BY KAREL LAMBERT
Within the scope of the course Postgraduate Studies in Fire Safety Engineering, I wrote a thesis on the use of power blowers for staircase ventilation.1 The project included a study of the literature and also experimental work, and this information produced the framework for a manual on ventilation. The ninth chapter discusses the various ventilator configuration possibilities and is the basis for this article.
Firefighters and the literature differ substantially on how ventilators should be configured. Two important factors play a role in this decision: the type of ventilator and the type of ventilation.
AIM OF VENTILATION
There is a distinction between horizontal and other types of ventilation. In horizontal ventilation, the inlet and outlet openings are on the same floor. A flow of air is created from the inlet opening to the outlet opening. In such an application, it is undesirable to have a two-way flow in the inlet because smoke can also flow out at the inlet. If that includes hot smoke, it creates a risk for firefighters who must enter through this (door) opening. The solution is to make an air seal and cover the entire door opening with a flow of air.
A similar problem arises when a room is put under positive pressure. Here, too, it is not advisable that an outflow be at the top of an inlet opening. The outflow would then be a type of leak, which will cause less positive pressure to develop in the room.
When configuring ventilation in an apartment, the entrance door may be the inlet opening. The outlet opening can then be a window on the second floor. In this configuration, a two-way flow in the inlet opening is no problem. As a matter of fact, no smoke will be drawn in from the second floor to the ground-floor inlet opening. The question here is, “How do we position a ventilator so that the highest yield can be obtained on the second floor (regardless of what happens on the ground floor)?”
There are two options: The inlet opening must be completely inflow, or a two-way flow may arise at the inlet if that improves ventilation.
A second area of tension in ventilation is weighing tactics against yield. To achieve high yield, the ventilator must be positioned where it will be in the fire crew’s way. This is not advisable from a tactical standpoint: When the attack team advances, the hose that goes through the door will move and will probably cause the ventilator to move. From the safety perspective, the ventilator can interfere with the egress of firefighters should they have to leave the premises quickly.
DEPENDENCE ON THE TYPE OF VENTILATOR
The first power blowers produced a cone-shaped flow of air. It has always been said that a conventional ventilator should be placed far enough from the door so that an air seal is formed. This means that the door opening is fully covered by the cone of air formed by the ventilator. Firefighters are taught that they must test whether the cone covers the door completely by feeling with their bare hand where the reach of the air flow is. In that way, the flow of air travels in the same direction over the full inlet and no smoke comes out at the top.
You can use the following rule of thumb to achieve this effect: The distance of the ventilator from the door should be equivalent to the height of the door. Many entrance doors are approximately two meters high. In those cases, the ventilator must be positioned approximately two meters from the door in a tilted position to completely cover the door.
A disadvantage of this method is that an important part of the air flow hits the wall around the door opening and does not contribute to ventilation.
Mark Yates2 states that the rule of thumb was questioned in 2002 because of the yield loss as a result of the air’s hitting the wall. Kriss Garcia3 describes this method in his book published in 2006 and concludes that covering the door opening is not absolutely necessary. However, it is a positive effect when applying positive pressure attack. My research leads me to think that an air seal is required only to ventilate the ground floor. According to Dr. Martin Thomas,4 the configuration depends on the aim to be achieved. He says there is up to 50 percent less inflow if the ventilator is two meters away from the door. The pressure inside will be 10 percent lower.
National Research Council of Canada research5 shows that the efficiency of a ventilator increases the closer it is positioned to the door. Also, the less a ventilator is tilted, the more efficient it is.
A tactical consideration must also be made in addition to the yield of the ventilator. A ventilator standing right in front of the door opening can provide the highest yield. However, it will stand in the way of the firefighters who go in or come out. It will quickly be pushed by the movement of the water hoses, which means that it will no longer be in the optimum position.
Weighing both elements (yield and tactics), the ideal solution seems to be to position the ventilator approximately 1.6 meters from the door. (1) The ventilator is not tilted in this case. Tilting will cause an air flow to originate in the direction of the door opening, producing the venturi effect on the top and sides of this flow of air, and additional air will be drawn in. It is possible to deviate from this by covering the door opening to avoid the outflow of air at the top. This is the case in horizontal ventilation.
The last element that must be taken into account is the in-situ situation. Firefighters sometimes use too many rules without knowing the bases for the rules. Sometimes, in practice, the ventilator must be tilted. For example, often the pavement is in a bad state and the blower can be better positioned if it is placed half a meter forward or backward. Also, there may be a doorstep or even a number of steps, which means that the entrance door to a house is half a meter above the level of the pavement. In such cases, it is best to tilt the blower.
