Which type of compressor is described as having a two-stage design in the APU?

The correct choice of a centrifugal type compressor for a two-stage design in the Auxiliary Power Unit (APU) is based on its methodology and efficiency in compressing air.

Centrifugal compressors operate by converting kinetic energy into pressure energy. They utilize a rotating impeller to accelerate the air, directing it into a diffuser where the velocity decreases, resulting in an increase in pressure. This type of compressor can be designed in stages, where each stage consists of an impeller and a diffuser. This two-stage design allows for significantly higher pressure ratios, which is beneficial for the efficiency and performance of an APU, especially in aviation where space and weight are critical considerations.

In contrast, piston type compressors typically utilize a reciprocating action to compress air and do not inherently adopt a two-stage design in a manner that is commonly associated with APUs. Axial flow compressors, while also efficient, are generally utilized in gas turbine engines rather than in APUs, and they have a more complex structure suited for high flow and higher pressure ratios required in propulsion. Scroll type compressors use a different mechanism (oscillating scrolls) and are less common in aviation applications, making them unsuitable for this particular question.

Thus, the efficiency and design characteristics of centrifugal