Compressors are machines used to compress and transfer fluids (i.e., air or gas) from one place to other. There are multiple types of compressors, and a centrifugal compressor is one of them. Centrifugal compressors are most widely used all over the world. The main reason is that the construction of these compressors is very easy. This article explains the centrifugal compressor working principle, types, and components.
What is a Centrifugal Compressor?
A centrifugal compressor is a mechanical device that compresses a fluid with the help of the impeller’s radial acceleration, which is surrounded by the compressor housing.
In a centrifugal compressor, the air or gas enters axially in the impeller and discharges radially. Therefore, it is also called a radial compressor.
In the centrifugal compressor, the impeller increases the speed of the working fluid (gas or air) by converting the kinetic energy of the air/gas into speed. But the diffuser further converts the speed of the air or gas into pressure energy.
Radial centrifugal compressors have a higher pressure ratio at a low flow rate which is the major advantage over axial compressors.
A centrifugal compressor is a device where a specific vapor or specific gas compresses with the help of the compressor impeller. These compressors may arrange in multiple stages if you want high-compressed gas. You can increase the pressure of the compressed gas by increasing the number of stages of the compressor.
Depending on the requirements of the pressure for different functions, several stages can be connected in series to attain the required pressure. These multi-stage compressors use for applications in the processing, gas & oil industries. On the other hand, sewage treatment plants use single-stage low-pressure applications to attain the required pressure ratio.
In these compressors, the speed increases with the help of an impeller that provides kinetic energy to the gas or air. Then, when the flow rate by the diffuser decreases, this kinetic energy converts into the velocity of the fluid. Then diffuser converts this velocity into an increase in pressure energy.
Centrifugal Compressor Working Principle
The centrifugal compressor working principle is a little bit different than a reciprocating compressor.
A centrifugal compressor works in the following way:
- At the start-up, the air introduces from the air tank or any other source into the centrifugal compressor.
- After entering the compressor, the air strikes the impeller. This impeller has multiple radial blades that rotate with the rotation of the impeller.
- As the air strikes the radial blades of the impeller, the air pushes by centrifugal force into the center of the impeller.
- After striking, the impeller blades provide kinetic energy to the air; due to that, its speed (velocity) increases.
- After passing through the impeller, the air enters the diffuser area. This diffuser has stationary vans. After entering the diffuser area, the speed or velocity of the flow of the air starts decreasing.
- According to Bernoulli’s principle, the velocity square is inversely proportional to pressure. The volute casing or diffuser converts the increased velocity of the air into pressure energy before the air is drawn into the impeller center. In most conditions, the increase in the impeller’s pressure will roughly equal the increase in diffuser pressure.
Actually, the diffuser is a static (stationary) part of the compressor that accompanies the flow of air as it exits the rotor. This speed slowdown eventually leads to a further rise in pressure.
The diffuser and the impeller account for approximately 35% and 65% of the total pressure generated by the compressor. The centrifugal compressor strongly affects the air compression process.
For a better understanding, watch the below-given video:
Do you know: How does a Reciprocating Compressor work?
Centrifugal Compressor Components
The centrifugal air compressor has the following major components:
- Impeller
- Casing
- Diffuser
- Inlet and Outlet valves/ports
- Collector
1) Impeller
The rotor or impeller is one of the most important parts of a centrifugal compressor. A centrifugal air compressor impeller or rotor is a disc with a key assembly attached to the shaft of the compressor. Different numbers of curved blades attach to this disc.
These blades provide a diffusion channel for the working gas or the air. The numbers of these curved blades vary from about 15 to 20 in a single impeller of the centrifugal compressor.
The impeller of the compressor gives speed or velocity to the air or gas through the vans attached to the rotating disk. These blades can be tilted backward, radially, or forwards depending on the required performance. Most multi-stage compressors use backward-sloping blades for maximum efficiency.
2) Casing or Housing
The above-given centrifugal compressor components (given in the schematic diagram) are mostly protected by housing. Basically, the housing is a tight way of the air ( or any other fluid) around the impeller.
The casing is designed in such a way that the air kinetic energy discharges at the impeller’s discharge port but this kinetic energy converts into pressure before the air exits the housing. The housing consists of several bearings that support the rotor axially and radially. It makes of steel or cast iron.
The housing has two main types:
- Vertically splits
- Horizontally splits
3) Diffuser
The impeller sends the air into the diffuser channel at very high speeds. This diffuser normally affects the two walls that make up the radial channel. These arrangements slow down the air or gas and convert dynamic pressure to static pressure.
The diffuser channel is a small space between adjacent membranes that normally rotate the air flow 180 degrees to guide it to the next runner. This is a centrifugal compressor component before the outlet section. After the diffuser section, the air or gas goes into the collector.
