Flow Paths Aren't Real?
Some really good friends who happen to be excellent firefighters told me that there wasn’t any such thing as flow path. Their position was that it was really convection and flow path was something someone made up to make a name for themselves. I initially said no, it’s real but it’s just a choice of words to describe what is happening. Because I couldn’t out right explain it in a better way I had to do some research:
Flow (verb) –
move along or out steadily and continuously in a current or stream: “from here the river flows north”
the action or fact of moving along in a steady, continuous stream:
a way or track that is built or is made by the action of people walking:
a line along which somebody or something moves; the space in front of somebody or something as they move:
a plan of action or a way of achieving something
The process in which heat moves through a gas or a liquid as the hotter part rises and the cooler, heavier part sinks
Prior to recent fire dynamics research the term flow path was mostly found when describing rivers or the movement of water. The flow path is not the gravity causing the liquid to move from a high point to a lower point but rather the description of where it goes. The same holds true for heat. Heat rises until it is blocked and then it moves horizontally until another barrier is encountered and then changes direction with the barrier. If it can’t go up, it goes across, if it can’t go across or up, it goes down. (Take that you science geeks!)
So by saying there is no such thing as flow path in the movement of heat and air within a structure fire would be the same as saying there is no flow path for a river. The logic there would be that it is gravity and not a flow path. So as you can see the debate is silly. The term flow path does not replace or dispute the scientific laws that create the situation it just describes the direction of the movement of air or water.
If you are fishing below the dam of a lake there are warning signs letting you know the times of water release because the water will rapidly rise and sweep or wash you away. If you are in the river when this occurs you will be caught in the rapid flow of water and be forced along the path of the river until you drown or escape the flow path. If you are between the fire and an opening heat is coming your way. How long it take depends on how far you are from the fire and the obstacles that the heat must negotiate up, across, down, across etc… but it is coming your way. The only thing that will stop it is that it cools off to the point it no longer has the energy to move or it’s blocked by a physical barrier. It will keep changing directions until it gets to a place in which it can travel upwards vertically unobstructed. If you are in this path the heat will be there directly and you could be burned unless you move out of this area.
This analogy is not quite exact because the river would typically have water moving away from the source only. An exception might be a hydraulic in a particular spot in the river (for you water rescue types). The movement of the heated air however, (convection) creates a draft (or draught if you are speaking the Queens English) which means to draw or pull something along. This would be the cool air entering feeding oxygen to the fire. All of it together is the complete “flow path” or you could look at it as two separate flow paths.
So my message here is don’t get so caught up on the terminology that you lose sight of the principles. If someone throws a baseball at you and it hits you then understand that you were in the travel path of the ball and if you had moved or held something up in front of you, you would not have been hit. And please never, ever walk between a urinal and someone taking a leak or you will have stepped right into the flow path and you are sure to get wet. The fact here is that my friends had a point and we need to do a better job at fully explaining some of this buzz word terminology in terms that we can all understand. Information travels so fast now days that the text books for much of this information won’t be out for years.