Cartridge Filter

Cartridge filters are available in various lengths and diameters as well as construction materials (woven, nonwoven, and membranes). The flow is outside-to-inside. Therefore, these filters require a strong core to be able to handle the increased pressure differential during operation. Some manufacturers have outer cages for increased stability and capacities. The cartridges are installed in pressure vessels with guide rods for easy installation. The orientation can be vertical or horizontal. The cartridge to vessel seal can be single o-rings, double-o-rings, tie rods, or other designs.

Cartridges are available in cleanable, back-washable metal media or in disposable form. A vast variety of materials, textures, pore sizes, and physical sizes are available. Disposable cartridges can be woven, nonwoven, or pleated and constructed of many types of fibers. The fibers can also be charged to aid in filtration.

High capacity cartridge filters, designed to have increased filter area based upon pleated configuration, flow channels, and flow chambers, are also available. The benefits are increased filter area in small housings when there are space constraints and other process requirements where there are increased solids in the fluid.

Filter cartridges are available in a wide variety of materials, and they provide a range of performance options. Cartridges are available with manufacturers’ particle size ratings from 0.25 μm to any larger particle size. When selecting a filter cartridge, the designer must determine what the manufacturer’s rating actually means in terms of removal percentage.

Filter cartridge solids removal performances and allowable flow rates vary greatly from manufacturer to manufacturer, even if the cartridges are made of the same material. Therefore, it is difficult to develop generalized relationships between the water flow rate and filter area. As a result, it is necessary to rely on manufacturers’ information when selecting and sizing a cartridge filter system.

In designing a water treatment system that includes cartridge filters, it may be desirable to select a fixed-pore filter medium and absolute rated filters. The fixed-pore cartridges provide more consistent particle removal efficiencies from one cartridge to the next than do nonfixed-pore cartridges. The fixed-pore type also prevents solids unloading and media migration during periods of high differential pressure. Fixed-pore filters are usually given absolute ratings by their manufacturers.

Nonfixed-pore cartridges may be used, but the differential pressure across the filters must be monitored closely. High differential pressures may cause solids unloading and media migration. If either occurs, the pressure drop through the filter will decrease and may be below the limit when the cartridge is scheduled to be changed. Therefore, the operator checking the pressure drop will believe that the cartridges are functioning correctly, even though large amounts of solids may have been released to the downstream water.

Solids unloading may be avoided by using a high differential pressure switch to continuously monitor the pressure drop or by changing the cartridges when the pressure drop is still small compared to the maximum pressure drop recommended by the manufacturer. The resulting frequent changing of the cartridges may result in excessive operating costs if the early change-out method is used.

Typically, cartridge filters have low solids-loading limits, so the cartridges can absorb only a relatively small amount of solids before they must be changed. Manufacturers have developed special cartridges to improve solids loading. Pleated construction of a thin filter medium such as paper or cotton fabric greatly increases the effective filter surface area of the cartridge. The increased surface area provides for higher flow rates and solids-loading capacities than a cylindrical cartridge of the same medium.

Some cartridges use a multi-layered design of media such as fiberglass, which provides in-depth filtration. The layers of media have progressively smaller pores as the water moves from the outside to the inside of the cartridge. As the pore size changes, particles are trapped at different depths within the filter, allowing higher solids loadings but typically decreasing flow rates slightly.

Since cartridge filters have low solids-loadings capacities, it is common to install primary solids removal equipment upstream of the cartridge filters. Typical systems include either a hydrocyclone or a sand filter followed by the cartridge filter. The upstream equipment removes the larger solids and reduces the amount of solids that the cartridges must remove, therefore extending the time between cartridge changes.

A spare filter vessel may be provided so that cartridges may be changed without reducing water flow rates. Any number of vessels can be used to provide the required number of cartridges, but the most common system arrangements include three 50% vessels or four 33% vessels. The number of filter vessels selected depends on a cost analysis and on operating preference.

Other factors to consider in the selection of cartridge filters are the type of filter medium and its characteristics. As an example, polypropylene cartridges are a better selection than cotton for water service, since cotton swells. The compatibility of filter membranes and binders with chemical additives or impurities in the water should be checked. The designer should contact specific manufacturers for detailed information.

When specifying a cartridge filter unit the following information should be included:

maximum water flow rate,

particle size to be removed by filtration and the percentage of removal required,

solids concentration in the inlet water,

design working pressure of the filter vessel, and

maximum pressure drop available for filtration.