A separate circuit that draws oil from the reservoir, filters it and returns it, running independently of the machine's own hydraulics. The name comes from the analogy with a kidney continuously cleaning a bloodstream rather than filtering at one point in a flow.

An in-line filter must not restrict system flow, which limits how fine it can be, and it only works when the machine runs. An off-line loop carries a small flow at low pressure through a much finer filter with no effect on the machine, and continues while the machine is idle.

No - it supplements them. In-line filters protect components from debris generated within the circuit and must remain. The off-line loop lowers the overall system contamination level, which in-line filtration alone can only maintain rather than reduce. The two do different jobs and a system generally wants both.

Yes, and that is one of its main uses. A kidney loop drives the particle count down over hours and holds it there, which frequently brings a system back into specification without an oil change - saving both the oil cost and the disposal.

By vacuum dehydration or coalescing units fitted in the off-line loop, since no particulate filter removes dissolved or emulsified water. Water is the second contaminant that destroys hydraulic components, so removing it is often as valuable as removing particles. Removing it is often as valuable as removing the particles.

A cart serves several machines and suits commissioning, transferring new oil into a system, and remedial clean-up. A permanent loop suits critical systems where cleanliness must be held continuously and where the cost of a contamination-related failure justifies dedicated equipment.

Yes - new oil is frequently dirtier than the system it is being poured into, which surprises people, because it picks up contamination in handling and storage. Transferring it through a filter cart rather than straight from the drum is one of the cheapest contamination controls available.