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

ROTARY

COMPRESSORS

1.

Physical motion

These employ piston

cylinder device with

to and fro motion.

These employ

an impeller

with rotary

motion.

2.

Pressure rise per stage

Discharge pressure is

high and gives a pressure

ratio of 4 to 7 per stage.

Discharge

pressure is

low and gives

a pressure

ratio of 3 to 5 per stage.

3.

Pressure range and discharge

Used for high pressure

and less volumetric

flow rates.

Used for less

pressure rise

and large

volumetric flow

rates.

4.

Upper limit of high pressure

These can produce

pressures up to 1000

bars

Normally these

produce pressures

up to 10 bars. With

large number of

stages, these can

produce pressures

up to 400 bars.

5.

Volumetric flow rate or capacity

These can handle volume flow rate of 50 m3/sec.

Rotary compressors can handle volume

flow 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 supply

compressed 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 the

lubricating 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 rate

and 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 

  1. Engine driven superchargers

  2. Rotary blowers

  3. Centrifugal compressors

  4. Turbochargers driven by exhaust gases of the engine

Advantages of supercharging

  1. Increase in power output

  2. Reduction in weight to power ratio

  3. Overcomes over loss of power in high altitudes

  4. Reduction in the size of the locomotive engine

  5. Reduction in the size of the marine engine

Disadvantages of supercharging

  1. Increase in Detonation in Spark ignition engines

  2. Increase in friction loss

  3. Increases load on the bearing

  4. Increased weight of the entire assembly

  5. Melting possibility of piston top

  6. Possibility of pre-ignition increases

  7. 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

  1. It is very suitable for high speed engines.

  2. There is no gearing between the gas turbine and the supercharger.

  3. Use energy of exhaust gases for some useful purpose.

  4.  Exhaust gases contain one third of the total heat produced.

  5. Increase power output of the engine at a cheaper rate and hence is economical.

  6. Noise pollution of exhaust gases decreases.

  7. Overall maintenance reduces

  8. Turbochargers occupy less space and hence are compact.

Disadvantages of Turbochargers

  1. Total initial cost increases.

  2. It increases fuel consumption at lower speeds.

  3. 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.