Positive Pressure for a Positive Advantage
PART 1
Recently, this author was looking up the definition of “progress” in Webster's dictionary for a non-fire service project. As I pondered the word “progress” (which is defined as “moving forward or onward, forward course; development”, etc.), I thought about the definition as applied to positive pressure ventilation (PPV) and how far the fire service has progressed with this specific type of ventilation. Specifically, we have moved from "You can't be serious" to numerous departments saying, "How did we ever fight fires without PPV?" In reflection, I can distinctly remember the following milestones as applied to PPV:
1973: A letter was circulated to all officers in the Los Angeles Fire Department about a new type of pressurized ventilation that was going to be adopted that placed fans outside buildings for smoke removal.
1983: An article appeared in American Fire Journal that was titled “Positive Pressure Ventilation For A Positive Advantage.” In the article, PPV was detailed as an alternate method to negative-pressure ventilation.
1984: After reading the 1983 American Fire Journal article, the Austin (Texas) Fire Department, conducted an “in-house” test and determined that PPV “applied in a way to give it’s maximum output can be over 300% as efficient at ventilating a structure than negative pressure ventilation.”
1986: An article appeared in the 1986 issue of Fire Command magazine that detailed the Austin Fire Department implementation of the “Smokebuster.” This was a 5-foot prop powered by a VW engine and mounted on a 1-ton truck. This PPV unit responded to all structure fires with positive success.
1989: PPV is introduced to the “Cold Storage Industry” and is immediately embraced as an effective way to minimize losses due to ammonia leaks.
1990: The German fire magazine Blaulicht printed an article on the merits of PPV. Germany adopted PPV for smoke removal and fire attack soon after.
1991: Fire Chief magazine conducted a survey on PPV. Fifty-seven persent of respondent departments were using PPV. Thirty-one percent of these respondents used PPV for smoke removal only, and 67 percent used PPV for both smoke removal and fire attack.
1991: A lead article in Military Firefighter (a European magazine) was titled “PPV — As Important As The Axe & Hose?” The article outlined the success of PPV in structure, marine, and aircraft fires.
1994: IFSTA Ventilation manual, Seventh Edition, recognizes PPV as more effective, versatile, and safer than negative pressure ventilation.
1997: The United Kingdom Fire Service publication, Fire Service Manual, Volume 2, includes PPV as an effective ventilation tool for smoke removal and fire attack. PPV is implemented in the British Fire Service soon after.
2000-2001: Most fire conferences in the United States (FDIC, Firehouse Expo, etc.) will normally have a workshop on the successful theory/use of PPV ventilation.
With this brief overview of the history of PPV, it is easy to see that we have progressed from a fireground operation that has been historically controversial to a powerful fireground operation that, if used correctly like any fireground tool, is capable of dramatically improving fireground operations and firefighter safety. So, let's spend several articles in reviewing PPV and cover a few different applications that have proven successful.
When fireground personnel determine ventilation is necessary to assist in the mitigation of an incident, plans for ventilation operations will normally begin by identifying the direction (horizontal or vertical) contaminants must travel to exit a contaminated area. Once the direction of travel is established, a method for moving the contaminants must be determined. There are two methods of pressurizing a contaminated area to enhance and direct the travel of contaminants to the exterior of a contaminated area: natural and mechanical (pressurized) ventilation. Because we are going to review the basics of PPV and possibly interject some additional controversy (what's a little controversy between firefighters?), let's focus on pressurized ventilation (Note: Smoke ejectors can also be referred to as fans or blowers. For this and the following articles, the term “blowers” will be used):
PRESSURIZED VENTILATION
The current methods that are used to provide pressurized ventilation with portable blowers are negative pressure and positive pressure ventilation. To examine these two methods, assume a simple room with a door and window in a structure will be ventilated. In this example, the room is filled with various products of combustion from a fire. The warmer gases have risen to the top of the room, and the cooler gases have settled at the bottom of the room. The door and window are closed.
Negative Pressure Ventilation
To ventilate this room, the door is opened and a blower is placed inside the room to exhaust the contaminants. This method will draw the contaminants within the room through the blower and exhaust the contaminants to the exterior of the room by creating a negative pressure (suction) within the room. By opening the window, the exhausting contaminants will be replaced with fresh incoming air. Although this method can perform satisfactorily, it has the following disadvantages:
Positive Pressure Ventilation
To ventilate the room in our previous example, the door is opened and a blower is positioned outside the room. This method will force clean, fresh, pressurized air inside the room and create a positive pressure (similar to blowing up a balloon) inside the room. The positive pressure will be equal at the top, bottom, and corners of the room. When the window is opened, the contaminants from all parts of the pressurized room will exhaust to the exterior (similar to piercing a hole in a blown up balloon). Compared to negative-pressure ventilation, positive pressure ventilation has the following advantages:
IMPLEMENTATION
The effective implementation of positive pressure ventilation is dependent on the proper utilization of the following factors:
1. ENTRANCE OPENING
A cone of pressurized air issued from a blower should completely cover an entrance opening, and can be accomplished with a single blower or multiple blowers as follows:
Single Blower
A single blower should be positioned so the cone of pressurized air just covers the entrance opening. Remember that blowers of the same size but from different manufacturers can produce varying sizes of cones of pressurized air. Therefore, it is important to know the operational capabilities of your equipment. Blowers are manufactured so they can be tilted back about 20-30 degrees to enhance sealing an entrance opening. Although it is important to seal an entrance opening with a cone of pressurized air, it is not absolutely necessary.
