W10

Learning Journey

  • Course: MEC2405 - Thermodynamics

  • Period: S2 2025

  • Lab: Actual Vapour-Liquid Power Cycles / Refrigeration System Lab

  • Learning Tasks:

    • Own-time: 1 Task

    • Real-time: 10 Tasks

    • Wrap-up: 4 Tasks

  • Schedule:

    • Week 10: Mon 6 Oct 25 - Sun 12 Oct 25

Overview

  • Topic: Steam power plants

  • Components:

    • Multistage turbines

    • Reheating mechanisms

    • Regeneration techniques using closed/open feedwaters

Increasing the Efficiency of the Ideal Rankine Cycle

  • Processes Involved:

    • Process 1-2: Limiting the heat transfer processes to two-phase systems restricts the maximum temperature at which heat can be used in the cycle (maximum: 374°C for water).

    • Process 2-3: Turbine challenges include handling steam with high moisture content; liquid droplet impingement on turbine blades leads to erosion and wear.

    • Process 4-1: Compressors cannot effectively handle two phases, leading to the necessity for superheating steam in the boiler and complete condensation in the condenser.

  • Resulting Cycle: The Rankine Cycle is illustrated for vapour-liquid power plants.

Modifications from Brayton Cycle to Rankine Cycle

  • Brayton Cycle Improvements:

    • Techniques such as regeneration and reheating/intercooling enhance efficiencies.

    • Improvements originate from the hypothetical Ericsson Cycle, which achieves Carnot Cycle efficiency.

  • Application to Rankine Cycle: Similar strategies improve the simple Rankine Cycle.

Recap of the Rankine Cycle

  • Phase Change:

    • During the vapour-liquid phase change, temperature remains constant, suggesting potential for effective energy conversion in Carnot-like cycles.

  • Carnot Cycle Suitability:

    • The direct Carnot Cycle model is unsuitable for power cycles due to impractical isentropic compression and temperature variances above critical points.

Real-World Factors in Vapour Power Cycle

  • Non-isentropic Operations:

    • Actual operations are influenced by fluid friction and heat loss, leading to non-isentropic behavior in pumps and turbines.

    • Effect of friction accounted via isentropic efficiencies, while maintaining quasistatic assumptions for analysis.

  • Efficiency Improvements:

    • Increase average heat transfer temperature to working fluid in the boiler.

    • Decrease average temperature for heat rejection in the condenser.

Reheat and Regeneration in Rankine Cycles

  • Gas-Turbine Cycle Regeneration:

    • Saves heat using exhaust gases to pre-heat gases coming from the compressor, preserving network delivered without increasing external heat input.

    • Enhance performance by approaching isothermal operations during expansion and compression.

  • Rankine Cycle Adaptation:

    • In Rankine Cycles, the pump work is significantly smaller than turbine work, with the specific volume of liquid state approximately one hundred times smaller than vapour state.

    • High efficiency improvements occur via reheating the steam during multistage turbine operation.

Technical Description of Reheat Rankine Cycle

  • Single Reheat and Two-Stage Turbine:

    • Steam returns to the boiler for reheating and has an independent tubing system to avoid contamination from the main stream.

    • Optimal reheat pressure is about one-fourth of the maximum cycle pressure, ensuring peak efficiency.

Regeneration Techniques in Rankine Cycles

  • Exhaust Steam Utilization:

    • Final turbine exhaust does not permit direct pre-heating of feedwater due to temperature equalization with the condenser.

    • Steam is extracted from various turbine locations and utilized to heat feedwater in methods between specific thermodynamic States (States 2 and 2').

  • Feedwater:

    • Water entering the boiler is termed "boiler feed water" or "feedwater".

  • Regenerator/Feedwater Heater Devices:

    • Open Feedwater Heater (OFH):

    • A mixing chamber where extracted steam from the turbine blends with pump-exiting feedwater.

    • Operates at steam pressure, exiting as a saturated liquid leading to lower pressure than boiler's.

    • Requires additional pumping to suitable boiler pressure.

    • Closed Feedwater Heater (CFH):

    • Transfers heat without mixing streams, allowing pressure variance between steam and feedwater.

    • Design complexity increases, but it offers greater design flexibility.

  • Combination of Feedwater Heaters:

    • Many plants utilize both OFHs and CFHs to maximize efficiency.

  • Throttle Valve Assembly (Trap):

    • Used in configurations where steam from CFHs is redirected back, maintaining necessary system pressure and flow.