Vapour Absorption System

Definition

A Vapour Absorption System (VAS) is a refrigeration cycle that uses thermal energy (heat) instead of mechanical energy (work) to produce a cooling effect. Unlike the Vapour Compression System, which uses a compressor, the VAS utilizes a secondary fluid (absorbent) to absorb the refrigerant vapour, creating a pressure difference that drives the cycle.

Main Content

1. Thermodynamic Principle

• The system relies on the affinity of an absorbent (like water) for a refrigerant (like ammonia).
• It operates on the principle of changing the concentration of a solution to shift the boiling point of the refrigerant at constant pressure.

2. Primary Components

• Generator: Uses a heat source to separate the refrigerant from the absorbent solution.
• Absorber: Where the low-pressure refrigerant vapour is absorbed back into the weak solution.
• Pump: A small mechanical device that circulates the strong solution to the high-pressure side.

3. The Refrigerant-Absorbent Pair

• Ammonia-Water: Ammonia acts as the refrigerant and water acts as the absorbent. Used for industrial freezing.
• Water-Lithium Bromide: Water acts as the refrigerant and Lithium Bromide acts as the absorbent. Used primarily for air conditioning.

Working / Process

1. Generation and Condensation

• The generator receives heat, which boils the refrigerant out of the strong solution.
• The high-pressure refrigerant vapour travels to the condenser, where it releases heat to the environment and turns into a high-pressure liquid.

2. Expansion and Evaporation

• The liquid refrigerant passes through an expansion valve, causing its pressure and temperature to drop significantly.
• Inside the evaporator, the cold refrigerant absorbs heat from the space being cooled, turning into vapour.

3. Absorption and Pumping

• The low-pressure refrigerant vapour enters the absorber, where it is absorbed by the weak solution coming from the generator.
• This creates a vacuum in the absorber, drawing in more vapour. The resulting strong solution is then pumped back to the generator to repeat the cycle.

       [Generator] ----(Heat In)----> [Condenser]
           ^                              |
           |                              |
      [Absorber] <---(Expansion Valve)---[Evaporator]
           |                              |
           -------(Pump)-------------------

(Cycle flow diagram showing the movement of refrigerant between components)

Advantages / Applications

• Energy Efficiency: Can utilize waste heat or solar energy, making it highly economical where electricity is expensive or unavailable.
• Low Wear: Since the pump is the only major moving part, these systems have fewer mechanical failures and quieter operation compared to compressors.
• Applications: Commonly used in large-scale industrial air conditioning, solar-powered refrigeration units, and chemical processing plants where steam or waste heat is abundant.

Summary

The Vapour Absorption System is a heat-operated refrigeration cycle that replaces the compressor with an absorber and generator to cycle the refrigerant. It is an environmentally friendly alternative that leverages thermal energy to achieve cooling.

Important terms to remember: - Absorbent: The fluid that pulls the refrigerant vapour into a liquid state. - Strong Solution: A solution with a high concentration of refrigerant. - Weak Solution: A solution with a low concentration of refrigerant after heating. - Thermal Energy: The primary energy input used to drive the refrigerant cycle.