The most important task for firefighters is to save lives. In the case of fires in multi-story residential buildings, this objective can be best achieved by using the stairways as the preferred route of attack. This method of entering a multi-story dwelling ensures that the most important escape route for the inhabitants is immediately controlled.
One of the most important aims for fire crews is to have a smoke-free and safe stairway. However, choosing the stairways as the preferred route of attack to a fire in a building means opening doors. Firefighting operations sometimes enable smoke to travel through a building, entering stairways, which can lead to inhabitant endangerment.
The problem of smoke spreading in a stairway is unfortunately not sufficiently solved by using a positive pressure fan in front of the building entrance. It’s only when the fan is in an optimal position, with all windows and all other doors to the stairway closed and with an outlet of sufficient size in the fire compartment, that the complete ventilation process might be ensured. But what can firefighters do when this is not the case? What can be done in larger and more complex buildings where positive pressure ventilation is not possible?
The problem is illustrated in Figure 1. This figure presents results from a series of fire simulations and shows the calculated smoke spread in a multi-story building with mainly open windows and doors either with or without a positive pressure fan in front of the main building entrance. Figure 1 shows the results if the entrance door to the fire compartment is open and not controlled. Using a positive pressure fan, there is a better airflow through the stairway and therefore a reduced density of smoke, but there is still smoke entering the stairway from the fire area through the upper part of the door. This flow will result in some amount even if the entrance door is somehow controlled (e.g., opened slightly while being controlled by a firefighter). Considering these flows, the situations depicted in Figures 1a and 1b are not very efficient. Furthermore, there is also a lot of turbulence around the entrance to the fire compartment because of the bi-directional flow through the doorframe.
A potential solution to this problem is depicted in Figure 2, which shows the same situation as Figure 1 but with the use of a portable smoke-blocking device. The smoke-blocking device blocks the flow—at least in the upper part of the entrance door—and hinders smoke from flowing out into the stairway, but allows a small amount of fresh air to enter the fire compartment at floor level.
The smoke-blocking device improves both situations depicted in Figure 1 (either with positive pressure ventilation or without). Figure 2 demonstrates that flows in a building are much simpler and easier to control if the upper half of the entrance door is blocked. Ideally, the stairway remains smoke-free, or the smoke can be cleared more easily. Turbulence around the entrance to the fire area is reduced, and fresh air enters the fire area in a smaller amount and only at floor level. Therefore, the work of firefighters in the fire compartment becomes easier and safer, and smoke can leave the fire area through open windows.
Once the problem and the solution were defined, the obvious next step is how to block the upper half of the door. If firefighters forced entry through the door using saws or other forcible entry tools, one possibility would be to cut only the lower half of the door. Indeed, this idea was tested in training fires but obviously has some disadvantages and risks. Therefore, firefighters set to designing and testing a smoke-blocking device that would be fast and easy to deploy.
Such a device must meet many requirements for safety and practical reasons, including sufficient resistance to high temperatures, flames and mechanical damage. The portable smoke-blocking device should be able to be cleaned very easily and therefore needs to be impregnated against dirt and water.
Beginning in 2005, German firefighters tested several designs for a smoke-blocking device. The best results to control all flows were achieved by using a combination of a metal frame with a spreader and a special textile fiber curtain. In addition, firefighters found that results improved significantly if the curtain also influences the characteristics of the fresh airflow (flowing into the fire compartment).
Looking at the flow of smoke shown in Figure 2, the curtain must prevent smoke from travelling through the upper part of a door, but must still allow a small amout of fresh air to travel into the fire area through the lower part of the door (this fresh air, however, flows with much less turbulence, which prevents excessive mixture of fresh air and smoke). This design keeps the volume of smoke expelled from the building to a minimum and improves the conditions and safety for firefighters and civilians by allowing temperatures to drop and better visibility in the lower layer. Separating the cold air in the lower half from the smoke in the upper half should also result in a smaller smoke layer that is premixed with the necessary amount of oxygen from the fresh air flow.
Figures 3 and 4 show portable smoke-blocking devices in use.
Supported by Science
In recent years, multiple studies from UL and the National Institute of Standards and Technology (NIST) have shown the influence that fresh airflow has on fire development. Because of the higher heat release rate of modern furnishings, the time span from starting ventilation (e.g., opening a door) to a flashover in the fire compartment decreases to about 1–2 minutes. For the safety of all life in the fire compartment and the adjacent rooms in the building, this development must be prevented. Therefore it’s strongly recommended that we make every effort to control/reduce the flow of fresh air.
