Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2000-05-19
2002-10-08
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S323200, C210S444000, C210S443000
Reexamination Certificate
active
06461513
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to the field of filtration systems. More specifically, the present invention is directed to a filtration system combining cross-flow currents and secondary flow currents such as Dean-Flow currents to assist in surface cleaning of the filter unit.
BACKGROUND OF THE INVENTION
The art has seen various filtration devices employing different methods for removing particulate or impurities from a feed fluid. For example, so-called dead end filtration systems force all of the feed fluid through a filter to separate impurities therefrom. Dead end filters designs may place a filter either directly across a flowpath or at an oblique angle to the flowpath. U.S. Pat. No. 1,822,006 to Bull discloses a dead-end filter wherein the feed fluid enters a cylindrical chamber housing a cylindrical filter and follows a spiral flowpath from one end of the cylindrical filter to the other. All of the feed fluid eventually forced through the cylindrical filter as the fluid flowpath terminates adjacent the far end of the filter from the input end. A sump chamber is provided below the helical fluid flowpath and filter for collecting material separated by the filter component. As all of the fluid must pass through the filter and as the sump chamber is not open throughout operation, the velocity of the fluid about the filter will continuously decrease to zero unless the filter is cleared. U.S. Pat. No. 3,637,078 to Hollar discloses an oil filter having a spiral guide positioned about a pleated filter. The Hollar filter is another example of a dead-end filter design as all of the oil entering the filter must pass through the filter cartridge. Particulate and other contaminants collect on the filter surface within the expanses spanning between the adjacent pleats and the spiral guide.
Other filter designs employ a spiral fluid flowpath to separate heavier particulate from the fluid medium. These centrifugal particulate separation devices employ a spiral path to generate centrifugal forces which force the heavier particulate to the outside of the spiral flowpath. For example, U.S. Pat. No. 3,402,529 to Franz provides a spiral flowpath down along a cylindrical non-porous sleeve having a significantly wider mouth portion at one end. The fluid medium air flows within the spiral path to the inside of the flow of the heavier contaminants. The Franz filter collects the separated particles at the bottom of the unit opposite the open mouth portion which then acts as an intake for the air. The Franz design is impractical for applications where the fluid medium is a liquid such as a fuel, oil, or water, however, as many types of colloidal particulate are known which have a lower specific gravity than the fluid medium. Such lighter particulate will tend to be forced to the inside of the spiral path by the heavier fluid medium. These lighter particulate may collect in the filter itself and require the filter unit to be shut down and the filter replaced or cleaned.
Cross-flow filtration is yet another alternative method for filtering particulate from a fluid medium. Cross-flow filtration differs from dead-end filtration in that the feed fluid provided to the filter unit actually passes across the enclosed filter membrane or filter media. Cross-flow filtration describes the condition of fluid flow past a membrane while the fluid is being pressurized against the surface. Cross-flow filtration performance has been found to be governed the pore size of the filter media, the generated fluid shear force across the surface of the filter media, and the deposit layer and the control of the deposit layer formation. Only a portion of the feed fluid passes through the filter to become filtrate, or permeate, fluid. The other portion of the feed fluid continues past the filter media and exits the filter unit as concentrate, or retentate, fluid. Flow velocity is of fundamental importance to the performance of a cross-flow filter. Should the flow velocity across the surface of a filter media become zero, the cross-flow ceases and the dead-end filtration begins. Additionally, the cake which forms on the filter media at zero velocity becomes thicker as the flow velocity, parallel to the medium, decreases. The thickness of the cake layer in a flow channel is determined by the shear force on the membrane surface which is roughly in direct proportion to the feed viscosity and the feed flow velocity. Therefore, higher fluid velocity entails a thinner deposit layer, a lower hydraulic resistance, and a higher filtrate flux.
In almost every filtration process a ‘secondary membrane’, also called a ‘dynamic membrane’ will be created. The contaminants which constitute the secondary membrane first fill up the pores and then form a very thin cake of constant thickness. The transition time for pore filling may be very short. Particulate has been observed on first use of a filter to immediately enter into the pores of the filter media, although only to a limited extent. The result is that a cross-flow filtration system typically experiences a rapid flux drop at the beginning of its use for filtration. Thereafter, the flux is stabilized at a relatively satisfactory level, and remains almost constant with a very slow decline as the process continues. This is unlike dead-end filtration where the flux rate drops continuously from the time the filter is operated until complete clogging. The rate of flux drop depends on the selection of membrane pore size and the nature of the contaminants. Filter pore-size must therefore be selected with a view towards the expected contaminants in order to control the formation of the deposit layer.
It is further known that as fluid flows through a curved channel about a normal or longitudinal axis that a secondary flow, which is the flow perpendicular to the main direction of flow, occurs. The secondary flow phenomenon is caused by centrifugal force which forces the fast moving fluid in the channel core toward the outside wall from where it journeys back along the floor and roof of the channel to the inside wall. When the fluid is forced through the channel at a critical velocity, a double-vortex flow known as Dean-Flow currents is formed.
The phenomenon of Dean-Flow was first observed by W. R. Dean who studied the secondary flow created by the motion of fluid in a curved pipe. Flow in a curved channel appears unstable for small disturbances, compared with a sudden increase in the loss of head when flow passes through a straight pipe at a critical velocity, i.e., the transition from laminar to turbulent flow. No such sudden increase in the loss of head is generally observed in a pipe of significant curvature, even though flow rate is much higher than the critical flow 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. The flow in a curved channel has been characterized as a double vortex flow, as shown in FIG.
3
. The Dean number, K, is the characteristic parameter used to describe the formation of vortices in this situation:
K
=(
v·d
/&ugr;)·(
d/R
)
0.5
Where v is the tangential velocity of the fluid, d is the diameter of the pipe, R is the radius of the pipe curvature, and &ugr; is the kinematic viscosity of the fluid. The higher the Dean number, the stronger the vortices induced.
Early studies on this secondary flow phenomenon were mainly focused on the heat transfer in a coiled heat exchanger. These studies showed that the heat transfer coefficient was much higher for a curved pipe than for a straight pipe. In recent years, studies on the double-vortex secondary flow show that the secondary flow may be employed to greatly reduce the filtered material concentration polarization in filters. As the fluid spins in a curved channel, a control mass of fluid travels radially, eventually reaching the outer wall where it must change direction towards a return path. The resulting flow profile takes the form of a toroidal vortex in which a fluid particle moves in three dimensions. The
Filtration Solutions, Inc.
Fortuna Ana
Williamson III Thomas R.
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