- Online (available or ready to use) compressors – all compressors that are available to serve peak load. Online compressors do not include back-up compressors whose only purpose is to be available when a compressor fails. Online compressors are all compressors that are physically connected to compressed air piping excluding back-up compressors.
- Back-up (standby) compressors: are those compressors not used to meet peak compressed air flow loads. Back–up compressors can be physically connected to the compressed air piping system and can be automatically controlled to turn on if one of the other compressors on the system fails. Back-up compressors do not normally operate.
- Trim compressor: is a compressor that is designated for part-load operation, handling the short term variable trim load of end uses, in addition to the fully loaded base compressor. In general, the trim compressor will be controlled by a VSD but it also can be a compressor with good part load efficiency. If the trim compressor does not have good part load efficiency broadly across its operating range, then it will take more compressors to meet the (best efficiency) Energy Standards requirements.
- Base compressor: the opposite of a trim compressor, a base compressor is expected to be mostly loaded. If the compressed air system has only one compressor, the requirements of the Energy Standards require that the single compressor be treated as a trim compressor.
(Ref.: 2016 Building Energy Efficiency Standards – Reference Ace v31, California Energy Commission)