Aircraft refrigeration: How many heat exchangers are employed in a standard boot-strap air-cooling system used for high-speed aircraft?

Introduction / Context:
Air-cycle (reverse Brayton) refrigeration systems are common in aviation. The boot-strap arrangement improves cooling capacity at high flight speeds by using compressor-driven pressure rise and staged heat rejection.

Given Data / Assumptions:

• High-speed aircraft with ram air available for heat rejection.
• Boot-strap system incorporates an auxiliary compressor driven by a turbine.
• Standard schematic includes primary and secondary heat exchangers.

Concept / Approach:
In the boot-strap system, ambient (ram) air first cools compressed bleed air in the primary heat exchanger. The air is then further compressed by an auxiliary compressor and cooled again in a secondary heat exchanger before expansion in a turbine to produce refrigeration.

Step-by-Step Solution:
Ram air provides sink for heat rejection.Primary heat exchanger: cools bleed air after main compression.Secondary heat exchanger: cools air after auxiliary compression (boot-strap stage).Air then expands in turbine to deliver low-temperature supply to cabin.

Verification / Alternative check:
System block diagrams consistently show two distinct heat exchangers labeled “primary” and “secondary.”

Why Other Options Are Wrong:

• One HX: insufficient for staged cooling after both compression steps.
• Three or four HX: not standard for classical boot-strap; would be heavier and more complex.
• None: contradicts the need for rejecting compression heat.

Common Pitfalls:
Confusing boot-strap with simple air-cycle (one HX) or regenerative variants; diagrams differ by configuration.

Final Answer:
Two heat exchangers