compressor mechanic Interview Questions and Answers

1. What are the main types of compressors?

   • Answer: The main types of compressors include reciprocating, centrifugal, rotary screw, and scroll compressors. Each has its own advantages and disadvantages regarding capacity, efficiency, and application.

2. Explain the difference between positive displacement and dynamic compressors.

   • Answer: Positive displacement compressors increase pressure by reducing the volume of a fixed mass of gas. Examples include reciprocating and rotary screw. Dynamic compressors use centrifugal force or other means to accelerate the gas, increasing its pressure and velocity. Examples include centrifugal and axial compressors.

3. Describe the function of a compressor's suction valve.

   • Answer: The suction valve allows refrigerant or air to enter the compressor cylinder during the suction stroke, preventing backflow during the compression stroke.

4. What is the role of the discharge valve in a compressor?

   • Answer: The discharge valve opens to allow the compressed gas to exit the cylinder and enter the discharge line, preventing backflow into the cylinder.

5. Explain the importance of compressor oil.

   • Answer: Compressor oil lubricates moving parts, cools the compressor, and seals the clearances between moving components, preventing leaks and wear.

6. How do you check the oil level in a compressor?

   • Answer: The method varies depending on the compressor type. Typically, it involves checking the oil sight glass or dipstick, ensuring the level is within the manufacturer's specified range.

7. What are the common causes of compressor overheating?

   • Answer: Overheating can be caused by low refrigerant charge, insufficient lubrication, restricted airflow, faulty motor, or excessive ambient temperature.

8. How do you troubleshoot a compressor that won't start?

   • Answer: Troubleshooting involves checking power supply, motor connections, overload protection, and starting components like capacitors or starters. Inspecting for tripped breakers or fuses is also crucial.

9. What are the signs of a failing compressor?

   • Answer: Signs include unusual noises (knocking, rattling, squealing), excessive vibration, overheating, low or no output pressure, and leaking refrigerant.

10. Describe the process of replacing a compressor.

    • Answer: The process involves recovering refrigerant, disconnecting electrical and piping connections, removing the old compressor, installing the new compressor, connecting the piping and electrical components, charging the system with refrigerant, and testing for leaks and proper operation.

11. What safety precautions should be taken when working on a compressor?

    • Answer: Always wear safety glasses, gloves, and appropriate clothing. Ensure the power is disconnected before working on electrical components. Use proper handling techniques for refrigerant, and be aware of high-pressure systems.

12. What is the purpose of a pressure relief valve in a compressor system?

    • Answer: The pressure relief valve protects the system from excessive pressure buildup, preventing damage or rupture.

13. Explain the function of a compressor's unloader valve.

    • Answer: The unloader valve bypasses the compressor during startup, reducing the load on the motor and preventing damage.

14. What is the difference between single-stage and two-stage compressors?

    • Answer: Single-stage compressors compress the gas in one step, while two-stage compressors use two stages of compression to achieve higher pressures more efficiently.

15. How do you identify the refrigerant used in a compressor system?

    • Answer: The refrigerant type is typically indicated on a label on the compressor or system components. The system's specifications or documentation should also list the refrigerant.

16. What tools are commonly used for compressor maintenance and repair?

    • Answer: Common tools include wrenches, screwdrivers, pressure gauges, refrigerant recovery equipment, vacuum pumps, leak detectors, and specialized compressor tools.

17. Explain the importance of proper refrigerant handling.

    • Answer: Refrigerant is often harmful to the environment and can be hazardous to health if mishandled. Proper handling involves using appropriate recovery and recycling equipment and following all safety regulations.

18. What is a compressor's volumetric efficiency?

    • Answer: Volumetric efficiency is the ratio of the actual volume of gas compressed to the theoretical volume of the cylinder's displacement.

19. What are the common causes of compressor pulsation?

    • Answer: Pulsation, or uneven pressure delivery, can be caused by problems with the valves, insufficient air intake, or problems within the compression chamber itself.

20. How do you diagnose a compressor with low discharge pressure?

    • Answer: Diagnosing low discharge pressure involves checking for refrigerant leaks, ensuring proper operation of the valves, verifying the motor's performance, and examining the overall system for blockages or restrictions.

21. What is the purpose of a crankcase heater?

    • Answer: A crankcase heater prevents refrigerant migration into the crankcase oil, which can damage the compressor.

