Understanding Refrigerant Vapor in Vapor Absorption Systems

In a vapor absorption cooling system, the refrigerant circulates through various components to produce a cooling effect. A critical step is obtaining pure refrigerant vapor at a high pressure so it can be condensed in the condenser.

Source of Pure Refrigerant Vapor for the Condenser

Let's analyze the typical process in a vapor absorption system, such as an ammonia-water system:

• Evaporator: The liquid refrigerant absorbs heat from the space to be cooled, turning into low-pressure vapor. This low-pressure vapor then flows to the absorber. So, the vapor from the evaporator is low pressure and goes to the absorber, not the condenser.
• Absorber: The low-pressure refrigerant vapor from the evaporator is absorbed by a weak solution (e.g., water in an ammonia-water system), forming a strong solution. Heat is exchanged during this process, but it's absorption, not generation of pure vapor for the condenser.
• Pump: The strong solution is pumped from the low-pressure absorber to the high-pressure generator.
• Generator: This is where heat is supplied to the strong solution. The added heat boils off the refrigerant (like ammonia) from the absorbent (like water). This process separates the refrigerant as a high-pressure vapor from the weak solution. This high-pressure, relatively pure refrigerant vapor is then directed towards the condenser. The remaining weak solution flows back to the absorber via a heat exchanger and expansion valve.
• Condenser: The high-pressure refrigerant vapor from the generator enters the condenser, where it releases heat to a cooling medium (like water or air) and condenses into a high-pressure liquid.

Based on this cycle, the pure refrigerant vapor that goes to the condenser originates from the generator.

Analyzing the Options

Let's look at the given options in light of how a vapor absorption system works:

• Option 1: By exchanging heat with the weak ammonia-water solution - This typically describes the heat exchange process in the absorber or in the solution heat exchanger, not the generation of pure vapor for the condenser.
• Option 2: Directly from the evaporator - The evaporator produces low-pressure vapor that goes to the absorber, not the high-pressure vapor needed for the condenser.
• Option 3: By supplying the heat to the generator where ammonia gets released - This accurately describes the function of the generator, which receives heat and boils off the refrigerant (ammonia in this case) from the solution, producing the high-pressure vapor sent to the condenser.
• Option 4: By feeding from an external reservoir - This is not part of the standard operating cycle of a vapor absorption system, which uses an internal circulation of refrigerant and absorbent.

Therefore, the pure refrigerant vapor obtained for the condenser in a vapor absorption cooling system comes from the generator where heat is supplied to release the refrigerant.