Book - School of Science and Technology

488 Fans and air treatment equipmentRelative efficienciesFigure 17.11 indicates trends of efficiencies which may be expected from the first fourtypes of particulate filter referred to above.It will be seen that filter 1, depending on impact, has no arrestance on small particlesizes. Filter 2 has a dust spot test efficiency of about 50% for particles of about 1 micron,diminishing rapidly for the smaller particles. The electrostatic filter, curve 3, has a dustspot test efficiency of about 90% while the paper filter 4 achieves virtually 100% forparticles of 0:1 mm and above.All filters vary in efficiency according to the velocity of air through them. Velocities andresistances of various filters are given in Table 17.3.In the case of fabric and paper filters, owing to the large surface areas involved, designsare usually based on forming the material into a zigzag formation.Electron m’scopeSmokesMicroscopeFogMistNaked eyeDrizzle100Collectionefficiency %4 3 2 1908070Figure 17.11 Efficiencies of air filters: (1) viscous coated metal; (2) fabric; (3) electrostatic; (4) paperabsolute6050Table 17.3 Typical characteristics of particulate filter mediaFilter typeFacevelocity(m/s)CleanResistance (Pa)DirtyApproximateefficiency/arrestance(%)Viscous impingementcleanable panel 1.5±2.5 20±60 100±160 65±80 arrestanceautomatic curtain 2±2.5 30±60 100±190 80 arrestanceDry fabric or fibrouscleanable panel 1.5±2 30±50 75±100 70±80 arrestancedisposable panel 1.5±3.5 45±90 150±250 70±90 arrestancelow efficiency bag 1.5±3.5 25±50 200±250 30±50 dust spotmedium efficiency bag 1.5±3.5 55±140 200±350 50±90 dust spothigh efficiency bag 1.5±2.5 55±140 200±350 Up to 95 dust spotAutomatic roll 2.5±3.5 30±80 160±200 30±45 dust spotElectrostatic, plus bag filter 1.5±2.5 120±200 250±400 Up to 95 dust spotAbsolutelow efficiency up to 1.5 up to 150 up to 400 95 sodium flamemedium efficiency up to 1.5 up to 280 up to 625 99.7 sodium flamehigh efficiency up to 1.5 up to 280 up to 625 99.997 sodium flame