The Marine Rankine Cycle: A Quick Review
Why a marine steam plant is a closed loop of boiler, turbine, condenser and feed pump — and how enthalpies around that loop set its thermal efficiency.
A steam ship moves by boiling water into high-pressure steam, letting that steam expand through a turbine, condensing it back to water, and pumping the water back to the boiler. That same four-step loop — unchanged in principle for well over a century — is the Rankine cycle, and it is the starting point for every marine steam plant.
A Closed Loop, Not a Single Burst
Most engines burn fuel in a single open sequence — air in, fuel burned, exhaust out — but a marine steam plant works differently. It does not throw its working fluid away after one use. Instead it recycles the same water endlessly around a closed loop of four components, adding heat at one point and taking work out at another. That closed loop is the Rankine cycle, and it is the foundation of every marine steam plant, from an oil-fired auxiliary boiler driving a turbo-generator to a large steam-turbine propulsion plant.
The four components, in order, are the boiler (which turns feedwater into high-pressure steam by burning fuel), the turbine (which lets that steam expand and extracts shaft work from it), the condenser (which cools the spent steam back into liquid water), and the feed pump (which pumps that water back up to boiler pressure to start the loop again). Nothing is consumed except fuel — the water simply goes round and round, changing phase and pressure as it goes.
Because the loop is closed, we can analyse it by looking only at four fixed state points — the places where the working fluid enters each component — and the enthalpy (energy per unit mass) of the fluid at each of those points is enough to tell us how much heat goes in, how much work comes out, and therefore how efficient the plant is.
The Four State Points
Label the fluid state at the exit of each component:
- State 1 (h1) — saturated liquid condensate leaving the condenser at low pressure. For typical condenser conditions this is around 190 to 210 kJ/kg.
- State 2 (h2) — the same water, now raised to full boiler pressure by the feed pump. Pumping liquid water takes very little energy, so h2 is only slightly above h1.
- State 3 (h3) — high-pressure steam leaving the boiler (often superheated), carrying the bulk of the energy added by the fuel. This is the highest-enthalpy point in the cycle, typically above 3000 kJ/kg.
- State 4 (h4) — expanded steam leaving the turbine at low pressure, having given up much of its energy as shaft work.
Heat is added between states 2 and 3 (in the boiler), and work is extracted between states 3 and 4 (in the turbine). The pump adds a small amount of work between states 1 and 2, and the condenser rejects heat between states 4 and 1 to close the loop.
This lesson treats the Rankine cycle as a review: it states the four state points and shows how they combine into a thermal efficiency, but it does not re-derive the thermodynamics of phase change, saturation, or superheat. The full derivation of the steam cycles — including the temperature–entropy (T–s) diagram and why efficiency rises with steam pressure and temperature — is covered in the Material & Energy Balances course. The marine value added here is simply reading enthalpies off a steam plant and turning them into the numbers that characterise the machinery.
- Turbine work: wt = h3 − h4 = 3350 − 2100 = 1250 kJ/kg
- Pump work: wp = h2 − h1 = 200 − 192 = 8 kJ/kg
- Heat added in the boiler: qin = h3 − h2 = 3350 − 200 = 3150 kJ/kg
- Thermal efficiency: η = (wt − wp)/qin = (1250 − 8)/3150 = 1242/3150 ≈ 0.394 (39.4%)
- wt = h3 − h4 = 3200 − 2200 = 1000 kJ/kg
- wp = h2 − h1 = 210 − 205 = 5 kJ/kg
- qin = h3 − h2 = 3200 − 210 = 2990 kJ/kg
- η = (wt − wp)/qin = (1000 − 5)/2990 = 995/2990 ≈ 0.333
Why the Pump Barely Matters
One feature of the Rankine cycle is striking on a ship: the feed pump does almost no work compared with the turbine. The turbine expands gas-like steam from high pressure down to a near-vacuum, extracting over a thousand kilojoules per kilogram. The feed pump, by contrast, only squeezes an incompressible liquid back up to boiler pressure — and because liquids are nearly incompressible, that takes only a handful of kilojoules per kilogram.
That asymmetry is why the cycle's thermal efficiency is often approximated by dropping the pump work entirely: efficiency is roughly turbine work divided by heat added. The exact formula keeps the pump term for correctness, and we will too — but it is worth remembering that for a steam plant the pump work is a rounding error next to the turbine work.
Check your understanding
- The marine Rankine cycle is a closed loop: boiler (heat in) -> turbine (work out) -> condenser (heat out) -> feed pump (work in)
- Thermal efficiency η = (wt − wp)/qin = ((h3 − h4) − (h2 − h1))/(h3 − h2), with all enthalpies in kJ/kg
- Pump work is tiny next to turbine work, so the efficiency is often close to turbine work divided by heat added