433A Pumps and Compressors 3 — Questions and Answers
Question 1: What is the difference between a single-stage and a multi-stage centrifugal pump?
- Single-stage pumps are newer designs
- A single-stage pump has one impeller while a multi-stage pump has multiple impellers in series, each adding head (pressure) to the fluid (Correct answer)
- Multi-stage pumps have multiple suction inlets
- Single-stage pumps operate at higher speeds
Correct answer: A single-stage pump has one impeller while a multi-stage pump has multiple impellers in series, each adding head (pressure) to the fluid
A single-stage pump has one impeller and produces head in a single step. A multi-stage pump has multiple impellers arranged in series, with the output of each stage feeding the next, progressively adding head to achieve much higher discharge pressures.
Question 2: What is the purpose of the volute casing in a centrifugal pump?
- To hold the bearings
- To gradually increase the cross-sectional area of the flow path, converting the velocity energy from the impeller into pressure energy (Correct answer)
- To protect the impeller from debris
- To mount the pump to the baseplate
Correct answer: To gradually increase the cross-sectional area of the flow path, converting the velocity energy from the impeller into pressure energy
The volute is a spiral-shaped casing that surrounds the impeller. Its gradually increasing cross-sectional area slows the high-velocity fluid leaving the impeller, converting kinetic (velocity) energy into pressure energy according to Bernoulli's principle.
Question 3: What is a reciprocating compressor and where is it commonly used?
- A compressor that moves back and forth on rails
- A compressor using pistons driven by a crankshaft to compress gas in cylinders, commonly used for high-pressure applications (Correct answer)
- A turbine-type compressor
- A compressor used only for refrigeration
Correct answer: A compressor using pistons driven by a crankshaft to compress gas in cylinders, commonly used for high-pressure applications
A reciprocating compressor uses pistons driven by a crankshaft to compress gas in cylinders, similar to an internal combustion engine. It is commonly used in high-pressure applications including industrial air supply, natural gas processing, and refrigeration.
Question 4: What is the function of an intercooler between stages of a multi-stage compressor?
- To increase the gas temperature
- To cool the compressed gas between stages, reducing its volume, lowering discharge temperature, and improving compression efficiency (Correct answer)
- To add lubricant to the gas
- To filter contaminants between stages
Correct answer: To cool the compressed gas between stages, reducing its volume, lowering discharge temperature, and improving compression efficiency
Compression heats the gas significantly. An intercooler removes this heat between stages, reducing the gas volume entering the next stage, lowering the overall discharge temperature, reducing power consumption, and bringing the compression process closer to isothermal (ideal) efficiency.
Question 5: What is the purpose of a compressor unloader?
- To remove the compressor from service
- To reduce or eliminate the compressor's pumping capacity during low-demand periods without stopping the motor (Correct answer)
- To increase compressor speed
- To drain moisture from the compressor
Correct answer: To reduce or eliminate the compressor's pumping capacity during low-demand periods without stopping the motor
A compressor unloader allows the compressor motor to continue running while reducing or eliminating the compression workload during low-demand periods. Methods include holding suction valves open, closing suction ports, or bypassing discharge to suction.
Question 6: Why is it important to check the direction of rotation before starting a centrifugal pump?
- Direction does not matter for centrifugal pumps
- Reverse rotation produces little or no flow and pressure, and can damage the pump, loosen the impeller, or cause the mechanical seal to fail (Correct answer)
- To ensure the motor warranty is valid
- Reverse rotation increases the pump's efficiency
Correct answer: Reverse rotation produces little or no flow and pressure, and can damage the pump, loosen the impeller, or cause the mechanical seal to fail
A centrifugal pump impeller is designed to work in one direction only. Reverse rotation produces drastically reduced performance, can unscrew threaded impellers from the shaft, damage mechanical seals, and cause the pump to overheat from churning fluid without producing useful flow.
433A Pumps and Compressors 3
What is the difference between a single-stage and a multi-stage centrifugal pump?