose level of performance is on par with what you’re out to get.
What is a Compressor?
A compressor is also known as a Heating, Ventilation & Air-Conditioning (HVACR) machine.
It is a mechanical device that reduces the volume of a fluid such as gas or liquid while at the same time increasing its temperature and pressure.
A compressor features two major components and these are the power source and a compressing mechanism (for example piston and vanes).
What’s more, these machines are similar to gas pumps because they transport compressed gas through pipes.
The latter has aided in the compression of natural gas in the oil and gas industry where the gas is pressurized in order to meet with the standards of certain jurisdictions that require at least 95 percent of the gas in petroleum to be compressed.
It is also worthy to note that certain factors influence a compressor’s performance and these are:
- Speed of rotation
- Pressure at suction
- Pressure at discharge
- Type of refrigerant used
The Basic Types of Compressors
A list of the major types of compressors by mechanical design has been outlined below and the feature of each, aids in its functionality.
Now, the best way to get a good idea of these devices is to compare them side by side and as such, a comparison between different types of compressors, how they work, and when to use them has also been given in this section.
Therefore, the two basic types of compressors are:
- Positive displacement compressors
- Dynamic compressors
1. Positive Displacement Compressors
In positive displacement compressors, gases are compressed due to the displacement of a mechanical linkage which reduces its volume.
First off, a certain amount of gas is passed into a confined space and the volume or space is subsequently reduced which helps to boost the gas’ pressure levels.
The gas is then released into a discharge piping or vessel system once the pressure has been raised.
If you’re wondering why this displacement is called positive in the first place, then reference can be made to thermodynamics where a displacement caused by the movement of a piston (as is the case of a reciprocating compressor) is known to be positive.
The movement can also be caused by rotation as is the case of a twin helical screw-rotating machine.
Consequently, the types of positive displacement compressors are:
- Reciprocating compressors
- Rotary compressors
a.) Reciprocating Compressors
Reciprocating compressors or piston compressors feature one or more pistons which are driven by a crankshaft; a component that also drives the piston rod, and connecting rod.
As the piston within the cylinder moves back and forth, the pressure of the gas is increased. This, in turn, helps in its compression. The compressed gas is then discharged into high pressure receiving tanks.
On the other hand, this positive displacement compressor can also be driven by electric motors or internal combustion engines.
They can be fixed to a particular location or portable enough to be moved around.
In terms of their horsepower, small compressors operate within the range of 5 to 30 horsepower (hp) and they are mostly used in the automobile sector of the economy.
Large compressors, on the contrary, have a horsepower above 1,000 hp (750 kW). They are available in the oil and gas industry and generally in large industrial applications.
– Types of Reciprocating Compressors
The various types of Reciprocating compressors are:
- Single-cylinder: A single cylinder reciprocating compressor features a suction, discharge area and compression. A double cylinder comes with dual suction, discharge areas, and compression, and it helps to achieve higher gas pressures.
- Multi-cylinder: While double cylinders are prevalent, there are instances where compressors are designed with as many as six cylinders.
- Multi-stage design: As the name implies, more stages are incorporated to arrive at the final processed gas. Here, the gas is compressed multiple times in several compression cylinders to increase pressure levels.
- Diaphragm compressor: This differs from the conventional reciprocating compressor since the compression of gas is brought about by the to and fro movement of a flexible membrane. The movement is facilitated by a rod and the crankshaft.
b.) Rotary Compressors
Rotary compressors also have a positive displacement. These low capacity types of equipment have applications in home freezers and refrigerators.
They can either have a single vane that is located within the cylinder and kept away from the rotor, or multiple vanes located in the rotor.
The various types of rotary compressors include:
– Rotary Screw Compressors
It uses two meshed helical screws in rotation to force the gas into a smaller space.
They can be employed in industrial and commercial purposes and their application can range between 3 horsepower (2.2 kW) to about 1,200 horsepower (890 kW).
Likewise, the discharge pressure can range between low to moderately high pressure (>1,200 psi or 8.3 MPa).
– Rotary Vane Compressors
These machines feature a rotor that is mounted in a larger housing which has either a circular or complex shape.
The rotor also has several blades which are inserted in radial slots within the rotor.
As the rotor moves, the blades move in and out of the slots. This increases and decreasess the volume of the gas.
