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Postitive Pressure for a Positive Advantage
Part 2 - Structural Considerations
To achieve the maximum efficiency with positive pressure in ventilating a structure, do not "open the structure up," or ventilate all rooms simultaneously, as is the common practice in structure fires. Multiple openings can diminish the flow of air through each room and increase the time necessary for ventilation. Normally, the most effective method for most structures is sequential ventilation of contaminated areas. Sequential ventilation, or targeting specific areas, and ventilating them in a strategic sequence maximizes the flow of pressurized air from a blower into and through a contaminated area, which also minimizes the time required for ventilation. Remember that closing a door or window in an area will isolate the area from the ventilation operation by eliminating the flow of pressurized air to an exhaust opening. Additionally, an area that has open walls or ceilings (from a fire), or other large openings, can be isolated from a ventilation operation by closing an appropriate door, if applicable.
BASEMENTS
If a basement has multiple openings that can be used for entrance and exhaust openings (e.g., interior door, exterior cellar door, windows, etc), position a blower to cover an appropriate opening to the basement (such as the interior doorway to the basement) with pressurized air and use the appropriate basement opening(s) as an exhaust opening. If a basement does not have suitable exhaust openings, several methods can be used to provide them:
If a frame-wood structure has shiplap siding, the two bottom shiplap boards (next to the plate on the foundation) can be easily removed, exposing the opening between the ends of the floor joists and between the floor and plate, creating an exhaust opening (it will be necessary to remove the basement ceiling to complete the opening).
If the preceding method is not practical, position a blower to pressurize the structure above the basement (e.g., front door to the first floor), creating a flow of air through the structure to an appropriate exhaust opening (e.g., rear door). Next, place an additional blower in the interior doorway to the basement. This will pressurize the basement and force contaminants out of the upper portion of the doorway, which will then be carried out of the structure by the flow of pressurized air through the first floor.
DWELLINGS
Raised porches are commonly encountered in dwelling-type occupancies and can present a challenge when they make it difficult to place a blower back far enough to seal the doorway with a cone of pressurized air. Remember that if the size of a porch does not allow proper placement, move the blower as far back from the door as the porch will allow. If the door is not properly sealed, contaminants can exhaust from the unsealed portions of the door and be carried back into the pressurized airflow entering the doorway. Although this condition can lengthen the operation due to the re-circulation of some smoke, the effectiveness of the positive pressure ventilation is not effected.
Single Story
Use sequential ventilation and any prevailing wind to horizontally ventilate contaminants. These structures offer excellent opportunities to experiment with the number and size of blowers to determine their effectiveness with areas of varying sizes.
Multiple Story
If a multiple story dwelling needs to be ventilated, start at the lowest level and ventilate towards the top of the dwelling. To ventilate the first floor, ensure that all exterior windows on the upper floor are closed, or a stairwell door to the upper floor is closed (whichever is appropriate). Position a blower at an appropriate entrance opening, and sequentially ventilate the contaminated areas on the first floor of the structure. To ventilate the second floor, leave the blower in the same position and ensure that all exterior windows and/or doors are closed on the first floor. If a stairwell door has been closed, open the door and sequentially ventilate the contaminated areas on the second floor.
MULTI-HABITATIONAL (apartments, condos etc.)
Apartment buildings, condominiums and other multiple-occupancy dwellings can have single or multiple floors, and enclosed central hallways that provide access to numerous rooms within the building. The hallways may be of considerable length (depending on the size of a building) and may incorporate fire doors at various intervals within the hallways. If fire doors are present, they may be opened to allow pressurized air to travel to a specific location, or they may be closed to compartmentalize specific sections of a building. This may be useful to keep contaminants from spreading to uncontaminated areas, or to divert pressurized air to a specific area.
Multi-story habitational occupancies can be effectively ventilated with positive pressure by:
First, pressurizing and ventilating appropriate hallways. This can be accomplished by directing pressurized air from the appropriate entrance opening to the selected hallway from a pressurized stairwell. Contaminants can then be directed to the appropriate exhaust opening.
Second, ventilating contaminated rooms or other areas that are common to a pressurized hallway.
As an example, consider a recently constructed three-story apartment complex with a center hallway on each of the three floors, an enclosed stairwell that is common to each floor, and fire doors separating each floor from the stairshaft.
A fire has occurred in an apartment at the far-end of the third-floor hallway resulting in the apartment and third-floor hallway charged with smoke:
Position a blower(s) outside the building to cover the entrance to the appropriate stairshaft with pressurized air. Ensure that fire doors to the first and second floors are closed and open the third-floor hallway door to the pressurized stairshaft. With an open exhaust opening (window or exterior door) at the end of the third-floor hallway, the hallway will be cleared of contaminants.
