COMPRESSORS, SUPERCHARGERS & TURBOCHARGERS
COMPRESSORS, SUPERCHARGERS
& TURBOCHARGERS
Rotary compressors handle more volume
with lesser increase in pressure.
Centrifugal compressors handle very large
volume with less increase in pressure.
Superchargers are high speed rotary
compressors. Turbochargers are high
speed turbines. Exhaust gases of internal
combustion engines run turbochargers.
Turbochargers run superchargers.
Sr.No. |
ASPECT |
RECIPROCATING COMPRESSORS |
ROTARYCOMPRESSORS |
1. |
Physical motion |
These employ pistoncylinder device withto and fro motion. |
These employan impellerwith rotarymotion. |
2. |
Pressure rise per stage |
Discharge pressure ishigh and gives a pressureratio of 4 to 7 per stage. |
Dischargepressure islow and givesa pressureratio of 3 to 5 per stage. |
3. |
Pressure range and discharge |
Used for high pressureand less volumetricflow rates. |
Used for lesspressure riseand largevolumetric flowrates. |
4. |
Upper limit of high pressure |
These can producepressures up to 1000bars |
Normally theseproduce pressuresup to 10 bars. Withlarge number ofstages, these canproduce pressuresup to 400 bars. |
5. |
Volumetric flow rate or capacity |
These can handle volume flow rate of 50 m3/sec. |
Rotary compressors can handle volumeflow rate of 500 m3/sec. |
6. |
Speed |
These run at low RPM. |
These run at high RPM. |
7. |
Gas supply |
These need a receiver as these give an intermittent supply of compressed gas. |
These do not require a receiver as these supplycompressed gas at a uniform rate. |
8. |
Lubrication |
System for lubrication is complex. |
Lubrication system is simple. |
9. |
Unit size |
These are bigger in size for a certain flow rate. |
These are relatively more compact. |
10. |
Unit weight |
These are having a heavy structure. |
These are of light structure. |
11. |
Stability |
Vibrations cause frequent balancing problems. |
There are no balancing problems. |
12. |
Repair and maintenance |
Their repair and maintenance is more & expensive. |
Their repair and maintenance is less& inexpensive. |
13. |
Purity of compressed gas |
Compressed gas is in contact with the lubricating oil. Gas contaminates. |
Compressed gas does not come in contact with thelubricating oil. Gas does not contaminate. |
14. |
Ideal efficiency |
Isothermal efficiency is the ideal efficiency because of slow speed, cylinder cooling & inter-cooling in between the stages |
Adiabatic efficiency is the ideal efficiency. |
15. |
Mechanical efficiency |
It is low due to number of sliding parts. |
This is high as there are no sliding parts. |
16. |
Flexibility |
Flexibility is possible in flow rate and pressure range. |
There is no flexibility is possible in flow rateand pressure range. |
Sr. No. |
Centrifugal Compressor |
Axial Compressor |
1. |
Flow at inlet is axial and flow outlet is radial. |
Fluid flows parallel to the axis of rotation—Inlet and outlet axial. |
2. |
Motion imparted to gas with inertia forces by the rotating impellers. Pressure rise with velocity reduction in the diffuser |
Motion imparted to gas with torque exerted by the rotor blades. Uses several rows of airfoils to achieve pressure rise. It makes these complex and expensive. |
3. |
Higher stage pressure ratio |
Lower stage pressure ratio |
4. |
Simple in construction |
Complex in construction |
5. |
Strong in construction |
Less strong in construction |
6. |
Shorter in length and larger in diameter |
Larger in length and smaller in diameter |
7. |
More resistant to foreign materials |
Less resistant to foreign particles |
8. |
Less chances of blockade |
More chances of blockade |
9. |
Handles large volume flow |
Handles larger volume flow |
10. |
Improved matching characteristics |
Average matching characteristics |
11. |
Less cost of construction |
More cost of construction |
12. |
Lesser efficiency and less flow rate |
High efficiency and more flow rate |
13. |
Less weight |
More weight |
14. |
Less starting power requirement |
More starting power requirement |
15. |
Number of stages are less but the pressure rise per stage is high |
Number of stages are more. The pressure rise per stage is low. Requires large of stages for certain pressure. It becomes complex. |
16. |
Less Flow rate is up to 200000 cfm |
High Flow rate is 30000 to 500000 cfm |
17. |
Discharge pressure up to 10000 psi |
Discharge pressure up to 250 psi |
