by Stephen Kerber and Daniel Madrzykowski
The Building and Fire Research Laboratory at the National Institute of Standards and Technology (NIST) started a Firefighting Technology Group in order to focus on the research needs of the fire service. One key objective of the group is to provide information to improve firefighter safety.
NIST recently completed a multi-year study on positive pressure ventilation. A wide range of experiments were conducted including laboratory experiments, high-rise buildings and a high school. The reports all total more than 1,000 pages.
This article will focus on the practical results of the studies. The results will be listed with the title of the report they came from and the Web link if you want to download and review it for further information.
Positive Pressure Ventilation
Positive pressure ventilation (PPV) is a ventilation technique used by the fire service to remove smoke, heat and other combustion products from a structure. This allows firefighters to perform tasks in a more tenable environment. PPV fans are commonly powered with an electric or gasoline engine and range in diameter from 0.30 m to 0.91 m (12 in to 36 in). More recently, fans up to 2.1 m (84 in) have been manufactured and mounted on trucks and trailers. Typically, a PPV fan is placed about 1.2 m to 3.0 m (4 ft to 10 ft) outside the doorway of the structure. It is positioned so that the “cone of air” produced by the fan extends beyond the boundaries of the opening. With the doorway within the cone of air, pressure inside the structure increases. An exhaust opening in the structure, such as an opening in the roof or an open window, allows the combustion products to escape due to the difference between the inside and outside air pressure. The smoke, heat and other combustion products are pushed out of the structure and replaced with ambient air.
Another use of PPV is to increase the pressure in a portion of a structure by not providing a vent location. This increase in pressure, if adequate, will prevent smoke flow to a “protected” area. This is most useful in larger structures such as schools, hospitals, malls and high-rise buildings.
Impact of Positive Pressure Ventilation on Heat Release Rate
Two experiments were conducted with nearly identical fuel loads, examining the effect of the PPV fan on the room fire. The first experiment used a fan to forcibly ventilate the room just after the window was opened. The second experiment was similar to the first experiment except that it was naturally ventilated.
The fan used was a 0.75 m (18 in), variable speed, electric positive pressure ventilator. The fan was positioned 2.44 m (8 ft) from the open doorway to the corridor at an angle of approximately 15 degrees from horizontal to create the cone of air around the doorway.
The heat release rate of the fire was increased by the PPV fan for the 200 s following the peak heat release rate. This is critical because this is the time window in which the fire department would typically be advancing to extinguish the fire. The peak heat release rate for the two experiments occurred at approximately the same time and the rate with the PPV fan was 2 MW higher. The PPV fan caused a 60 % increase in burning rate during this time of initial fire department attack. This reinforces the importance of selecting a ventilation location close to the seat of the fire that allows for the majority of the combustion products to be ventilated to the exterior of the structure. Even though the heat release rate was increased, the combustion zone was being forced out of the window opening.
The PPV ventilated experiment forced the flames at least 1.83 m (6 ft) out of the room as compared to the 0.91 m (3 ft) by the naturally ventilated experiment. Flame extension out of the building openings may pose a potential ignition hazard to materials nearby.
While the use of PPV in this particular configuration caused an increase in the room’s fire burning rate, it lowered the temperatures in the room, forced the combustion products to flow out of the room without affecting the corridor and improved the visibility leading up to and in the room itself.
In this experimental configuration, a firefighting team would likely have been able to attack the PPV ventilated fire more easily than the naturally ventilated fire. This data set demonstrates that coordination of firefighting crews is essential to carry out positive pressure ventilation in the attack stages of a fire. In this experiment, ideal coordination was simulated as the window was ventilated in the correct location and the fan was initiated five seconds later. Once the fan was turned on, it took approximately 60-90 seconds (s) for the fire to reach its peak burning rate and the flow of gases was forced away from the entrance. After this transition, the fire remained at a steady burning rate until the fuel was consumed. This would indicate that for the conditions in this experiment firefighters should delay at least 60 s after ventilation and fan start before advancing towards the fire. This would allow the flows to stabilize, temperatures to decrease and visibility to improve. The burning rate of the fire could become steady at the rate determined by the modified air flow and would be less likely to rapidly change as the firefighters approach.