In comparison with a piston compressor, a rotary vane compressor operates more quietly and is best suited to the electric motor drive.
Like piston compressors, they can also be single or multi-staged, as well as stationary or portable.
Their discharge range can be between 29 psi as is the case of dry vane machines and 190 psi for oil-injected machines.
– Scroll Compressors
These are also known as scroll pump or scroll vacuum pump and they feature two spiral vanes that are interwoven.
While one of the vanes is fixed, the other moves around it which help in compressing the gas.
Scroll compressors also operate even more quietly and smoothly than other types of compressors in the lower volume range.
2. Dynamic Compressors
Dynamic compressors are also known as turbo compressors and they depend on a fluid’s inertia and momentum to bring about its increased pressure levels.
In their mode of operation, velocity energy is impacted to a stream of gas and this energy is then converted to pressure energy.
There are two basic types of dynamic compressors and these are:
- Centrifugal compressors
- Axial compressors
a.) Centrifugal Compressors
Centrifugal compressors make up about 80 percent of the entire dynamic processors, therefore, leaving 20 percent to axial compressors.
That being the case, they are widely used in oil refineries, natural gas processing plants, chemical and petrochemical plants.
Due to their high output pressures of over 1,000 psi (6.9 MPa) and horsepower range of a 100 to 1000, these machines can be used in:
- Snowmaking operations
- Air conditioning systems
For their mode of operation, compression is made possible by exerting inertial force on the gas with the use of high-speed rotating impellers
The gas is forced to the rim of the impeller which helps to increase its velocity. This velocity is then converted to pressure energy by a diffuser.
The process can also be carried out in a single stage or multi-stage where each stage takes advantage of an impeller (a rotating disk) and diffuser (a stationary element).
Both single and multistage machines are generally made up of standardized components. However, the multistage helps in improving the compression ratio since centrifugal compressors generally have lower compression ratios in comparison to displacement compressors.
Centrifugal compressor also features two casing designs and these are:
– Horizontally Split Casing Design
This compressor has an outer casing which can be split horizontally to aid in the maintenance of its internal component.
Within the compressor, the rotating disk or impellers are connected to one rotating shaft to form a multi-stage structure.
As the gas passes through the intake nozzle, a centrifugal force created by the high-speed movement of the impellers causes it to be compressed and pressurized before it is sent out to an ejection nozzle.
– Vertically Split Casing Design
While the internal components of this machine have a similar design to that of the horizontal split type casing, its outer design differs.
Here, the rotor bundle and the diaphragm seals are axially arranged in a steel barrel casing.
Generally, this design depends on the working pressure and the type of gas that is to be compressed.
b.) Axial-flow Compressors
Another type of dynamic rotating compressors is the axial-flow compressor. They are mostly employed where compact design or high flow rates (large flow volumes) is desired.
These compressors have a pressure range between low to medium and you’ll find their application in jet engines, natural gas pumping stations, chemical plants, and large gas turbine engines.
When it comes to how this compressor works, gas is compressed with the use of an array of airfoils which are arranged in rows.
The airfoils can exist as pairs, where one of the set is a rotating airfoil known as the blade or rotor and the other is a stationary airfoil also known as stators or vanes.
While the rotating airfoil accelerates the fluid; the stationary airfoil decelerates and also redirects its direction in preparation for the rotor blades of the next stage.
– Pros and Cons
What this means is that the velocity of the gas is first increased before it is slowed down and passed through the blades which help to increase the gas pressure.
In comparison with other compressors, axial machines are relatively expensive since they require more parts and materials of high quality.
They, however, have high efficiencies and employ multi-stages where the cross-sectional area of the gas passing along the compressor diminishes to give an optimum axial Mach number.
c.) Hermetically Sealed, Open, or Semi-hermetic
There are also compressors that are specifically designed for refrigerators. These types can either be classified as hermetically open, sealed, or semi-hectic.
Each description refers to the way the motor drive is positioned in relation to the gas that is being compressed.
The main types of compressors and how they work as outlined above will help you to understand the mode of operation of these machines.
They have found an application in a wide range of industries and that being so, they are still relevant in 2019.
If you’ll also like to use one of these, it all starts with having a good idea of the machine configuration, operation type, as well as their limitations.
The result is being able to ascertain which will meet your needs best depending on the design or volume that is required.
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