Secondary filters
Illustrated Guide to Ventilation (BG 2/2009), compiled by Kevin Pennycook and published by BSRIA in 2009, states: ‘Secondary filters (classes F5 to F9) have an average efficiency ranging from 40 – 95%. Average efficiency is defined at the weighted average of the efficiencies for the different specified dust loading levels up to final pressure drop. The air velocity through the filter media is typically 0·02 – 0·1 m/s. Secondary filters are effective in trapping pollen, bacteria and dust. Secondary filters can be of the extended media bag type, pocket type or a rigid bag.’
--BSRIA
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