Basics of Filtration

Pressure Drop

There must be a difference in pressure between the inlet and outlet sides of a filter in order to push a liquid through the filter. This pressure differential is largely influenced by the resistance to flow of the filter or medium. The pressure differential is the difference in pounds per square inch (PSI) of KPa between the inlet and outlet ports. Pressure differential may be referred to as PSID,

P, pressure drop, or differential pressure.

System Pressure Drop

The actual system pressure drop (difference in pressure between the inlet and the outlet) is due to loss of PSI, resulting from loss of flow through the cartridge and loss of flow through the housing. Both losses contribute to total P.

NOTE : Cartridge P increases throughout the filtration process as the cartridge collects dirt and becomes more resistant to flow. Housing P remains constant (assuming constant flow rate and fluid density).
Total System Pressure Drop
P = P Cartridge + P Housing

System Pressure Drop

Fluid flows through channels created by pores in the filter medium. This is called laminar flow, moving in orderly layers, rather than in a turbulent manner. During laminar flow, pressure loss resulting from flow through the cartridge is dependent upon.

Micron Rating
Viscosity (centipoise-cPs, centistokes-cSt, second saybolt Universal-SSU)
Flow Rate (gallons per minute-gpm)

Change in pressure drop can be calculated with the following equation: P = AuQ

Where:
P = Pressure Drop
A = Cartridge (laminar) flow constant
U = Viscosity (cps)
Q = Flow rate (gpm)

Housing Pressure Drop

All flow in housing must pass through the same inlet and outlet port restrictions, which represent only a few square inches in area. Flow through the cartridge filters may be divided among several square feet of area.

Thus, the flow rate per unit area through filter housing ports is typically higher than the flow rate per unit area through cartridge media. This high flow rate produces turbulent flow in the housing as fluid disperses through the inlet port or seat cups and into the less restrictive housing cavities. Housing pressure drop increases as flow rate and/or fluid density increase but decrease as port size and the number of seat cups increase (seat cups/plates hold column of cartridges).
Housing pressure drop is affected by four main variables: