Power cycles — Rankine, Brayton, and combined

Power cycles are one of the highest-value — and highest-trap — areas on the Thermal & Fluid Systems exam. They're multi-step, so one wrong state point flows through to the wrong final answer. Master the state-by-state bookkeeping and they become reliable points.

Rankine (steam)

The steam power cycle: pump → boiler → turbine → condenser. Thermal efficiency is net work over heat in. You'll lean on steam tables to get enthalpy and entropy at each state, track quality in the two-phase region, and account for reheat and regeneration (feedwater heating) when the problem adds them. Pump work is small but not always negligible — don't drop it when the problem wants net work.

Brayton (gas turbine)

The gas-turbine cycle: compressor → combustor → turbine. Performance keys on the pressure ratio and the isentropic efficiencies of the compressor and turbine. The back-work ratio (compressor work as a large fraction of turbine work) is a favorite concept — gas turbines spend a lot of their output just running the compressor.

Combined cycle and cogeneration

A gas-turbine (Brayton) topping cycle whose exhaust heat raises steam for a Rankine bottoming cycle — high overall efficiency, and a common "why combined cycle?" question. Cogeneration/CHP uses the rejected heat as a useful product.

Where people lose points

• State-point errors — wrong enthalpy/entropy off the steam tables, or the wrong gas relation.
• Ideal vs. actual — forgetting to apply isentropic efficiency to the compressor or turbine.
• Dropping pump work, or confusing net work with turbine work.
• Losing track of states across a multi-component cycle under time pressure.

How to study it

Build a fixed routine: number every state, fill a property table for each, then balance component by component. Drill Rankine (with reheat/regeneration) and Brayton (with real efficiencies) until that bookkeeping is automatic — the discipline is what protects you when one cycle has six or seven states.

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