Question

In a mechanical refrigeration system, the highest temperature of refrigerant occurs

• A. Between condenser and evaporator
• B. In evaporator
• C. Before expansion valve
• D. Between compressor and condenser

Hint:

In any vapor-compression cycle (refrigeration or heat pump), the compressor's discharge line always carries the hottest fluid, while the expansion valve's exit carries the coldest.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question focuses on the Thermodynamics of the Vapor Compression Refrigeration Cycle (VCRC). We need to track the state of the working fluid (refrigerant) as it moves through the four primary components of the system: the compressor, the condenser, the expansion device, and the evaporator. Understanding where energy is added as work and where it is rejected as heat is key to identifying the points of maximum and minimum temperature in the cycle.
Step 2: Key Formulas and approach:
The approach is based on the analysis of the T-s (Temperature-Entropy) or P-h (Pressure-Enthalpy) diagrams for a standard refrigeration cycle. In an ideal cycle:
1. Compression is isentropic ($s = const$), which significantly raises both pressure and temperature.
2. Condensation is isobaric ($P = const$), where heat is rejected, and temperature typically drops or stays constant during phase change.
3. Expansion is isenthalpic ($h = const$), which causes a drastic temperature drop.
4. Evaporation is isobaric, where heat is absorbed at a low temperature.
Step 3: Detailed Explanation:

Low-Temperature Stage: The refrigerant leaves the evaporator as a saturated or slightly superheated vapor at low pressure and low temperature.

Compression Stage: As the vapor enters the compressor, mechanical work is performed to compress it. This compression increases the kinetic energy of the molecules, resulting in a "discharge" temperature that is the highest in the entire system.

Post-Compression State: At the exit of the compressor (and before entering the condenser), the refrigerant is a high-pressure, superheated vapor. This is the peak temperature point.

Condensation Stage: In the condenser, the refrigerant begins to release heat to the surroundings. The temperature remains high but starts to decrease as the superheat is removed and the fluid turns into a liquid.

Expansion Stage: Passing through the expansion valve causes the temperature to plummet due to the Joule-Thomson effect or flash evaporation.

Conclusion: Therefore, the segment of the piping containing the hottest refrigerant is the discharge line located between the compressor outlet and the condenser inlet.

Step 4: Final Answer:
The maximum temperature is found between the compressor and the condenser, corresponding to option (D).

Answer 2

A simple way to locate the hottest point in a refrigeration cycle is to track where energy is being added to the refrigerant, since adding energy is what raises its temperature.

1. Compressor: This is the only component in the whole cycle that adds energy to the refrigerant, through the mechanical work done on it during compression, similar to how a bicycle pump heats up as you compress the air inside it. This work goes directly into raising the refrigerant's temperature.
2. Condenser: Here the refrigerant only loses energy, releasing the heat it picked up plus the extra heat from compression, to the surrounding air or water. Temperature falls through this component.
3. Expansion valve: No energy is added here either; the refrigerant simply expands rapidly, and its temperature falls sharply as a result.
4. Evaporator: Energy comes into the refrigerant here too, but only enough to absorb heat from the space being cooled while staying at a constant, low temperature during the phase change, nowhere close to the temperature reached during compression.

Since the compressor is the only place where mechanical work is pumped directly into the refrigerant with nothing yet released, the refrigerant temperature peaks right as it leaves the compressor, in the line running to the condenser.

Therefore, the correct answer is Between compressor and condenser.