Book - School of Science and Technology

486 Fans and air treatment equipmentTests for filtersThe filter efficiency is a measure of its ability to remove dust from the air, expressed interms of the contaminant concentrations upstream and downstream of the filter, thus: ˆ 100 C 1 C 2C 1where ˆ filter efficiency (%)C 1 ˆ upstream concentrationC 2 ˆ downstream concentrationWeight or gravimetric testWith this method, a carefully metered quantity of air containing a known quantity ofsynthetic dust is drawn through a filter paper from the unfiltered intake, and a similarquantity of air is drawn through another filter paper downstream from the filter. Theseare weighted on an accurate balance and a comparison of the two weights gives thegravimetric efficiency. The heavier particles, as explained, are the most easily collectedand these constitute the greater part of the weight, hence even a poor filter will give a highefficiency of perhaps 90% by the gravimetric method. In consequence, the weight test haseffectively been superseded.Dust spot test*Using this method, sample quantities of air are drawn, as before, from upstream anddownstream of the filter under test through filter papers and the resultant stains areviewed optically. The light penetration is measured by a photo-sensitive cell and acomparison of the relative intensities then provides dust-spot test efficiency. This, knownpreviously as the blackness test, is a much more stringent criterion and many filters whichmay provide a 90% result by the gravimetric method are able to produce no more than50% following a dust-spot test.Arrestance testsThese establish the ability of the air cleaning medium to remove injected dusts from an airstream and may be carried out as a part of the dust spot test procedures. Arrestance isexpressed as a percentage, calculated as for efficiency, but using mass in place of concentrationvalues. Both the dust spot and arrestance tests may be used for on-site testing(see BS 6540: Part 1: 1985) and may use either atmospheric dust, i.e. that present in the airat the site of the test, or a synthetic dust consisting of 72% by weight fine dust, 23% byweight coal dust and 5% cotton fibres. Methylene blue particle sizes closely resemble thedistribution found in atmospheric pollution and the test going under this name is similarin principle to the old blackness test and, like it, has been superseded.Sodium flame testThis is used for testing high efficiency filters, i.e. with a penetration of less than 2%.The method is described in BS 3928: 1969 and involves generation of an aerosol ofsodium chloride of particle size between 0.02 and 2 mm. Samples of the air upstream* BS 6540: Part 1: 1985.