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Difficulty: MediumHeat Engines and Refrigeration Principles

In an automotive air conditioning system, high-pressure liquid refrigerant passes through an expansion valve just before entering the evaporator coils. This component causes a sudden, significant drop in the fluid's pressure. What is the primary thermodynamic purpose of this pressure drop?

  1. To lower the boiling point of the refrigerant, allowing it to vaporize and absorb heat from the passenger cabin.Answer
  2. B
    To compress the liquid refrigerant into a smaller volume, forcing it to absorb surrounding thermal energy more effectively.
  3. C
    To cause the refrigerant to contract in volume as it warms up, reducing the physical space it occupies in the evaporator.
  4. D
    To steadily increase the sensible temperature of the liquid while preventing any phase change from occurring.

Answer

To lower the boiling point of the refrigerant, allowing it to vaporize and absorb heat from the passenger cabin.
The core principle of a vapor-compression refrigeration cycle is using a phase change to move heat. By dropping the pressure at the expansion valve, the boiling point of the refrigerant is drastically lowered. This allows it to easily boil into a gas when exposed to the relatively warm air of the passenger cabin in the evaporator. Boiling is an endothermic process that absorbs massive amounts of latent heat, thereby effectively cooling the cabin air.

Step-by-Step Solution

1
Analyze the state of the refrigerant at the expansion valve.
The refrigerant enters the valve as a high-pressure liquid.
Understanding the starting state is necessary to determine what physical changes will occur.
2
Determine the physical effect of a sudden pressure drop on a fluid.
A drop in pressure significantly lowers the boiling point of a fluid.
This is a fundamental thermodynamic principle governing phase changes.
3
Evaluate the result in the context of the next component (the evaporator).
Because the boiling point is now lower than the cabin temperature, the liquid boils (vaporizes) in the evaporator, absorbing massive amounts of latent heat.
This phase change is the primary mechanism by which the air conditioning system removes heat from the air.

Key Concept

Phase changes in a refrigeration cycle
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Topics

Heat Engines and Refrigeration Principles

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