Aircraft environmental control: In a boot-strap air-refrigeration system equipped with evaporative cooling, where is the evaporator (for the evaporative subcooling stage) installed relative to the main components (first heat exchanger, secondary compressor, second heat exchanger, cooling turbine)?

Introduction / Context:
A boot-strap air-refrigeration system is widely used in aircraft environmental control systems. It employs two heat exchangers, a secondary compressor, and a cooling turbine. In the evaporative cooling variant, an evaporator is added to improve temperature reduction beyond what sensible heat exchange alone can achieve.

Given Data / Assumptions:

• System elements in order: first heat exchanger → secondary compressor → second heat exchanger → cooling turbine.
• Evaporator provides additional cooling via phase change (latent heat absorption).
• Goal: reduce air temperature before it reaches the second heat exchanger to raise overall effectiveness.

Concept / Approach:
Placing the evaporator after the secondary compressor but before the second heat exchanger reduces the compressed air temperature by latent cooling. This lowers the approach temperature in the second heat exchanger, increases its effectiveness, and ultimately yields a lower turbine-inlet temperature, improving the refrigeration effect at the cabin.

Step-by-Step Solution:

Verification / Alternative check:
Lowering the temperature upstream of the second heat exchanger increases log-mean temperature difference on the air side, enhancing heat rejection and reducing turbine inlet temperature, which increases the specific refrigeration effect.

Why Other Options Are Wrong:

• Combustion chamber location is irrelevant in an air cycle ECS; there is no combustor in the refrigeration loop.
• Before the secondary compressor: this is intercooling, not evaporative subcooling for the second exchanger.
• Between second exchanger and turbine: too late to benefit the second exchanger and turbine inlet temperature optimally.

Common Pitfalls:
Confusing evaporative subcooling with intercooling; the former targets lowering temperature ahead of the second exchanger, not compressor work reduction.

Final Answer:
Between the secondary compressor and the second heat exchanger