A centrifugal compressor is a radial flow rotodynamic fluid machine that uses mostly air as the working fluid and utilizes the mechanical energy imparted to the machine from outside to increase the total internal energy of the fluid mainly in the form of increased static pressure head.
During the second world war most of the gas turbine units used centrifugal compressors. Attention was focused on the simple turbojet units where low power-plant weight was of great importance. Since the war, however, the axial compressors have been developed to the point where it has an appreciably higher isentropic efficiency. Though centrifugal compressors are not that popular today, there is renewed interest in the centrifugal stage, used in conjunction with one or more axial stages, for small turbofan and turboprop aircraft engines.
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Characteristics Features of Centrifugal Compressors
a) It occupies a smaller length than an equivalent axial flow compressor.
b) It has better resistance to foreign object damage.
c) Because of the relatively short passage length, loss of performance due to buildup deposits on blade surfaces will not be as great as the axial flow compressors.
d) It can work reasonably well in a contaminated atmosphere as compared to axial flow compressor.
e) It has the ability to operate over a wide range of mass flow rates at any particular rotational speed.
f) Its efficiency under the most favorable circumstances, are less than those of axial compressors designed for the same duty, by as much as 3 or 4 %.
g) However, at very low mass flow the axial flow compressor efficiency drops, blading is small and the advantage appears to lie with the centrifugal compressor in its relative simplicity and cost.
Centrifugal Compressor Parts :
Parts of centrifugal compressor and its function
The principal components of a centrifugal compressor are shown in Fig. and detail of each part is given below.
1. Inlet casing with accelerating (converging) nozzle
2. Inlet guide vanes (IGV)
5. Scroll or volute
6. Inducer section
Centrifugal Compressor Diagram :
1. Inlet casing with accelerating nozzle
- The function of the inlet casing is to accelerate the fluid from its initial condition to the entry of inlet guide vanes and to provide uniform velocity at the eye.
- The inlet flange is axisymmetric and the inlet duct takes the form of a simple converging nozzle.
- The outlet of the inlet casing is known as the impeller eye.
2. Inlet guide vanes (IGV)
- The function of inlet guide vanes is to direct the flow in the desired direction at the entry of the impeller.
- The inlet guide vanes should be chosen so as to obtain a minimum relative Mach number at the eye tip.
- The function of the impeller is to increase the energy level of fluid by whirling it outwards by increasing the angular momentum of the fluid. (Refer Fig.)
- Both static pressure and velocity of fluid are increased in the impeller.
- The impeller vanes help to transfer the energy from the impeller to the fluid.
- The hub is the curved surface of revolution of the impeller A-B.
- The shroud is the curved surface C-D forming the outer boundary to the flow of fluid.
(Shrouding an impeller eliminate tip leakage losses but at the same time increases friction losses.)
- Impellers may be enclosed by having the shroud attached to the vane ends (called shrouded impellers) or unenclosed with a small clearance gap between the vane ends and the stationary wall.
4. A diffuser consisting of a number of fixed diverging passages in which the air is decelerated with a consequent rise in static pressure.
5. Scroll or Volute
- The air leaving the diffuser is collected in a spiral passage known as volute or scroll and the volute discharges the air through the delivery pipe.
- Different cross-sections are employed for the volute passage are rectangular, circular, and trapezoidal.
6. Inducer section
- At the entry to the impeller the relative flow has a velocity Vr1, at angle α1 to the axis of rotation as shown in Fig.
- This relative flow is turned into the axial direction by the inducer section or rotating guide vanes.
- The inducer starts at the eye and usually finishes in the region where the flow is beginning to turn into the radial direction.
Working Principle of Centrifugal Compressor :
Air is sucked into the impeller eye and whirled outwards at high speed by the impeller disk. At any point in the flow of air through the impeller the centripetal acceleration is obtained by a pressure head so that the static pressure of the air increases from the eye to the tip of the impeller. The remainder of the static pressure rise is obtained in the diffuser, where the very high velocity of air leaving the impeller tip is reduced to almost the velocity with which the air enters the impeller eye.
