The required filter size is based on the permissible flow resistance of the oil filter at the nominal flow rate.
The selection of the filter size is affected by:
- The selected filtration grade,
- The flow rate absorbed (very different on flow and return filters),
- The filter area
- The filter design
- The contamination in the atmosphere in which the system is operating,
- The permissible pressure drop
- The viscosity of the hydraulic fluid and thus its temperature at that moment
For the selection of the size the figures are available from the manufacturers on the nominal flow that is reduced by the pressure loss Δp. The actual size must be selected significantly larger depending on the filtration grade: for a filtration grade of 25 μm the size must around twice as large and for 5 μm five to six times as large.