1. Power Consuming Fluid Machines - IIBY
Dr. P M V Subbarao
Mechanical Engineering Department
I I T Delhi
Another set of Small Things !!!

2. Classification of Pumps

3. Classification of Compressors
Rotodynamic Compressors
Positive Displacement of Compressors
Centrifugal
Compressors
Mixed
Compressors
Axial
Compressors
Reciprocating
Compressors
Rotory
Compressors

4. Thermodynamic Comparison of Pump & Compressor
Work input for unit mass in a Pump:
Work input for unit mass in a Compressor:

5. Centrifugal Pump
                                           

7. Centrifugal Compressors
The centrifugal compressor consists essentially of a stationary casing containing a rotating impeller which imparts a high velocity to the air, and
A number of diverging passages (Diffuser) in which the air is decelerated with a consequent rise in static pressure.
Air is sucked into the impeller eye and whirled round at high speed by the vanes on the impeller disc.
Centripetal acceleration leads to increase in rise of some amount of static pressure.
The remaining increase in rise of pressure occurs in diffuser.

10. Anatomy of A Centrifugal Compressors

11. Va2
Vr2
Vw2
Va2
Vf2
Vr2
Vw2 b1
Vf1=Va1
Vr1

12. Work Consumed & Pressure or Head Rise
In general air enters the impeller eye in axial direction.
The axial portion of impeller of the vanes are curved so that the air can pass smoothly into the eye.
Due to inertia, the air trapped between the impeller vanes is reluctant to move round with the impeller and prevents air acquiring a whirl velocity equal to the impeller speed (Vw2 < U).
This is called slip and quantified as slip factor, s.
where, n is number of vanes
Theoretical Torque :
Theoretical Power Consumed:

13. Power input factor, y
Typical values for the power input factor, y : – 1.04.

14. 3 2 1

15. Stagnation Pressure
Va22/cp
Stagnation Temperature
Stagnation Enthalpy

16. Work consumed by A compressor = Increase in Stagnation Enthalpy of gas
Work consumed by A Pump = Increase in Stagnation Enthalpy of liquid

17. isentropic pressure rise due to (T03-T01).
For an irreversible compression or pumping, the actual pressure rise is less than
isentropic pressure rise due to (T03-T01).
For a compressor:

19. Slip Factor, Power Input Factor & Efficiency
Power input factor and slip factor are neither independent of one another nor of efficiency.
The power input factor represents an increase in the work input, the whole of which is absorbed in overcoming frictional loss and therefore degraded into thermal energy.
Power input factor should be as low as possible.
Low values of Power input factor imply that the values of efficiency are high.
However, the value of efficiency also depends on friction losses in the diffuser which does not affect power input factor.
The slip factor limits the capacity of the compressor and this should be as high as possible.
A high value of slip factor require higher number of vanes.
Higher number vanes will increase frictional losses and hence increase the value of power input factor and decrease the value of efficiency.
A suitable compromise must be found, and present day practice is :
19 – 21 vanes to get a slip factor value of 0.9.

20. The inlet mach number is controlled by Using the angle of prewhirl.
Inlet Mach Number for Impeller and Diffuser
At intake the air is deflected a certain angle before it passes into the radial channels on the impeller.
The inlet Mach number:
Vw1 b1
Vr1
The inlet mach number is controlled by
Using the angle of prewhirl.
Va1

21. The role of A Pump in a Fluid Handling System
Delivery Pressure or Head
Static Pressure or Head
Suction Pressure or Head

22. Similarity Analysis
Differential head of pump:

23. Similarity Analysis
Pressure Ratio of A Compressor:

24. Affinity Laws for Centrifugal Pumps

25. Pump Characteristics
Q, Discharge

26. Compressor Characteristics

30. Axial Flow Compressors

31. Axial Flow Compressors

32. Compressor Stage
Va32/cp
Va22/cp
Va12/cp

33. Vr1
Va1
Va1
Vf1
Vw1
Vr2
Va2
Va2
Vf2
Vw2
Va3

34. Power input to the compressor:
Current Practice:
Theoretical Power input to the compressor:

35. For an isentropic compressor:

36. A high pressure rise per stage will decrease the number of stages for a given overall pressure rise.
A high pressure rise per stage is obtained using:
High blade speed.
High inlet flow velocity.
High fluid deflection in rotor blades.

37. Blade Speed
For a given rotor speed the velocity of the blade at the tip will be maximum.
The centrifugal stress in the rotor blades depends on the rotational speed, the blade material and length of the blade.
The maximum centrifugal stress is given by,
b, hub-tip diameter ratio.
K varies in the range 0.55 – 0.65.

38. INLET CONDITIONS
Vr1
Va1
Vr1
Va1

39. Fluid Deflection
Vr1
Vr2
Vrmax

40. Frictional Losses

41. Multi Stage Compression
Vr1 Vf Vf
Mean Vf
Mean Vf
Fourth Stage
First Stage
Work done factor, l, decrease with number of stages.

43. Radial Variation of Blade Angles