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What Is a Compressor? | Process Engineering Glossary
What Is a Compressor?
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume, delivering the gas at a higher pressure and typically higher temperature than the inlet condition. Compressors serve essential functions in process facilities, including raising gas pressure for downstream processing, providing pneumatic energy for instrument air systems, driving gas through pipelines, and supplying feed gas to reactors operating above atmospheric pressure.
Compressor types include centrifugal machines that use rotating impellers to impart velocity and then convert it to pressure, reciprocating machines that use pistons to physically compress trapped gas volumes, and screw or rotary machines that use meshing helical elements, with each type well suited to different combinations of flow rate, pressure ratio, and gas properties.
Applications of Compressor
Process Gas Compression
Process compressors raise gas pressure between process units, providing the driving force for gas to flow through reactors, separation equipment, and heat exchangers at the pressures required by the process design.
Refrigeration and Cooling Systems
Refrigeration compressors circulate refrigerant through cooling systems, compressing low-pressure refrigerant vapor to the high pressure needed for condensation and subsequent cooling duty delivery.
Instrument Air and Utility Gas Supply
Air compressors supply the instrument air that powers pneumatic control valves and the utility gas systems that support various facility operations.
Benefits of Knowing Compressor
Enables understanding of a critical process equipment category. Compressors are among the most critical and expensive equipment items in many facilities, making their proper design, reliable operation, and proactive maintenance significant to overall facility performance.
Connects thermodynamic principles to practical equipment design. Compressor performance is governed by thermodynamic principles including compression ratio, polytropic efficiency, and discharge temperature, connecting theory to practical engineering decisions.
Supports effective troubleshooting and performance monitoring. Understanding compressor operating characteristics, including surge, stonewall, and efficiency curves, supports meaningful performance monitoring and troubleshooting.
Limitations to Consider
Compressor surge can cause rapid, severe damage. Centrifugal compressors operating below their minimum stable flow can enter surge, a destructive oscillating flow condition that must be prevented through anti-surge control systems.
Discharge temperature limits constrain compression ratio per stage. The temperature rise during compression limits the pressure ratio achievable in a single compression stage, potentially requiring multi-stage compression with intercooling.
Liquid carryover can cause catastrophic damage. Compressors are designed for gas service, and ingestion of liquid slugs can cause immediate, severe mechanical damage, making upstream separation and knockout drum protection essential.
Compressor FAQ
How does compressor performance relate to pump curves?
Like a pump curve for liquid service, a compressor performance map shows the relationship between flow, pressure ratio, speed, and efficiency, supporting correct operating point selection.
How is a compressor shown on process documentation?
Compressors appear on the PFD and P&ID with driver type, design conditions, and all associated instrumentation including anti-surge controls clearly defined.
How does compressor selection relate to process safety?
Compressor hazard scenarios are evaluated during process hazard analysis, addressing seal leakage, surge protection, and the consequences of compressor trip on the overall process.
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