Vapour absorption refrigeration system working, components, definition

The Vapour Absorption Refrigeration System (VARS) is a refrigeration technology that uses thermal energy instead of mechanical energy to provide cooling. It operates on the principle of absorption and desorption of a refrigerant–absorbent mixture.

Unlike a vapour compression system, which uses a compressor driven by electricity, a VARS uses heat energy (from steam, waste heat, solar energy, or gas flame) to circulate the refrigerant. This makes it a highly energy-efficient and eco-friendly system, ideal for industries where heat is readily available.

In most VARS systems, ammonia acts as the refrigerant, and water acts as the absorbent. However, for air conditioning and chilled-water applications, water–lithium bromide combinations are also used.

Let’s explore this system in depth — including its components, working cycle, types, advantages, disadvantages, and modern developments.

Also read: Types of Vapour absorption refrigeration system

Basic Principle of Vapour Absorption Refrigeration System

The principle of VARS is based on absorbing and releasing refrigerant vapour using a suitable absorbent solution.

Here’s how it works in simple terms:

1. The refrigerant (ammonia) evaporates at low pressure and temperature by absorbing heat from the cooling space.
2. The ammonia vapour is absorbed by water, forming a strong ammonia–water solution.
3. This strong solution is pumped to a higher pressure and sent to the generator, where heat is applied.
4. The applied heat separates the ammonia vapour from water.
5. The ammonia vapour is then condensed into a liquid and expanded again in the evaporator, repeating the cycle.

Thus, the mechanical compression used in traditional systems is replaced by absorption, pumping, and thermal separation, making the system more sustainable and less dependent on electricity.

Vapour absorption refrigeration system line diagram

Main Components of Vapour Absorption Refrigeration System

The Vapour Absorption Refrigeration System is made up of several key components that work together to form a closed thermodynamic cycle.

1. Evaporator

• The evaporator is the section where the actual refrigeration occurs.
• Low-pressure liquid ammonia absorbs heat from the area or substance to be cooled and evaporates into vapour.
• The absorbed heat causes a cooling effect — this is the useful refrigeration output of the system.

2. Absorber

• The absorber contains water, which acts as the absorbent.
• It absorbs the ammonia vapour from the evaporator and forms a strong ammonia–water solution.
• The process is exothermic (releases heat), so the absorber is cooled using water to maintain absorption efficiency.

3. Pump

• The pump raises the pressure of the strong ammonia–water solution and sends it to the generator.
• The energy required by the pump is very small compared to the compressor used in vapour compression systems.

4. Generator (Desorber)

• The generator receives the high-pressure strong solution from the absorber.
• Heat energy (from steam, solar collectors, or waste heat) is applied to the generator.
• The ammonia is separated from the water as vapour, leaving behind a weak solution (with low ammonia concentration).

5. Condenser

• The ammonia vapour from the generator passes through the condenser, where it rejects heat to the cooling medium (usually water or air).
• The vapour condenses into high-pressure liquid ammonia, which is ready to produce cooling.

6. Expansion Valve

• The liquid ammonia from the condenser passes through an expansion valve, where its pressure and temperature drop suddenly.
• A part of the liquid evaporates (flash evaporation), and the mixture enters the evaporator.

7. Heat Exchanger (Solution Heat Exchanger)

• Used to improve system efficiency.
• The hot weak solution leaving the generator transfers heat to the cold strong solution going to the generator.
• This reduces the required heating load on the generator.

8. Auxiliary Components

• Analyzer and Rectifier – remove water vapour from the ammonia before it enters the condenser.
• Pressure Reducing Valve – reduces the pressure of the weak solution before it re-enters the absorber.

Line Diagram Explanation

The line diagram of the Vapour Absorption Refrigeration System illustrates the flow of refrigerant and absorbent through all components.

1. Ammonia vapour from the evaporator enters the absorber and is absorbed by water.
2. The resulting strong solution is pumped to the generator.
3. Heat is supplied to the generator to release ammonia vapour.
4. The vapour flows to the condenser, where it becomes liquid.
5. The liquid ammonia expands through the expansion valve and re-enters the evaporator.
6. The weak solution from the generator returns to the absorber, completing the cycle.

This closed-loop operation ensures continuous cooling as long as a heat source is available.

Working of Vapour Absorption Refrigeration System

Types of Vapour Absorption Refrigeration Systems

1. Ammonia–Water System

• Refrigerant: Ammonia
• Absorbent: Water
• Suitable for low-temperature refrigeration, ice plants, and industrial cooling applications.

