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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