Pumps, Compressors and Fluid Machinery
This section covers Pumps, Compressors and Fluid Machinery. The exam tests pump and compressor classification, head and power calculations, priming, cavitation, intercooling and safety devices.
Core concepts
- A pump moves liquid; a compressor moves and pressurises gas. Centrifugal machines use a rotating impeller and suit large flows; positive-displacement machines trap a fixed volume per cycle and suit high pressure or metering.
- Pump head is energy per unit weight, expressed in metres. Pump power P = ρgQH ÷ η, where Q is flow in m³/s, H is head and η is efficiency.
- Priming fills a centrifugal pump casing with liquid so it can create suction. Running dry damages seals. Cavitation occurs when local pressure falls below vapour pressure; it causes noise, vibration and erosion, and is avoided by adequate NPSH.
- Compressors need a relief valve; multistage compression uses intercoolers to reduce work and temperature.
Worked example
A pump delivers 0.01 m³/s against 20 m head with efficiency 0.8. Water density is 1000 kg/m³ and g is 9.81 m/s². Power = (1000 × 9.81 × 0.01 × 20) ÷ 0.8 = 2,452.5 W, about 2.45 kW. The reasoning divides hydraulic power by efficiency; the motor must supply more than the water receives.
Traps that cost marks
Do not use litres per second directly in the power formula without converting to m³/s. Do not say a centrifugal pump is self-priming in all installations. Cavitation is not air binding. A positive-displacement compressor must not run with a closed discharge and no relief.