What type of compressor is classified as oil-free and uses liquid to seal spaces between impeller vanes?

The classification of the compressor as oil-free and its use of liquid to seal spaces between impeller vanes points specifically to the liquid ring compressor. This type of compressor operates by creating a liquid seal using water or other liquids that form a ring around the impeller, facilitating the compression process without the introduction of oil, which is crucial for applications where oil contamination must be avoided.

In a liquid ring compressor, the liquid not only seals the spaces but also aids in cooling the compressor, making it efficient in handling gases that might contain moisture or are sensitive to oil exposure. This feature allows for a cleaner operation and is particularly valuable in industries such as pharmaceuticals, food processing, and electronics, where oil-free air is a necessity.

The other types of compressors mentioned do not fit the criteria of being oil-free or use liquid for sealing in the same manner. Dynamic compressors, while often oil-free, do not utilize liquid sealing mechanisms, and rotary and reciprocating compressors usually involve oil for lubrication and sealing purposes. Hence, the liquid ring compressor is the most appropriate choice given its unique characteristics that align with the question.