Fire Service Irony – Creating Wind Driven Fires
After spending a few days last week at UL for the final PPV / PPA experiments I couldn’t resist pulling out a retro Hump Day from last year. Now let me put a disclaimer here, the results of all the experiments are not fully developed and analyzed and I am not representing any organization or officially commenting on behalf of anyone. This is The Hump Day for goodness sake. A slightly satirical look at ourselves. O.K. maybe not always slightly. These are just my personal observations, comments and opinion.
One of the discoveries during the experiments is that with a PPV fan deployed at the front door a fire will build enough energy and pressure to move towards and overcome the pressure created by the fan. (Not a breakthrough discovery, most of us have seen this happen on the scene) Yes let me say it again. The positive pressure created by the fan is no match for the pressure created by the intensified fire growth. Not even with two fans. So one must be extremely careful when using fans in an attempt to control the fire and not be in the mindset that the positive pressure is strong enough to control the direction of the fire growth. It’s not all (the fire, heat and smoke) going out the vent and it’s all about how much time you have before the fire comes back to the fan. Why does it come back to the fan you might wonder? Well because that’s where the oxygen is and in these modern fuel load ventilation limited conditions the too rich to burn fuel seeks oxygen. We purposely tried to spread the fire from one end of the house to the opposite end by venting the opposite end. The result was that the fire only spread back to the fan at the front door.
So for now just remember all venting is about timing and coordination. Fire is coming to where ever you open the structure. Your objective is to get WATER on the fire or close the door between the fire and opening before the time is up whatever vent strategy you deploy in these vent limited modern fuel load fires. Big oxygen big fire…..big fire big water! Please don’t create a wind driven fires on purpose! Remember if you’re using a fan and the fire is still burning make sure it has someone with a radio stuck by their ear and with a hand on the kill switch or you might just cut your crew off if they have gone in the opposite direction of the fire. Hey fan man STAY LOW its headed your way fast!