Aircraft air-cycle systems: The “reduced ambient” air-cooling system configuration contains which combination of turbines and heat exchangers?

Introduction / Context:
Aircraft environmental control systems use variants of the air-cycle (reversed Brayton) process: simple, bootstrap, regenerative, and reduced ambient. Each configuration uses different numbers of turbines and heat exchangers to meet cabin cooling at varying flight conditions.

Given Data / Assumptions:

• Reduced ambient system is intended to achieve lower turbine inlet temperatures by further cooling the compressed air.
• Ram air availability and pressure ratios are typical of transport aircraft.
• Components are arranged to maximize precooling before expansion.

Concept / Approach:
Reduced ambient systems employ an additional cooling/expansion stage to reduce the effective “ambient” temperature seen by the main expansion turbine. This requires two heat exchangers (primary and secondary) and two turbines arranged to stepwise drop the temperature and extract work.

Step-by-Step Solution:

Verification / Alternative check:
Cycle schematics used in textbooks show two HXs and two turbines for the reduced ambient variant, distinct from simple (1 HX, 1 turbine) and bootstrap (2 HXs, 1 turbine + secondary compressor) systems.

Why Other Options Are Wrong:
Options with only one turbine or one heat exchanger describe simpler cycles that cannot attain the same low supply temperatures under comparable conditions.

Common Pitfalls:
Mixing up “bootstrap” and “reduced ambient”; the former uses a secondary compressor driven by a turbine, while the latter uses additional cooling and expansion stages.

Final Answer:
Two cooling turbines and two heat exchangers