22. Explain the concept of compressor capacity.

    • Answer: Compressor capacity refers to the volume of gas it can compress and deliver per unit of time, typically measured in CFM (cubic feet per minute) or m³/h (cubic meters per hour).

23. What is the significance of the compressor's pressure ratio?

    • Answer: The pressure ratio is the ratio of the discharge pressure to the suction pressure. It indicates the compressor's ability to increase pressure.

24. Describe the function of a receiver tank in a compressor system.

    • Answer: A receiver tank stores compressed air, smoothing out pressure fluctuations and providing a reserve supply.

25. How do you perform a leak test on a compressor system?

    • Answer: Leak testing involves pressurizing the system with nitrogen and using a leak detector to identify any leaks.

26. What is the difference between a hermetic and open compressor?

    • Answer: A hermetic compressor has the motor and compressor sealed within a single housing, while an open compressor has the motor and compressor as separate units.

27. What are some common causes of excessive vibration in a compressor?

    • Answer: Excessive vibration can result from worn bearings, imbalance, loose mounting, or internal mechanical problems within the compressor.

28. How do you determine the proper refrigerant charge for a compressor system?

    • Answer: The proper charge is determined by the system's specifications, often using a superheat and subcooling measurement method with pressure-temperature charts.

29. What is the importance of regular compressor maintenance?

    • Answer: Regular maintenance extends the compressor's lifespan, improves efficiency, reduces downtime, and ensures safe operation.

30. Explain the function of a suction strainer in a compressor system.

    • Answer: A suction strainer filters out contaminants from the gas entering the compressor, preventing damage to internal components.

31. What are the signs of a failing compressor bearing?

    • Answer: Signs include excessive vibration, unusual noises (growling, rumbling), and increased operating temperature.

32. How do you prevent compressor oil contamination?

    • Answer: Preventing contamination involves using clean oil, regularly changing the oil, and ensuring proper system filtration.

33. What is the purpose of a moisture indicator in a compressor system?

    • Answer: A moisture indicator detects the presence of moisture in the refrigerant, which can cause damage to the system.

34. Describe the process of evacuating a compressor system.

    • Answer: Evacuation removes air and moisture from the system before charging with refrigerant, using a vacuum pump.

35. What is a scroll compressor? What are its advantages and disadvantages?

    • Answer: A scroll compressor uses two spiral-shaped plates to compress refrigerant. Advantages include quiet operation, compact size, and high efficiency. Disadvantages can include higher initial cost and limited capacity compared to other types.

36. What safety measures should be taken when handling high-pressure refrigerant?

    • Answer: Always wear appropriate safety gear including eye protection and gloves. Use proper pressure relief valves and follow all safety guidelines for handling high-pressure systems. Ensure adequate ventilation.

37. Explain the importance of proper system design in preventing compressor failures.

    • Answer: Proper design ensures optimal refrigerant flow, reduces stress on the compressor, prevents overheating, and maximizes efficiency.

38. What is the role of a thermal overload protector in a compressor?

    • Answer: The thermal overload protector shuts off the compressor if it overheats, preventing damage.

39. How do you interpret a compressor's performance data?

    • Answer: Analyzing performance data involves examining pressure readings, temperature readings, current draw, and efficiency metrics to identify potential problems.

40. What is the significance of compressor head pressure?

    • Answer: Head pressure is the pressure at the compressor's discharge. It reflects the compressor's ability to compress the gas and is crucial for proper system operation.

41. Describe the different types of compressor valves.

    • Answer: Compressor valves include reed valves, plate valves, and poppet valves. Each type has different advantages and disadvantages regarding durability, efficiency, and cost.

42. What is the purpose of a suction line accumulator?

    • Answer: A suction line accumulator traps liquid refrigerant that might enter the suction line, preventing it from reaching the compressor.

43. How do you maintain a compressor's electrical components?

    • Answer: Maintenance includes checking connections for tightness, inspecting wiring for damage, ensuring proper grounding, and verifying the operation of overload protectors.

44. What are some common causes of compressor noise?

    • Answer: Noise can indicate worn bearings, loose components, valve problems, or other mechanical issues within the compressor.

45. How do you troubleshoot a compressor with high current draw?

    • Answer: High current draw can signal mechanical problems, problems with the motor, or restrictions within the system. Check for proper lubrication and inspect for any blockages or restrictions.

