Filter Sizing Principles

Hydraulic filter sizing involves matching the filter to the system flow rate, required cleanliness level, and expected dirt load. Key parameters include flow capacity, beta ratio (filtration efficiency), and dirt holding capacity.

Beta Ratio (ISO 16889)

The beta ratio indicates filter efficiency at a specific particle size:

Beta(x) = Particles upstream (>x um) / Particles downstream (>x um)

• Beta 75: 98.7% efficiency - General industrial
• Beta 200: 99.5% efficiency - Proportional valves
• Beta 1000: 99.9% efficiency - Servo valves
• Beta 2000: 99.95% efficiency - High precision

ISO 4406 Cleanliness Code

The ISO code indicates particle counts per 100ml at three sizes:

XX/YY/ZZ = particles >4um / >6um / >14um

• 21/19/16: New oil, no filtration
• 18/16/13: General hydraulics
• 16/14/11: Proportional valves
• 14/12/9: Servo valves

Filter Location

• Pressure line: Protects specific component, highest dP capacity
• Return line: Catches all system contamination, common choice
• Suction line: Protects pump, must be low dP
• Kidney loop: Offline filtration for continuous cleaning

Sizing Formula

Filter housing rated flow should exceed system flow:

Filter Flow Rating >= System Flow x Safety Factor (1.5-2.0)