Unit
- Positive displacement compressors trap a fixed volume of gas and reduce its volume to increase pressure. Reciprocating compressors use pistons in cylinders, while rotary types include screw, vane, and lobe designs.
- Dynamic compressors impart kinetic energy to the gas via rotating impellers or blades, then convert that energy into pressure through diffusers. Centrifugal and axial compressors dominate high flow applications.
- Reciprocating compressors are used in applications requiring high pressure ratios, such as natural gas transmission, air separation, and refrigeration. Stroke length and speed determine capacity.
- Rotary screw compressors use meshing helical rotors to continuously compress gas. They are common in industrial air systems and medium-pressure gas boosting due to smooth flow and low vibration.
- Centrifugal compressors are preferred for large-volume, constant-head applications like pipeline transport and process gas compression. Multiple stages can achieve high discharge pressures.
- Axial compressors are primarily used in gas turbines and large-scale petrochemical plants. They handle massive flow rates with relatively moderate pressure ratios per stage.
- Application-driven selection considers factors such as gas composition, required discharge pressure, flow rate consistency, and environmental conditions. For example, oil-free reciprocating compressors are chosen for oxygen service to avoid combustion risk.
- Materials of construction must withstand corrosion, erosion, and thermal stresses. Stainless steel, titanium, or nickel alloys are used for corrosive gases; cast iron and carbon steel suffice for dry air. Valve materials are selected for fatigue resistance at high cycle rates.
- Sealing systems prevent gas leakage between compression chambers and the environment. Labyrinth seals, carbon ring seals, and mechanical face seals are common; dry gas seals are preferred in centrifugal compressors handling hazardous or expensive process gases.
- Lubrication and cooling are critical for reliability. Flooded screw compressors inject oil for sealing and cooling; reciprocating compressors use forced feed or splash lubrication. Interstage coolers and jacket water systems maintain temperature limits and improve efficiency.
- Control and protection systems manage capacity, surge, and safety. Variable speed drives, inlet guide vanes, and suction throttling regulate flow. Anti-surge controllers, pressure relief valves, and vibration monitoring prevent damage during upset conditions or start-up.
- Performance testing and certification validate compressor output under standardized conditions. ASME PTC 10 and ISO 5389 define test codes. Shop tests measure flow, power, and head; field tests verify installation integrity and compliance with contractual guarantees.
Process Flow
- Stage 1 - Gas Intake: Ambient or process gas enters the suction side through a filter and inlet valve, isolating the compression chamber from upstream conditions.
- Stage 2 - Compression: The moving element (piston, rotor, impeller) reduces volume or imparts velocity, raising gas pressure and temperature according to the thermodynamic cycle.
- Stage 3 - Discharge: High-pressure gas exits through a check valve into the delivery line. Interstage cooling may occur for multi-stage configurations.
- Stage 4 - Aftertreatment: Downstream separators, dryers, and receivers condition the compressed gas for end-use. Lubricant and moisture removal ensure quality.
Quick Revision
- Compressors are classified as positive displacement or dynamic, each with distinct subcategories.
- Reciprocating units excel in high pressure; rotary screws offer smooth flow; centrifugal and axial handle high volume.
- Selection depends on gas properties, pressure, flow, and application safety.
- The basic compression process involves intake, compression, discharge, and aftertreatment.