Selection factors, filter media

All filters require a filter medium to retain solids, whether the filter is for cake filtration or for filter-medium or depth filtration. Specification of a medium is based on retention of some minimum parficle size at good removal efficiency and on acceptable hfe of the medium in the environment of the filter. The selection of the type of filter medium is often the most important decision in success of the operation. For cake filtration, medium selection involves an optimization of the following factors ... 

The most commonly used filter medium is woven cloth, but a great variety of other media is also used. The main types are listed in Table 10.2. A comprehensive discussion of the factors to be considered when selecting filter media is given by Purchas (1971) and Mais (1971) see also Purchas and Sutherland (2001). Filter aids are often used to increase the rate of filtration of difficult slurries. They are either applied as a precoat... 

Proper selection of the filter medium is more of an art than a science. The filter cutoff must be chosen to capture the smallest particles of interest. Other factors that must be considered are the type of filter (bulk or surface), the required flow rate, and the size of the membrane. These parameters are not independent and the best choice will usually involve trade-offs. Finally, the material from which the filter is made must be considered. It must be selected for compatibility with the intended postfiltration processing. Glass-fiber filters, for example, often have very high blanks for common ions such as chloride and sodium. 

The choice of filter medium and fabric construction is important to baghouse performance. The fabric filters are constructed from both natural and synthetic materials, and the selection is usually made on the basis of the following factors [2] ... 

The filter medium is that critical component which determines whether or not a filter will perform adequately. Within the context of solid/Uquid separation the term filter medium can be defined as any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension, and is impermeable to the remaining components (Purchas and Sutherland, 2002). The principal role of a filter medium is to cause a clear separation of particulates (which may be solid particles, liquid droplets, colloidal material, or molecular or ionic species) from the liquid with the minimum consumption of energy. In order to achieve this, careful selection of the medium must take into account many factors criteria by which a medium is assessed include the permeability of the clean medium, its particle retention capability and the permeability of the used medium. Serious loss of permeability may follow plugging or blinding of pores in the filter medium, and can determine the lifetime of the medium if an uneconomic filtration rate results. Permeability and particle retention are dependent on the structure of the medium, but interaction of media structure with the shape and size distribution of the particles challenging the medium is also of crucial importance. 

A wide variety of filter media are available to suit most applications. Selection of a medium must take into account many factors and requires testwork to be undertaken to evaluate the suitability of a medium for any application. Bearing in mind the critical role played by the medium in a filtration process insofar as an incorrect selection may prevent correct functioning of the filter or lead to high running costs, or it may cause too frequent downtimes, or lead to a liquid product of low quality or to overly wet cakes selection of an appropriate medium becomes self-evident. An investment in cloth selection can prevent avoidable costs after installation of the filtration system. 

From the above, it is clear that the operator has many cloth selection factors to consider in order to obtain optimum performance. Often the only way is by trial and error, guided by broad general principles. Once the optimum cloth is found, the following benefits are obtained a clean filtrate with no loss of solids by bleeding, an economic filtration time (production rate), an easily discharged filter cake, no deterioration of the medium by sudden or gradual blinding and an adequate cloth life. 

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 compatibihty 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. 

Polystyrene capacitors have exceptionally low tan S values (< 10 q, making them well suited for frequency-selective circuits in telecommunications equipment. Polymer capacitors are widely used for power-factor correction in fluorescent lighting units, and in start/run circuitry for medium-type electric motors used in washing machines, tumble-dryers and copying machines for example. They are also used in filter circuits to suppress radio frequencies transmitted along main leads. Such interference noise may originate from mechanical switches, furnace controllers and switch mode power supplies it not only spoils radio and television reception but can also cause serious faults in data-processing and computer equipment. 

It is, therefore, necessary to relate basic filtration requirements to possible types of filter and then to study the specific performance of individual designs of filters, of suitable types, against system requirements. In the case of straightforward plications, this generally leaves only a suitable filter size to be selected. Where other or more critical factors are involved, close co-operation between potential user and equipment manufacturer may be necessary in order to arrive at an optimum solution, particularly as regards the choice of medium. 

Among the more important factors that must be taken into consideration when choosing a filter for a particular application are the size, shape, and hardness of the particles to be removed, the quantity of those particles, the nature and volume of the fluid to be filtered, the rate at which the fluid flows, whether the flow is steady, variable, and/or intermittent, the system pressme and whether that pressme is steady or variable, the available differential pressme, the compatibility of the medium with the fluid, the fluid temperatme, the properties of the fluid, the space available for particle collection, and the degree of filtration required. Let s examine how some of these factors affect filter selection. 

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