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Many processes require a plastic melt free of contaminates larger than a specified size. For example, the fiber industry usually filters down to 5 micron particle size to protect the melt spinning machines from filament breaks. The fiber process typically operates at 2000 to 4000 meters per minute. A filament break at these velocities is costly to both the product quality and the process efficiency.

The media used for filtration has included sand packs, wire screens, sintered metal powder sheets, and sintered metal fiber sheets. Recent innovations in media include some sandwich combination of wire screens and sintered fiber sheets which in theory provide the best properties of each component.

One gear pump manufacturer (1) states, "Because running clearances can be as low as .00025 about the periphery and on either side of the metering gears, it is understandable that the slightest burr, nick, or particle of foreign matter will cause scoring and possible seizure (of the pump)". If one remembers that .001" equals 25 microns, it is obvious that filtration down to 6 microns is required to meet this pump manufacturers stated requirement.

Wire screens have dominated the extrusion filtration market with square woven screen the most important media. A 500 mesh square woven screen, fine by most standards, will usually have an opening of approximately .001 inch on a diagonal opening of requirement for particle retention, a twilled Dutch weave of the appropriate construction may have to be considered if wire screen is preferred.

Fine mesh screens are, of necessity, manufactured from fine wire. They are therefore weak in tension. A satisfactory fine mesh screen must be supported by a series of sequentially more coarse mesh screens in order to reduce the unsupported length without wire strength failure. In addition, the pressure drop across the total screen pack must be monitored carefully to prevent a pressure differential which will rupture the fine mesh screen. Carley and Smith (2) have given us a valuable tool to compute the screen pack pressure drop as a function of percent of open area.

Sintered metal powder and fiber filter sheets are considered an "in-depth porous media" by Marcus (3). He states that "gel captured and holding capacity of screen packs is primarily a function of surface area. Sintered powder or fiber media are better for gel capture since they can have, properly designed, a significantly higher area in contact with the polymer due to the thickness dimension.

Porous media usually has a strength to pressure drop ratio which is considerably higher than that of most equivalent screen packs. The advantage of small pore size coupled with the greater thickness leads to potentially a higher allowable pressure drop than does a screen pack on an equal area basis. Edie (4) offers a method of computing pressure drop through a porous media.

It is obvious that the subject of filtration is too complex to cover in one short article. Future issues of the Newsletter will cover various screen changer designs, continuous filtration and screen changers, and back flush units.

We welcome your comments on filter media as outlined in this article. Please write to the author c/o Robert Gregory Associates, Inc., Route 16A, Box 130, Intervale, NH 03845.

1. Zenith. "Care and Maintenance of Zenith High Temperature Metering Pumps", Bulletin 7W14609, 1.
2. Carley, J., Smith, W., "Design and Operation of Screen Packs" Polymer Engineering and Science 18,408 (1975).
3. Marcus, J. "Gel Removal Through Porous Media" SPE 323rd Annual Technical Conference Papers XXI 599, 81975.
4. Edie, D. "Calculation of Filter Pressure Drop and Shear" Fiber Producers Conference 1 23, (1983).

- Robert B. Gregory

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