After the hallway is cleared, close the exhaust opening at the end of the third floor hallway and open the door to the contaminated apartment. By creating appropriate exhaust openings, sequentially ventilate the apartment of all contaminants.
If an exhaust opening cannot be opened at the end of the hallway, open the door to the contaminated apartment and ventilate the hallway through the contaminated apartment. This is most effectively accomplished by opening a patio sliding glass door, large window, or using a room with the greatest number of windows as the initial exhaust opening. When the hallway and that portion of the apartment utilized as an exhaust opening is cleared, sequentially ventilate the other portions of the apartment.
COMMERCIAL BUILDINGS
Commercial buildings are diverse in their size, height, and type of occupancy. However, the following factors should be applied when considering positive pressure ventilation operations:
Sealed Areas
Typical modern multi-story sealed office buildings (exterior glass panels that cannot be opened) present a unique challenge. Normally, offices and similar types of areas in these buildings have only one opening that can be used for an entrance and exhaust opening (unless windows are broken). To ventilate these types of areas, use a blower to provide a flow of air past the contaminated opening(s). This blower can be located outside or inside to provide pressurized air to the interior of the building through a hallway and past the area to be ventilated. Place an additional blower in the doorway of the area to be ventilated. The blower will provide pressurized air that will create a positive pressure within the area to be ventilated and force the contaminants out of the upper portion of the entrance opening. The pressurized air flowing past the entrance opening will force the exhausting contaminants to follow its direction to the exterior of the building.
Area
Depending on the type of building, determine:
The effectiveness of removing contaminants horizontally or vertically
The path that pressurized air must travel to remove contaminants within the building.
Large structures or large areas within structures require increased air volume to remove large quantities of contaminants. Consider the use of larger blowers for these applications. Multiple-series blowers or parallel blowers can provide additional air flow and enhance the removal of contaminants. Use multiple blowers to pressurize large entrance openings. Some large doors (garage doors, for example) can be partially closed to facilitate the pressurization of a smaller entrance opening. Remember that office areas should be considered prime locations for ventilation (if the records of a business are saved, so is the financial cornerstone of the business). When possible, large areas of a building should be divided into smaller areas by closing partition doors, rolling or sliding fire doors, etc., and then sequentially ventilating each contaminated area. When large ventilators, broken skylights, or ventilation openings in a roof (that have been created by fire suppression operations) will negatively effect positive pressure operations, consider using these openings as vertical exhaust openings. Openings such as doors and windows that are below these exhaust openings must be closed to ensure the flow of pressurized air is maximized and directed to appropriate vertical exhaust openings.
Multiple Floors
Buildings with multiple floors can be ventilated by sequentially ventilating each floor. Position blower(s) on the exterior of the building and pressurize the opening to a stairshaft that is common to contaminated floors. Start from the bottom floor and work toward the top floor using the pressurized stairshaft to channel pressurized air to each floor as necessary. Sequentially ventilate each floor by opening a door to the pressurized stairshaft and an appropriate window as an exhaust opening.
HIGH RISE
Of the various types of structures that are encountered by fire service personnel, high-rise structures can be the most challenging. This is due to several factors:
Most fire suppression operations occur above the ground.
The migration of smoke and toxic combustion products throughout a high-rise building can often present a greater hazard to life and fire suppression efforts than the spread of the fire itself. As smoke normally travels upward, floors above a fire become prime exposures from stratification and mushrooming.
Inherent factors such as "poke-thru-construction", heating, ventilation, air conditioning (HVAC) systems, stairshaft pressurization systems, vertical stairshafts, and curtain construction can enhance the spread of fire and smoke throughout these types of buildings.
If a fire is above the reach of aerial devices, the only method of vertical access is elevators and stairshafts. As stairshafts can become contaminated with smoke, heat, and fire gases (i.e., Meridian One Plaza fire, etc), and elevators can be dangerous to personnel if they inadvertently travel to the fire floor (i.e., Memphis high rise fire, First Interstate fire, etc), safe access to upper floors can be challenging.
Building and Air Movement Characteristics
The following characteristics are common to high-rise buildings and can effect the travel of contaminants within a building and assist or hamper ventilation operations:
Construction
Smoke can travel between floors due to unsealed openings, spaces or cracks between floors, and "poke through" construction (openings utilized for pipes, electrical alleys, etc.). Additionally, the pressure that is generated by a fire (up to three times over atmospheric pressure) will force smoke through these openings that may contaminate other floors or stairshafts within a building (as was graphically demonstrated in the MGM fire in Las Vegas).