The time to reach this new steady condition could vary with building layout, fire size and fan capacity. After the fans are in place and turned on, the fire conditions and flow from the exhaust vent should be assessed to ensure that the fan are having the desired impact, prior to deploying the firefighters into the structure.
NISTIR 7213, Effect of Positive Pressure Ventilation on a Room Fire.
S. Kerber and W.D. Walton, National Institute of Standards
and Technology, Gaithersburg, MD 20899, March 2005.
(1) Comparison of corridor with PPV (top) vs the naturally ventilated corridor (bottom).
(2) Comparison of the flame length leaving the window opening. PPV on the bottom and natural ventilation on the top
Using PPV fans to Pressurize Stairs
One hundred and sixty experiments were conducted in a thirty-story vacant office building in Toledo, Ohio, to evaluate the ability of fire department positive pressure ventilation (PPV) fans to pressurize a stairwell in a high-rise structure in accordance with established performance metrics for fixed stairwell pressurization systems. According to NFPA 92A, 25 Pa is required for fixed smoke control systems in non-sprinklered buildings.
Variables such as fan size, fan angle, setback distance, number of fans, orientation of fans, number of doors open, and location of vents open were varied to examine capability and optimization of each. Fan size varied from 0.4 m (16 in) to 1.2 m (46 in). Fan angle ranged from 90 degrees to 80 degrees. The setback distances ranged from 0.6 m (2 ft) to 3.6 m (12 ft). One fan to as many as nine fans were used which were located at three different exterior locations and three different interior locations. Fans were oriented both in series and in parallel configurations. Doors throughout the building were opened and closed to evaluate the effects. Finally a door to the roof and a roof hatch were used as vent points. The measurements taken during the experiments included differential pressure, air temperature, carbon monoxide, metrological data and sound levels.
PPV fans used correctly can increase the effectiveness of firefighters and survivability of occupants in high-rise buildings. In a high-rise building it is possible to increase the pressure of a stairwell to prevent the infiltration of smoke if fire crews configure the fans properly. Although many factors contribute and need to be considered for effective PPV operations, properly configured PPV can achieve stairwell pressures that are high enough to meet or exceed the performance metrics for fixed smoke control systems.
Proper configuration requires the user to consider a range of variables including, fan size, set back, and angle, fan position inside or outside of the building, and number and alignment of multiple fans. The data collected during this limited set of full-scale experiments in a 30-story office building demonstrated that in order to maximize the capability of PPV fans the following guidelines should be followed:
- Regardless of size, portable PPV fans should be placed so that the cone of air “seals” or extends over the open inlet doorway. In these experiments 1.2 m (4 ft) to 1.8 m (6ft) set back from the doorway and angled back at least 5 degrees maximized the flow through
- the fan shroud and air entrainment around the fan shroud as it reaches the doorway.
- Placing fans in a V-shape is more effective than placing them in series, Figure 3.
- When attempting to pressurize a tall stairwell, portable fans at the base of the stairwell or at a ground floor entrance alone will not be effective. Placing portable fans inside the building below the fire floor is a way to generate pressure differentials that exceed the NFPA 92A minimum requirements. For example, if the fire is on the 20th floor, placing at least one fan at the base of the stairwell and at least one near the 18th floor blowing air into the stairwell could meet the NFPA 92A of 25 Pa for most of the stair, Figure 4.
- Fans used inside the building should be set back and angled just as if it were positioned at an outside doorway, Figure 5.
- Placing a large trailer mounted type fan at the base of the stairwell is another means of generating pressure differentials that exceed the NFPA 92A minimum requirements of 25 Pa., Figure 6.
The experiments also documented that PPV fans can be loud (> 100 dB) which may have an impact on fire ground and command post communications. Gasoline powered fans generate carbon monoxide but the magnitude has to be compared to that of the hazard created by the fire in the building, this is addressed in a section below. Overall, when properly setup and correctly operated, positive pressure ventilation is a tool which the fire service can use to improve the safety and effectiveness of fire ground operations.
(3) Putting fans in series does provide more pressure than a single fan, however placing the fans side by side provides a larger increase.
(4) The graph above shows the impact of a single fan outside the entry door to the stairwell on floor 1compared with adding additional fans inside the building. While the pressure from the single fan on floor 1 decreases with the height of the building, the fans placed within the building are able to maintain a pressure in excess of 20 Pa throughout the stairwell.
