EPC School

1. What is the working principle of rotodynamic/ positive displacement compressors?

2. What are isothermal, adiabatic and polytropic compression processes?

3. In a reciprocating compressor what is the function of an unloader and how does it work?

4. In a double acting reciprocating compressor cylinder how is gas prevented from escaping from one side of piston to another?

5. Why is lubrication needed in the cylinder?

6. What is the function of rider ring?

7. Why do we use double acting reciprocating compressor?

8. What kind of sealing system is used for piston rods in recip machines?

9. What is the principle of operation of suction and discharge valves in a reciprocating compressor?

10. What is a labyrinth compressor?

11. What is the difference between fans, blowers and compressors?

12. In case of centrifugal compressors what types of seals are used?

13. Why are intercoolers used?

14. Where is the head developed in a centrifugal compressor?

15. What type of seals is used for air compressor?

16. How does a centrifugal compressor performance vary with molecular weight for a fixed volume flow rate?

17. How are critical speeds avoided in case of flexible rotors?

18. What would be the effect of closing the discharge valve in a centrifugal compressor?

19. Why do we go for reverse flow arrangement in a centrifugal compressor?

20. What is surging?

21. Under what condition does the compressor surge?

22. What precautions can be taken to prevent surging?

23. What are the types of screw compressors?

24. Does a screw compressor develop pulsating or continuous flow?

25. What is the need for timing gears in dry screw compressors?

26. Which rotor (male/female) is the drive shaft in a screw compressor?

27. What are the benefits of screw compressors over reciprocating?

28. What are the various protection devices on centrifugal compressors?

29. What are the various protection devices on reciprocating compressors?

30. What is the working principle of balancing drum and why is it used?

31. What is the method of capacity control for centrifugal compressors?

32. What is the method of capacity control for reciprocating compressors?

33. What is the method of capacity control for screw compressors?

34. What are the various auxiliary systems for a centrifugal compressor?

35. What are the various auxiliary systems for a reciprocating compressor?

36. Why are rundown tanks required?

37. What are the symptoms of surging?

Answers to Above Questions-

1. Rotodynamic type compressors are machines in which air or gas is compressed by the dynamic action of rotating vanes or impellers imparting velocity and to the flowing gas. The velocity head is converted into pressure, partially in the rotating element and mostly in the stationary diffusers or blades.

Positive displacement type compressors are machines in which successive volumes of air or gas are confined within a closed space. The pressure is increased as the volume of the closed space is decreased. 

2. Isothermal Process – a process in which temperature is constant.

Adiabatic Process – a process during which there is no external exchange of heat.

                                    PVk = Constant

Polytropic Process – a process for perfect gases, following the law

                                    PVn = Constant                        n=1 for isothermal process.

3. Unloader is a form of capacity control. It unloads by keeping the suction valve forcibly open and thereby avoiding compression by recirculating gas back to suction line. It is also used to start the machine in no load condition

4. Piston rings acts as pressure breakers and prevent gas from escaping

5. Prevent wear of cylinder and rider ring  and to take away the heat of friction between rider ring and cylinder and

6. To take the load of  piston and safe guarding the piston and also guide along the cylinder

7. To smoothen the forces on the crank shaft and reduce the no of cylinders

8. PTFE, carbon packing rings

9. Differential pressure across them.

10. These are vertical type reciprocating machines. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. In labyrinth piston compressors, an extremely large number of throttling points provide the sealing effect around pistons and piston rods. 

11. Discharge pressure generated

Fans                 < 0.5 psi

            Blowers            0.5 >P< 1.65 psi

Compressor     > 1.65 psi

12. Labyrinth seal ( incase of air compressor)

Carbon rings (incase of air compressor)

Wet mechanical seal, dry gas seals (incase of process gas compressor)

13. To reduce the work required for compression inturn increasing the efficiency of compressor

To avoid using higher metallurgy which increases cost and thickness (needed for high temperature applications)

14. Velocity is imparted to gas in the impeller and the velocity head is converted into pressure, partially in the rotating impeller and partially in the stationary diffusers

15. Labyrinth seal and Carbon rings

16. For the same volume flow rates power consumption increases for higher molecular weight gases

17. By quickly ramping up through critical speed range. Ramping windows are provided in governors

18. Reduce the flow and increase the head hence operating point is shifted to the left in characteristic curve, compressor may start to surge

19. To counter the axial thrust generated in the compressor

20. When the compressor flow goes below certain value reversal of flow take place, this phenomenon is called as surging

21. Low flow, high back pressure

22. Antisurge system is provided, having recycle (spillback) valve, as soon as compressor approaches the surge control line, this valve opens and increases the flow through compressor thereby prevent surging, for Air Compressors blow off / vent valves are used to prevent surging

23. Dry screw compressor & Wet screw compressor

24. Pulsating flow of very high frequency

25.  Screw compressors have two rotors male and female. Both the rotors are synchronized by timing gear, so that constant gap is always maintained between the rotors and prevent them from contact

26. Male rotor

27. More flow, less pulsation, more reliability, low maintenance cost, high compression efficiency compared to reciprocating compressor, can handle change in molecular weights, can handle liquid, gas and solids to an extent

28. Antisurge system, Alarm & trips given on certain parameters such as High bearing temperature, high shaft vibration, low lube oil pressure, axial displacement of shaft, overspeed trips and process related trips.

29. Alarms & trips on High bearing temperature, Piston rod drop, stuffing box temperature, process related trips.

30. Balancing drum is used to reduce the axial thrust load. One side of balancing drum see the discharge pressure and other side it see the suction pressure, so axial thrust produced by the pressure difference across the impeller is balanced by the axial thrust across the balancing drum.

31. Suction throttling

 Variable speed

 Inlet guide vanes

 Spill back valve

32. Unloaders on suction valves.

   Clearance volume

   Variable speed

33. Spill back valve

   Slide valve opening

   Speed control

34. Lube oil system

   Seal oil/gas system

   Surge control system

   Cooling water system

   Machine protection system

35. Lube oil system

      Cooling water system

      Machine protection system

      Sealing system

36. Basically to supply lube oil during emergency shutdown/trip and for safe coast down

37. High vibration, noise, flow reversal in compressor, fluctuations in suction/discharge pressures.