CONFINED SPACE VENTILATION - TRACER GAS-ANALYSIS OF MIXING CHARACTERISTICS

Dilution ventilation is often used to control potential airborne health hazards, fire, or explosive conditions by mixing contaminated air with uncontaminated (e.g., outdoor) air. Dilution ventilation of confined spaces is used to displace oxygen-deficient, explosive, or toxic atmospheres prior to worker entry. Application of dilution ventilation requires the selection of an empirical mixing factor This study sought an objective means of measuring mixing effectiveness for application to selected petrochemical industry vessels. The tracer gas sulfur hexafluoride (SF6) was metered into the air entering the vessel of interest, and the SF6 concentration was monitored at the vessel exhaust with an infrared analyzer modified to sample at 90 L/min for rapid response. The rate of SF6 concentration rise and decay was recorded using a datalogger The mixing factor was calculated on the basis of measured ventilation rates, vessel dimensions, and SF6 rise and decay curves. A ir in the vessels analyzed did not exhibit rapid mixing; mixing factors calculated from experimental observations were less consistent than hypothesized-an indication of the heterogeneous nature of actual ventilation patterns. Qualitative ventilation analysis (smoke tubes, anemometry) was employed to assess air circulation within the vessels and confirmed shunting, large eddy formation, and an apparent effect of ambient (outside) wind on internal airflow patterns. It is concluded that tracer gas studies provide useful information on air mixing within vessels, which can assist the, industrial hygienist in optimizing dilution ventilation for specific confined spaces.