Odour, covering and ventilation

A technique is described based on the decay in concentration of added SF₆ to measure L₀, the rate of leakage from an enclosure with no extraction of air. It is believed this measurement is much more precise than measurements of E₀, the minimum rate of extraction which just prevents leakage. Three out of four enclosures studied had L₀ values equating to residence times of air that were well under one hour. Relationships were developed between extraction rate and concentration and emission rate for enclosed odour sources based on mass transfer from water to air. These could be used to assess the benefits of minimising extraction rates while remaining within concentration limits set on the grounds of corrosion or toxicity. From these relationships a critical flow can be identified, termed Q₅₀, at which both the emission rate and concentration of a particular species are at 50% of their maximum value. In any particular system, Q50 for one species, such as H₂S, will in general not be the same as for another species, nor for odour concentration. As a consequence the benefit of reducing extraction rates based on H₂S may not appear as good as it would based on an assessment of odour concentration. A second consequence is that as the rate of air extraction is varied, the ratio between two species or between H₂S and odour concentration, is likely to vary.