A French company developed a smaller fan, called a turbo fan, through which the airflow is shaped more as a cylinder than a cone. The speed of the airflow is higher than that of a conventional fan. Such a fan is not intended to attempt to cover the entire inlet opening. The fan is designed in such a way that the venturi effect yields maximum results, meaning that a smaller appliance can still generate large airflows. The German fire engineer Christian Emrich6 suggests that such ventilators must be positioned slightly farther away from the inlet opening.
ALTERNATIVE CONFIGURATIONS WITH ONE VENTILATOR
In principle, it is possible to let a power blower work inversely. Its yield will be much, much lower. Yet, it can be a way to use the means available to extract smoke from a room where there is only one (door) opening. An example of this is the extraction of smoke from a pub of approximately 538 square feet after a fire. If the room does not have any other opening, natural ventilation will take a long time. It will be faster to let the fan do the extraction.
The same principle can also be used when damping down an underground parking garage (removing residual smoke left after extinguishment to decrease the carbon monoxide level a lot before the garage can be reopened to the public). Airflow will be created by letting the fan blow outside, which will contribute to extracting the smoke from the parking garage.
However, when using this method, remember that smoke gases will go through the fan. If an appliance with a combustion engine is used, it can stall because of a lack of oxygen. Moreover, these appliances were not designed to be used in this way. It is, therefore, recommended that this configuration be used only if there are no other options.
If there is a fire in a building, several teams of firefighters would probably respond to the scene. This implies that more ventilators will be available for the ventilation. It is possible to combine these ventilators to achieve better results.
The recommendations below are the results of tests with conventional ventilators. Research must be done to determine if these conclusions also apply to the turbo fans and straight stream air flow fans.
Two Blowers One Behind the Other, or in Series
Some authors in their literature propose positioning two blowers one behind the other. The idea is that the front blower will reinforce the flow of air generated by the back blower. In addition, the back blower ensures that the inlet opening is covered completely. Another way to view this system is that the blower at the back feeds the one in front in the same way as occurs in a dual booster water pump system.
Most literature recommends that the larger blower be positioned in front. However, Garcia (3) prefers that the smaller blower be in front one meter from the door, which, he says, produces 30 percent more yield than by using one blower. Koen Desmet7 mentions 10 percent additional yield in his work.
In my experiments, I presumed that the front blower had to be far enough from the door so that it would not be an obstacle for the firefighters. A fan that stands one meter away from a door opening is in the firefighters’ way. The best result with two blowers one behind the other was achieved when the front blower was tilted as much as possible and the back blower was not tilted. This method produced more yield than using one blower. I did only one series of experiments with blowers in a series; I did multiple series with blowers in a “V” pattern. Therefore, I can say with a high degree of certainty that my results about “V” patterns are correct.
There is, therefore, no agreement in the literature on the method to be used when putting one blower behind the other. However, it is indeed certain that two ventilators produce a higher yield than one ventilator. It is definitely the case that the yield will be even higher if the first ventilator is positioned one meter from the door. However, that is not so convenient for the firefighters working. Where a door is not used to enter or leave the building, the ideal configuration would be in a series and would be as follows: one ventilator positioned just in front of the door and not tilted and a second ventilator positioned at approximately 1½ meters from the door and tilted.
Two Blowers Next to or Parallel to One Another
A second possibility when combining two blowers is to position them next to one another. This can be done in the case of a normal door and a garage door. In the case of the door, the blowers can be positioned in a series or in the shape of a “V.” Pertaining to the garage door, the width of the opening plays a major role. The blowers are positioned some distance apart to cover the entire opening. If there is a high sectional door, it may be convenient to lower part of the door so that the inlet opening is slightly smaller. If the inlet opening is 13 feet high, no configuration can cover the entire opening. By lowering the door two meters, the inlet opening can be brought down to the level that can be fully covered by the two blowers.
For the purposes of my thesis, I obtained a yield of 51 percent more at a normal door opening than when using one blower, which is close to the additional yield obtained with two blowers in series. Note, however, that two parallel blowers in front of a normal door opening form even more of an obstacle for firefighters than one blower.
This type of configuration is not as well known but is, nevertheless, referred to in a number of works. (1, 3) 8,9 In this configuration, the blowers are positioned in such a way that they form a “V” (Figure 1). Both blowers are aimed at the door opening. The idea behind this is that a venturi effect is caused between the two blowers. The two airflows hit the door opening and draw in additional air. The distance between each of the blowers and the door opening is approximately five feet.
In Australia, people are taught that they must leave one blower standing straight and the other blower must be tilted. The idea behind this is that one blower covers the bottom of the door opening while the second blower covers the top of the door opening. My research showed that a higher yield is obtained if neither of the blowers is tilted. The same remark can be made here as in the case of the configurations for one blower-that is, that firefighters have to take the local situation into account. Sometimes tilting is necessary because of a couple of steps that give access to the building or because the pavement is not horizontal.
