Combustion Processes (con’t from Unit 4) Objectives Discuss the B-SAHF model and what each letter stands for. Explain B-SAHF model for each phase of fire development and the use of the model for each phase. Discuss if the students think this is a useful tool for teaching fire dynamics to fire service professionals. What is B-SAHF? Acronym used to help size up critical fire behavior indicators. Consists of the following: Building Factors Smoke Air Track Heat Flame Let’s discuss each critical factor! Building Can be pre-planned to determine fire loads Inventory fire behavior indicators present before a fire begins Construction and features Will help determine fire behavior and spread Smoke, the second critical factor Smoke Along with air track the two most important indicators of fire behavior. Provides valuable cues as to the location of fire. Its burning regime (ventilation or fuel controlled). Stage of burning throughout the structure. Air Track and its impact Observe air movement from outside and continue while on the interior. Watch how smoke changes with fluctuations in the air track. Make air tracks work for you not against you! Heat While heat cannot be observed directly, observe its effect on air track (i.e. velocity of smoke discharge), the building or exposures, and sensation of changes in temperature. Can be a significant fire behavior indicator It is important to remember that our personal protective equipment provides significant insulation and slows the transfer of heat and resulting sensation of changes in temperature. Flame, the visible part! The most visible part of the B-SAHF model. Do not get so focused on visible flames that you miss important, but subtle building, smoke, air track, and heat indicators! Flame indicators such as location, volume, color, etc. provide help more complete picture but must be integrated with other factors to see the big picture! Other parts of the puzzle Ventilation controlled fires Operations conducted by firefighters can cause a ventilation controlled fire to enter back into the growth phase. Firefighters can be caught in vent induced extreme fire behavior. Watch the B-SAHF indicators to help recognize signs of impending extreme fire behavior. Fire events Rollover or flameover Flashover Backdraft Fire stages Incipient or ignition stage: Occurs after ignition Flames are small and are contained within material first ignited. No oxygen reduction (21%) Convection brings in additional oxygen Fire stages Growth stage: Fire grows in intensity and more fuel is involved releasing additional energy. Increased fuel consumption and heat generation. Heat carried up by convection and spreads horizontally until reaching the walls. Fire stages Fully developed stage: After reaching walls, heated gases bank downwards into lower portions. Heat is radiated which brings materials in the room up to ignition temperature. Oxygen is reduced to 9 – 12%. Upper portions 1000 degrees F. If flashover occurs it is between growth and fully developed stages. Hot temperatures but heat released slowed due to lack of oxygen. Backdraft conditions may be present. Fire stages Backdraft Plenty of heat waiting on introduction of additional oxygen. Proper ventilation can prevent/reduce backdraft. Transfer of heat Conduction Convection Radiation Direct flame Is it useful? How do you feel as a class after reading and reviewing the B-SAHF model? Is it a useful tool to train fire service professionals?