4) Collector
After the diffuser, the air or the gas is collected and directed toward the outlet section. The component uses to accumulate the air or gas emitted by the diffuser is known as a collector. It also knows as a scroll or a volute.
A collector can also have valves and other devices for controlling the compressor. The collector is the last component of the centrifugal compressor. A delivery pipe connects with the collector from where the pressurized air or any other fluid discharges and transfers to desired location or section.
5) Impeller Blades
These blades connect with the impeller rotation. The main objective of these blades is to provide kinetic energy to the working fluid and increases its speed.
Read More: Types of Positive Displacement Compressors
6) Inlet and Outlet Ports
The inlet port of the compressor uses to suck the air or gas inside the compressor. In contrast, the outlet port uses to discharge the pressurized air or gas.
Types of Centrifugal Compressors
The centrifugal compressor has the following major types:
- Single-stage compressor
- Multi-stage compressor
1. Single-stage Compressor
A single-stage compressor has a single runner that moves air or gas in pressure ratios of up to 3:1 for pressure or vacuum work. It is believed that these types of compressors have a beam structure or a suspended impeller arrangement.
In this type of compressor, the impeller is at the shaft’s non-driving. One of the main advantages of these compressors over multistage compressors is that they offer high efficiency and the produced gas is completely surge-free and oil-free.
The single-stage compressor also has the following further types:
i) Overhung single-stage compressors
In this compressor, the impeller locates on the shaft’s non-driven end (outside of the radial bearing on the non-drive end). These compressors have axial flow suction nozzles that offer a wider working range and superior efficiency.
These centrifugal compressors are primarily designed for high-flow and low-head applications. The flow of the compressor is regulated by butterfly valves, suction/discharge valves, or inlet guide blades.
Read More: Working of Single Stage Compressor
ii) Integrally geared single-stage compressors
This single-stage compressor with an integrated gearbox has a suspended semi-open impeller. The overhung impeller is located directly on the gearbox’s high-speed shaft.
It can process gas flow rates of equal to 300,000 m³/h, delivery pressures of up to 50 bar, and pressure ratios of up to 3.5. These compressors are used for dealing services with clean process air or gas.
2. Multistage-stage Compressor
The multi-stage compressor consists of 1 to 10 impellers and can arrange according to various flow path patterns. In each stage, it is assumed that the compression ratio and the temperature are constant.
These compressors can be settled in dual-flow, compound, and straight-through arrangements. This compressor also considers to have a beam-type design, but the runner is located between the radial bearings.
Multi-stage compressors also have the following major types:
- Integrally geared compressors
- Pipeline compressor
- Compressors with bell casing
- Vertically split casing compressors
- Horizontally split casing compressors
i) Horizontally split casing compressor
In a compressor with a horizontally split casing, the cylindrical casing or housing divides into two parts:
- Lower halve
- Upper halve
The advantage of these types is that the maintenance of the internal parts can be completed by just removing the upper part. Due to this, most of the pipe connections, except for the main process I/O connections, are shown in the lower part. The key process in/out connections are best used in the lower half, but can also be used in the upper half.
These types of centrifugal compressors have high reliability as well as high efficiency. These have an easy design. The horizontal split casing compressors use for the applications such as metallurgy, mining, chemical, and oil & gas industries.
Read More: Types of Positive Displacement Compressors
ii) Vertically split casing compressors
The vertically split housing compressor consists of a closed cylinder with two end caps (also known as a barrel). The benefit of the vertically split joint is that it has high-pressure resistance because of its one-piece structure with conventionally tested flange seals at both ends.
These compressors use for food, energy, and oil & gas industries. These also use for offshore platforms.
iii) Compressor with bell casing
This is a vertical split or barrel compressor. At high pressure, this compressor has a bell-shaped housing that is closed with shear rings instead of screws.
iv) Pipeline compressor
They have a bell-shaped housing with a single vertical end cap. Mostly, these compressors use to transport natural gas. These typically have side inlet and outlet nozzles that are located in opposite directions to simplify installation in a natural gas pipeline. Axial inlets can also use if the pressure ratio permits for a single runner.
These compressors generally design to control pressure up to 100 bar. The pipeline compressor has a high capability to control a proper flow of at high efficiency. These have a high flow rate and low compression ratio.
v) Integrally geared multistage compressors
This centrifugal air compressor is usually used for high-flow/low-pressure or low-flow/high-pressure applications. This compressor has large gears and 1 to 4 high-speed pinions. You can attach one or two impellers to each pinion shaft.
Advantages and Disadvantages of Centrifugal Compressor
The advantages and disadvantages of the centrifugal compressor are given below:
Advantages of Centrifugal Compressors
- At a low flow rate, it offers a high-pressure ratio.
