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An axial-flow compressor is a high-speed and very large-volume compressor that works, as you may have already guessed, by the flow entering the compressor in the axial direction. They are a form of dynamic compressor that contains rotary and stationary components through which the air flows and then becomes compressed.
This page will serve as an overview of axial flow air compressors, providing you with all the relevant information to help you better understand them!
Table of Contents
- What Are Axial Flow Compressors?
- How Does an Axial Compressor Work?
- Axial Compressor Design
- Performance Comparison vs. Other Compressor Types
- Axial Flow Compressor Applications
- How Reliable Are Axial Compressors?
- Axial Flow Compressors Manufacturers & Where to Buy
- FAQs (Frequently Asked Questions)
- Other Types of Air Compressors
What Are Axial Flow Compressors?
Axial flow compressors move airflow in the axis of rotation of the driving shaft. This driving shaft rotates the rotor compressor blades around it causing an increase in kinetic energy and thus static pressure through a process called diffusion.
Axial compressors are typically made up of many alternating rows of rotating (rotors) and stationary (stators) blades. The compressor’s rotating and stationary blades are similar to that of an aircraft wing as they have small airfoil cross-sections.
Given that airfoils are employed in the acceleration and diffusion of air in an axial compressor, you will find the majority of theory and research regarding the flow of air in axial compressors based on studies of isolated airfoils.
Axial Flow Compressor Advantages and Disadvantages
| Axial Compressor Advantages | Axial Compressor Disadvantages |
|---|---|
| High peak efficiency | Performance can be sensitive to mass flow rate |
| Suitable for multi-staging | Complex design and typically expensive |
| Ability to handle large volumes of air | |
| A high-pressure ratio can be achieved |
How Does an Axial Compressor Work?
The first stationary row at the front of the rotor is called the inlet guide vane or IGV. The intake needs to be a row of stationary vanes to ensure air uniformly enters the compressor. The stator rows not only work to raise the pressure but also help, due to their airfoil design, direct the airflow in the correct direction so that it reaches the next row of rotors.
A combination of a row of stator and rotor blades is called a stage in axial compression, and each stage could potentially lead to a pressure rise of up to 25%. But as this is no guarantee, you may only reach a 5% pressure rise. Multiple compressor stages are required to ensure you’re able to reach the high-pressure ratios required.
Axial Compressor Design
Axial flow compressors are designed to withstand several off-design operations like differing air flows, rotational speed, and pressure ratios. It is important for the axial compressor to not exceed limits on any of these variables, as that could provide poor results on the successful operation of the compressor.
Potential issues include choking of the compressor at low airflows and stalling of the compressor at high air flows above the design limit. It is therefore important to consider designing the axial compressor with flexibility so that it may be better at handling any potential off-design operations.
The compressor rotors within the axial compressor design have two possible configurations, drum and disk type. That being said, both configurations ensure that air velocity is kept at a constant value which is important for the sections of the compressor that follow. They do so by contracting the space between the shaft and the casing moderately.
Drum-type configurations have their rotors attached to a constant-diameter driving shaft with the casing approaching the shaft as the flow goes further towards the end stages. They’re commonly used within numerous industrial applications.
Disk type configurations are different in that the casing has a constant diameter whilst the driving shaft increases in diameter towards the outlet, this lowers the air volume and therefore increases the air pressure energy. These configurations are a very lightweight design and hence are best suited to aircraft engine compressors.
Nonetheless, for either of the rotor configurations, the axial component of the air velocity is to be kept at a more or less constant value using such gradually contracting the space between the shaft and casing. Steady air delivery after the compressor’s job has been done is crucial for the later sections of the system, hence why this is such an important design consideration.
In order to achieve the intentions of these design configurations, you need to use specific materials for manufacturing the major components of the axial flow compressors. For example, rotors and stators should be made from aluminum, titanium, nickel alloys, steel, or some fibrous composites. While the casing is made from a composition of aluminum, magnesium, titanium, steel, or iron.
Performance Comparison vs. Other Compressor Types
As axial flow compressors are a type of dynamic compressor, it is only right to compare them against the other form of the dynamic compressor. Introducing the centrifugal compressor.
Centrifugal compressors have a far simpler design than axial flow compressors which leads to easier manufacturing operation, fewer components, reduced price per unit, and significantly lower maintenance costs. As you already know, the flow direction in the axial compressor is parallel to the axis of the shaft. The flow in the centrifugal compressor radially recedes from the driving shaft perpendicular to its motion.
One thing that should be known is the difficulty centrifugal compressors are designed with multi-stages. On the other hand, axial compressors are very suited to this design and have smaller frontal areas and high mass flow rates.