18. |
Efficiency 70 to 85 % |
Efficiency 85 to + 90 % |
19. |
Operating speed up to 50000 RPM |
Operating speed up to 10000 RPM |
20. |
Overall Less pressure rise because of limited or less number of stages |
More pressure rise because of large number of stages |
21. |
More stresses and life is less |
Relatively less stresses and more life |
22. |
These supply relatively less continuous flow of air. |
These supply a continuous flow of compressed air |
23. |
Uses of Centrifugal compressors(i) gas turbines(ii) turbo-shaft(iii) auxiliary power units(iv) micro-turbines. |
Applications of Axial compressors(i) large gas turbines in jet engines(ii) high speed ships(iii) blast furnaces(iv) aerospace engines. |
CENTRIFUGAL COMPRESSOR
STALLING IN A CENTRIFUGAL COMPRESSOR
Stalling is complete separation of flow & reverse flow. This is normally a local disruption of the airflow in a gas turbine or turbocharger compressor. It occurs only on any one blade of an axial compressor. This is related to compressor surge which is a complete disruption of the flow through the compressor. It is an unstable operation of the compressor. This causes vibrations, noise and causes failure of any one blade. It reduces due to
(i) Improved design
(ii) Using of hydro-mechanical and electronic control systems such as Full Authority Digital Engine Controls(FADEC).
The process of stalling in a compressor is of two types.
(a) Rotating stall
(b) Axis-symmetric stall or compressor surge
SURGING IN A COMPRESSOR
It happens due to sudden fall of supply pressure to the compressor inlet. The rate of pressure decrease is less in the outgoing stream than in the incoming stream of air. Consequently there is a tendency of back flow. A surging is a global phenomenon. It is due to stalling occurring in all the blades at the same time. In this, flow chocks or reverses. It is also an unstable operation of the compressor. This also causes severe vibrations and may cause the overall failure of the unit.
PRE-WHIRL IN A CENTRIFUGAL COMPRESSOR
It is a special type of motion to air at entry of centrifugal compressor.
ADVANTAGES OF PRE-WHIRL
(a) This reduces shock wave formation
(b) It reduces losses which were to be there due to shock wave formation
DISADVANTAGES OF PRE-WHIRL
(i) Output reduces due to Pre-whirl.
(j) Pressure ratio also reduces.
To overcome the disadvantage
(i) Twist the pre-whirl vanes (between the root and the tip)
(i) First in a linearly way and then in a parabolic manner.
SUPERCHARGERS AND TURBOCHARGERS
DEFINITION
Supercharging is the process of increasing the density of inlet air in internal combustion engines.
This increases the power output of the engine.
Methods of supercharging
-
Engine driven superchargers
-
Rotary blowers
-
Centrifugal compressors
-
Turbochargers driven by exhaust gases of the engine
Advantages of supercharging
-
Increase in power output
-
Reduction in weight to power ratio
-
Overcomes over loss of power in high altitudes
-
Reduction in the size of the locomotive engine
-
Reduction in the size of the marine engine
Disadvantages of supercharging
-
Increase in Detonation in Spark ignition engines
-
Increase in friction loss
-
Increases load on the bearing
-
Increased weight of the entire assembly
-
Melting possibility of piston top
-
Possibility of pre-ignition increases
-
Possibility of overheating of exhaust valves
DEFINITION OF A TURBOCHARGER
A turbocharger is a high speed turbine. Exhaust gases of the IC engine run the turbocharger.
Turbocharger runs the supercharger to increase overall power output of the IC engine.
Advantages of Turbochargers
-
It is very suitable for high speed engines.
-
There is no gearing between the gas turbine and the supercharger.
-
Use energy of exhaust gases for some useful purpose.
-
Exhaust gases contain one third of the total heat produced.
-
Increase power output of the engine at a cheaper rate and hence is economical.
-
Noise pollution of exhaust gases decreases.
-
Overall maintenance reduces
-
Turbochargers occupy less space and hence are compact.
Disadvantages of Turbochargers
-
Total initial cost increases.
-
It increases fuel consumption at lower speeds.
-
In the turbocharger, the temperature of air increases during compression. Which in turn reduces the density of the air. It reduces the mass of air entering the cylinder. Hence it reduces the power available.