2. Water–Lithium Bromide System

• Refrigerant: Water
• Absorbent: Lithium Bromide
• Used for large-scale air conditioning and chilled water production (cannot be used below 0°C because water freezes).

Based on Effect

1. Single-Effect VARS:
   • One generator and absorber.
   • Simple design but low efficiency (COP 0.6–1.0).
2. Double-Effect VARS:
   • Two generators operating at different pressure levels.
   • Utilizes waste heat more efficiently.
   • COP 1.0–1.5.
3. Triple-Effect VARS:
   • Three generator stages for maximum efficiency.
   • Used in large industrial applications.

Based on Heat Source

• Direct-fired: Uses natural gas or oil burner.
• Steam-operated: Uses steam from industrial processes.
• Solar-powered: Uses solar collectors as heat input.
• Waste-heat-driven: Uses exhaust gases from engines or turbines.

Thermodynamic Performance and COP

The Coefficient of Performance (COP) of an absorption system is given by:

COP= Qg / ​QE​e

Where:

• Qe = Heat absorbed in the evaporator (refrigerating effect)
• Qg = Heat supplied to the generator

For a reversible absorption system,

COP= {[Te / (Ts – Te)] x [(Tg-Ts)/Tg]}

where:

• Te = Evaporator temperature
• Ts = Absorber/condenser temperature
• Tg​ = Generator temperature

In practice:

• Single-effect system COP: 0.6 – 1.0
• Double-effect system COP: 1.0 – 1.5
• Triple-effect system COP: up to 1.8

Thus, even though the COP is lower than that of compression systems (2.5–6), VARS uses inexpensive or waste heat, making it economically viable.

Comparison: Vapour Absorption vs Vapour Compression System

Applications of Vapour Absorption Refrigeration System

1. Industrial Cooling:
   • Used in chemical, fertilizer, pharmaceutical, and food industries.
   • Ideal where process heat or waste heat is available.
2. Cold Storage:
   • For preserving fruits, vegetables, dairy, and frozen foods.
   • Ensures long shelf life and controlled temperature.
3. Air Conditioning:
   • Large commercial buildings, hospitals, and hotels use LiBr-based VARS for centralized cooling.
4. Solar Cooling Systems:
   • Absorption systems powered by solar energy are becoming popular for sustainable refrigeration.
5. Combined Heat and Power (CHP) Plants:
   • Waste heat from power generation is used to operate absorption chillers.
6. Marine and Remote Applications:
   • Ideal for ships and remote areas where electrical power is limited but heat energy is available.

Advantages of Vapour Absorption Refrigeration System

• Low electricity consumption: Only a small pump uses electricity.
• Can utilize waste or solar heat: Makes it highly energy-efficient.
• Eco-friendly: Uses natural refrigerants like ammonia and water.
• Low maintenance: Few moving parts reduce wear and tear.
• Quiet operation: No compressor noise.
• Long lifespan: Robust and durable for industrial use.

Disadvantages of Vapour Absorption Refrigeration System

• Large and heavy system: Requires more space.
• Lower COP: Efficiency is less compared to vapour compression systems.
• High initial cost: Components like generator and absorber add to cost.
• Sensitive to ambient temperature: Efficiency decreases in hot climates.
• Slow operation: Takes longer to start and respond to load changes.

Future Developments and Research Trends

Modern research focuses on improving VARS through:

• Hybrid absorption-compression systems combining both cycles for better efficiency.
• Nano-fluids in heat exchangers to enhance heat transfer rates.
• Advanced refrigerant–absorbent pairs like ammonia–lithium nitrate.
• Compact modular designs for portable cooling units.
• Integration with renewable energy such as solar or geothermal heat.

These advancements aim to make VARS more efficient, smaller in size, and suitable for a broader range of applications.

Conclusion

The Vapour Absorption Refrigeration System (VARS) is a sustainable alternative to the vapour compression system, especially in industries where waste heat or solar energy is available.

By replacing mechanical energy with thermal energy, it reduces dependence on electricity and helps lower greenhouse gas emissions. Although it has limitations such as size and slower response, ongoing innovations are making it more efficient and practical.

With the growing global demand for eco-friendly and energy-saving cooling systems, VARS is poised to play a key role in the future of sustainable refrigeration and air conditioning technologies.