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 hopefully provide you with all the necessary information required for you to be able to understand axial compressors and whether they’re right for your application.
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 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 blades are similar to that of an aircraft wing as they have small airfoil cross-sections.
How does an Axial Compressor Work?
The first stationary row at the front of the rotor is called the inlet guide vanes 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 flow of air 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 the axial compression, and each stage could potentially raise the pressure up to 25%. But as this is no guarantee, multiple stages are required in the compressor to ensure you’re able to reach the high pressure required.
Axial Compressor Design
Axial flow compressors are designed to withstand several off-design operations like differing air flows, speeds, 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 both 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. These configurations are a very lightweight design and hence are best suited to aircraft engine compressors.
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, 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 that 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 against 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 compressors have numerous applications within a variety of industries with the most notable being the following:
- 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
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 in to 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.
If you’re having any issues with your axial compressor please check out our troubleshooting page.
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.