Chapter 5
COMPRESSORS
OBJECTIVES
After studying this chapter, the student will be able to:
▪ Explain the principles of compression.
▪ Describe how centrifugal compressors operate.
▪ Describe how axial flow compressors operate.
▪ Identify and describe centrifugal and positive displacement compressors.
ANSWERS TO REVIEW QUESTIONS
1. List two purposes for which industrial manufacturers use compressors. Answer: Compressors are
used to transfer granular powders and BB size plastic pellets from place to place. In natural gas
2. List four gases that compressors are used to compress. Answer: Nitrogen, hydrogen, carbon
dioxide, air, and chlorine.
3. List the two types of compressors. Answer: Dynamic and positive displacement.
4. List three rotary compressors. Answer: Rotary screw, lobe, liquid ring.
5. List two reciprocating compressors. Answer: Piston, reciprocating.
6. Compare and contrast the operation of an axial compressor with a multi-stage centrifugal
compressor. Answer: Axial compressors transfer axially, while centrifugal compressors use
7. Describe a rotary screw compressor. Answer: This device operates with two helical rotors that
rotate towards each other causing the teeth mesh. As the left rotor turns clockwise, the right
8. Describe a sliding vane compressor. Answer: The sliding vane compressor uses a slightly
off center rotor with sliding vanes to compress gases. The gas inlet port is positioned so
gas flows into the vanes when they are fully extended and form the largest pocket. As the
vanes turn toward the discharge port the gases are compressed. The body of the
9. Describe a lobe compressor. Answer: Lobe compressors are characterized by the two kidney
bean shaped impellers used to trap and transfer gases. The close clearances between the casing
and impellers is maintained by a set of timing gears. During operation the two impellers move in
3. Describe a liquid ring compressor. Answer: A liquid ring compressor combines the centrifugal
action of the liquid with the rotary action of the curved vanes. A liquid ring compressor has one
11. Describe parallel arrangement in a reciprocating compressor. Answer: Parallel cylinder
12 Describe a multistage arrangement in a reciprocating compressor. Answer: The first cylinder in a
multistage compressors is usually referred to as the first stage. Two stage multistage
13. Describe a double-acting arrangement in a reciprocating compressor. Answer: Double-acting
compressors use a common cylinder and piston to discharge and take in gases on each side of the
cylinder. Double-acting compressor cylinders are equipped with two (2) spring loaded intake
14. List the basic components of a centrifugal compressor. Answer:
a. impeller(s)
b. vanes
15. List the main differences between a single-stage and a multistage compressor. Answer: The
16. What are the components of a typical compressor system? Answer: The basic components of a
compressor system include; a compressor, storage drum, piping and valves, dryer, water removal
17. Contrast intercooler and aftercooler heat exchangers. Answer: Heat exchangers are used
18. Describe how to startup a dynamic compressor. Answer: See procedure at end of chapter.
19. Describe how to startup a positive displacement compressor? Answer: See procedure at end of
20. List four types of compressor fans. Answer: Fans can be classified as centrifugal, propeller,
20. Define thermal shock. Answer: Thermal shock is caused by extreme variations in temperature
22. Describe the term, “compression ratio” and explain how to use it.
Answer: Compression ratio—the ratio of discharge pressure (psia) to suction pressure
23. Explain the operation of a pulsation dampener. Answer: A common problem found with
reciprocating compressors is pulsation. This inherent problem occurs because the suction
24. List the major components of a sliding vane compressor. Answer: See Figure 5.8 Sliding
25. Explain the primary differences between operating a positive displacement compressor
system and a centrifugal compressor system. Answer:
Compressors usually can be classified into two groups: positive displacement and