Which type of compressors is characterized by a fixed volume?

Positive displacement compressors are characterized by a fixed volume because they operate by trapping a specific amount of air or gas and then forcing it into a smaller volume, thereby increasing pressure. This method creates a constant volume of gas that is compressed, making it ideal for applications where a consistent and high-pressure output is necessary. They function by displacing a set volume of air or fluid with each cycle, regardless of the downstream pressure.

Dynamic compressors, on the other hand, operate based on the principle of imparting velocity to the air or gas for compression, allowing for a continuous flow which does not have a "fixed volume" in the same way that positive displacement compressors do. Rotary compressors can refer to a specific type of positive displacement compressor, but the distinction lies in their mechanism rather than the characteristic of fixed volume. Lastly, centrifugal compressors, a subset of dynamic compressors, rely on high-speed rotation to compress gas, thus they also do not operate on a fixed volume mechanism. This makes positive displacement compressors unique in their operation and application in various industrial settings.