--- title: "Classification of air compressors" subject: "Applied Thermodynamics" chapter: 2 key: "semester_3/applied_thermodynamics:air-compressor-classification" source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/air-compressor-classification site: BCT Engineering --- # Classification of air compressors Applied Thermodynamics · Chapter 2: Air Compressors - Asked in: year not given, 5 marks - Video: [Classification of Air Compressor (Reciprocating Air Compressors & Rotary air compressors) Explained](https://www.youtube.com/watch?v=LY3qEUSXNVs) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/air-compressor-classification - Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-2 ## Question What is an air compressor? Classify air compressors and state the applications of compressed air. ## Short answer Air compressors raise pressure by reducing volume (positive displacement: reciprocating, rotary) or converting kinetic energy (dynamic: centrifugal, axial). Further classification includes stages, action, delivery pressure, capacity, cooling, and cylinder arrangement. Applications: pneumatic tools, braking systems, engine starting, spray painting, cleaning, process instrumentation. ## Answer An **air compressor** is a machine that raises the pressure of air by reducing its volume (positive displacement type) or by converting kinetic energy into pressure (dynamic type). #### Classification 1. **Principle of operation** - *Positive displacement:* reciprocating (piston) and rotary (Roots blower, sliding vane, screw, lobe). - *Dynamic (non-positive displacement):* centrifugal and axial flow compressors. 2. **Number of stages:** single-stage, two-stage and multi-stage. 3. **Action:** single-acting (air compressed on one side of the piston) and double-acting (both sides). 4. **Delivery pressure:** low (up to 10 bar), medium (10 to 80 bar) and high (80 to 1000 bar). 5. **Capacity:** low (below 0.15 m³/s), medium (0.15 to 5 m³/s) and high (above 5 m³/s). 6. **Cooling:** air-cooled (fins) and water-cooled. 7. **Cylinder arrangement:** vertical, horizontal, V, W and radial. #### Applications of compressed air - Pneumatic tools: drills, hammers, riveters, spray painting. - Operating pneumatic brakes in trucks and trains. - Starting diesel engines and supplying air to gas turbines. - Cleaning, sand-blasting, air-lift pumps and mine ventilation. - Process industries, food packaging and instrumentation air. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An air compressor is a machine that A. lowers the pressure of air to vacuum B. converts heat into shaft work using air C. only cools the air D. raises the pressure of air, by reducing its volume or converting kinetic energy into pressure Answer: D. Positive displacement types reduce volume; dynamic types convert kinetic energy into pressure. (Key point: Definition) 2. Which pair is correctly classified by principle? A. Reciprocating and screw are positive displacement; centrifugal and axial are dynamic B. Reciprocating and screw are dynamic C. Centrifugal and axial are positive displacement D. Roots blower is dynamic Answer: A. Positive displacement: reciprocating and rotary (roots, vane, screw); dynamic: centrifugal and axial. (Key point: Principle classification) 3. Medium-pressure compressors deliver at A. up to 1 bar gauge B. 1 to 10 bar C. 10 to 100 bar D. above 100 bar Answer: B. Low is up to 1 bar, medium 1-10 bar, high 10-100 bar, very high above 100 bar. (Key point: Delivery pressure) 4. Which is an application of compressed air? A. Making ice directly B. Generating steam in boilers C. Operating pneumatic brakes and starting diesel engines D. Increasing feed water temperature Answer: C. Compressed air drives pneumatic tools, brakes, engine starting and more. (Key point: Applications) ## Related in Applied Thermodynamics - [Components of reciprocating air compressor](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Areciprocating-compressor-components) — Applied Thermodynamics, chapter 2 - [Work of single-stage compressor without clearance](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Asingle-stage-compressor-work-derivation) — Applied Thermodynamics, chapter 2 - [Single-stage compressor power numerical](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Asingle-stage-compressor-numerical) — Applied Thermodynamics, chapter 2 - [Two-stage compressor with perfect intercooling](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Atwo-stage-compressor-intercooling-numerical) — Applied Thermodynamics, chapter 2 - [Air-conditioning: definition and scope](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aair-conditioning-definition-scope) — Applied Thermodynamics, chapter 4 - [Classification of steam boilers](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aboiler-classification-examples) — Applied Thermodynamics, chapter 1 ## Related in other subjects - [Air compressor: heat transfer rate](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Acompressor-air-heat-transfer-rate) — Fundamental of Thermodynamics & Heat Transfer, chapter 4 - [Compressor work and polytropic efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acompressor-exit-temperature-polytropic-efficiency) — Turbo Machines, chapter 1 - [Classification and sources of air pollutants](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aair-pollutants-classification-sources) — Environment and Pollution Control, chapter 1 - [Compressor and turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acompressor-turbine-performance-characteristics) — Turbo Machines, chapter 3