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Difficulty: HardHeat Engines and Refrigeration Principles

In a standard vapor-compression refrigeration cycle, the condenser relies on a cooling fan to move ambient air across its coils. Based on thermodynamic principles, which of the following is the most likely consequence if the condenser cooling fan completely fails while the compressor continues to operate?

  1. The high-pressure refrigerant will remain a gas instead of condensing into a liquid, because it cannot adequately dissipate thermal energy to the environment.Answer
  2. B
    The system will automatically switch from convection to conduction, causing the expansion valve to absorb the excess heat directly from the compressor.
  3. C
    The trapped thermal energy will accelerate the phase change process, causing the high-pressure gas to condense into a liquid much faster than normal.
  4. D
    The hot gaseous refrigerant will rapidly become an incompressible fluid inside the coils, forcing the condenser to act as a secondary hydraulic pump.

Answer

The high-pressure refrigerant will remain a gas instead of condensing into a liquid, because it cannot adequately dissipate thermal energy to the environment.
In a vapor-compression refrigeration cycle, the condenser's primary job is to reject heat absorbed from the interior space (and heat added by the compressor) to the outside environment. It does this via forced convection using a cooling fan. When the high-pressure, high-temperature gas loses this heat, it condenses into a high-pressure liquid. If the fan fails, the heat cannot be dissipated into the ambient air. Consequently, the refrigerant retains its thermal energy and remains a gas, entirely disrupting the cooling cycle.

Step-by-Step Solution

1
Identify the normal function of the condenser in the refrigeration cycle.
The condenser removes heat from the high-pressure, high-temperature gaseous refrigerant coming from the compressor.
Understanding baseline component function is necessary to analyze the system failure.
2
Determine the thermodynamic result of normal heat removal in the condenser.
As the gaseous refrigerant loses thermal energy to the cooler outside air, it undergoes a phase change (condenses) into a high-pressure liquid.
Phase changes are driven by the transfer of latent heat.
3
Analyze the impact of the cooling fan failure on heat transfer.
Without the fan blowing air across the coils, forced convection stops. Heat dissipation drops drastically.
Airflow is required to carry away the heat; still air acts as an insulator.
4
Conclude how the lack of heat transfer affects the state of the refrigerant.
Because the thermal energy cannot be adequately rejected to the environment, the refrigerant cannot shed enough heat to condense. It remains a hot, high-pressure gas as it exits the condenser.
A substance cannot condense into a liquid until sufficient thermal energy has been removed.

Key Concept

Thermodynamics of the Vapor-Compression Cycle
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Topics

Heat Engines and Refrigeration Principles

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