Pumps, Compressors, and Prime Movers
What this domain tests
This domain covers the machines that move fluids and generate power: centrifugal and positive-displacement pumps, centrifugal and reciprocating compressors, steam turbines, and internal combustion engines. Questions test operating principles, performance curves, cavitation, NPSH, and efficiency.
Core rules to have at your fingertips
- Centrifugal pump: flow varies with head; performance curve shows head falling as flow rises. Best efficiency point (BEP) is where the pump should operate. Throttling moves the operating point along the curve.
- NPSH: Net Positive Suction Head available must exceed NPSH required to avoid cavitation. NPSHA = (p_suction/ρg) + z − (p_vapour/ρg) − h_friction.
- Positive-displacement pump: delivers essentially constant flow regardless of head; requires a relief valve on the discharge.
- Compressors: centrifugal compressors have surge and choke limits; reciprocating compressors need unloaders and intercoolers.
- Steam turbine: impulse (pressure drop across nozzles) vs reaction (pressure drop across moving blades). Condensing turbines exhaust to vacuum; back-pressure turbines exhaust to process steam.
- Efficiency: pump efficiency η = (ρgQH)/P_shaft; turbine isentropic efficiency η = (h1 − h2)/(h1 − h2s).
Worked example
A centrifugal pump delivers 30 L/s against 25 m head. Water density 1 000 kg/m³, g = 9.81 m/s². Shaft power input = 9 kW.
Hydraulic power = ρgQH = 1 000 × 9.81 × 0.030 × 25 = 7 358 W = 7.36 kW. Pump efficiency = 7.36 / 9 = 0.818, i.e. 81.8 %.
If NPSHA were 4.5 m and NPSHR at this flow were 5.0 m, the pump would cavitate - the answer would be to raise suction head or reduce flow.
Traps that cost marks
- Confusing NPSHA (system) with NPSHR (pump).
- Forgetting to convert L/s to m³/s in the power formula.
- Assuming a positive-displacement pump can be throttled on the discharge - it cannot.
- Mixing up surge (low flow) and choke (high flow) in centrifugal compressors.
- Using the wrong enthalpy for turbine efficiency - must use the isentropic end point, not the actual.
Exam strategy
For pump/compressor questions, state the operating point, check NPSH, and confirm the efficiency formula. For turbine questions, draw the h-s diagram and label the isentropic drop.