The CCDB technique is a special technology developed by FSI to combat filter media fouling. Filter media fouling is the main cause of filtrate flux decline and loss of product quality in filtration systems. This patented technology is especially valuable for handling challenging process fluids, such as fluids with high concentrations of suspended solids and colloidal solids. Successful applications of this technology include pre-treatment for Navy coastal seawater RO desalination, filtration of compressible gellular types of solids in the food processing industry, and oily wastewater recycling with high solids content.
The purpose of centrifugal separation is to provide a rough screening for mechanical protection of the filter media surface. It is not effective for the removal of suspended solids or any colloidal solids that have low specific gravity. The separation of suspended solids or any colloidal solids will rely on the other features of this design.
The feed flow in this design is driven within the filter in a downward spiraling direction relative to the filter surface. CFF is formed because the flow is parallel with the filter surface and at the same time it is being pressurized against the filter surface.
Flow velocity is of fundamental importance to the performance of a cross-flow filter. If the flow velocity becomes zero, the cross-flow stops and dead-end filtration (with its various drawbacks) begins. The cake layer that forms on the filter media at zero velocity becomes thinner when the flow velocity (parallel to the medium) increases. The thickness of the cake layer in a flow channel is determined by the shear force on the filter media surface, which is roughly in direct proportion to the feed viscosity and the feed flow velocity. Therefore, higher flow velocity results in a thinner deposit layer, a lower hydraulic resistance, and therefore a higher filtrate flux rate.
Most of the filter cartridges for micro-filtration on the market are depth cartridge filters. These filters function through a dead-end pressure feed, and therefore are prone to plugging due to cake formation. The figure to the left illustrates both "dead-end" filtration and CFF.
Further problems arise when "dead-end" filters are allowed to dry. In such a case, particles are usually cemented inside the flow passages, which greatly reduces the flux rate during subsequent operation. These filters are only useful in light-duty applications.
A technique which further enhances Cross-Flow Filtration is the creation of vortices generated by Dean Flow on the filter media surface. The high shear forces created by the circular motion of the vortices constantly clean the filter media surface. This results in a higher flux rate and prolongs the service life or the permissable interval between backwashes. (if backwashes are required in the system)
This Dean Flow phenomenon was first discovered by W. R. Dean while studying the secondary flow created by the motion of fluid in a curved pipe. Under normal conditions, when flow passes through a straight pipe at a critical velocity (the transition between laminar and turbulent flow), a sudden increase in the loss of head pressure is experienced. Dr. Dean's study shows that flow in a significantly curved channel appears unstable and secondary flow is present. However, there is no evidence of the sudden increase in the loss of head, even if the flow rate in the curved pipe is much higher than the critical rate. This phenomenon suggests that the pressure drop is much smaller in a curved pipe than in a straight pipe at the same flow rate. This flow in a curved channel has been characterized as double-vortex flow.
The animation shown above simulates the secondary flow traced by small particles, which occurs when liquid flow passes through a curved passage. The direction of the secondary flow is perpendicular to the main direction of flow. The secondary flow (shown as double-vortex flow) is also known as Dean Flow. Some of FSI's filter designs take advantage of the vortices generated by Dean Flow to keep the filter membrane surface clean.
Shown to the right is a cartridge tested in tap water with 0.1% test dust. The lower 2/3 of the cartridge was located in the filter housing with spiral guides to promote Dean Flow. The upper 1/3 of the cartridge was located in a part of the housing which had no spiral guides. The lower part has no visible cake formation due to the vortices generated by the Dean Flow, which kept the surface clean. This test result clearly reveals the advantage of Dean Flow.
A chart comparing filter performance data with and without the Dean Flow effect from another test in shown at left. In this test, a 3 micron Nylon 66 membrane was used as the filter media and the feed was tap water with 0.2% ISO fine test dust.
Frequent backwashing with backpressure pulses to loosen and penetrate the cake layer formed by contaminants is a good method to maintain a high level of flux. This can be achieved by different methods, using a diaphragm pump, a piston pump, or hydraulic cylinders, combined with various fluid flow control components. Regular backwashing minimizes the maintenance necessary for the filtration unit in tough applications.