- These compressors are easy to design as compared to positive displacement compressors.
- These are more reliable than reciprocating or rotary compressors.
- They don’t need any particular foundation.
- These compressors are oil-free in nature.
Disadvantages of Centrifugal Compressors
- The centrifugal compressor is less flexible than positive displacement compressors.
- These have cavitation problems.
- It doesn’t have the self-priming capability.
- They operate at relatively high speeds and require clarification or long-term installation.
Applications of Centrifugal Compressors
- In power plants, these compressors are attached to the gas turbine, which is used for air compression before transferring it to the combustor.
- They are used in automotive engines.
- These compressors are used in superchargers.
- Centrifugal compressors are used in the petrochemical industry for different processes, including nitrogen generation, air separation, and the production of different chemicals, such as ethylene, methanol, and ammonia.
- They are used in the oil and gas industry for gas injection, gas lift, and gas compression processes. These compressors assist to maintain the pressure of gas transmission systems, gas storage, and natural gas pipelines.
- It uses in air separation plants to manufacture purified end-product gases.
- Pipeline compressors transfer natural gas from the workshop to the consumer.
- Centrifugal compressors are most commonly used in large-scale refrigeration and air conditioning systems like commercial HVAC systems and industrial cooling plants.
Difference Between Centrifugal Compressor and Centrifugal Pump
The working principle of the centrifugal pump and centrifugal compressor is almost the same. The main difference is that the pump uses to pump different liquids from one place to another place while the compressor uses to compress the gases or air.
Read More: Working of Centrifugal Pump
Difference between Centrifugal Compressor and Reciprocating Compressor
|
Reciprocating Compressor |
Centrifugal Compressor |
|
Air is drawn in different chambers. |
Air is drawn between the blades of an impeller. |
|
Pressure is variable. |
In this type, pressure remains constant. |
|
These compressors cant deliver pulsating free flow rate. |
It delivers pulsating free flow rate. |
|
Single-acting and double-acting compressors its examples. |
Axial and radial compressors are examples of centrifugal compressors. |
|
It is less efficient than the centrifugal compressor. |
It is more efficient. |
|
It has less reliability. |
This dynamic compressor has more reliability. |
|
Mass flow rate and intake temperature do not affect performance. |
Mass flow rate and inlet temperature directly affect the performance of the compressor. |
|
The velocity of fluid need not be high. |
The velocity of fluid must be high. |
|
Fluid is directly imparted into pressure energy. |
The fluid provides kinetic energy which converts into pressure energy. |
|
A reciprocating compressor increases the pressure of air by reducing its volume. |
The centrifugal compressor compresses the air by transfer of energy from a rotating impeller. |
|
It has a high initial cost. |
It has a low initial cost. |
|
This compressor requires high maintenance costs than a centrifugal compressor. |
It needs a low maintenance cost. |
|
These compressors don’t have cavitation problems. |
These have cavitation problems. |
|
A reciprocating compressor requires high maintenance. |
It needs low maintenance. |
FAQ Section
What is Surging in Centrifugal Compressor?
A surge is a working stage at which the maximum head capacity and the lowest flow limit of the centrifugal compressor are reached.
Actually, during the operation of the centrifugal compressor, the impeller blades increase the kinetic energy of the fluid. As a result, this fluid slows down by the volute casing or diffuser, and this process is known as plenum. During the plenum process, the kinetic energy of the fluid converts into pressure energy and increases the fluid pressure.
When the plenum pressure on the back of the compressor is more than the outlet pressure of the compressor, the fluid inclines to inverse or even moves backward inside the compressor. Due to this, the inlet pressure starts increasing, the plenum pressure starts decreasing, and the fluid flows back again. This mechanism is known as the surge.
What are centrifugal compressors used for?
- It uses for the compression of gases.
- It uses in the chemical, food, and oil & gas industries.
- Centrifugal compressors are also commonly used in oil refineries.
- These are used in the superchargers.
Who Invented the Centrifugal Compressor?
In 1905, the centrifugal compressor was invented by Professor Auguste Rateau.
What is the major advantage of a centrifugal compressor?
One of the main advantages of the centrifugal compressor is its high efficiency in compressing gases, which achieves by its continuous flow process and capability to control large volumes of gas with the lowest mechanical losses. This feature of these compressors makes them ideal for different industrial applications that need energy-efficient and reliable compression.
What are the components of the centrifugal compressor?
- Impeller
- Housing
- Diffuser
- Collector
- Impeller Blades
- Inlet and Outlet Ports
What are the two types of centrifugal compressors?
- Single-stage compressor
- Multi-stage compressor