Question

Carnot cycle is different from Rankine cycle in steam power plant during the following process.

• A. Heat Addition
• B. Expansion Work
• C. Heat Rejection
• D. Pump Work

Hint:

The Carnot cycle represents the theoretical maximum efficiency for any heat engine operating between two temperature reservoirs. Real cycles like the Rankine cycle deviate from the Carnot cycle due to practical limitations in achieving isothermal heat transfer during phase change and other irreversibilities.

Answer 1

The Correct Option is A

Step 1: Understand the Carnot Cycle in a steam power plant context.
The Carnot cycle consists of four reversible processes:
1. Isothermal heat addition at high temperature.
2. Isentropic expansion in a turbine.
3. Isothermal heat rejection at low temperature.
4. Isentropic compression in a pump.
In the context of a steam power plant, the isothermal heat addition would ideally involve heating water to its boiling point, evaporating it completely at a constant high temperature to become saturated steam, and then further expanding it isothermally (which is practically difficult to achieve in a real boiler). Step 2: Understand the Rankine Cycle in a steam power plant.
The basic Rankine cycle also consists of four processes:
1. Isobaric heat addition in a boiler (heating water to saturation, evaporating it at constant pressure, and often superheating the steam). 2. Isentropic expansion in a turbine.
3. Isobaric heat rejection in a condenser (condensing the steam at constant pressure).
4. Isentropic compression in a pump (pumping the condensed water back to the boiler pressure).
Step 3: Compare the heat addition processes in both cycles.
Carnot Cycle: Heat addition occurs isothermally at the highest temperature. This would require a complex heat transfer mechanism to ensure the temperature of the working fluid remains constant while its phase changes and expands.
Rankine Cycle: Heat addition occurs isobarically (at constant pressure) in the boiler. This process involves heating water through different phases (liquid, saturated liquid, saturated mixture, saturated vapor, superheated vapor), and the temperature of the working fluid increases during the sensible heating of water and superheating of steam, remaining constant only during the phase change (boiling).
The heat addition process is significantly different because the Carnot cycle requires isothermal heat addition at the highest temperature, which is not practical in a real boiler where the temperature of the working fluid generally increases during heat addition (except during phase change at constant pressure). Step 4: Compare other processes (Expansion, Heat Rejection, Pump Work).
Expansion Work: Both cycles ideally involve isentropic expansion in a turbine.
Heat Rejection: Both cycles ideally involve heat rejection at a lower temperature (isothermal in Carnot, isobaric condensation in Rankine).
While the nature of the constant temperature/pressure process differs, the fundamental purpose of heat rejection is similar.
Pump Work: Both cycles ideally involve isentropic compression of the working fluid in the liquid phase.
The most significant and fundamental difference between the Carnot and Rankine cycles in a steam power plant lies in the heat addition process.