Reducing airflow can be achieved by keeping the door closed as much as possible even after the fire crew accesses the fire compartment—for example, using a chock to fix the door in a nearly closed position. However, it’s best to keep a firefighter at the door to ensure egress of the interior crews. Unfortunately, many departments don’t have the required staffing to position a firefighter for door control at this point in the fire attack.
A portable smoke-blocking device doesn’t require a firefighter to remain by the door, as the lower half of the door remains open for egress. Further, there’s another reason that smoke-blocking devices may be more effective than keeping the door nearly completely closed with a chock. When placed in the upper half of the door, the smoke-blocking device can positively influence the flow pattern in the lower half of the door, forcing the flow of fresh air to go low into the fire compartment. This reduces the chance that smoke and fresh air will create an ignitable mixture, thus giving firefighters and potential victims in the fire room better conditions in the layer closest to the floor—exactly where it’s needed.
It must be emphasized at this point that under-ventilating a fire in a compartment can lead to a higher amount and concentration of combustible gases. Entering a fire compartment even with the mandatory PPE is still dangerous, and smoke layers that are hot and contain combustible gases should be cooled and inerted to further reduce their ability to ignite.
Part of the reason smoke curtains work is because they transform the airflow from bi-directional flow (hot smoke out, high; fresh air in, low) to a one-directional flow (fresh air in, low). Firefighters then enter in the one-directional flow path.
This limited airflow improves visibility and reduces the temperature in the lower area. It also motivates the attack team to extinguish flames that they see more clearly—especially on the floor—which then leads to a reduced out-gassing and flaming combustion. Firefighters are therefore advised to use their hoseline to extinguish the fire, cool and inert the smoke layer and possibly cool other surfaces as they enter the fire compartment. In addition, the reduced flow of fresh air increases by several minutes the time between the start of ventilation (opening the door) and flashover in the fire compartment. This should allow firefighters more time to get water on the seat of the fire.
Following are some of the many situations where portable smoke-blocking devices could be used:
- Keep smoke out of stairways in multi-story buildings: Smoke curtains can keep stairways smoke-free, and/or allow firefighters to more quickly clear smoke that is already in the stairway. A smoke-free stairway enables the rescue team to stay closer to the fire area and therefore communicate more easily with the attack team. This results in a shorter escape route and improved safety for firefighters.
- Close open or damaged doors: Because they are easily set up, smoke curtains can serve to close an open or damaged door to prevent the spread of smoke in a building.
- Conduct positive pressure ventilation: With a smoke curtain in place, the positive pressure ventilation of a stairwell is much easier to achieve, and ventilation operations can be affected section by section.
- Limit airflow to the fire: Smoke curtains can reduce heat release rates (by depriving the fire of oxygen) and lead to less severe fire development. By depriving a fire of oxygen (under-ventilated fire), the time between the opening of an entrance door and a flashover in the fire compartment can be increased to allow sufficient time for firefighters to extinguish and control the fire.
- Stabilize conditions in the case of window failure or inadvertent opening of doors: Windows or doors opening unexpectedly can lead to rapid changes in the flow of smoke and hot fire gases, creating life-threatening hazards for firefighters. Smoke curtains can prevent the interior attack crew from being caught in the flow path between the fire and its outlet, or can at least reduce the air speed in this flow path.
- Reduce smoke damage: Firefighters often underestimate the damage caused by the spread of smoke. Smoke-blocking devices prevent smoke damage and create a highly visible separation of “black” and “white” areas, which in turn makes firefighters more conscious of the need to reduce smoke damage.
In hundreds of real fires, firefighters in central Europe have successfully used smoke curtains. Figure 5 shows one example: a fire in a residential building in Heilbronn, Germany, that occurred on Feb. 6, 2006. The fire area itself (a child’s room) was totally damaged by heat and smoke and the corridor suffered considerable damage. The portable smoke-blocking device placed in the doorway to the apartment effectively prevented smoke from entering the stairway. The firefighters entering the fire compartment were able to enter with a one- directional flow at their backs.
The bottom line: Using a portable smoke-blocking device can prevent the spread of smoke in buildings, resulting in improved safety for both occupants and firefighters, easier rescue operations and less smoke damage. This is a simple and efficient method for preventing the spread of smoke in residential buildings and modern structures with an open architecture. More information for the many uses of smoke curtains in building fires can be found at www.smokeblockingdevice.com.