PPV Fan Sizing and Clearing Time

A PPV fan's rated flow is measured in free air at the fan face, and what reaches the structure is less: the cone has to seal the doorway, some air spills, and the exhaust opening has to be sized to let the air out. An entrainment efficiency somewhere around 50% to 70% is the working assumption, and the effective flow is what the arithmetic should use. The clearing time is a dilution problem rather than a displacement one. Air entering a room mixes with what is already there instead of pushing it out as a plug, so the contaminant falls exponentially and each air change removes about 63% of what remains: getting to 90% clear takes 2.3 air changes and 99% takes 4.6. That is why a crew that gives the fan one air change and goes in finds the building still charged. An 18,000 cubic ft single story with a 12,000 cfm fan at 60% entrainment moves 7,200 effective cfm, so one air change is 2.5 minutes and clearing to 10% of the starting concentration takes 5.8 -- and getting from there to 99% clear costs another 5.8, as long again as the whole clear took to that point. Halve the exhaust opening so the effective flow drops to 3,600 cfm and the same fan takes 11.5 minutes: the opening, not the fan horsepower, doubled the time. A planning estimate; the incident commander, the department's own SOPs, and a gas meter reading govern when a structure is clear.

effective_cfm = fan_cfm x entrainment_efficiency; air_change_min = volume_cf / effective_cfm; clearing_min = air_change_min x ln(1 / remaining_fraction); required_cfm = volume_cf x ln(1/f) / (efficiency x target_time_min).

Positive-pressure ventilation clearing time as a well-mixed dilution problem -- the contaminant falls exponentially, so clearing to a remaining fraction f takes ln(1/f) air changes -- with the fan's rated free-air flow reduced by an entrainment efficiency, by name. Public physics. The incident commander, department SOPs, and a gas meter reading govern when a structure is clear.

The dilution relation is public physics. The structure volume, fan rating, entrainment efficiency, and target are the crew's own values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: volume_cf, fan_cfm, entrainment_efficiency, remaining_fraction, target_time_min, effective_cfm, air_change_min, clearing_min, required_cfm

• Dilution, not displacement well-mixed exponential decay; ln(1/f) air changes to reach a remaining fraction fpublic ventilation physics
• Entrainment efficiency roughly 0.5 to 0.7 of the fan's free-air rating actually reaches the structurefireground ventilation practice
• The opening governs the exhaust opening sets the effective flow; halving it doubles the clearing time with the same fanfireground ventilation practice