FIRE ATTACK WITH POSITIVE-PRESSURE VENTILATION
SUCCESSFUL INTERIOR fire attack must include good ventilation techniques Ventilation is the planned and systematic removal of hot gases and smoke with replacement by fresh air. It has many advantages Maximum effectiveness is thought to occur when the ventilation and fire attack operations are coordinated with hoselines in readiness before ventilation is initiated.
Vertical ventilation is achieved by creating openings above the fire, making use of natural convection currents to allow heat and smoke to escape. By venting horizontally, cool air enters the structure from one side, allowing escape of combustion products on the other side either by natural draft currents, mechanical fans, or fog streams which, when directed out of windows, create a venturi and pull the products of combustion with it.
Recently, a new ventilation technique has developed. Called fvositii e-pressu re ventilation, it s an adaptation of the fire-protection engineering principle of pressurized stairwells for evacuation of high-rise buildings. It employs large-volume fans to blow air into the building from outside and force out smoke and fire gases through selected channels Not only has there been a growing interest in positive-pressure ventilation for venting and assisting overhaul and salvage operations, but its applications are being broadened to include its use prior to and during fire attack itself.
Many fire officers will find the idea of beginning ventilation before fire attack hard to accept. For those officers, it s recommended that these procedures be tried out during live fire training so that the benefits can be clearly recognized and the proper technique acquired.
For a dwelling fire, positive pressure will require the use of at least two small fans or one large one. Larger buildings or fire areas will require additional fans or blowers. The fans should be placed back from the opening approximately six feet, covering the opening with in-rushing air to eliminate churning. The height of the fan should be at a midpoint between the top and the bottom of the opening. This position will aid normal horizontal ventilation procedures. It may be desirable to move the blowers closer to the seat of the fire as the operations proceed, or for a fire in large-area structures.
The initiation of positive-pressure ventilation, by pushing the heat, smoke, and firegases ahead of the attack team, will alleviate the need to advance into the area with a wide-angle fog pattern. Forcing air in from the unburned side while making exhaust openings directly to the outer air from the fire area of a confined structure will speed the attack and eliminate considerable water damage. The heat and smoke will have been pushed ahead by the blowers, enhancing visibility and maneuverability, reducing possible fiashover, and speeding fire-suppression activities.
Positive-pressure ventilation prior to attack can give an initial increase of oxygen to the fire. Thus, it should not be used if the signs of a backdraft are evident. However, since it allows a faster attack on the seat of the fire with earlier knockdown and extinguishment. the end result is less fire, heat, and smoke damage. Attack lines should enter the building not more than 15 seconds after starting positive-pressure ventilation.
There are numerous benefits from using positive-pressure ventilation prior to the fire attack. Visibility improves as the heat, smoke and fire gases are forced out ahead of the attack crew-. With the heat moving out, room temperature drops, reducing the possibility of fiashover and allow ing the attacking firefighters to advance to the seat of the fire more quickly. With these improvements, the attack team can move much faster, thus placing extinguishing water on the seat of the fire in a shorter period of time. This brings earlier control, limits fire spread, restricts damage, and leads to effective extinguishment. The fans, continuing to operate, force fresh air in behind the firefighters, and the steam generated is moved away from them. This lessens the chance of steam burns.
Remember to apply positive-pressure ventilation effectively; firefighters must be trained in the technical aspects and operations to use w ith positive pressure. Blowers must be of adequate size and properlyplaced. An opening must be made in the confined structure near the seat of the fire to allow the heat, smoke, and fire gases to escape. Information on this subject can be obtained from Ventilation Methods and Techniques from Fire Technology Services or the Positive Pressure Training Manual from Tempest Technology Corporation.