Thermal Engineering Lecture: Vapor Compression Refrigeration Cycle

Introduction

• Topic: Vapor compression refrigeration cycle
• Previous discussions: Vapor power cycles (power generation)
• Today's focus: Refrigeration or refrigerating effect, not power production

Concept of Refrigeration

• Purpose: Produce a refrigerating effect (cooling)
• Example: Household refrigerator maintains a cold compartment
• Cold space maintained at sub-zero temperature (e.g., -10°C)
• Heat transfer from cold space to maintain low temperature
• System Components:
  • Heat absorber: Evaporator
  • Working fluid: Refrigerant

Working Fluid: Refrigerant

• Special working fluid: Refrigerant (not water)
• Properties to be discussed later (e.g., R-134a, R-22, etc.)

Cycle Components

1. Evaporator: Heat Absorption

   • Space to be cooled at sub-zero temperature
   • Refrigerant absorbs heat, evaporates to saturated vapor
   • Point 1: Saturated refrigerant vapor

2. Compressor: Pressure Increase

   • Work absorbing device (W in)
   • Inlet: Saturated vapor (point 1)
   • Outlet: High pressure vapor (point 2)

3. Condenser: Heat Rejection

   • High pressure vapor releases heat
   • Outlet: Return to original state (point 3)
   • Pressure maintained at condenser level

4. Throttle Valve: Pressure Reduction

   • No work production, used for pressure drop
   • Inlet: High pressure from condenser (point 3)
   • Outlet: Low pressure to evaporator (point 4)
   • Process: Throttling (rapid, non-equilibrium)

Thermodynamic Representation

1. T-S Diagram

   • Process mapping for energy measurement
   • Point transitions (1-2, 2-3, 3-4, 4-1)
   • 1-2: Reversible adiabatic compression
   • 2-3: Constant pressure condensation
   • 3-4: Throttling (constant enthalpy), shown as a dotted line due to fast process
   • 4-1: Constant pressure evaporation

2. P-H Diagram

   • Mapping on pressure-enthalpy plane
   • 1-2: Reversible adiabatic (dh > 0)
   • 2-3: Constant pressure condensation
   • 3-4: Throttling (h3 = h4)
   • 4-1: Constant pressure evaporation

Analysis: Ideal vs. Carnot Cycle

• Carnot cycle: Less preferred due to wet compression issues
• Comparison:
  • Ideal cycle: Dry compression preferred (saturated vapor at inlet)
  • Carnot cycle: Wet compression (liquid-vapor mixture)

Performance Metrics

1. Coefficient of Performance (COP)

   • Desired Effect/Input Energy
   • Formula: COP = (h1 - h4) / (h2 - h1)
   • Higher COP indicates better performance

2. Refrigeration Effect

   • Amount of energy extracted from the cold space (Q_in)
   • Formula: q_in = h1 - h4 (kJ/kg)

Important Details

• Flash Gas Fraction (X4): Quality of refrigerant at evaporator inlet
  • X4 = (h4 - hf) / hfg
  • Important for evaporator design

Conclusion

• Reviewed Vapor compression refrigeration cycle processes and performance metrics
• Discussed practical issues and importance of dry compression
• Next class: Continuation of the discussion