LESSON 31 POWER CYCLE (20241203095500)-merged

Lesson Overview

• Focus on Power Cycles:

  • Heat engines operate in cycles producing net work.

  • Defined as power cycles.

• Categories:

  • Vapor Power Cycle: Working fluid exists in vapor and liquid phases.

  • Gas Power Cycle: Working fluid remains in gaseous phase.

Vapor Power Cycle

• Example: Steam power plant.

• Key Processes in Steam Power Plant:

  • Process 1-2: Heat energy from fuel vaporizes water into steam in the boiler.

  • Process 2-3: Steam expands in turbine/engine producing work.

  • Process 3-4: Vapor condenses in condenser.

  • Process 4-1: Condensed steam is pumped back to boiler, completing the cycle.

• Characteristics:

  • Alternation between vaporization and condensation.

  • Considered a closed vapor power cycle.

Ideal Cycle for Vapor Power Cycle Analysis

• Importance of Ideal Cycle for Analysis:

  • Actual vapor power cycles have irreversible and non-quasi-equilibrium processes.

  • Apply thermodynamics principles to reversible and quasi-equilibrium processes.

  • Utilize idealized vapor power cycle for performance analysis (theoretical).

Carnot Vapor Cycle

• The Carnot cycle:

  • Most efficient cycle between two temperature limits.

• Processes:

  1. Process 4-1: Constant pressure heat addition in the boiler.

     • Saturated liquid to dry saturated steam.

     • Heat added: qA = hg,1 – hf,4.

  2. Process 1-2: Isentropic expansion in turbine.

     • Dry saturated steam expands to wet steam, doing work.

     • Steam turbine work: wt = hg,1 – h2.

  3. Process 2-3: Heat rejection in condenser.

     • Exhaust steam condensed by rejecting heat qR.

     • Heat rejected: qR = h2 – h3.

  4. Process 3-4: Isentropic compression in pump.

     • Pumping from wet steam to saturated liquid.

     • Pump work: wp = hf,4 – h3.

• Net Work Done (wnet):

  • wnet = Heat added (qA) – heat rejected (qR).

  • Expressed in terms of turbine and pump work.

• Thermal Efficiency:

  • Carnot cycle indicates the maximum possible efficiency but is impractical.

Rankine Cycle

• Overcomes Carnot Cycle Limitations:

  • Complete condensation of vapor eliminates issues with wet steam.

• Processes in Rankine Cycle:

  1. Boiler:

     • Water converted to steam by heating.

     • Heat addition: qA = hg,1 – hsub,4.

  2. Steam Turbine:

     • Dry saturated steam expands, producing work.

     • Steam turbine work: wt = hg,1 – h2.

  3. Condenser:

     • Exhaust steam condensed by heat rejection.

     • Total heat rejected: qR = h2 – hf,3.

  4. Feed Pump:

     • Pumps condensate to boiler pressure.

     • Pump work: wp = hsub,4 – hf,3.

• Thermal Efficiency:

  • Rankine cycle efficiency ranges from 35% to 45%.

  • Pump work can often be neglected due to its small size.

Pure Substance

• Definition:

  • Homogeneous composition, aggregation; does not change despite phase changes.

• Phases of Pure Substance:

  • Solid, Liquid, Gas.

• Important Terms as Applied to Steam:

  1. Wet Steam: Contains moisture.

  2. Dry Saturated Steam: No moisture content.

  3. Superheated Steam: Heated beyond saturation temperature.

  4. Quality of Steam: Ratio of mass of dry steam to total mass; x (0 to 1).

  5. Critical Point: No distinct vaporization process; saturated states are identical.

  6. Triple Point: States with the same pressure and temperature.

  7. Sensible Heat: Heat to raise temperature at constant pressure (hf).

  8. Latent Heat: Heat for phase change at saturation temperature (hfg).

  9. Total Heat or Enthalpy: Combination of sensible and latent heat.