Thermodynamics RANKINE CYCLE in 10 Minutes!

Summary

This video provides a comprehensive explanation of the Rankine cycle, a vapor power cycle where water transitions between liquid and gas states. It contrasts the Rankine cycle with gas power cycles, highlighting key differences such as pump work and the use of property tables for non-ideal substances. The cycle involves a boiler, pump, turbine, and condenser, with heat addition and rejection processes occurring at constant pressure. Efficiency can be improved by superheating the vapor and adjusting system pressures. The video also covers real-world deviations from ideal conditions and offers insights into calculating thermal efficiency and net power output in steam power plants.

Takeaways

• 😀 Vapor power cycles differ from gas power cycles because the substance used transitions between gas and liquid states.
• 😀 The ideal vapor power cycle is the Rankine Cycle, which is the primary representative cycle for vapor cycles.
• 😀 Heat exchangers in the vapor power cycle are called boilers (for heat addition) and condensers (for heat rejection).
• 😀 A pump is used instead of a compressor in the Rankine Cycle, as the work involved is much smaller due to the liquid's small specific volume.
• 😀 In the Rankine cycle, energy calculations are based on changes in enthalpy, and boundary work and flow work are considered in steady-state devices.
• 😀 The turbine's work output can be calculated by the change in enthalpy, assuming an ideal isentropic process with no kinetic energy change.
• 😀 The work required by the pump is calculated using the liquid's specific volume and the pressure difference, as enthalpy changes are minimal.
• 😀 The main differences between vapor power cycles and gas cycles are the methods for calculating pump work and turbine expansion (no isentropic relationships for liquids).
• 😀 To improve efficiency in Rankine cycles, strategies like superheating, increasing boiler pressure, and decreasing condenser pressure are used.
• 😀 In actual Rankine cycles, inefficiencies like non-isentropic turbines and pumps, friction, and heat losses lead to performance reductions, requiring adjustments like increased heat addition.

Q & A

What distinguishes vapor power cycles from gas power cycles?

Vapor power cycles differ from gas power cycles in that they use a substance that transitions between gas and liquid phases. The substance undergoes phase changes as it moves between a saturated liquid and a saturated vapor, which is not the case for gas power cycles.

What is the Rankine Cycle, and why is it significant in vapor power cycles?

The Rankine Cycle is the ideal vapor power cycle, and it is the primary representative cycle used in most vapor power cycles. It involves a fluid that goes through phase changes between liquid and gas when heat is added or removed.

What are the components of a vapor power cycle?

The main components of a vapor power cycle include the boiler, condenser, pump, and turbine. The boiler adds heat to turn the liquid into gas, the condenser removes heat to condense the gas into liquid, the pump increases the pressure of the liquid, and the turbine extracts work from the energy in the vapor.

How does the work done by the pump in a Rankine Cycle differ from that in a gas cycle?

In the Rankine Cycle, the pump does not compress a gas but increases the pressure of a liquid. Since the specific volume of the liquid is much smaller than that of the gas, the work required by the pump is significantly smaller compared to that required in a gas cycle.

What is meant by isentropic expansion in the context of the Rankine Cycle?

Isentropic expansion refers to a process where the entropy remains constant. In the ideal Rankine Cycle, this expansion happens in the turbine, which is assumed to be perfectly efficient and adiabatic (no heat exchange).

Why is the pump's work calculated differently from that in gas cycles?

The pump's work in the Rankine Cycle is calculated using v Delta P (specific volume multiplied by pressure change) instead of enthalpy because the change in enthalpy is small for liquids. This is the first major difference from gas cycles, where enthalpy changes are more significant.

How do the isentropic relationships for ideal gases differ from those used for the Rankine Cycle?

For the Rankine Cycle, which uses liquids and vapors (not ideal gases), isentropic relations for gases do not apply. Instead, we use property tables to find entropy values and other properties that correspond to those specific entropy values.

What are the factors that affect the efficiency of a Rankine Cycle?

The efficiency of a Rankine Cycle is influenced by several factors, such as the superheating of the vapor, the pressure of the boiler, the pressure of the condenser, and the use of reheating and regeneration. Superheating is particularly important for improving efficiency.

What challenges arise when analyzing non-ideal Rankine Cycles?

In non-ideal Rankine Cycles, deviations occur due to factors such as non-isentropic pumps and turbines, fluid friction in components, and heat losses during the steam flow. These issues cause deviations from the ideal cycle, requiring adjustments to efficiency calculations and other aspects.

How do the properties of steam at different pressures and temperatures affect the Rankine Cycle?

The properties of steam at different pressures and temperatures, such as enthalpy and entropy, are crucial for calculating the work done and the heat added or removed in the Rankine Cycle. The state of the steam (e.g., saturated liquid, superheated vapor, or mixture) determines the efficiency and performance of the cycle.