Usually, about half of the total pressure rise occurs in the impeller and the other half in the diffuser. Owing to the action of the vanes in carrying the air around with the impeller, there is a slightly higher static pressure on the forward side of the vane than on the trailing face. The air will thus tend to flow around the edge of the vanes in the clearing space between the impeller and the casing. This results in a loss of efficiency and the clearance must be kept as small as possible. Sometimes, a shroud attached to the blades as shown in Figure (d) may eliminate such a loss, but it is avoided because of increased disc friction loss and of manufacturing difficulties.
The straight and radial blades are usually employed to avoid any undesirable bending stress to be set up in the blades. The choice of radial blades also determines that the total pressure rise is divided equally between impeller and diffuser.
Pressure and velocity variation across a centrifugal compressor:
- Air enters the compressor at a mean radius with a low-velocity V1, and atmospheric pressure P1 as shown in Fig.
- It is then accelerated to a high-velocity V2, and pressure P2, depending upon the centrifugal action of the impeller.
- The air now enters the diffuser where its velocity is reduced to some value V3, and pressure increases to P3.
- In practice, about half of the total pressure rise per stage is achieved in the impeller and the remaining half in the diffuser.
Velocity Diagram of Centrifugal Compressor
- The velocity diagrams at the inlet and outlet of the impeller of a centrifugal compressor are shown in Fig. (a) and (b).
- In the analysis of centrifugal compressor the following assumptions are made:
(i) The flow phenomenon is steady and uniform throughout.
(ii) There is no separation of flow.
(iii) The flow through the impeller is frictionless.
(iv) There are no shock waves occurring anywhere.
The following are the notations used in the analysis of a centrifugal compressor.
α1 = Exit angle from the guide vanes at entrance = absolute angle at inlet
β1 = Inlet angle to the rotor or impeller
β2 = Outlet angle from the rotor or impeller
α2 = Inlet angle to the diffuser or the stator
u1 = Mean blade velocity at inlet
u2 = Mean blade velocity at exit
V1 = Absolute velocity of air at inlet to the rotor
V2 = Absolute velocity of air at the exit to the rotor
Vr1 = Relative velocity of air at inlet to the rotor blade l
Vr2 = Relative velocity of air at the exit to the rotor blade
Vw1 = Velocity of whirl at inlet (tangential component of absolute velocity 1 V )
Vw2 = Velocity of whirl at exit (tangential component of absolute velocity 2 V )
Vf1 = Velocity of flow at the inlet (Component of 1 V perpendicular to the plane of rotation)
Vf2 = Velocity of flow at exit (Component of 2 V perpendicular to the plane of Rotation).
m = Mass flow rate, kg/sec
(i) If no pre-whirl, the air enters the impeller eye in an axial direction, α1= 90 degree, Vf1= V1, Vw1= 0 and air will be leaving the impeller in the radial direction β2= 90 degree, Vf2=Vr2 and Vw2 = Vu2
(ii) If the air enters the impeller eye in an axial direction α1= 90 degree but air will not leaving the impeller in radial direction β2 < 90 degrees, Vr2 ≠ Vf2, and Vw2 <Vu2
Centrifugal Compressor Efficiency :
- The compression process in a reciprocating compressor may approach isothermal compression due to slow speed, cooling of the cylinder, and interstage cooling.
- But in centrifugal compressor; running at high speed, there is a lot of friction between molecules of air, between air and blade passages, eddies formation, and shocks at inlet and exit.
- These factors cause the internal generation of heat and consequently the maximum temperature reached would be more than that for adiabatic compression.
- Therefore, the index of compression of uncooled rotary compression may be as high as 1.7 and the high value of compression index demands a large amount of compression work and this value may be reduced by surrounding the air passages with cold water jackets and by the use of intercoolers.
