Marine Engineering Knowledge and Principles: Applied Thermodynamics, Heat Transfer, Fluid Flow and Materials
This domain tests the applied physics behind every machinery decision on board: thermodynamic cycles, heat transfer, fluid flow and strength of materials. Questions are usually short calculations or greater-than/less-than judgements rather than pure theory, so the marks sit in correct units and correct assumptions.
Core rules to hold
- Diesel cycle efficiency: eta = 1 - (1/r^(gamma-1)) x [(rho^gamma - 1)/(gamma(rho - 1))], with r the compression ratio and rho the cut-off ratio. Raising r raises efficiency; raising rho lowers it.
- Heat exchangers use LMTD, never the arithmetic mean temperature difference. Q = U x A x LMTD, and U must include fouling resistances in series with the metal wall.
- Pump affinity laws for a fixed impeller diameter: Q proportional to N, H proportional to N squared, P proportional to N cubed.
- NPSH available must exceed NPSH required with margin; NPSHa falls as suction temperature, suction lift or suction friction rises.
- Restrained thermal stress: sigma = E x alpha x deltaT, independent of the length of the member.
Worked example
A lube-oil cooler has oil in at 55 C and out at 47 C; sea water in at 32 C and out at 40 C. Counter-flow terminal differences are 55 - 40 = 15 K and 47 - 32 = 15 K, so LMTD equals 15 K with no correction factor. For Q = 300 kW and U = 500 W/m2K: A = 300000 / (500 x 15) = 40 m2. Reason the units first: 300 kW must become 300000 W before dividing.
Traps that cost marks
Averaging terminal temperatures instead of using LMTD; quoting U without fouling allowance; and confusing indicated with brake power when an efficiency is requested.
Use the same discipline with flow problems: write the energy equation, list every loss, then substitute numbers.