Turbomachine

1. Chapter 3 Compressor  A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. It performs work on the gas to raise its pressure and temperature, making it useful for transportation, storage, or industrial processes. Basic Principle • The compressor imparts kinetic energy to the gas using rotating elements (like impellers or pistons). • This energy is then converted into pressure energy. By:Bikila Gadissa 1

2. Classification of Compressors  Compressors are generally classified into two main categories: By:Bikila Gadissa 2

3. Positive Displacement Compressors • These compressors trap a fixed amount of gas and then reduce its volume to increase pressure. • Types: • Reciprocating Compressors (piston type) • Rotary Compressors • Characteristics: • High pressure ratio per stage • Lower flow rate • Used in small-to-medium capacity systems By:Bikila Gadissa 3

4. Dynamic Compressors (Velocity Type)/Centrifugal • These compressors increase the velocity of gas and then convert it into pressure energy through diffusion. • Types: • Centrifugal Compressor • Axial Flow Compressor • Characteristics: • Continuous flow • Suitable for large volumes of gas • Common in gas turbines, HVAC, and process industries By:Bikila Gadissa 4

5. Applications of Compressors Application Area Example Use Power Generation Gas turbines Refrigeration & Air Conditioning Refrigerant circulation Oil & Gas Industry Gas pipeline transport, refining Manufacturing Pneumatic tools, air supply Automotive Turbochargers, superchargers Aerospace Jet engines (axial/centrifugal compressors) By:Bikila Gadissa 5

6. Centrifugal Compressor • A centrifugal compressor is a dynamic (velocity-type) compressor that increases the pressure of gas by imparting kinetic energy through a rotating impeller, which is then converted to pressure energy in a diffuser. Centrifugal compressor parts By:Bikila Gadissa 6

7. 1.By the type of flow, • Radial flow • Axial flow 2. By the type of energy conversion, • Volute casing • Volute casing with guide vanes 3. By the method of drive, • Gear drive • Belt drive

8. Principle of operation of Radial Compressor • Gas enters the compressor axially at low velocity and pressure. • The rotating impeller accelerates the gas radially outward due to centrifugal force. • As the gas passes through the diffuser, its velocity decreases, and pressure increases (energy conversion). • The casing collects the high-pressure gas and directs it to the discharge outlet. By:Bikila Gadissa 8

9. Pressure and velocity variation across a centrifugal compressor Fig x. Pressure and velocity variation across centrifugal compressor By:Bikila Gadissa 9

10. Principle of operation of Radial Compressor … The pressure and velocity variation across the compressor is as shown in the figure. By:Bikila Gadissa 10

11. 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 x. • 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. By:Bikila Gadissa 11

12. Main Components/Elements • Impeller • The main rotating component. • Consists of blades or vanes mounted on a shaft. • Converts mechanical energy from the motor into kinetic energy of the gas. • Diffuser • Stationary passage surrounding the impeller. • Converts the high velocity (kinetic energy) of gas leaving the impeller into static pressure. • Casing (Volute or Scroll) • Collects and directs the compressed gas from the diffuser to the discharge. • Shaped to maintain smooth flow and minimize energy losses. • Inlet Guide Vanes • Control the amount and angle of air entering the impeller. • Used for performance control. • Shaft and Bearings • Support and drive the impeller. • Bearings reduce friction and maintain alignment. • Drive Mechanism • Usually an electric motor or turbine drives the impeller via a shaft. By:Bikila Gadissa 12

13. Work Done and Pressure Rise • No work is done on the gas in the diffuser because the diffuser simply converts one type of energy into another type. • Work absorbed by the gas depends upon the condition of the gas at inlet and outlet of the impeller. • 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. By:Bikila Gadissa 13

14. Work Done and Pressure Rise … By:Bikila Gadissa 14

15. Work Done and Pressure Rise … (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 ,ideal case Fig.Velocity triangles and blade shape of a 3D radial compressor impeller By:Bikila Gadissa 15

16. Work Done and Pressure Rise … (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 By:Bikila Gadissa 16

17. Work Done … From Euler’s energy equation, ideal case, By:Bikila Gadissa 17

18. …cont’d at the outlet • In ideal case, as shown in Figure, air leaves the impeller tip at an angle of 900. Hence, Vw2= U2. But in actual case due to slip between the impeller and the fluid, Vw1is somewhat less than U2as shown in Figure ‘. By:Bikila Gadissa 18

19. Slip Factor (Real Condition) In a real centrifugal compressor, the air does not leave exactly at the blade angle due to: • fluid inertia • blade curvature • flow separation • finite number of impeller blades This causes the actual whirl velocity to be less than the ideal/ theoretical whirl velocity. By:Bikila Gadissa 19

20. Slip Factor (σ) impellers, the formula for   is given by Stanitz as For radial vaned follows: By:Bikila Gadissa 20

21. Work Done with Slip Factor By:Bikila Gadissa 21

22. Power Input Factor • The Euler equation with slip gives the idealized mechanical energy imparted to the fluid by the rotor: But actual shaft power must account for several additional real-world effects: • aerodynamic losses (diffuser, blade profile, wakes, secondary flows) • thermodynamic irreversibilities (actual enthalpy rise vs. ideal work) • leakage and recirculation • mechanical losses in bearings, seals, gearbox, couplings To capture these, we introduce a power input factor that scales the Euler work to the actual shaft power required. By:Bikila Gadissa 22

23. Power Input Factor ψ By:Bikila Gadissa 23

24. Temperature-Equivalent Work Done in a Compressor • For an adiabatic compressor, the specific work input can also be expressed in terms of the total (stagnation) temperature rise across the compressor. Total Temperature Definition ??? : Inlet total (stagnation) temperature ??? : Exit total (stagnation) temperature • The energy required to compress the fluid is proportional to the increase in stagnation temperature. By:Bikila Gadissa 24

25. Temperature-Equivalent Work Done in a Compressor … By:Bikila Gadissa 25

26. Relating temperature rise to pressure ratio … By:Bikila Gadissa 26

27. Relating temperature rise to pressure ratio … By:Bikila Gadissa 27

28. Relating temperature rise to pressure ratio … By:Bikila Gadissa 28

29. Relating temperature rise to pressure ratio… By:Bikila Gadissa 29

30. Relating temperature rise to pressure ratio… By:Bikila Gadissa 30