In regards to the ever-so-important pressure ratio. The centrifugal compressor achieves the highest per-stage ratio in comparison to the axial which develops a very low-pressure ratio per stage. This leads to the requirement for more stages, which may become a hassle. The positive displacement compressors have a very high-pressure ratio when compared to these two dynamic compressors.
Axial compressors hold the best efficiency rating, higher than centrifugal and positive displacement compressors. This is primarily at large capacities because positive displacement compressors are generally the best for small capacities.
Axial Flow Compressors vs Centrifugal Compressors
| Axial Flow Compressors | Centrifugal Compressors | |
|---|---|---|
| Flow Direction | Parallel to the axis of shaft rotation | Radially receding from the driving shaft |
| Operation | – Narrow off-design operation limits between choking and surging situations – Poor partial load performance | – Extended off-design operation limits between choking and surging situations – Better partial load performance |
| Pressure Ratio | 1.25:1, requires multiple stages | 4.5:1, requires a lower number of stages for the same overall pressure ratio |
| Starting Power | High | Low |
| Design and Manufacturing | Challenging blade and compressor design and manufacturing along with more components. However, is suitable for multi-staging | Simpler design and manufacturing with fewer components. However, is far less suitable for multi-staging |
| Isentropic Efficiency | 86% – 88% | 80% – 82% |
Axial Flow Compressor Applications
- Power generation
- Sewage treatment
- Compressed air energy storage systems
- Large-volume air separation
- Nitric acid
- Blast furnaces that produce industrial metals like steel, pig iron, and possibly even lead and copper
- Refinement in catalyst regeneration air machines
- Industrial wind tunnels and test chambers
Axial compressors are predominately used for power generation purposes. This power generation could be at power stations or large industrial plants where gas turbines are used to generate electric power. You will also find axial compressors being applied this way in large marine gas turbine power plants.
Apart from their power generation purposes, axial flow air compressors are also being used across many chemical plants such as air separation plants, nitric acid plants, and also blast furnaces that produce industrial metals such as pig iron, or even lead, and copper.
They are also crucial in the design of large gas turbines such as high-speed ship engines, jet engines, and even relatively small-scale power stations. Due to their very high performance and reliability they can also be found and almost are the only compressor used within the aerospace industry on airplane engines.
Here they compress the air that moves into the engine, releasing higher-pressure air into the combustion area where the fuel is injected and burned. This creates a significant amount of energy from the hot flow of air through the stages which then powers the compressor and then out the back as thrust to move the plane forwards.
How Reliable Are Axial Compressors?
Axial compressors, like any other type of air compressor, certainly undergo faults and may require maintenance and servicing of high quality at times.
However, they offer a high volume capability in a relatively compact scenario and they can be extremely reliable compressors when applied properly. This can be said for any of the varying dynamic compressors.
If you’re having any issues with your axial compressor please check out our air compressor troubleshooting guide.
Axial Flow Compressors Manufacturers & Where to Buy
Though I couldn’t find any Axial flow air compressors direct on Amazon, your best bet is to visit a manufacturer’s website directly.
An example, you can visit the Siemens Energy website to gain a better understanding of their axial flow product offerings. Other popular axial compressor manufacturers include:
FAQs (Frequently Asked Questions)
An axial compressor is a gas compressor that is capable of continuously pressurizing gases. Axial compressors move airflow in the axis of rotation of the driving shaft. The driving shaft rotates the rotor compressor blades around it which results in an increase in kinetic energy and thus static pressure through a process called diffusion.
The key difference between axial compressors and centrifugal compressors is the flow direction. The former’s is parallel to the axis of shaft rotation, while the latter is radially receding from the driving shaft. Axial compressors generally have higher starting power and are suitable for multi-staging, while centrifugal have lower starting power and are less suitable for multi-staging.
An axial-flow compressor compresses its working fluid by first accelerating the fluid via a row of rotating blades called rotors, and then diffusing it via a row of stationary blades called stators, allowing for a pressure increase.
One of the best advantages of axial flow compressors is that they have the ability to handle large volumes of air. In addition to this, they also provide a high-pressure ratio due to them being capable of being multi-staged.
Other Types of Air Compressors
If you’re interested in finding out more information on other types of air compressors please visit our specific pages:
- Reciprocating Air Compressors Explained
- Rotary Vane Compressors Explained
- Rotary Compressors Explained
- Centrifugal Compressors Explained
- Scroll Air Compressors Explained
- Oilless Air Compressors vs. Oil Compressors
If you have any questions regarding axial flow air compressors, please leave a comment below with any photos, if applicable, so that someone can help you!
