Off-line filtration systems (also referred to as a kidney loops or recirculation loops) are typically found in variable volume pump systems where system filters do not see continuous flow or where the return flow volume makes a return filter impractical. These systems typically consist of a separate pump and filter plumbed to the reservoir along with associated components to provide a continuous re-circulating flow of fluid from the reservoir through the filter and back to the reservoir.
There are many advantages to incorporating an offline filtration package. Because the system is at a low pressure and the flow is continuous with little or no flow surges, economical low pressure filter housings can be used. In addition, because temperature control is necessary to ensure that the components see the proper viscosity and that the oxidation rate of the fluid is optimized, heat exchangers can be used within the off-line system to achieve the necessary operating parameters. The off-line system can also be used as the make-up point to add fluid to the system and help ensure that only clean fluid is introduced to the system. The off-line system can also be shutdown to change filter elements without securing the main system.
Rarely can an off-line filtration system be used as the sole contamination control solution. It is normally used to supplement the main system filters and a typical recommended size for the offline system flow rate is approximately 20% of the total main system volume.
In addition to hydraulic systems, off-line filtration systems can be used on gearbox applications to clean the fluid where there are no other lubrication circulation systems present.