Occasionally, conditions may be encountered that will not allow a blower to be positioned the correct distance back from an entrance opening. As an example, consider a raised porch that has a width of 4 feet, but it is necessary to position a blower 6 or 7 feet back from the entrance opening. In these cases, position the blower as far back from the entrance opening as the limited conditions will allow. If a blower is being used for smoke removal only, the smoke can recirculate from the unsealed portions of the entrance opening to the blower and back into the contaminated area, lengthening the time for ventilation. However, if a blower is being used for fire attack and fire is near an entrance opening, fire can recirculate from the unsealed portions of the entrance opening to the blower, and back into the fire area, creating a hazard to personnel and the blower. This fact is graphically illustrated in the popular video “FIRE” from the TV series Discovery. Interestingly, the efficiency of positive pressure in these cases is not compromised, only the time necessary to remove contaminants, or safety considerations of an operation when fire is near a blower (which is noteworthy).
Multiple Blowers
For standard entrance openings (i.e., door openings of 3 feet by 6½ feet), maximum effectiveness can be achieved by placing two blowers in line (series) with each other. The blower nearest the doorway should be positioned about 2 feet from the entrance opening. This ensures that all of the pressurized air from the blower enters the building, yet allows sufficient room for ingress-egress of personnel. The blower furthest from the doorway is positioned behind the aforementioned blower and is used to cover the entrance opening with pressurized air, and force additional pressurized air into the building. If two blowers of unequal size are used in the series position, place the larger blower about 2 feet back from the entrance opening and utilize the smaller blower behind the larger blower. Some blowers have been constructed to allow “stacking” or placing one blower on top of another. In this case, place the “stacked” blowers approximately 2 to 3 feet back from an entrance opening for maximum efficiency and ease of ingress-egress for personnel. For large entrance openings, multiple blowers in a parallel configuration should be used due to their combined ability to cover the larger opening with pressurized air. The size of the opening will dictate the number of blowers that will be necessary to cover the opening with combined cones of pressurized air. Remember that some openings (i.e., vertically moving loading-dock doors) can be reduced in size by partially closing the door which will reduce the size of the entrance opening that must be covered by pressurized air.
2. FLOW OF PRESSURIZED AIR
It is imperative that the flow and path of pressurized air between an entrance and exhaust opening be controlled to achieve maximum ventilation. Simultaneously opening unwanted windows and/or doors will not facilitate a successful positive pressure ventilation operation.
3. EXHAUST OPENING
Exhaust openings can be selected to provide horizontal or vertical ventilation of contaminants. Always create the exhaust opening first (if there ever was an “always” in the fire service, this is it!), and then pressurize the structure or area to be ventilated. If you reverse this order, where will the pressurized airflow go until an exhaust opening is created? The size of an exhaust opening can vary and is dependent upon:
As an example, let's apply this general guideline to a single-story, single-family dwelling of about 1,800 square feet:
A single 18-inch blower powered by a 2- to 3-horsepower engine would be most efficient when used with exhaust openings that are three-fourths to about the same size as the entrance opening.
A single 18- to 24-inch blower powered by a 5-or-more-horsepower engine would require an exhaust opening between one and one and one-half the size of the entrance opening.
Multiple 18- or 24-inch blowers powered by approximately 5-horsepower engines and in series or stacked configurations would require exhaust openings up to one-and-three-fourths the size of the entrance opening.
4. TRAINING AND COMMUNICATION
The key to effective positive pressure ventilation is similar to any fireground tools and/or equipment and is dependent on controlling the entrance opening, the path of the interior air flow, and the exhaust opening. These factors can only be achieved in their proper relationship if all fireground personnel have been properly trained and are aware of the goal of the intended operation.
MISCELLANEOUS CONSIDERATIONS
Carbon Monoxide
If a gasoline powered blower is being used and an exhaust odor is noticeable inside the area to be ventilated, this is an indicator that the exhaust opening is not large enough. An insufficient exhaust opening will allow pressurized air and the blower exhaust contaminants to accumulate inside the area to be ventilated. The exhaust odor should disappear by increasing the size of the exhaust opening (opening another window, door, etc.) and will also reduce the amount of carbon monoxide in the area to be ventilated. As carbon monoxide in the exhaust from gasoline-powered blowers can be a major consideration, the following considerations can alleviate this problem:
Weather
Temperature, humidity, snow, and rain do not have any appreciable effect on positive pressure ventilation. Although cold-damp weather conditions may limit the ability of smoke to rise, it will not limit the ability of blowers to move contaminants horizontally, and in most cases, vertically. Wind can have an adverse effect on positive pressure ventilation, but it's effect is dependent on direction and velocity. If it is not possible to utilize the prevailing wind as an advantage, positive pressure has proven effective against winds (leeward to windward) of up to about 25 mph. As winds exceed 25 mph, efficiency will be reduced accordingly.
SOP’s
Solid ventilation Standard Operational Procedures, in concert with proper training scenarios, form a foundation for the safe and effective use of all ventilation operations (including PPV operations). The SOP’s should spell out the specific intent of each ventilation procedure, how they should be implemented, and what should be considered before implementation. Without this guidance, your department runs the risk that any ventilation operation will be haphazardly used, potentially making conditions in the hot-zone exactly the opposite of what proper ventilation is designed to accomplish.
In Part 2, we will cover structural considerations.
EDITOR'S NOTE: Portions of this article appeared in National Fire and Rescue Magazine and in Tempest Training Package. Chief Mittendorf’s book Truck Company Operations, published by Pennwell, can be purchased by clicking here.
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