Principles of Absorption Refrigeration Explained
Refrigeration systems are essential for cooling applications in homes, industries, and commercial spaces. While the mechanical vapor compression cycle is the most common, another important method is the absorption refrigeration cycle. Both achieve cooling by evaporating a refrigerant at low pressure and rejecting heat at higher pressure, but they differ in how the refrigerant is circulated.
π Vapor Compression vs. Absorption Cycle
- Mechanical Vapor Compression Cycle → Uses a mechanical compressor to create the pressure difference and circulate the refrigerant.
- Absorption Refrigeration Cycle → Uses a liquid absorbent and a group of components (absorber, pump, generator, and valves) that act as a thermal compressor.
π In both systems, the condenser, expansion valve, and evaporator remain the same. The difference lies in how the refrigerant is pressurized and moved.
π§ͺ Working Principle of Absorption Refrigeration
In absorption systems, the refrigerant vapor is absorbed by a liquid absorbent instead of being mechanically compressed.
- Absorber → The refrigerant vapor from the evaporator is absorbed by the absorbent, releasing heat to the surroundings.
- Pump → Moves the weak solution to the generator at high pressure.
- Generator (Boiler) → Heat is supplied to boil off the refrigerant, concentrating the solution.
- Condenser → High-pressure refrigerant vapor is cooled and condensed into liquid.
- Expansion Valve → Reduces pressure and temperature before entering the evaporator.
- Evaporator → Refrigerant absorbs heat from the cooled space, evaporates, and the cycle repeats.
π‘️ Common Absorbent-Refrigerant Combinations
- Ammonia-Water System → Ammonia is the refrigerant, water is the absorbent. Suitable for low temperatures below 0°C.
- Lithium Bromide-Water System → Water is the refrigerant, lithium bromide is the absorbent. Limited to temperatures above 0°C.
π Thermodynamics of Mixtures
The performance of absorption systems depends on the composition of the absorbent-refrigerant mixture.
- Mass Fraction (ΞΎ) → Expresses the ratio of one component in the solution.
[ \sigma = \frac{m_{LiBr}}{m_{LiBr} + m_{H2O}} ] - Vapor Pressure → Determined by both temperature and mass fraction.
- In LiBr-water systems, vapor pressure decreases as LiBr concentration increases, enabling water to evaporate at lower temperatures.
⚙️ Basic Lithium Bromide-Water Absorption System
A typical H₂O–LiBr absorption refrigeration system consists of eight main components:
- Evaporator – Refrigerant absorbs heat (Qe) from the cooled space, changing from liquid to vapor.
- Absorber – Vapor is absorbed by LiBr solution, releasing heat (Qa) to surroundings.
- Pump – Moves weak solution to generator at high pressure.
- Generator – Heat (Qb) boils off refrigerant (water), concentrating the solution.
- Condenser – Refrigerant vapor condenses, rejecting heat (Qc).
- Expansion Valve 1 – Reduces pressure, refrigerant enters evaporator.
- Heat Exchanger – Improves efficiency by exchanging heat between strong and weak solutions.
- Valve 2 – Allows strong solution to return to absorber, completing the cycle.
π Note: The pump consumes only a small amount of mechanical work (Wp) compared to a mechanical compressor, making absorption systems energy-efficient when waste heat is available.
✅ Advantages of Absorption Refrigeration
- Uses heat energy instead of large mechanical work.
- Can operate on waste heat, solar energy, or steam, reducing electricity consumption.
- Quiet operation (no mechanical compressor).
- Suitable for large industrial cooling and air-conditioning applications.
π Conclusion
Absorption refrigeration is an alternative to mechanical compression, especially useful where waste heat or renewable energy can be utilized. By understanding the principles, components, and thermodynamics of absorption systems, learners and professionals can appreciate how these systems achieve cooling with minimal mechanical work.
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