Positive Pressure Ventilation

Positive pressure fire attack has been under great scrutiny from veterans of the fire service in the local area that I serve. Blowers have been used in the fire service to help blow smoke to ventilate structures for many years. Ventilation on the fire ground offers many challenges. The use of blowers has had some controversy as to the overall benefits, and only recently has the National Institute of Standards and Technology (NIST) and Underwriter Laboratories (UL) conducted studies to see if positive pressure attack on the fire ground can help in making fire attack safer and more effective. Positive pressure ventilation as a whole can help reduce some of these ventilation challenges when coordinated with rescue and fire attack, creating a safer more efficient environment for firefighting crews. Positive Pressure Ventilation is the best ventilation tactic to use while on the fire ground. 

The Underwriters Laboratory has been conducting live-fire experiments to learn what the effects of various forms of ventilation tactics have on fire behavior. Working closely with the National Institute of Standards and Technology (NIST), positive pressure ventilation experiments have been conducted on high-rise buildings, floor collapse in various floor configurations, natural ventilation, vertical ventilation, and residential structures. (Halton 2012) Through these experiments and the recent interest in PPV, fire-service organizations have been able to learn a great deal about what happens when we force-ventilate a structure. 

If a bulkhead door or vent is opened to remove smoke, the stairwell becomes a flow path for fire gases. (Kerber, Madrzykowski 2007) Hot gasses through stairwell doorways can create extreme hazards for firefighters. Positive Pressure Systems are recognized as useful tools and are one system available when designing newer buildings such as environmentally friendly green schools.

Ventilation is a key component in safe firefighting tactics. If a large fire is not properly ventilated it is much harder to fight. Mechanical fans can be used to create positive pressure ventilation in coordination with existing openings such as windows and skylights, or by cutting new exhaust vents in the building if needed. Vertical ventilation can increase survival time for evacuating, trapped or unconscious persons. Proper vertical ventilation relieves the upper portions of a building from heat, gas, and smoke. (LaFemina 2006) Even building design engineers realize the importance of planning for proper ventilation in the event of a fire. 

Some newer structures have positive pressure ventilation systems already built into the stairwells. For older structures, or those not equipped with PPV systems, fans have been engineered to provide flow capacities easily comparable to fixed or mounted modern stairwell pressurization systems. A PPV van is placed generally 4 to 10 feet outside a doorway of the structure. A “cone of air” produced by the fan extends beyond the boundaries of the opening. The pressure inside the structure increases with the doorway within the cone of air. An opening like a roof or window becomes the exhaust opening, allowing the flow to escape due to the difference in pressure between the inside and outside air. (Hall, Adams 1998) Gases, smoke, and heat are then pushed out of the structure and replaced with ambient air.

Kerber and Madrzykowski produced another study of positive pressure ventilation in 2008. The 2008 study focused on an entirely different type of structure than the high-rise 2007 study. This study evaluated positive pressure ventilation in masonry educational buildings, and the ability of PPV fans to limit the smoke spread and remove smoke from areas where occupants might be located. The building used for the experiments was single story. The study examined high pressure created by portable fans mounted in different configurations and locations. The study focused on two scenarios, one in a long hallway, and one in a gymnasium. Instruments showing temperature, pressure, thermal imagery and video were placed to access positive pressure ventilation tactics. (Kerber, Madrzykowski 2008) The study took a look at how these tactics may increase or decrease the rationale behind use of PPV in fire situations. The main focuses of the 2008 study were how it contributed to the benefit of forensic science and how PPV can help save occupant lives.

PPV can lower building occupant exposure to fire gases, so safe evacuation can take place. The 2008 study points out that “Many fire departments don’t have staffing levels that allow them to affect multiple rescues in a highly populated structure. These tactics (PPV) provide the fire department the ability to remove the hazard from the occupants as opposed to removing the occupants from the hazard which is much more time consuming and labor intensive.” (Kerber, Madrzykowski 2008 p. 3) 

Pressure experiments were conducted prior to the fire experiment to look at the effect the fans had on pressurization independent of the fire. The volume of pressure was tested and changed by opening and closing various doors along the different hallways and classrooms. The pressures were graphed and averaged over set time configurations. The portable fans used pressurized the hallway 7 Pa, this pressure decreased to 2 Pa when a ventilation point opened. Mounted fans increased pressurization to 17 Pa above ambient, when a ventilation door or window opened, that pressure decreased to an average of 10 Pa. (Kerber, Madrzykowski 2008) These experiments demonstrated the average change in pressure when fans were used in the hallway and gymnasium without fire present. When fire was presented into the experiment, pressure was measured in the fire rooms prior to fan introduction. Opening a window dropped the pressure an average of 4 Pa. In almost every one of the experiments, the pressure created by the fans exceeded peak pressure created by the fire. This was the goal of the experiment. Portable fans were not as effective as larger mounted fans. 

