- Rankine Cycle Definition: The Rankine cycle is defined as a thermodynamic process that converts heat into work, primarily used in power plants.
- Closed Feed Water Heaters: These heaters use an indirect method to transfer heat from steam to feed water without direct mixing.
- Heat Transfer Efficiency: In a closed feed water heater Rankine cycle, the feed water temperature ideally matches the steam’s saturation temperature but is usually slightly lower in practice.
- Cogeneration Concept: Cogeneration (Combined Heat and Power, CHP) uses steam’s heat for industrial processes, improving overall energy efficiency.
- Industrial Benefits: Cogeneration plants provide both power and process steam, meeting the needs of industries like paper, chemical, and textile with high efficiency.
Rankine Cycle with Closed Feed Water Heaters
A regenerative Rankine cycle can use closed feedwater heaters in power plants. Extraction steam condenses on one side of a surface heat exchanger, while feedwater flows separately on the other side. The streams do not mix and can remain at different pressures. The closed feed water heater arrangement is shown on a T-s diagram in Figure 1.
In an idealized closed feed water heater, the feedwater outlet can approach the extraction-steam saturation temperature. An actual exchanger needs a finite terminal temperature difference to transfer heat.
Real feed water therefore leaves below the extraction steam’s saturation temperature. The actual temperature gradient, pressure losses and drain cooling affect heater performance.
Condensed extraction steam can cascade through a trap to a lower-pressure heater, flow to the condenser or be pumped forward. The drain arrangement depends on the cycle design.
Differentiate Between Open and Closed Feed Water Heater
The open and closed feed water heaters can be differentiated as follows:
| Open feed water heater | Closed feed water heater |
| Direct-contact mixing vessel | Surface exchanger with separate streams |
| Can approach outlet equilibrium at the heater pressure | Needs a finite terminal temperature difference |
| Extraction steam mixes with feedwater, so both enter at a common heater pressure | Extraction steam and feedwater do not mix and can operate at different pressures |
| A pump is normally required after the heater to raise the mixed feedwater to the next pressure level. | Feedwater pressure is maintained through the tubes; condensed steam drains by pressure difference, traps or a drain pump according to the design. |
| Also provides deaeration when designed for that duty | Does not itself provide direct-contact deaeration |
| Cost depends on vessel, pump and system duties | Usually adds tubes, drains and controls; total cost is design-specific |
Many large steam plants combine open and closed heaters. The number, type and extraction pressures are optimized for efficiency, turbine output, reliability and cost rather than chosen by one universal rule.
Cogeneration
Engineering thermodynamics distinguishes energy quantity from its ability to produce work. A condensing steam turbine must reject heat to complete its cycle. Cogeneration changes the system boundary by supplying useful heat at a temperature and pressure that match a nearby demand.
Cogeneration, or combined heat and power, produces electricity and useful thermal energy from the same fuel input. The useful heat can be steam, hot water or another thermal service.
Combined Heat and Power (CHP) serves a site that has simultaneous electric and thermal loads. A well-matched cogeneration plant can achieve total system efficiency above 80%, but typical systems span about 60% to 80%. Approaching 90% is possible for some designs, not guaranteed. Fuel, equipment, load matching and the calculation boundary determine the claimed energy savings.
Pulp and paper, chemical, textile, food and other industries can use CHP process steam. Required pressure and temperature are set by each process. A range such as 4 to 5 kg/cm2 and 150 to 180oC is only an example and is not suitable for every plant.
Facilities with steady demand for both electric power and process steam may be good CHP candidates. A feasibility study must compare load profiles, fuel and grid prices, emissions, water, reliability, operating staff and interconnection requirements.
A topping-cycle steam CHP plant generates high-pressure steam in a boiler, expands it through a turbine to generate power, then supplies the exhaust or an extraction to a thermal load. Conditions such as 105 bar and 535oC are examples for a particular high-pressure system, not standard values for all CHP plants. Boiler furnace temperature, stated here as 800oC to 900oC, also varies with fuel and design.
A steam-turbine cogeneration plant coordinates power production with the process steam requirement.
The figure shows an idealized back-pressure arrangement. If the process needs 100 kW of useful heat at 5.0 kg/cm2, steam can expand through the turbine to that pressure before entering the process heater. The stated output of about 20 kW at 5.0 kg/cm2 and an exhaust pressure of 5.0 kg/cm2 is only valid for the assumed mass flow and inlet and outlet enthalpies; pressure alone does not determine turbine power.
Where water quality and process design permit, condensate returns from the process heater to the boiler cycle. Ideal analysis may neglect pump work because it is small relative to turbine work, but real performance and equipment sizing must include it.
The total-system efficiency or utilization factor compares net power plus useful process heat with fuel energy input. Energy delivered to a steam turbine is not all converted into those useful outputs because boilers, turbines, pipes, generators and auxiliaries have losses:
Where:
Qout is the heat rejected rather than delivered as a useful thermal output.
Removing a separate condenser does not make utilization 100%. Stack loss, radiation, blowdown, mechanical loss, generator loss and auxiliary power remain, and the process may not use all available heat.