(5) Typical fan placement in the building, pressurizing the stairwell.
(6) Data from a 1.2 m (46 in), trailer mounted fan, pressurizing the stair from outside of an entry door on floor 1. The graph shows how the pressures in the stairwell increased in relation to the increase in fan speed. In this case, the single fan provided adequate pressure for the full height of the building.
NISTIR 7412, Evaluating Positive Pressure Ventilation In Large Structures: High-Rise
Pressure Experiments. By S. Kerber; D. Madrzykowski, and D.W. Stroup. National Institute of Standards and Technology, Gaithersburg, MD 20899, March 2007.
High-rise Fire Tests in Chicago
While the pressure experiments in Toledo were informative and provided a wealth of information, our colleagues from the Chicago and New York City Fire Departments wanted to see the PPV fans in action against hot smoke created by realistic apartment fires. The Chicago Fire Department obtained permission to test in a16 story highrise that was slated for demolition. NIST instrumented the building and working with experienced chiefs and firefighters from Chicago, New York, Toledo and Ottawa conducted a series of experiments to determine if PPV fans could be successfully deployed and implemented in a high rise, apartment building as a means to increase the safety and effectiveness of firefighters.
Apartments were carpeted and furnished in order to develop realistic fire conditions. The building had two stairs. One stair was used as the “attack” stair and one was used as the protected stair for building occupant evacuation. The fan placements and strategies were based on the previous Toledo experiments. The door between the apartment and the public corridor was kept open during the experiments, so the public corridor and the stairs on each end of the corridor would be exposed to combustion products from a post-flashover fire.
The results of the experiments showed that the portable fans were effective at ventilating the 16-story stairwell and keeping it free of smoke while pressurizing. In most cases the single portable fan at the base of the stairwell improved conditions in the stairwell. The increased pressures greatly reduced the amount of smoke that was able to flow into the stairwell under natural ventilation conditions. However, when a second fan was added two floors below the fire floor, smoke was kept completely out of the stairwell, even with the fire floor door open or with an additional door open. In order to protect both stairwells completely four PPV fans were used.
The large trailer or truck mounted fans which were positioned at the front of the structure were able to clear the stairwell quickly when vented and were able to keep smoke out of the entire stairwell with the fire floor door open. In fact the mounted fans were able to keep both stairwells free of smoke by flowing into the front door and opening the doors to each stairwell on the 1st floor.
These experiments demonstrated that the fans could decrease the temperature in the stair and if the apartment had self vented to the outside, the fans could also provide cooling and fresh air in the public corridor on the fire floor.
(7) The thermal images from an IR camera show the hot gases entering the stairwell through the open doorway in the image on the left. The image on the right shows the conditions in the stairwell after the PPV fan was started, hot gases are no longer entering the stair.
Carbon Monoxide in Buildings
The thought of putting gasoline powered PPV fans inside of buildings has raised concerns due to potential build up of carbon monoxide (CO). A fire has the potential to produce a very large amount of CO. According to Purser, this amount could be on the order of 50,000 ppm (5 %) in an under-ventilated fire. Tenability limits for incapacitation and death for a 5 minute exposure are 6000 ppm (0.6 %) to 8000 ppm (0.8 %) and 12,000 (1.2 %) to 16,000 ppm (1.6 %) respectively. Using PPV fans to keep the CO produced by the fire along with the other harmful combustion products out of the stairwells greatly increases the chances of safe evacuation.
The internal combustion of a gasoline fan engine also produces CO. While the levels are much lower than the fire they have to be analyzed. CO meters were placed in both stairwells to monitor the fans impact on CO levels. The National Institute of Occupational Safety and Health (NIOSH) has established a recommended exposure limit for CO of 35 ppm (0.0035 %) as an 8-hour time weighted average (TWA) and 200 ppm (0.02 %) as a ceiling exposure. A reading of 1200 ppm (0.12 %) is considered immediately dangerous to life and health (IDLH). The National Research Council (NRC) also defines emergency exposure guidance levels of, 1500 ppm (0.15 %) for 10 minutes, 800 ppm (0.08 %) for 30 minutes, 400 ppm (0.04 %) for 60 minutes and 50 ppm (0.005 %) for 24 hours.