Keeping temperatures under control during a fire is key to survivability. Limiting the spread of hot gas throughout a building allows better safety and survivability for firefighters and victims. Temperature measurements were taken throughout the 2008 experiments as well. Firefighter protective clothing standards require that protective clothing withstand exposure to 260o C (500o F) for five minutes without substantial damage, the turnout gear may survive at this temperature, but the firefighter inside the gear may sustain injury or death. (USFA 1993) Natural ventilation did not have a large impact on ceiling temperatures in the experiment. There was either a slight increase in temperature because of introduction of oxygen as airflow was provided to the fire, or a slight decrease in temperature as heat escaped. The PPV ventilated scenarios, on the other hand, decreased the ceiling temperatures in every experiment regardless of ventilation location. The mounted fans had maximum impact but even the small single portable fans used in the experiment reduced temperatures. (Kerber, Madrzykowski 2008) As far as temperature control was measured, positive pressure ventilation in this experiment was a success. 

Smoke and Visibility observations were an important component of the 2008 analysis. The concentrations of toxic gas were not measured in this experiment, however, usefulness of PPV was assessed on the basis of visibility. Occupants exposed to smoke and the resulting reduction in visibility may become disoriented and evacuation slowed which in turn increases exposure to lethal gases and smoke. Firefighters on breathing apparatuses are not affected by toxicity, however their effectiveness in rescue and firefighting operations may be hampered by reduced visibility. In each of the experiments the smoke layer descended to the floor. The PPV fans were able to limit the spread of smoke, and force smoke back into the gymnasium, away from the lobby area. This was the desired outcome. (Kerber, Madrzykowski 2008) The PPV fans were able to keep smoke out of a designated area, in a real situation; this could have life-saving outcomes for occupants. 

Pressure, temperature and video data gathered during this experiment, as well as data gathered in the earlier high-rise structure fire,  demonstrate that PPV fans were able to create pressures that forced combustion products into a pre-arranged area. The ability to control where smoke and heat exit, removing the bi-products of fire from building occupants is well worth the effort of PPV systems. This gives occupants much needed extra time and a larger safety margin in the ability to evacuate a fire. 

Some of the positive effects noticed in these positive pressure ventilation experiments were: Ventilation fans do not negatively impact a structure. In the experiments sited here, fans never made conditions less tenable. Larger fans were more efficient than the smaller portable fans. Fans created pressure high enough to limit the flow of combustion gases down hallways. Conditions leading to the fire room always improved with the fans. Fans improved firefighter visibility in the experiment while conducting search and rescue operations. 

Positive pressure fire attack has been under great scrutiny from veterans of the fire services even though blowers have been used in the fire service to help blow smoke to ventilate structures for many years. Newly constructed buildings are often using PPV systems designed right into the stairwell and building construction, acknowledging the usefulness of this technology. Ventilation on the fire ground offers many challenges. Recently the National Institute of Standards and Technology (NIST) and Underwriter Laboratories (UL) have conducted studies to see if positive pressure attack on the fire ground can help in making fire attack safer and more effective. The two studies cited here are examples of successful experiments with a positive outcome for PPV. Positive pressure ventilation as a whole can help reduce some of these ventilation challenges when coordinated with rescue and fire attack, creating a safer more efficient environment for firefighting crews. Positive Pressure Ventilation is the best ventilation tactic to use while on the fire ground.

References

Hall, R., & Adams, B. (1998). Essentials of fire fighting (4th ed.). Stillwater, Okla.: Fire Protection Publications.

Halton, B. (1012, February 10). Vertical Ventilation and Firefighting: Inside the UL Tests. Fire Engineering Magazine, 2. Retrieved September 26, 2013, from http://www.fireengineering.com//articles/2012/02/vertical-ventilation-ul-tests.html

Kerber, S., & Madrzykowski, D. (2008). Evaluating Positive Pressure Ventilation in Large Structures: School Pressure and Fire Experiments. U.S. Department of Commerce NISTIR Tech note 1498, 1498, 2-187.

Kerber, S., & Madrzykowski, D. (2007). Evaluating High Pressure Ventilation in Structures: High Rise Fire Experiments. U.S. Department of Commerce NISTIR 7468, Nov., 2-72.

LaFemina, F. (2006, March 20). Ventilation Basics. Fire Service, Firefighters & Departments | Fire Rescue 1. Retrieved September 30, 2013, from http://www.firerescue1.com/fire-products/ventilation/articles/18506-Ventilation-Basics/

USFA minimum standards pdf. (n.d.). FA-137 Minimum Protective Gear Standards. Retrieved September 26, 2013, from www.usfa.fema.gov/downloads/pdf/publications/fa-137.pdf

number/title, d. (n.d.). NFPA 92A: Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences. National Fire Protection Association. Retrieved September 30, 2013, from http://www.nfpa.org/codes-and-standards/document-information-pages?mode=code&code=92A