Prime Movers, Pumps, Compressors, and Fluid Machinery
Domain scope
This domain tests pump and compressor performance, head, power, efficiency, affinity laws, NPSH and cavitation, specific speed, fans, turbines, and prime-mover matching. Questions often combine a performance curve with a system curve.
Core rules
- Pump hydraulic power: P_h = ρgQH. Shaft power: P_s = ρgQH/η.
- System head rises with flow squared; the operating point is where pump and system curves intersect.
- Affinity laws for a geometrically similar pump: Q ∝ N, H ∝ N², P ∝ N³ at constant diameter.
- Cavitation occurs when local pressure falls below vapor pressure; require NPSH_a > NPSH_r with margin.
- Positive-displacement compressors are used for high pressure and low flow; centrifugal compressors for large flow.
- Compressor work depends on whether the process is modeled as isothermal, adiabatic, or polytropic.
Worked example
A pump delivers 0.02 m³/s against 30 m head with efficiency 0.75 and water density 1000 kg/m³. Hydraulic power = ρgQH = 1000×9.81×0.02×30 = 5,886 W. Shaft power = 5,886/0.75 = 7,848 W ≈ 7.85 kW. The motor must be selected above this value with an appropriate service factor.
Traps
- Confusing NPSH available with NPSH required.
- Using affinity laws when impeller diameter changes as well as speed.
- Mixing head in metres with pressure in bar without converting.
- Using isothermal compressor work for a highly insulated machine.
- Reading efficiency as a percentage without dividing by 100.