- But it is generally impossible to provide sufficient cooling to bring the compression curve to the left of the isentropic line.
- It is because of this reason that the criterion of thermodynamic efficiency of a reciprocating compressor is isothermal while that for rotary compressors is isentropic compression.
- “Isentropic efficiency is defined as the ratio of isentropic work required to compress the air from P01, to P02 to the actual work required for the same pressure ratio.”
Isentropic Efficiency = Isentropic Compression Work /Actual Compression Work
Advantages of Centrifugal Compressor :
a.) When compared to other compressors, it is relatively agile and easy to manufacture.
b.) As this compressor does not require any special foundation it is highly energy-efficient and reliable.
c.) They consist of a small number of rubbing parts and are absolutely oil-free in nature.
d.) It generates a higher pressure ratio per stage than the axial flow compressor.
Disadvantages of Centrifugal Compressor.
a.) They produce a limited amount of pressure and are not suitable for very high compression.
b.) As they work at relatively high speed an enlightened or worldly mounting is required.
c.) They are very sensitive towards problems such as stalling and choking.
Applications of Centrifugal Compressor :
- In gas turbines and auxiliary power units.
- In automotive engine and diesel engine turbochargers and superchargers.
- In pipeline compressors of natural gas to move the gas from the production site to the consumer.
- In oil refineries, natural gas processing, petrochemical, and chemical plants.
- Air-conditioning and refrigeration and HVAC: Centrifugal compressors quite often supply the compression in water chillers cycles.
- In air separation plants to manufacture purified end product gases.
- In oil field re-injection of high-pressure natural gas to improve oil recovery.
- Large buildings with cooling loads in excess of 400 tons of refrigeration or 1,400 kW typically use water-cooled chillers with either centrifugal compressors or Turbocor compressors within the central plant cooling system.
Difference between Centrifugal Compressor and Axial Flow Compressor :
Comparison of Centrifugal and Axial flow compressor are as follows :
Sr. no. Centrifugal compressor Axial Flow Compressor 1. Flow is perpendicular to the axis of the compressor. The flow of air is parallel to the axis of compressor. 2. Low manufacturing and running costs. High manufacturing and running costs. 3. Requires low starting torque. Requires high starting torque. 4. Not suitable for multi-staging. Suitable for multi-staging. 5. Requires a large frontal area for a given rate of flow. Requires less frontal area for a given rate of flow. 6. Pressure ratio per stage is4:1. The pressure ratio is 1.1 to 1.2. 7. Isentropic efficiency is 70% Isentropic efficiency is 80%. 8. Used in supercharging I.C. engine and for refrigerants and industrial gases. Used universally with large gas turbines.
Difference between Centrifugal Compressor and Reciprocating Compressor :
|Sr. no||Parameters||Reciprocating Compressor||Centrifugal compressor|
|1.||Balancing||A poorly balanced and vibration problem occurs.||Better balanced; because no reciprocating part.|
|2.||Mechanical efficiency||Less efficiency due to more sliding and bearing members||More due to less bearing members.|
|3.||Pressure ratio||The pressure ratio per stage is high about 5 to 8.||The pressure ratio per stage is high about 3 to 4.5.|
|5.||Delivery pressure||Capable to deliver high pressure.||Capable to deliver medium pressure.|
|6.||Capacity||Handles small volume||Handles large volume|
|7.||Flexibility||More flexible with capacity and pressure range.||No flexibility in capacity and pressure range.|
|8.||Compression efficiency||Higher at compression ratio above 2.||Higher at compression ratio below 2.|
|9.||Speed||Adaptability to low-speed drive||Adaptability to high-speed low maintenance cost drivers such as a turbine.|
|11.||Suitability||For low, medium, and high pressure and low and medium gas volumes.||For low and medium pressure and large gas volumes.|
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