By: Rodger Broome'
Fire Training Specialist
West Jordan Fire/EMS
West Jordan, Utah
A written paper submitted to the Utah Fire and Rescue Academy as part of the Utah State Fire Officer Certification Program
July 9, 1999
Abstract
The foremost cause of death for people who die in structure fires is a result of smoke inhalation. The toxic gases and incompletely burned materials produced by fire makes the atmosphere lethal inside the compartment of the fire. Ventilation is useful in structure fires to release heat and ignitable fire gases out of the structure to control the fire spread and extinguish it eventually. Positive Pressure Ventilation is a method of ventilation that accomplishes that along with minimizing the risks of flash over, quicker location of the fire, and better visibility to accomplish a more complete and thorough search and rescue attempt of occupants. The nature of positive pressure ventilation helps the fire behave naturally while working against it by reducing its supply of ignitable gases and trapped heat. The outcome is the minimizing of some factors which make compartment fires more dangerous to the firefighters that outside of structure fires. Positive Pressure Ventilation should be used as early in a structure fire incident as possible to achieve the lifesaving benefits it offers.
The ventilation of a structure fire has advantages to the fire control efforts of the Firefighters fighting a fire. The advantages are found in the Rescue Operations, Fire Attack and Extinguishment, Property Conservation, slowing Fire Spread and reduction of Hazardous Fire Conditions to Firefighters. The advantages found in these aspects of Fire Operations are better understood by understanding fire behavior inside a compartment.
Compartment Fires are different from outside of structure fires in many ways. One of the significant differences is the trapping of heat around the seat of the fire. Fire produces heat, light, and fire gases, as products of a chemical reaction called Rapid Oxidation. A demonstration of slow oxidation of a material is the rusting of metal. The oxidation of a material that produces fire is made rapid by increased heat energy. A heat source capable of igniting a material is needed to start the Rapid Oxidation Reaction. The Rapid Oxidation Reaction called fire is dependant on having a Fuel to burn, oxygen to oxidized the fuel, and heat to accelerate the chemical reaction. If the fuel that is ignited needs to produce enough heat to continue to accelerate the oxidation process to make the fire grow. The initial stage of fire after ignition is the incipient state. When the incipient state fire grows to produce more heat than it needs to continue burning and it has an adequate amount of oxygen to be used in the oxidation reaction, it will become a free-burning fire. The free-burning fire is a fuel dependant fire. Fuel dependant fires will self-extinguish when the fuel source runs out or is otherwise eliminated. This is the type of fire found outside of structures.
The fires inside a compartment radiate their heat back into the seat of the fire from the walls and ceiling of the compartment raising the temperature of the fire more quickly the non compartment fires. The compartment fire will accelerate the oxidation process for a given fuel faster than non confined fires by feeding itself with its own heat. Usually the fire uses the oxygen that is limited by the compartment and becomes oxygen dependant if it is allowed to burn long enough. This creates a danger known as Back draft Explosion Danger. When the compartment is containing the heat necessary to ignite the fuel and the fuel is available to be oxidized, introducing oxygen into the process results in an instantaneous oxidation explosion. This is an extreme danger to firefighters if not dealt with properly. To reach a Back draft condition, a compartment fire will need to grow past the free-burning state to a total room and contents involvement of fire. This is called a flash over. Flash over is an extreme danger to firefighters and is responsible for many firefighter deaths. Flash over kills firefighters by its extreme temperatures which exceed the protection capabilities of the personal protective clothing they wear. Flash over will occur in a compartment when the extreme temperatures necessary to ignite the entire room and its contents are achieved by the fire growth. Ventilation of the compartment will remove the trapped heat and ignitable gases feeding the fire. A drop in the temperature will delay the flash over long enough to allow firefighters to enter the compartment and cool the fire more with water from hose streams and rescue trapped occupants. Ventilation allows the fire to not become oxygen dependant, which helps it to burn but, removes a lot of the fuel and heat accelerating its growth. Ventilation helps eliminate the factors of compartment fires that make them burn so hot and spread quickly.
The removal of heat makes the atmosphere more habitable than before and the threat to life is reduced. This makes the survivability of victims inside more likely. The removal of the gases and smoke make the toxic threat of smoke inhalation lesser too. The ventilation of the fire compartment minimizes the spread of the smoke to other portions of the structure making visibility better for the rescuers. The heat of the fire also causes and increase in atmospheric pressure. As the molecules of the fuels and oxygen are heated, they spread out away from each other causing a pressure in the confined space of the structure. The pressure continues to rise as the fire gets hotter. The structure can become pressurized to a degree that the windows will break outward. Prior to ventilation, the structure is full of smoke, hot and pressurized. The fire will most often "ventilate itself" by causing an opening by breaking a window or burning through a wall or ceiling. There will be a temporary reduction in the heat and therefore the atmospheric pressure of the compartment will drop.
Positive Pressure Ventilation is a method of blowing air with a fan into a doorway and adding to the atmospheric pressure of the structure. After the structure pressure is elevated with the fan, an exhaust hole near the fire is created to release the heat and gases from the structure. The easiest way to create an exhaust hole is usually a window. The exhaust hole shouldn't be larger than one and one-half times the size of the doorway being charge with ir pressure by the fan. Too large an exhaust hole, results in the inability to maintain the atmospheric pressure inside the compartment needed to push the smoke and heat out. The fan maintains the pressure in spite of the absence of the heat maintaining it before. Positive Pressure Ventilation removes the heat and smoke faster than any other method because of the pressure pushing them out of the compartment. The reduction of the smoke in the structure make the visibility in the structure better.
