Comparison Centrifugal and axial compessor

Gas turbine science

1. M3. Present the difference between centrifugal and axial compressors, their design criteria, usage, merits and demits in application along with proper examples.

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3. C entrifugal compressor The centrifugal compressor consists of an impeller which is rotated at high speed by a turbine. This causes air to be induced into the centre of the impeller and then accelerated radially outwards between the impeller vanes creating a rise in static pressure. This high velocity air is then decelerated through a number of fixed diverging passages, known as the diffuser, with a further consequent rise in static pressure. In practice, it is usual to design the compressor such that about 50% of the static pressure rise occurs in the impeller and 50% in the diffuser.

4. Axial compressor Compressed gas principally flows parallel to the rotation axis. Axial compressors have the benefits of high efficiency and large flow rates, particularly in relation to their sizes and cross sections. They offer the most compact and lightweight compressor design for large volumes and the lowest cost per flow rate for large flow rate applications. They do require several rows of blades (axial stages) to achieve large pressure rises, making them complex and fragile relative to other compressor designs such as centrifugal compressors.

5. A xial Compressors Design Criteria The axial-flow compressor is made up of a series of rotating airfoils called rotor blades and a stationary set of airfoil called stator vanes. As its name implies, the air is being compressed in a direction parallel to the axis of the engine. A row of rotating and stationary blades is called a stage. The entire compressor is made up of a series of alternating rotor and stator vane stages, with each stage constructed of blades shaped to provide the most lift for the least drag.

6. Centrifugal Compressors Design Criteria The centrifugal compressor consists basically of an impeller and a diffuser manifold. Other components such as a compressor manifold may be added to direct the compressed air into the combustion chamber. As the impeller revolves at high speed, air is drawn in at the eye or inducer. Centrifugal force provides high acceleration to this air and causes it to move outward from the axis of rotation toward the rim or exducer of the rotor, where it is ejected at high velocity and high kinetic energy. The pressure rise is produced in part by expansion of the air in the diffuser manifold by conversion of the kinetic energy of motion into static pressure energy. The total compression is shared between the rotor and the diffuser, but the diffuser does not work on the air.

7. Impellers: In order to maximize the airflow and pressure rise through the compressor, impellers are normally designed to operate at tip speeds approaching 600m/s. The local air temperature and component stresses associated with such operation require the impeller to be manufactured from high strength, aluminum, titanium or steel alloys. In the centre inlet, or eye, region of the impeller the vanes are curved in the direction of rotation to ease the flow of air into the compressor. The flow passages in the impeller and diffuser must be carefully designed to take account of the changing pressure, temperature, density and velocities of the air as it passes through them. The shape of the impeller channels between eye and tip is designed to obtain a uniform change in velocity thereby avoiding local decelerations up the trailing face of the vane which might lead to flow separation.

8. Diffusers A diffuser is used to reduce the velocity of the air exiting the impeller and thereby provide a further increase in its pressure. The diffuser assembly consists of a number of stationary vanes which are designed to accept high tangential velocity airflow, at an appropriate angle, from the exit of the impeller The air flow in the diffuser is decelerated, with a commensurate rise in static pressure, to provide a high pressure, increased temperature, reduced velocity airflow suitable for entry to the combustion chamber. Efficient deceleration of airflow is, a difficult process since there is a natural tendency for the air to break away from the walls of the diverging passages, reverse its direction, and flow back in the direction of the pressure gradient.

9. Centrifugal Compressor Usage With the development of aero engines, axial compressors have nearly replaced centrifugal compressors. Thus, centrifugal compressors are found in small turbofan engines. Centrifugal compressors are used in many industrial applications such as refineries, chemical and petrochemical plants, natural gas processing and transmission plants, very large-scale refrigeration, and iron and steel mills. These centrifugal compressors may have single or multiple stages (up to five stages) and can operate at very high inlet pressures. Rolls-Royce RFA36 and RFA24 giant compressors may have pressures up to 155 bar and flow rate of 200 m3/min at design speed from 3,600 to 12,800 rpm.

10. Axial Compressor Usages The most frequent application is in aircraft turbojet engines, nearly 100% of them use an axial flow compressor to compress the air going through the engine, resulting in higher pressure air entering the combustion area where fuel is injected and burned, creating energy via the hot airflow through the turbine stages, which are used to power the compressor stages, and then out the back as thrust . Axial compressors are used in applications where high mass flow rates are required. Axial compressors are exclusively used in jet engines of higher thrust. Also in land based gas turbines used for power generation. A few facilities in the world have axial compressors to generate high mach number flow for continuous wind tunnel testing.

11. Advantages of Axial Flow Compressor: High peak efficiency. Small frontal area for given flow . Straight through flow, allowing high ram efficiency. Increased pressure rise due to increased number of stages with negligible losses. Disadvantages of Axial Flow Compressor: Good efficiency over narrow rotational speed range. Manufacturing is difficult . Cost is very high. Heavy weight. High starting power requirements. 

12. Advantages of centrifugal compressor Centrifugal compressors are reliable, low maintenance. Generating a higher pressure ratio per stage as compared to axial flow compressor. Low weight, easy to design and manufacture. Suitable for continuous compressed air supply, such as cooling unit. High-flow rate than the positive displacement The oil free in nature. They have fewer rubbing parts compressor. Relatively energy efficient. Wide range of rotational speed. It does not require special foundation.

13. Disadvantages of centrifugal compressor Large frontal area for a given air flow rate compared to the axial flow compressor. Unsuitable for very high compression, limited pressure. They are sensitive to changes in gas composition. They work at high speed, sophisticated vibration mounting needed. Problem of surging, stalling and choking.

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