Doors
Doors that have been opened or inadvertently "left open" will allow smoke to travel horizontally and/or vertically to additional areas within a building. It is essential that the status of doors that may effect the travel of smoke from a contaminated area be monitored, particularly stairshaft doors.
Windows
Openings created by windows that have been removed will allow smoke to travel horizontally to the exterior of a building. Panels or windows in the exterior walls that are not openable may need to be broken to create ventilation openings to the exterior of a high-rise building.
Sealed buildings
Most high-rise buildings can be classified as "sealed buildings" (external glass panels that are not openable). These buildings control their internal environment (i.e., temperature, humidity, etc.) by HVAC systems, and therefore, are capable of retaining smoke, heat, and fire gases until they are manually ventilated to the exterior of a building.
HVAC systems
Heating, ventilation, and air conditioning systems are utilized to control the internal environment of high-rise buildings. Unless personnel are totally familiar with a HVAC system encountered at an incident, the HVAC system should be shut down until appropriate communications have been established with personnel familiar with the system.
Stairshafts
Natural vertical air currents that normally flow upward in stairshafts can be created by simply opening a door at the bottom and roof levels of a stairshaft. These air currents can be effectively utilized to remove contaminants that have collected or are entering a stairshaft from a fire within a building. Additionally, a natural vertical air flow in a stairshaft can be created or augmented by the following:
Stairshaft pressurization fans. When pressurization fans are activated (manually or automatically) within a stairshaft, an upward flow of air will be created when an opening (door, etc.) is created at the top of a stairshaft. The strength of this air flow will be dependent on the number and strength of the fans. These fans are capable of providing adequate air flows for ventilation operations.
Positive pressure. Positive pressure created by fire department blowers can effectively create or augment the natural flow of air in a stairshaft and result in the discharge of contaminants within a stairshaft by pressurizing the ground level entrance opening to the stairshaft and opening the roof level door. Additionally, positive pressure can be utilized with stairshaft pressurization fans for increased air flow and efficiency.
Elevators
Elevators (and stairshafts) may be located in a central core (known as a center-core building), or can be randomly located within a building. These vertical passageways provide excellent channels for the travel of smoke.
Mushrooming and Stratification
Stratification of smoke and fire gases occurs as heat and smoke travel vertically within a building. These products of combustion will rise through any available vertical openings until their temperature is equal to the temperature of the surrounding air. When stabilization of the temperatures occurs, smoke and fire gases will form layers or clouds within a building. This stratification of smoke and fire gases generally serves as a “lid” for other products of combustion which will tend to bank down below the stratified smoke and horizontally spread to other portions of a building. This process is common in vertical stairshafts. Mushrooming is a condition caused by smoke and fire gases that cannot vertically escape a building. Smoke and fire gases will rise to the highest level possible and then begin to bank down and fill all available spaces. Depending on the height of a high-rise building and the location of a fire, mushrooming smoke and fire gases can contaminate the upper floors, or contaminate the floors below stratified smoke in a stairshaft.
Ventilation Considerations
When a fire or contaminants from a fire have been verified within a high-rise building, positive pressure can be utilized (at the discretion of the Incident Commander) as follows:
Vertical ventilation
By opening the bottom and top door to a stair shaft, a natural upward flow of air is normally quickly developed. Blowers can be utilized to augment this flow of air, regardless of the height of a building. This process can be used to keep contaminants from accumulating in a stairshaft (if the roof opening is kept closed) or to remove accumulated contaminants and keep a stairshaft clear. This operation is currently utilized by the Los Angeles City Fire Department who requires that first alarm truck companies are responsible for pressurization of vertical stairshafts in high-rise buildings with fire.
Horizontal ventilation
Cross ventilation of a contaminated floor can be effectively accomplished by utilizing blowers to pressurize a stairshaft and directing pressurized air across a contaminated floor and out appropriate exhaust openings (i.e., windows). Remember to use windows on the leeward side of a building if possible. Contaminants can, if necessary, be also directed to an opposing stairshaft that opens to the roof. The exhausting contaminants from the affected area will be vertically exhausted from the opposing unpressurized stairshaft with the roof access door open. The preceding cross-ventilation operations are effective up to approximately 25 floors in a high-rise building. If necessary to cross ventilate floors above the 25th floor, it may be necessary to position an additional blower in the pressurized stairshaft to supply additional pressurized air to the floor to be ventilated. This is due to pressurized air that is lost through cracks, poke-through construction, ducts, and other similar openings.
Note: These ventilation operations may require personnel on a roof and ground for proper coordination of these operations.
In Part 3, we will look at additional PPV applications.
EDITOR'S NOTE: Portions of this article appeared in National Fire and Rescue Magazine and in Tempest Training Package. Chief Mittendorf's newest book, Truck Company Operations, can be purchased by clicking here.
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