This cheat sheet compares fans, blowers, and compressors as common machines used to move gases in engineering systems. Students need it because the three devices can look similar but are chosen for different pressure, flow, and density-change requirements. It helps connect classroom fluid mechanics to HVAC systems, pneumatic tools, turbines, engines, and industrial process equipment.
The core idea is pressure ratio, which is outlet absolute pressure divided by inlet absolute pressure. Fans produce high flow with very small pressure rise, blowers produce moderate pressure rise, and compressors produce high pressure rise with significant gas density change. The most important calculations use flow rate, pressure rise, power, efficiency, and gas laws such as P1 V1 / T1 = P2 V2 / T2.
Key Facts
- Pressure ratio is PR = P2 / P1, where P1 and P2 must be absolute pressures.
- A fan is usually used when PR is about 1.0 to 1.1 and the main goal is moving a large volume of gas.
- A blower is usually used when PR is about 1.1 to 2.0 and the gas has a noticeable but moderate pressure increase.
- A compressor is usually used when PR is greater than 2.0 and the gas density changes significantly.
- Volumetric flow rate is Q = A v, where A is flow area and v is average gas velocity.
- Fluid power for an incompressible estimate is Pfluid = ΔP Q, where ΔP is pressure rise and Q is volumetric flow rate.
- Shaft power is Pshaft = Pfluid / η, where η is efficiency written as a decimal.
- For an ideal gas, density can be estimated by ρ = P / (R T), so increasing pressure or decreasing temperature increases density.
Vocabulary
- Fan
- A gas-moving machine designed mainly for high flow rate and low pressure rise.
- Blower
- A gas-moving machine that produces a moderate pressure rise, usually higher than a fan but lower than a compressor.
- Compressor
- A machine that raises gas pressure enough to cause a major increase in gas density.
- Pressure Ratio
- The ratio of outlet absolute pressure to inlet absolute pressure, written as PR = P2 / P1.
- Volumetric Flow Rate
- The volume of gas passing through a cross section each second, written as Q = A v.
- Efficiency
- The useful fluid power output divided by the shaft power input, written as η = Pfluid / Pshaft.
Common Mistakes to Avoid
- Using gauge pressure in PR = P2 / P1 is wrong because pressure ratio must use absolute pressure. Add atmospheric pressure to gauge pressure before dividing.
- Calling every gas-moving device a compressor is wrong because fans and blowers may not create enough pressure rise for meaningful compression.
- Ignoring efficiency in power calculations is wrong because real machines need more shaft power than the ideal fluid power. Use Pshaft = ΔP Q / η.
- Treating gas density as constant in high pressure compression is wrong because compressors can greatly change density and temperature.
- Choosing a device only by flow rate is wrong because pressure rise, pressure ratio, efficiency, noise, temperature rise, and application limits also matter.
Practice Questions
- 1 A device takes air from 101 kPa absolute to 111 kPa absolute. Find PR and classify it as a fan, blower, or compressor using the usual ranges.
- 2 A blower delivers Q = 0.80 m3/s with a pressure rise of ΔP = 18,000 Pa and efficiency η = 0.60. Find the required shaft power.
- 3 Air flows through a duct with area A = 0.25 m2 at average velocity v = 12 m/s. Find the volumetric flow rate Q.
- 4 A system needs high gas flow but only a small pressure rise for ventilation. Explain why a fan is usually a better choice than a compressor.