Not all centrifugal compressors have what feature?

The characteristic that not all centrifugal compressors have is the feature of more than one air supply. Centrifugal compressors can operate with a single air supply, making it feasible for many applications without needing multiple sources of air. This is different from other common features found in these compressors.

For example, built-in compressors are indeed standard in many models, as they are integral to the functioning of the system. Similarly, variable speed drives are increasingly common in centrifugal compressors because they enhance efficiency by allowing the compressor to adjust its speed to match the demand for air. Discharge silencers are also frequently included to minimize noise during operation, making them a typical feature of many centrifugal compressor designs.

Thus, while multiple air supplies could be beneficial in certain configurations or applications, they are not a universal requirement for centrifugal compressors, which is why this option stands out as the correct answer.