Without the use of the fans the CO levels in the hallway and in the south stairwell during the fire almost always reached or exceeded the 800 ppm (0.08 %) limit of the meters used. It is conservative to say that many of the meter locations exceeded the IDLH threshold. In most cases, the use of the fans to vent the stairwell and to pressurize the stairwell to reduce smoke and CO levels allowed the CO levels to decrease from IDLH conditions to less than 200 ppm (0.02 %). Ultimately the CO produced by the PPV fans was at least one order of magnitude less than that created by the fire.
NISTIR 7468, Evaluating Positive Pressure Ventilation In Large Structures: High-Rise Fire Experiments. S. Kerber and D. Madrzykowski. National Institute of Standards and Technology, Gaithersburg, MD 20899, November 2007.
PPV in large single story structures
Working again with the Toledo Fire and Rescue Department and with the support of DHS and USFA, NIST had the opportunity to conduct fire experiments in a retired high school. The school was originally constructed in 1956 and was added on to substantially until 1988. The structure is oddly shaped with numerous sections and court yards, but overall has the dimensions of 210 m (700 ft) wide by 130 m (425 ft) deep by 9 m (30 ft) tall. The building was constructed of masonry bearing walls and steel column grids. All of the fires, gas flows, and measurements were limited to the first floor.
Both portable fans and large truck or trailer mounted fans were used. The two main fire scenarios were examined: 1) a fire in a classroom open to long hallways and 2) a fire in a gymnasium. Both scenarios included fires that produced a large amount of smoke and hot gases. Instrumentation was placed to assess tenability criteria and how PPV tactics could impact survivability. Measurements included temperature, pressure, thermal imaging and video views.
In this series of experiments the fans increased the pressure sufficiently to reduce temperatures throughout the hallways even up to the fire room as shown in Figure 8. The PPV fans would provide occupants a more survivable environment and increase firefighter safety, limit smoke spread, keep additional parts of the structure safe for occupants and undamaged and reduce the scale of the emergency for the firefighters, and increase visibility.
(8) The thermal gradient from the ceiling to the floor, in the hallway just outside the open classroom doorway is shown. As the fire develops the hot gas layer in the hallway develops and reaches a steady state at approximately 400 seconds. This is followed by the fire department opening a remote door to the hallway and then starting the PPV fans with using the remote doorway as an outlet vent. That had very little effect on the temperature. It did not make conditions worse, but close to the fire room it did not improve them. After 800 seconds, the window to the classroom was vented. The remote door was closed and the PPV fans were turned on first one then another was added. The impact on the fans on the temperature in the hall is clear.
NIST Technical Note 1498, Evaluating Positive Pressure Ventilation In Large Structures:
School Pressure and Fire Experiments. S. Kerber and D. Madrzykowski. National Institute of Standards and Technology, Gaithersburg, MD 20899, July 2008.
Wind and PPV
The impact of wind on the growth and movement of a structure fire can be significant. NIST with the support of the Fire Protection Research Foundation, DHS/FEMA Assistance to Firefighters Research and Development Grant Program and USFA conducted eight fire experiments to examine the impact of wind on fire spread through a multi-room structure.
The experiments were designed to expose a public corridor area to a wind driven, post-flashover apartment fire. As in the experiments in Chicago, the door from the apartment to the corridor was open for each of the experiments. The conditions in the corridor were of critical importance because the corridor is the portion of the building that firefighters would use to approach the fire apartment or that occupants from an adjoining apartment would use to exit the building. The fires were ignited in the bedroom of the apartment. Prior to the failure or venting of the bedroom window, which was on the upwind side of the experimental apartment, the heat release rate from the fire was on the order of 1 MW. The peak heat release rates from the post-flashover structure fire were typically between 15 MW and 20 MW. When the door from the apartment to the corridor was open, temperatures in the corridor area near the open doorway, 1.52 m (5.00 ft) below the ceiling, were in excess of 600 °C (1112 °F) for each of the experiments. The heat fluxes measured in the same location, during the same experiments, were in excess of 70 kW/m². These extreme thermal conditions are not tenable, even for a firefighter in fully protective gear. These conditions were reached within 30 s of the window failure.
These experiments demonstrated the thermal conditions that can be generated by a “simple room and contents” fire and how these conditions can be extended along a flow path within a structure when a wind condition and an open vent are present.
