Which of the following statements is true about rotary compressors?

Rotary compressors are often characterized by their design and functionality, which typically involve a mechanism where rotating components compress the refrigerant. When considering the capacity control of rotary compressors, it is true that they do not vary capacity effectively compared to other types of compressors, such as variable speed or scroll compressors. This limitation arises because rotary compressors tend to operate at a fixed displacement, which means that they do not adjust their capacity based on demand as efficiently as other designs.

This characteristic is particularly important in applications where precise temperature control is necessary. While rotary compressors excel in certain conditions, their inability to adapt capacity to changing load conditions can lead to less efficient operation when the demand for cooling varies significantly.

Other options present statements that are not representative of rotary compressor performance: the claim that they require constant maintenance is not accurate as they typically have relatively low maintenance needs; their efficiency can vary widely based on design and application, and while they are often efficient, they may not necessarily be the most efficient compared to other compressor types; finally, rotary compressors can operate effectively across a range of temperatures, not solely in cold environments.