Book - School of Science and Technology

Air filtration 487and downstream of the filter are passed through a flame photometer to determine theconcentrations captured. Where high filter efficiencies are vital to the installation thequality of the seal between the filter and supporting frame is as important as the efficiencyof the filter media. On-site testing of such high efficiency particulate air filter (HEPA)installations is in consequence necessary before the plant is put into service and atfrequent intervals during use. Tests used for on-site testing include the Di-octyl-phthalate(DOP) test and the sodium flame test.Practical filtersIn selecting a filter it is necessary to know by what method the maker's guarantee of testefficiency has been determined, and what type of dust was used, since it is unlikely that adetermination was made under the particular conditions of atmospheric pollution obtainingat the site in question.The dust holding capacity of a filter, that is the mass of dust a filter can retain betweenits `clean' and `dirty' condition, is also an important feature to consider when selecting afilter because this will affect the frequency of maintenance or replacement.Air filters fall into five main categories as follows:. Viscous impingement. Usually of some form of corrugated metal plates or metal coils orturnings or the like, in each case covered with a viscous oily liquid to arrest the particleson impingement. Other materials used instead of metal are glass fibres, similarly coatedwith a sticky fluid.. Fabric. The material used in this type of filter is some form of textile, normally glass orsynthetic fibres in a random matting bonded together. The dirt particles are in this casearrested partly by being trapped in the interstices of the material, and partly by beingcaught on the fibres.. Electrostatic. In this system of filtration, dust particles entering the filter are subjectedto an electrostatic ionising charge and, on subsequently passing through parallel plateswhich are alternately charged and earthed, the particles are repelled by the chargeplates and adhere to the earthed plates. The electrical charge is supplied from a powerpackcontainingthe necessary transformers and rectifiers to produce the high voltageDC required.. Paper or absolute. This filter uses a special form of paper made usually from wovenglass fibre.. Adsorption. Makes use of activated carbon (charcoal), activated alumina or otherchemicals to adsorb odours, gases and the like.Air washers will also act as air cleaning devices, but are unlikely to be suitable for use inmodern systems.Materials used should be inherently non-flammable or so treated that they retain suchqualities through their life. Viscous filter liquids should have a flash-point of not less than177 C. All such material should generate a minimum quantity of smoke and toxic gas.** Test Methods for Ignitability, Smoke and Toxicity of Air Filters. London Scientific Services, 1990.