Hence wind is a critical part of size-up and needs to be considered when using PPV. Wind speeds on the order of 10 mph to 20 mph are high enough to create a wind driven fire condition in the structure with an uncontrolled flow path.
If the fire has vented a window, important information can be gained by observing the behavior of the flame at the window. If the fire apartment has a high pressure relative to the outside due to an imposed wind, the flame will “pulse” out of the window to balance the overpressure. If the flames are being forced out of the window a flow path has been established through the building and the flow direction maybe favorable to interior firefighting. If the flames are pulsing or being forced into the window, condition may not be favorable to interior firefighting and caution should be used on the approach to the fire floor. Even if flames are being forced out of adjacent windows there could still be sufficient energy flows on the fire floor to create a hazard for firefighters.
NIST, the Fire Department of New York City (FDNY), and the Polytechnic Institute of New York University with the support of the DHS/FEMA Assistance to Firefighters Research and Development Grant Program and USFA, have conducted a series of wind driven fire experiments in a seven story building on Governors Island, New York. In several of these wind driven fire experiments the use of PPV fans was examined.
Only portable PPV fans were used in these experiments. Given the high pressures created by the wind, two 27 inch PPV fans alone could not overcome the effects of a wind driven condition. However when used in conjunction with door control or other means to mitigate the wind driven conditions the PPV fans were able to maintain tenable and clear conditions in the stairwell. The key to successful use of PPV fans was to mitigate the wind driven fire condition via door control or other tactics. Then the PPVs can be used to clear the stair and then pressurize the stairwell to provide a safe working environment. Although the PPV fans, when used alone, could not reverse the flow of a wind driven fire, PPV fans always improved conditions in the stairwell.
It is important to note that even though these experiments were examining large structures, wind can have a significant impact on smaller structures, such as single family, homes as well. Consideration should be given to using the wind as a natural PPV source by working to keep the wind at your back. If the wind is coming from the rear of a single family home that is on fire, making entry into the front of the home may not be the best approach, even if that is the unburned side.
Fire departments that wish to implement the PPV tactics examined in these studies will need to develop training and determine appropriate methods for deploying these tactics. Variations in the methods of deployment may be required due to differences in staffing, equipment, building stock, typical weather conditions, etc.
NIST Technical Note 1618, Firefighting Tactics Under Wind Driven Conditions:
Laboratory Experiments. D. Madrzykowski and S. Kerber. National Institute of Standards and Technology, Gaithersburg, MD., January 2009.
NIST Technical Note 1629, Firefighting Tactics Under Wind Driven Fire Conditions:
7-Story Building Experiments. S.Kerber and D. Madrzykowski. National Institute of Standards and Technology, Gaithersburg, MD., April 2009.
Purchase Fans with a Purpose
Much like all firefighting equipment, it is very important when making purchasing decisions for PPV fans to consider your response area and target hazards. Larger portable fans and even larger mounted fans are necessary to adequately pressurize and/or ventilate large foot print buildings and high-rise buildings. With some exceptions, small portable fans should be limited to house scale applications. While compartment space is at a premium these days special consideration should be given to what fans are needed. Don’t buy a compact fan because it takes up the least amount of compartment space and expect it will meet your needs at the next warehouse fire. Often times adding many small fans at multiple inlet points will not be able to accomplish what one or two well placed larger fans can do.
Summary
NIST has conducted numerous experiments examining the effectiveness of Positive Pressure Ventilation (PPV) for the fire service. These experimental studies ranged in scale from a single room to a 30-story high-rise office building. The research identified tactical considerations for the most effective use of PPV fans. The results of the studies provide insight to questions such as where to place the fans, how much larger can the fire grow with added oxygen from the fan and what size fans are needed to effectively pressurize a stairwell or a hallway in a large structure.
Large structures pose additional challenges to firefighter and building occupant safety: increased travel distance (exposure time), more complicated egress paths, and potentially larger fires. Large structures include high-rise buildings as well as large volume structures such as warehouses or schools. In all of these structures, PPV has the ability to remove or limit occupant exposure to fire gases, so safe egress can be accomplished. Many fire departments don’t have staffing levels that allow them to effect multiple rescues in a highly populated structure. These tactics may 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.
To download copies of the referenced reports or view videos from the experiments go to
