Centrifugal compressors work by accelerating air radially outward between impeller vanes via centrifugal force, increasing pressure. Axial compressors compress air flowing parallel to the axis using multiple rotor and stator stages. Centrifugal compressors are commonly used in industrial applications while axial compressors are used in jet engines due to their high efficiency at large flow rates. The key differences are that centrifugal compressors have higher pressure ratios per stage but are limited to lower pressures overall, while axial compressors can achieve higher pressures through multiple stages but are more complex, heavy, and expensive.

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

2. Welcome

3. Centrifugal 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. Axial 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:
i. High peak efficiency.
ii. Small frontal area for given flow.
iii. Straight through flow, allowing high ram efficiency.
iv. Increased pressure rise due to increased number of
stages with negligible losses.
Disadvantages of Axial Flow Compressor:
i. Good efficiency over narrow rotational speed range.
ii. Manufacturing is difficult.
iii. Cost is very high.
iv. Heavy weight.
v. 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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