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.
Understanding Fan, Blower & Compressor Comparison
The key physical difference is what happens to the gas after energy enters it. A fan blade mainly changes the direction and speed of air. Ducts, guards, or curved passages then convert part of that speed into a small pressure increase.
Many blowers use a similar rotating impeller, but their housing is designed to build more pressure before the gas leaves. Positive displacement compressors trap a fixed amount of gas and reduce its space.
This squeezing process raises pressure directly. Piston, screw, scroll, and rotary vane compressors all use this basic idea in different ways.
Gas flow is controlled by the whole system, not only by the machine. A long narrow duct, a dirty filter, a partly closed damper, or a sharp bend resists flow. This resistance is called pressure loss.
As flow rate increases, the losses usually rise quickly. A machine has a performance curve showing the pressure it can produce at different flow rates. The actual operating point occurs where the machine curve matches the system resistance.
This explains why fitting a more powerful fan does not always produce the expected airflow. The duct system may be the real limit.
Temperature matters much more during compression than during ordinary ventilation. When gas is squeezed, work is done on it. Much of that work becomes internal energy, so the gas becomes hotter.
A bicycle pump warms up for this reason. Hot compressed air can damage seals, reduce lubricant life, and hold more water vapour until it cools. Industrial systems often use aftercoolers to remove heat after compression.
Multi stage compressors may cool the gas between stages. Cooling reduces the work needed for the next stage and improves efficiency.
Students should distinguish volume flow rate from mass flow rate. A fan may move a large volume of air each second, yet the mass moved depends on air density. At high altitude, air is less dense, so the same fan can move roughly the same volume while moving less mass.
This affects cooling, combustion, and aircraft systems. For a gas, density equals pressure divided by the gas constant times absolute temperature. Absolute temperature must be used because gas behaviour depends on temperature measured from absolute zero, not from the freezing point of water.
Power calculations need careful unit checks. Pressure rise in pascals multiplied by volume flow rate in cubic metres per second gives fluid power in watts. Shaft power is larger because bearings, turbulence, leakage, electrical motors, and heat waste energy.
Efficiency describes how much input power reaches the gas usefully. Real machines can become noisy, hot, or unstable when operated far from their intended range.
Compressors can suffer surge, where flow repeatedly reverses or pulses. Good engineering selection considers required flow, pressure, temperature, noise, safety, maintenance, and energy use over the full operating condition.