## Lesson 28. Pressure Drop

The resistance to fluid flow imposed by a filter in the process of separating the components of a fluid stream. Pressure drop relates to the energy required to push a fluid through the filter. 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. The pressure differential is greatly influenced by the resistance to flow of the filter or medium. The pressure differential is the difference in pounds per square inch (PSI or kPa) between the inlet and outlet ports. Pressure differential may be referred to as PSID, ΔP, pressure drop, or differential pressure.

Terminal pressure: The maximum pressure drop the filter is designed to handle. Filter designers seek to reduce pressure drop when designing filter systems.

7.6 Pressure Drop

7.6.1 System Pressure Drop

The actual system pressure drop (difference in pressure between the inlet and outlet) is due to loss of PSI, resulting from loss of flow through the cartridge and loss of flow through the housing and any other component in the system. All losses contribute to total ΔP. It is important to note that the cartridge ΔP increases throughout the filtration process as the cartridge collects dirt and the flow becomes restricted.

7.6.2 Cartridge Pressure Drop

Water or fluid flows through channels created by pores in the filter medium called laminar flow. The water moves in orderly layers rather than in a turbulent manner. During the laminar flow, pressure loss resulting from flow through the cartridge is dependent upon:

a. Micron rating

b. Viscosity

c. Flow rate

The following equation can be used to calculate the change in pressure drop:

ΔP = AuQ                             (7.10)

where;

ΔP = Pressure Drop

A = Cartridge (laminar) flow constant

U = Viscosity

Q = Flow Rate

7.6.3 Housing Pressure Drop

All flow in housing must pass through the same inlet and outlet port restrictions, which is only a few square inches in area. The cartridge has several square feet of area that the flow can be divided upon.

Therefore the flow rate per unit area through the filter housing ports is typically higher than the cartridge media. As the flow rate increases, the port size should increase to keep the pressure drop increasing. Housing pressure drop is affected by four main variables:

i) Flow rate

ii) Fluid density, expressed as specific gravity

iii) Inlet and outlet port sizes

iv) Number of seat cups (seat plate) in the separator plate

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