Unit-II: Refrigeration Systems

 

1. Vapour Compression Refrigeration Cycle (VCRS)

The vapour compression refrigeration cycle (VCRS) is the most commonly used refrigeration system in domestic and industrial applications. It works by compressing, condensing, expanding, and evaporating a refrigerant to remove heat from a desired space.

Working of Vapour Compression Refrigeration Cycle

The cycle consists of four main components:

1. Compressor:

   • Increases the pressure and temperature of the refrigerant.

   • Converts low-pressure vapour into high-pressure, high-temperature vapour.

2. Condenser:

   • Releases heat to the surroundings, converting high-pressure vapour into a high-pressure liquid.

   • Typically air-cooled or water-cooled.

3. Expansion Valve (Throttle Valve):

   • Reduces the pressure and temperature of the refrigerant.

   • Converts high-pressure liquid into a low-pressure liquid-vapour mixture.

4. Evaporator:

   • Absorbs heat from the refrigerated space, converting low-pressure liquid into low-pressure vapour.

   • This completes the cycle as the refrigerant returns to the compressor.

Mathematical Expressions for VCRS

Using the first law of thermodynamics:

• Refrigerating Effect (RE):

  $$RE = h_1 - h_4$$

  (Heat absorbed in the evaporator)

• Compressor Work (Wc):

  $$W_c = h_2 - h_1$$

  (Energy input to the compressor)

• COP (Coefficient of Performance):

  $$COP = \frac{\text{Refrigerating Effect}}{\text{Work Input}} = \frac{h_1 - h_4}{h_2 - h_1}$$

where:

• $h_1$ = Enthalpy of refrigerant at evaporator outlet

• $h_2$ = Enthalpy at compressor outlet

• $h_3$ = Enthalpy at condenser outlet

• $h_4$ = Enthalpy at expansion valve outlet

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2. Representation of Vapour Compression Cycle on Different Diagrams

1. P-H (Pressure-Enthalpy) Diagram

• The most commonly used diagram for refrigeration cycles.

• Helps in enthalpy calculations for efficiency analysis.

• Stages:
  1 → 2: Compression (Pressure ↑, Enthalpy ↑)
  2 → 3: Condensation (Pressure Constant, Enthalpy ↓)
  3 → 4: Expansion (Pressure ↓, Enthalpy ↓)
  4 → 1: Evaporation (Pressure Constant, Enthalpy ↑)

2. T-S (Temperature-Entropy) Diagram

• Used for entropy analysis.

• Shows the phase changes of the refrigerant.

3. P-V (Pressure-Volume) Diagram

• Used for visualizing pressure and volume variations in the cycle.

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3. Types of Vapour Compression Cycles

1. Simple Vapour Compression Cycle

• Basic refrigeration cycle used in domestic refrigerators and air conditioners.

2. Multi-Stage Vapour Compression Cycle

• Used for large-scale refrigeration (e.g., industrial applications).

• Improves efficiency by using multiple compressors.

3. Cascade Vapour Compression Cycle

• Used for very low-temperature applications (e.g., liquefied gas production).

• Uses two or more refrigeration cycles operating at different temperature levels.

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4. Advantages & Disadvantages of Vapour Compression Cycle

✅ Advantages:

• High COP compared to other refrigeration cycles.

• More compact and efficient.

• Works over a wide range of temperatures.

❌ Disadvantages:

• Compressor consumes high power.

• Regular maintenance is required for compressor and refrigerant leaks.

• Uses harmful refrigerants (CFCs, HFCs), impacting the environment.

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5. Vapour Absorption Refrigeration System (Basic Introduction)

This system uses a heat source (instead of a compressor) to drive the refrigeration cycle.

Working Principle

1. Uses a liquid absorbent and refrigerant pair (e.g., Water-Ammonia or Lithium Bromide-Water).

2. Heat is supplied to separate the refrigerant from the absorbent.

3. The refrigerant evaporates, absorbs heat, and provides cooling.

4. The refrigerant then recombines with the absorbent, repeating the cycle.

6. Electrolux Refrigeration System (For Domestic Units)

• A gas absorption system used in domestic refrigerators.

• Uses no moving parts, making it silent and low-maintenance.

• Operates using ammonia, hydrogen, and water.

• Works on the principle of diffusion and absorption.

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7. Comparison: Vapour Absorption vs Vapour Compression System

Feature	Vapour Compression System	Vapour Absorption System
Energy Source	Uses electricity (compressor)	Uses heat energy (burners, solar)
Moving Parts	Uses compressor (moving part)	No moving parts (silent)
Efficiency	Higher COP	Lower COP
Maintenance	Requires regular maintenance	Low maintenance
Applications	Domestic refrigerators, ACs	Industrial cooling, gas refrigerators
Environmental Impact	Uses harmful refrigerants	Uses eco-friendly refrigerants

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8. Simple Problems on Vapour Compression Cycle

Example Problem

A refrigerator operates on a simple vapour compression cycle. The enthalpies at different points are:

• $h_1 = 180$ kJ/kg

• $h_2 = 210$ kJ/kg

• $h_3 = 100$ kJ/kg

• $h_4 = 100$ kJ/kg

Find:

1. Refrigerating Effect

2. Work Done

3. COP

Solution

1. Refrigerating Effect (RE):

   $$RE = h_1 - h_4 = 180 - 100 = 80 \text{ kJ/kg}$$
   

2. Work Done (Wc):

   $$W_c = h_2 - h_1 = 210 - 180 = 30 \text{ kJ/kg}$$
   

3. COP:

   $$COP = \frac{RE}{W_c} = \frac{80}{30} = 2.67$$
   

Thus, the system has a COP of 2.67.

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Conclusion

• VCRS is the most efficient and widely used refrigeration system.

• Absorption systems are useful where waste heat or solar energy is available.

• Understanding thermodynamic diagrams helps in analyzing performance.

• Simple problems help in practical applications of refrigeration cycles.