A centrifugal flow compressor is a form of dynamic compressor that contains a radial design. This radial design operates at a constant pressure rather than a constant flow like displacement compressors.
This page will serve as an overview of centrifugal air compressors, providing you with all the relevant information to help better understand them!
What are Centrifugal Compressors?
Centrifugal air compressors are reliable, efficient and compact devices that allow for the management of air compressor capacity at constant pressures due to their modern like controls.
They typically use gears but some modern configurations of the centrifugal compressor have started to adopt very high speed electric motors to drive the impellers. These configurations are suited to applications that require oil-free air as they operate without a gearbox and its accompanying oil-lubrication needs.
Their performance can be affected by numerous external conditions that may be difficult to control like the change in inlet temperature.
How does a Centrifugal Compressor Work?
Centrifugal compressors draw air into the centre of their rotating impeller using its radial blades. The flow in the centrifugal compressor radially recedes from the driving shaft perpendicular to its motion. This air is then pushed towards the centre due to the centrifugal forces present. The radial movement of air increases the kinetic energy which can be converted into pressure (a form of potential energy) thanks to a diffuser and volute. You could say that the kinetic energy itself causes an increase in pressure, but the process is not that simple.
A diffuser is a stationary component that acts as a radial passage near about the same width as the impeller blades. This primarily works to convert the velocity of the air (kinematic energy) into pressure (potential/static energy). The radial area of the diffuser expands and this expansion causes the desired diffusing effect to occur.
The blades essentially produce a pressure variation very similar to an airfoil of a spinning propeller on an aircraft or wind turbine. The key difference being that the blades are far closer together on a centrifugal compressor. This closeness of the blades and complexity of the design cause a serious alteration in flow between the blades which can be very unsteady,The centrifugal compressors are built in stages, and each stage has a part to play in the overall pressure increase of the system. The stages can vary depending on the application of course.
Centrifugal Compressor Design
Centrifugal compressors are designed to thrive in conditions that have higher capacities due to the continuous air flow through their multiple stages. In most cases, typical centrifugal air compressors have three stages within industrial plants. This is because air systems within plants need on average about an 8:1 pressure increase and centrifugal compressors offer about 2:1 or 3:1 per stage.
Between each stage in the compressor, intercoolers are found and at the exit of the final stage (in this case, the third stage) an after-cooler can be found. The intercoolers cool the air coming out of each stage and remove excess moisture before the air passing into the next stage. This can have a positive impact on the air quality and resulting efficiency of the system. The compressor is mounted on a base that contains:
- the driver – typically a motor
- gears – bull gear with pinion gear – these driver the air compressor stages
- stages – amount vary depending on the application
- coolers – intercoolers and after-cooler
- piping – connecting the system
- valves – inlet guide vanes, blow-off valve, check valve
- modern controls – in most cases, a control panel
Performance Comparison vs. Other Compressor Types
As centrifugal flow compressors are a type of dynamic compressor, it is only right to compare them against the other form of dynamic compressor. Now, introducing axial flow compressors.
Axial flow compressors have a far more complex design than centrifugal compressors which results in more difficult manufacturing operations, more components, higher price per unit and significantly higher maintenance costs. As you have learnt from this article, the flow in the centrifugal compressor radially recedes from the driving shaft perpendicular to its motion. On the other hand, axial compressors flow direction is parallel to the axis of its shaft.
Axial compressors are far more suited to designs with multi-stages and have smaller frontal area. Centrifugal compressors can boast difficulties if a multi-stage requirement is desired.
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 or difficulty.
Positive Displacement Compressors
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.
Specifically looking at reciprocating air compressors, a type of positive displacement compressor. It can be said that centrifugal compressors typically have a better isothermal efficiency and isentropic efficiency. Importantly, they require far less maintenance than reciprocating
compressors. However, if the pressure ratio is greater than two, it is likely that the reciprocating compressors will operate with far better efficiencies. And although they require more maintenance, they’re a lot easier and cheaper to repair.
Reciprocating compressors require more maintenance than centrifugal compressors due to their multistage operation. Though this may be worth it for a wide array of applications as it is certainly advantageous. Reciprocating compressors are far more flexible and work significantly better with varying pressures.
Centrifugal compressors are best suited for applications above 200 horsepower for reasons stated previously. They have numerous applications across a variety of industries with the most notable being:
- Pipeline booster services
- Onshore and offshore gas lift services
- Chemical plants
- Gas injection
- Transmission and storage of natural gas
- Jet engines
How Reliable are Centrifugal Compressors?
Centrifugal air pumps and compressors, like any other type of air compressor certainly undergo faults and may require maintenance and servicing of high quality at times.
They have very complex designs and therefore require great testing to determine their performance before manufacturing. Wind tunnels and advanced computational models are used to determine the designs performance, and therefore can result in more reliable compressors.
They offer high-volume capabilities in relatively compact scenarios and can be extremely reliable compressors when applied properly. This can be said for any of the varying dynamic compressors.
From:About Air Compressor.