When should negative pressure ventilation be used?
Ventry® Fans are not exhaust fans. Exhaust fans or smoke ejection fans use negative pressure to remove smoke from a structure during firefighting efforts. Ventry Fans are designed for positive pressure ventilation (PPV) and attack (PPA). Positive pressure is simply more effective than negative pressure for aggressive, coordinated attack. This can be easily observed: Stand in front of a fan and you will notice lots of air flow; stand on the other side and there is very little air flow.
Using a smoke ejector to ventilate a structure is like fighting a fire using buckets of water. It was done in the past, but there are much more effective methods now.
The same observation is apparent at a structure fire by observing how fast positive pressure pushes smoke and heat out versus how long it takes using negative pressure (sucking air through the fan and ejecting it outside).
On attack: Unlike negative pressure and vertical ventilation, PPV can be accomplished without entering the structure, requires fewer people, and takes much less time to implement. PPV does not damage your fan by sucking hot and flammable gases through the engine, as can be the case with negative pressure ventilation.
“I’ve never seen a firefighter fall through a flowerbed.” ~ Chief Kriss Garcia, 8/23/16 during PPA webcast.
Vertical ventilation requires a team on the roof of a burning building; that takes lots of time and manpower plus the roof of a burning building is an extremely dangerous location.
Although negative pressure ventilation can be accomplished from outside a building, in practice it is usually set up from inside, which again puts the ventilation crew at risk.
One instance where we do advocate using a Ventry PPV Fan for negative ventilation: Negative pressure ventilation can be a useful technique during cleanup and overhaul, once the smoke is gone, to replace fouled air inside the structure with 100% clean air. Set the fan inside the room, aiming out the exit door. As long as you are not pulling smoke through the fan—which would damage the fan engine—it can effectively move bad air out and pull fresh air in. This technique can negate the need to buy an electric fan.
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