Pumps, Compressors, and Prime Movers
Core concepts
Know centrifugal vs. positive-displacement pumps (reciprocating, rotary), their characteristics, and applications. Understand NPSH (available vs. required) and cavitation. Be able to interpret pump curves (head vs. flow, efficiency, power) and system curves - the operating point is where they intersect. Know affinity laws (Q ∝ N, H ∝ N², P ∝ N³). For compressors: reciprocating, rotary screw, centrifugal; understand intercooling, volumetric efficiency, and surge in centrifugal machines. Prime movers include steam turbines (impulse vs. reaction) and electric motors.
Traps to watch
- NPSH available must exceed NPSH required to avoid cavitation.
- Affinity laws: doubling speed quadruples head and octuples power.
- Confusing pump head (energy per weight) with pressure (head × ρg).
- Centrifugal compressors surge at low flow; do not confuse with choke at high flow.
- Reciprocating compressors need clearance-volume corrections for volumetric efficiency.
Worked example / rule of thumb
A pump running at 1,450 rpm delivers 100 m³/h at 40 m head with 15 kW. Doubling speed to 2,900 rpm: Q = 200 m³/h, H = 160 m, P = 120 kW. Rule of thumb: never overspeed a pump without checking motor and piping ratings - power scales with the cube of speed.