The increased visibility inside of the structure makes finding victims and the fire by firefighters much easier than before. The reduction of heat makes it safer and slows the fire spread in the compartment if the exhaust hole is near the fire. The slowing of the fire gives the search and rescue crews more time to look for victims before the fire makes the structure too dangerous to remain inside. The increased visibility of the structure makes duplication and missed areas less likely. The time spent on the pre-attack operations outside setting up PPV will be made up by the quicker search caused by increased visibility and reduced heat stress on the rescuers. The property in the structure will suffer less damage by smoke, heat and searching firefighters who are trying to find a victim without being able to see.
Positive Pressure Ventilation is quick and easy to set up and start. The fan is usually a pull-start motorized fan that will start easily if its maintenance is taken care of regularly. Creating the exhaust hole is usually only the breaking out of a window of the room involved in fire. A fan with wheels on the bottom for moving it easily can be set up by one person While the breaking out of the window requires only one person also. The person wheeling the fan to the door is still able to carry other equipment needed to enter the structure. The exhaust hole maker can use a tool to ventilate and use it for the interior operations also. This whole operation can be done by a single three person crew prior to and during a quick interior attack and search of a free burning state structure fire. When the first in Engine arrives with three people to a structure fire with a possible victim inside, the crew is not bound by the OSHA standard known commonly as the "two in two out rule." The Company Officer should dismount the apparatus in Full Personal Protective Equipment and don an SCBA. The Engineer can set the apparatus in pump, extend a pre-connected hose to the anticipated point of entry, and return to the pump panel to charge the line. The third Firefighter dismounts in full PPE and SCBA obtains a search light and the PPV fan and goes to the anticipated entry point. The Company Officer makes a walk around of the structure to locate the fire and size up the incident. The Company Officer takes a tool on the walk around and locates the ventilation exhaust port when the fire is located. The Firefighter should have the fan started on a closed door at the point of entry and the Company Officer creates the exhaust port by breaking a fireroom window once the fan is started. The Company Officer returns quickly to the point of entry to meet with the other Firefighter and they don their SCBA masks. The Firefighter may have already donned his mask. The door at the point of entry is opened and the fan set up is checked for accuracy by the Firefighter or Company Officer. The Company Officer working in a "working command" mode enters the structure with the Firefighter after waiting ten seconds for the PPV to become effective. The two-person-crew attacks what fire they find or rescues any victims found during this initial offensive attack and search. If the fire is located before occupants, the fire should be knocked down so the team can continue to search for occupants with a decreased threat from the fire.
A second-in-crew with a back up line can assist in search and rescue or fire control depending on what is in need of supplementation by the first-in- team. More people on the fire ground in the early stages of the operations makes this even easier. A Fire Company should train with Positive Pressure Ventilation. The Company Officer can practice its use when conducting routine training evolutions with the crew. The crew members need to get in the habit of being in full Personal Protective Equipment when arriving at the scene of a structure fire and being ready to work when dismounting the apparatus. They can be ready to gather tools, advance a hose line, and pull the fan with them to the door of the structure they plan to enter. The entry crew can start the fan and open the door of the structure to pressurize it upon receiving water in the hose from the Engineer. The Incident Commander can have one of the Firefighters go to make the exhaust hole and return, or he can make the exhaust hole while doing his size up walk-around. The latter is probably the most efficient. This type of operation can be effective if an immediate rescue of a trapped victim is necessary. If not, a team of firefighters from the next apparatus to arrive can be assigned to secure the utilities, make the exhaust hole, and set up as a rapid intervention team as prescribed by OSHA. The only time an Incident Commander should not use Positive Pressure Ventilation is when Back draft conditions exist. Vertical ventilation is the only safe operation to mitigate the Back draft conditions while minimizing the risks to the personnel. In the majority of structure fires that are still being contained by the structure, Positive Pressure Ventilation works well.
The practice of this type of evolution can be done at a training facility, abandoned houses with owners' written permission for use, or even just a parking lot or park with the space to stretch hose lines and simulate the openings of a structure with traffic cones. The Company Officer can simulate the proper pressurizing of the structure at the station. The fan can be set up and a window in the other side of the station opened as an exhaust hole. The Company Officer can hang paper strips or a light material in the window to gauge the effectiveness of the ventilation by examining the draft coming out of the window. The crew will become so comfortable with the use of Positive Pressure Ventilation, the operation will become second nature and efficient. The crew members will learn to coordinate their efforts to make the operation successful. The success of this operation will result in fewer injuries and deaths caused by structure fires.
Positive Pressure Ventilation is a very simple and effective way to make initial fire operations safer and more successful. The Search and Rescue of a structure on fire is the highest priority in the Fire Service. The rescue can be accomplished better by the increasing the visibility of the structure cause by rapid ventilation of the smoke. The reduction of the interior heat will allow firefighters to gain quick control and extinguishment of the fire while reducing the risks of death by flash over. There are other methods of ventilation at the disposal of the Incident Commander but no other type of ventilation is as safe or effective than Positive Pressure Ventilation when a fire is burning inside a structure.
References
DeHaan, John D.(1997), Kirk's Fire Investigation (4th Ed.) Prentice-Hall, Inc. NJ
IFSTA (1998), Essentials of Fire Fighting (4th Ed.) Fire Protection Publications, OK