Question

For an air refrigeration cycle, COP improves when

• A. Compressor exit temperature increases
• B. Turbine exit temperature decreases
• C. Refrigeration temperature increases
• D. Heat rejection temperature increases

Hint:

In gas refrigeration cycles, the turbine acts as an expander to lower the temperature.
The lower the temperature at the exit of this expander, the higher the capacity of the gas to absorb heat from the cold space, which enhances the COP.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question focuses on the performance optimization of an air refrigeration cycle (such as the Bell-Coleman or Reverse Brayton cycle).
We need to determine which temperature change improves the Coefficient of Performance (COP).

Step 2: Key Formula or Approach:
For an air refrigeration cycle, the COP is given by:
\[ \text{COP} = \frac{\text{Refrigeration Effect}}{\text{Net Work Input}} = \frac{T_1 - T_4}{(T_2 - T_1) - (T_3 - T_4)} \]
where:
$T_1$ is the compressor inlet temperature,
$T_2$ is the compressor outlet temperature,
$T_3$ is the turbine inlet temperature, and
$T_4$ is the turbine outlet (exit) temperature.

Step 3: Detailed Explanation:

• The refrigeration effect occurs as the air passes through the evaporator/cabin, absorbing heat. This effect is proportional to $T_1 - T_4$.

• Lowering the turbine exit temperature ($T_4$) directly increases the temperature difference ($T_1 - T_4$), thereby increasing the refrigeration effect.

• Additionally, a lower $T_4$ expands the temperature range over which heat is absorbed, leading to more effective cooling per unit of work done.

• Consequently, a lower turbine exit temperature improves the overall COP of the cycle.

Step 4: Final Answer:
The COP of an air refrigeration cycle improves when the turbine exit temperature decreases.