Single Ventilation Fan

Chemical building with cedar shake siding hidden roof crane lifting lugs and single exhaust fan on flammable room.

Gas detection sensor installed (cover removed) in duct chase of continuous running exhaust fan of low level ventilation system.

Low level continuous ventilation fan and on right thermostatically controlled upper level fan.

Outside exhaust fan at top of duct chase. Upper ventilation with motor operated louver controlled by a thermostat.

Lower & Upper Level Ventilation System Comparison


Air intake at lower level so heavier than air vapors are drawn up past where workers breathe and discharge at ceiling height. Pockets of fumes collect under fan. Cheap way to try to combine lower and upper level ventilation systems.


Air intake same as graphic “A” with air mixing with heavier than air vapors and exhausting at low level. Problem is vapors are directed downward and condense back into a liquid chemical state that infuses with plants and earth.


Air intake mixing with heavier than air vapors and exhausting at low level same as graphic “B.” Gasses are directed up outside duct chase and discharged at roof line. Even better, the fan could be above the roof to meet N.F.P.A.91 para. 2-4.5.1 as shown in graphic “D” on right. Dual ventilation systems have fan #1 running continuously with sensor turning on fan #2 and sounding malfunction alarm.


Same as graphic “C” plus the incoming air is forced down past elevated floor by an inside duct where it mixes with heavier than air vapors and is drawn out of the sump through a duct up to the exhaust opening in the wall. Hot air at ceiling is removed by a thermostat opening motorized dampers at vent openings located near ceiling. System works best when energizing fan #2 for the upper level ventilation system.


The previous Graphics “A” through “D” show ventilation systems attempting to remove heavier-than-air vapors or fumes from the total volume of air in the building. Explosive fumes can begin anywhere in the building as they come off a leaked or spilled chemical liquid. Gravity will eventually pull all heavier-than-air fumes down into the sump floor area. When the sump floor is sloped (U.S. Patent #6,305,131) downward toward a sloped trench (same patent), the liquid can be collected into a smaller exposed surface area reducing the amount of vapor coming off the liquid chemical. By placing a posi-ventilation tube (becomes part of the elevated floor supports) inches over the sloped trench, these fumes can be removed from the building. Graphic “E” (y-axis) shows the posi-vent tube (patent pending) with strategically placed holes on its bottom surface where air and fumes are picked up and directed out of the building wall, up a duct to a continuously running exhaust fan #1 on the roof. Graphic “F” (x-axis) shows a cross section view of the same floor of the building and click here to provide a close-up detail of posi-tube over the sloped trench. If fan #1 malfunctions, a flow sensor sounds an alarm and turns on fan #2. Both fans could be on an emergency generator or standby circuit. A posi-ventilation dual exhaust fan system eliminates costly non-fire rated explosion relief panels, fire rated drop down shutters and blast shafts (U.S. Patent #6,223,473B1).

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