Compressors in Piping Systems: Classification, Working, Applications, and Interview Questions
Compressors are one of the most important rotating equipment items used in modern industry and piping systems. Their main purpose is to increase the pressure of gases and provide compressed air or gas for different industrial operations. In many plants, compressors support pneumatic tools, control systems, air-operated valves, process gas handling, refrigeration, ventilation, and utility air networks. Without compressors, many industrial systems would not operate efficiently.
In simple terms, a compressor takes in low-pressure gas and delivers it at a higher pressure. This compressed gas may be air, nitrogen, refrigerant gas, process gas, or another industrial fluid depending on the application. Compressors are widely used in power plants, oil and gas facilities, chemical plants, refineries, manufacturing industries, air conditioning systems, and gas transmission pipelines.
In this article, we will study the classification of compressors, their working principles, major types, applications, and some important interview questions with answers.
Why Compressors Are Important in Piping and Process Industries
Compressors play a vital role in piping systems because they help move and store gases under pressure. Many industrial processes require compressed air as a utility service. Pneumatic instruments, automation systems, air starters, cleaning systems, and control valves depend on compressed air. In some industries, compressed gases are also required as feedstock or process media. In gas turbine plants and jet engines, compressors are used to compress air before combustion, which greatly improves performance and efficiency.
Compressed air is often called the “fourth utility” after electricity, water, and gas because of its widespread industrial use. This makes compressors essential equipment in both process plants and utility systems.
Classification of Compressors
Compressors are generally classified into three main groups:
- Positive Displacement Compressors
- Centrifugal Compressors
- Axial Compressors
Each type works on a different principle and is suitable for different operating conditions. Let us understand them one by one.
1. Positive Displacement Compressors
Positive displacement compressors work by trapping a fixed volume of gas and then reducing its volume mechanically to increase pressure. This is similar to the way a pump increases fluid pressure by displacement. These compressors are widely used where high pressure is needed at relatively low to moderate flow rates.
Positive displacement compressors include reciprocating compressors, rotary compressors, diaphragm compressors, screw compressors, vane compressors, scroll compressors, liquid ring compressors, and blower-type machines. Their operation is more directly related to volume reduction than high-speed velocity conversion.
2. Centrifugal Compressors
Centrifugal compressors work on the same basic principle as centrifugal pumps. Gas enters the impeller at the center, then the rotating impeller throws the gas outward at high velocity. This velocity energy is then converted into pressure in the diffuser or volute casing. These compressors are dynamic machines and are suitable for large flow rates.
Centrifugal compressors are widely used in gas processing, petrochemical plants, gas turbine systems, refrigeration plants, and air separation units. They are known for high reliability, smooth operation, and excellent performance in continuous service.
3. Axial Compressors
Axial compressors are used mainly where very high flow rates and high efficiency are required. Gas flows parallel to the axis of rotation, passing through a series of rotating and stationary blades. Each stage slightly increases the gas pressure, and multiple stages are used to achieve high compression ratios.
Axial compressors are commonly used in gas turbines, jet engines, and high-performance industrial power systems. They are highly efficient and capable of handling large volumes of gas. Because of their design, they are more complex than many other compressor types, but they offer outstanding performance in advanced applications.
Working of Centrifugal Compressors
The working principle of a centrifugal compressor is based on converting kinetic energy into pressure energy. Gas enters the compressor through the suction inlet and flows into the center of the impeller. The impeller rotates at high speed and accelerates the gas outward due to centrifugal force. As the gas moves through the diffuser, its velocity decreases and pressure increases.
This makes centrifugal compressors suitable for continuous operation, especially where large gas flow is required. Their operation is smooth, and they are often preferred in industries where reliability and low vibration are important. Compared to reciprocating compressors, centrifugal compressors typically provide a more continuous flow and fewer moving parts.
Working of Reciprocating Compressors
Reciprocating compressors operate by using a piston moving back and forth inside a cylinder. As the piston moves down, suction pressure causes gas to enter the cylinder through the inlet valve. When the piston moves up, the gas is compressed and discharged through the outlet valve.
These compressors are used when high pressure is required with lower flow rates. They are widely used in utility air systems, gas compression stations, refrigeration systems, and industrial plants. Their pressure range is usually higher than that of many rotary machines.
Single Acting Compressor
In a single acting compressor, compression occurs on only one side of the piston. One stroke is used for suction and compression. This design is simpler and commonly used in many utility applications.
Double Acting Compressor
In a double acting compressor, both sides of the piston are used for compression. This makes the compressor more efficient than a single acting compressor because compression occurs during both strokes. Double acting compressors are often used in industrial utility air systems and process gas services.
Diaphragm Compressor
A diaphragm compressor is similar in concept to a diaphragm pump. In this type, a flexible diaphragm compresses the gas without direct contact between the gas and the moving mechanism. This makes it suitable for handling toxic, corrosive, or ultra-clean gases.
Working of Rotary Compressors
Rotary compressors compress gas using rotating elements instead of pistons. They provide continuous flow and relatively smooth operation. These compressors are commonly used in air conditioning systems, refrigeration units, industrial plants, and many utility services.