46. What is the importance of proper system cleanliness during compressor installation and maintenance?

    • Answer: Cleanliness is crucial to prevent contamination of refrigerant and compressor oil, ensuring efficient and reliable operation.

47. Explain the concept of compressor efficiency.

    • Answer: Compressor efficiency refers to how effectively it converts electrical energy into compressed gas. Higher efficiency means less energy consumption for the same output.

48. How do you identify and repair a cracked compressor housing?

    • Answer: A cracked housing usually requires replacement. Repair is generally not feasible due to the high pressure involved.

49. What are the environmental considerations related to compressor operation and maintenance?

    • Answer: Environmental considerations include proper refrigerant handling to prevent ozone depletion and global warming, responsible disposal of waste materials, and energy-efficient operation to reduce carbon emissions.

50. How do you determine the correct size compressor for a specific application?

    • Answer: This depends on the required airflow or gas flow rate, pressure requirements, and other system parameters. Consult system specifications and manufacturer's recommendations.

51. What is the purpose of a check valve in a compressor system?

    • Answer: A check valve allows flow in one direction only, preventing backflow of refrigerant or compressed gas.

52. How do you troubleshoot a compressor with excessive oil carryover?

    • Answer: Excessive oil carryover can indicate worn piston rings, problems with the oil separator, or incorrect oil level. Check the oil level, inspect the oil separator, and check for wear on piston rings or other internal components.

53. What are the common types of compressor motors?

    • Answer: Common types include AC induction motors and DC motors. The choice depends on the application and power requirements.

54. How do you safely recover refrigerant from a compressor system?

    • Answer: Refrigerant recovery requires specialized equipment and adherence to safety regulations. Use a recovery machine to capture refrigerant and ensure it is properly recycled or disposed of.

55. What training and certifications do you have related to compressor mechanics?

    • Answer: [Candidate should list their certifications and training, e.g., EPA certification for refrigerant handling, specific manufacturer training, etc.]

56. Describe your experience with different types of compressor systems.

    • Answer: [Candidate should describe their experience with various types, including specific makes and models, applications, and maintenance procedures.]

57. How do you stay up-to-date on the latest technologies and best practices in compressor maintenance?

    • Answer: [Candidate should mention professional development activities, industry publications, training courses, etc.]

58. Describe a challenging compressor repair you have completed and how you solved it.

    • Answer: [Candidate should describe a specific situation, highlighting their problem-solving skills and technical expertise.]

59. What are your salary expectations?

    • Answer: [Candidate should state their salary expectations based on their experience and research of the market rate.]

60. Why are you interested in this position?

    • Answer: [Candidate should articulate their reasons, highlighting their skills and interests related to the job description and company.]

61. What are your strengths as a compressor mechanic?

    • Answer: [Candidate should list relevant strengths, such as troubleshooting skills, technical expertise, problem-solving abilities, teamwork, and communication skills.]

62. What are your weaknesses as a compressor mechanic?

    • Answer: [Candidate should honestly mention a weakness and explain how they are working to improve it. Avoid mentioning weaknesses that are critical to the job.]

63. Do you have experience with different brands of compressors?

    • Answer: [Candidate should list specific brands and models they've worked with.]

64. Are you comfortable working independently and as part of a team?

    • Answer: [Candidate should affirm their ability to work effectively in both environments.]

65. How do you handle stressful situations in a fast-paced work environment?

    • Answer: [Candidate should explain their approach to stress management, emphasizing their ability to remain calm and focused under pressure.]

66. What are your long-term career goals?

    • Answer: [Candidate should articulate their career aspirations, showing ambition and a desire for professional growth.]

67. Describe your problem-solving approach when dealing with a malfunctioning compressor.

    • Answer: [Candidate should outline a systematic approach, including gathering information, diagnosing the problem, and implementing a solution.]

68. Do you have experience with preventative maintenance programs for compressors?

    • Answer: [Candidate should describe their experience with developing and implementing preventative maintenance schedules.]

69. How do you ensure the safety of yourself and others while working on compressors?

    • Answer: [Candidate should detail their commitment to safety protocols and procedures.]

70. What are your computer skills?

    • Answer: [Candidate should list their relevant computer skills, e.g., use of diagnostic software, maintenance scheduling software.]

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