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Reheat Rankine Cycle and Methods of Increasing the Efficiency of the Rankine Cycle

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14 Questions

Decreasing the condenser pressure in the Rankine cycle results in a decrease in the average temperature during heat addition.

False

Superheating steam to a higher temperature in the Rankine cycle results in a decrease in net work output.

False

Increasing the boiler pressure in the Rankine cycle results in a decrease in the moisture content of the steam.

False

The process of reducing condenser pressure is depicted by shifting the process from 2-3 to 2'-3' on a T-s diagram.

False

Superheating steam to a higher temperature in the Rankine cycle results in a decrease in the steam temperature and heat rejection temperature.

False

The net work output increase resulting from superheating steam to a higher temperature is represented by the gray area on the T-s diagram.

False

Increasing the boiler pressure in the Rankine cycle results in a net increase in work output.

True

In an ideal reheat Rankine cycle, the steam is expanded isentropically to the condenser pressure in the high-pressure turbine.

False

The pump work in the reheat Rankine cycle is given by WP = (h6 - h5), kJ/kg.

False

The thermal efficiency of the reheat Rankine cycle is given by e = WT / QA.

False

The heat added in the reheat Rankine cycle is given by QA = (h2 - h6) + (h3 - h1), kJ/kg.

False

The turbine work in the reheat Rankine cycle is given by WT = (h1 - h4) + (h3 - h2), kJ/kg.

False

The reheat process in the reheat Rankine cycle takes place at constant temperature.

False

The heat rejected in the reheat Rankine cycle is given by QR = (h5 - h4), kJ/kg.

True

Study Notes

Methods to Enhance Efficiency of Rankine Cycle

  • Reducing condenser pressure:
  • Decreases steam temperature and heat rejection temperature
  • Increases net work output (blue area on T-s diagram)
  • Steam exits as saturated mixture at lower pressure's saturation temperature

Superheating Steam to Higher Temperature

  • Increases average temperature during heat addition
  • Increases net work output (blue area on T-s diagram)
  • Illustrated by superheating steam from state 3 to state 3'

Increasing Boiler Pressure

  • Increases steam boiling temperature in Rankine cycle
  • Shifts process from 2-3 to 2'-3' on T-s diagram
  • Increases net work output (blue area) but decreases net work output (gray area)
  • Increases moisture content of steam from state 4 to state 4', a negative effect

Reheat Rankine Cycle

  • Increasing boiler pressure can increase thermal efficiency, but it also increases moisture content at the turbine exit, making it unacceptable.
  • To correct this, the simple Rankine cycle is modified with a reheat process.

Ideal Reheat Rankine Cycle

  • Steam is expanded isentropically to an intermediate pressure in a high-pressure turbine (stage I).
  • Steam is then sent back to the boiler, where it is reheated at constant pressure to the inlet temperature of the high-pressure turbine.
  • The steam is then sent to a low-pressure turbine and expands to the condenser pressure (stage II).

Formulas

  • Heat Added (QA): (h1 – h6) + (h3 – h2), kJ/kg
  • Turbine Work (WT): (h1 – h2) + (h3 – h4), kJ/kg
  • Pump Work (WP): (h6 – h5), kJ/kg or v5 (P6 – P5), kJ/kg
  • Enthalpy (h6): v5 (P6 – P5) + h5
  • Heat Rejected (QR): (h4 – h5), kJ/kg
  • Thermal Efficiency (e): WT - WP / QA

This quiz covers methods to improve the efficiency of the Rankine cycle, including reducing condenser pressure and superheating steam, and their effects on temperature and work output.

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