Rotary Screw Compressor
Rotary screw compressors use two meshing helical rotors to trap and compress gas. As the rotors rotate, the gas volume gradually decreases and pressure increases. These compressors are highly popular because they offer continuous flow, high reliability, and low maintenance.
Rotary Vane Compressor
Rotary vane compressors use vanes mounted in a rotating rotor. The vanes move in and out of the slots and trap gas in chambers that reduce in volume as the rotor turns. This type offers smooth and compact operation and is useful for moderate pressure applications.
Scroll Compressor
Scroll compressors use two spiral-shaped elements, one fixed and one orbiting. Gas is trapped in pockets and compressed as these pockets move toward the center. Scroll compressors are commonly used in air conditioning, refrigeration, and small to medium-capacity systems because they are quiet, efficient, and reliable.
Liquid Ring Compressors
Liquid ring compressors are a special type of rotary compressor. In this design, a liquid ring forms around the rotor and creates compression chambers. These compressors are suitable for handling wet, saturated, or contaminated gases. They are also commonly used as vacuum pumps in many industries.
They are especially useful where gas may contain moisture or small amounts of liquid, since the liquid ring helps seal and compress the gas. Their operation is quiet and reliable, and they are used in chemical plants, vacuum systems, and process industries.
Axial Compressors in Gas Turbines and Jet Engines
Axial compressors are widely used in gas turbines and jet engines because they can handle a large amount of air with high efficiency. In these machines, air passes through multiple stages of blades, and each stage increases pressure slightly. The final compressed air is then used for combustion or thrust generation.
The major advantages of axial compressors are high efficiency, high flow capacity, and suitability for continuous high-speed operation. This is why they are commonly found in aerospace and power generation applications.
Applications of Compressors
Compressors are used across many industries and applications. Some common examples include:
- Compressed air supply for pneumatic tools and instruments
- Air conditioning and refrigeration systems
- Gas turbines and jet engines
- Oil and gas processing plants
- Petrochemical and chemical industries
- Air separation and industrial gas systems
- Vacuum systems and specialized process operations
- Boiler air supply and utility services
Because of these wide applications, compressors are considered a core part of mechanical, process, and piping engineering systems.
Advantages of Compressors
Compressors provide many operational advantages. They make automation possible through pneumatic systems, support process requirements, and improve the efficiency of many industrial operations. Compressors can also store energy in the form of compressed air, which can later be used when needed. In many plants, they help maintain stable and reliable operation of instruments and equipment.
Important Interview Questions and Answers on Compressors
1. What is a compressor?
Answer: A compressor is a machine used to increase the pressure of a gas by reducing its volume. It is widely used to supply compressed air or process gas in industrial systems.
2. What are the main types of compressors?
Answer: The three main types are positive displacement compressors, centrifugal compressors, and axial compressors.
3. How does a centrifugal compressor work?
Answer: Gas enters the impeller, gains velocity due to high-speed rotation, and then pressure increases in the diffuser as velocity decreases.
4. What is the difference between a reciprocating compressor and a centrifugal compressor?
Answer: A reciprocating compressor uses a piston to compress gas in a cylinder, while a centrifugal compressor uses a rotating impeller to increase gas velocity and pressure. Reciprocating compressors are better for high pressure and lower flow, while centrifugal compressors are better for high flow and continuous operation.
5. What is a double acting compressor?
Answer: A double acting compressor compresses gas on both sides of the piston, making it more efficient than a single acting compressor.
6. Why are axial compressors used in gas turbines?
Answer: Axial compressors are used because they can handle large air flow with high efficiency and are suitable for high-speed continuous operation.
7. What is the use of a liquid ring compressor?
Answer: A liquid ring compressor is used for compressing wet or contaminated gases and is also commonly used as a vacuum pump.
8. What is the advantage of rotary compressors?
Answer: Rotary compressors provide continuous flow, smooth operation, compact design, and low vibration compared to many other compressor types.
9. Where are compressors commonly used?
Answer: Compressors are commonly used in air conditioning, refrigeration, gas turbines, pneumatic systems, refineries, chemical plants, and industrial utility networks.
10. Why are compressors important in piping systems?
Answer: Compressors support pneumatic instruments, process gas handling, utility air networks, and many plant operations. They are essential for safe and efficient industrial performance.
Conclusion
Compressors are essential machines in engineering and piping systems. They provide compressed air and gas for a wide variety of industrial operations. The main compressor categories are positive displacement compressors, centrifugal compressors, and axial compressors. Each type has its own working principle, advantages, and applications.
Reciprocating compressors are suitable for high pressure and lower flow. Rotary compressors are compact and reliable for continuous service. Centrifugal compressors are ideal for large flow applications, while axial compressors are widely used in gas turbines and jet engines. Special types such as diaphragm compressors, scroll compressors, and liquid ring compressors serve specific industrial needs.
Understanding compressor classification, working, and applications is important for students, engineers, interview preparation, and plant operation professionals. A clear knowledge of these machines helps in better selection, maintenance, and troubleshooting in real-world projects.
Compressors remain one of the most important pieces of rotating equipment in the industrial world, and their role in process plants, utility systems, and energy generation will continue to grow.