If horizontal ventilation is required and it is necessary to avoid a two-way airflow in the doorway, then it is advisable to tilt one of the blowers. The door opening is then fully covered, but there is a slight yield loss. Garcia (3) states that a “V”-shaped configuration produces 10 percent more yield than a series or parallel configuration. I found that this difference was more likely 20 percent. This configuration was tested in different places. The additional yield for one blower varied from 52 percent to 164 percent. This means that the “V”-shaped configuration generated an airflow that was at least 11/2 times greater than the airflow caused by one blower standing right in front of the door. In the case of the maximum yield, the airflow was found to be 2.6 times greater than it was for one blower. If one blower straight in front of the door sends a certain debit of air through the building, you would expect a combination of two blowers to generate an air debit that is, at most, twice this debit. However, in the best test, a debit of 2.6 times as much was measured. The difference in the increase is attributed to the venturi effect between the blowers.
During my research, I also varied the angle between the two blowers. The angle is clearly marked in Figure 1. It is the angle between the centerline of the door and the connection between the middle of the door and the middle of the blower. In Figure 1, the angle is 30°. An angle of 45°, as in photo 1, is too wide. A better result is achieved with a smaller angle. A configuration of 30° or 20° (photo 2) produces similar results.
|(1) Two blowers in a “V” shape at a 45° angle. (Photo by author.)|
|(2) Two blowers in a “V” shape at a 20° angle; firefighters cannot pass in and out easily. (Photo by author.)|
|Figure 1. “V”-Shaped Configuration|
|Illustration by Bart Noyens.|
The much higher yield of the “V”-shaped configuration makes it extremely worthwhile if it is necessary to ventilate on higher floors. The longer the distance between the ground floor and the fire floor, the more loss in the staircase to the floor that requires ventilation. With a “V”-shaped configuration, a stronger “pump” at the bottom of the staircase is formed, as it were. Because higher velocity occurs, there will also be more loss in the case of the “V”-shaped configuration than in the case of a configuration with one blower. For more detailed information on this, refer to my thesis. (1) Tactically, a “V”-shaped configuration is a worthwhile configuration. In contrast to the configuration with one blower, the blowers are not in the way of the attack teams. The space between the two blowers can be used to enter or leave the building. It is perfectly possible to put one or more attack hoses between the two blowers. That is why the position with the 30° angle is regarded as the optimal position. A high yield is achieved, but the space between the two blowers is also adequate. In the case of a 20° angle, the space is too small to allow firefighters to pass easily.
The V-shaped configuration does not work if there are side walls hindering good airflows. Two blowers have been configured in a “V”-shape in a garage. Both blowers draw in air, which they then subsequently blow in the direction of the door opening. Drawing in this air is hindered by the side walls of the garage, which causes the yield of the blowers to decrease. The blower on the left in photo 3 is especially hindered by the side wall. I established in the experiments that one blower positioned in front of the door produces a higher yield than two blowers in a “V”-shaped configuration if side walls hinder their drawing in of the air.
|(3) A “V”-shaped configuration in a room where the side walls are too close to each other; the blowers cannot draw in sufficient air. (Photo by author.)|
It is also possible to make additional configurations. One configuration I tested is the combination of two blowers in a “V” shape at the entrance door and one blower at the bottom of the stairs (photos 4-5). This type of configuration is convenient in buildings where there is a lobby on the ground floor. In high-rise buildings, often the entrance door and the stairs are some distance apart. In most cases, there are also a number of doors between the entrance and the stairs. All elements cause the ventilation yield to drop. Such an experimental configuration, nevertheless, produces a high yield.
|(4) In tests in Oostkamp, two blowers were positioned in a “V” shape at the entrance door of an apartment building.|
|(5) A third blower was added at the bottom of the staircase. (Photos by author.)|
This experimental configuration was compared with the configuration of one blower and a “V”-shaped configuration of two blowers. The results are listed in Table 1. The experimental configuration produced approximately 3.5 times the yield of one single blower.
|(6) An aerial photograph of a building housing several companies. The second building from the left was hit by a very large fire. (Photos courtesy of www.SERF.se.)|
In Sweden, blowers are also used on a grand scale to put rooms adjoining the burning rooms under positive pressure. This is often combined with applying the cold-cutting extinguishing technology. Cold-cutting technology refers to a cutting extinguisher that enables firefighters to make s small hole (0.25 inch) in a construction element (wall, door, floor). After this has been done, water is applied. This water cools the gases. This method was applied remarkably efficiently in a fire in an industrial complex in Boräs (photo 7).10
|(7) In a fire in Boräs, the fire brigade used 11 power blowers in six different places to put the adjoining premises under positive pressure. The smoke gases at the edge were cooled by a cutting extinguisher, preventing the fire from spreading.|
A fully developed fire was raging in one of the companies in a large industrial complex. The fire occupancy was 118.11 feet × 262.467 feet. The fire brigade was striving to save the adjoining occupancies. Eleven power blowers were used. All adjoining companies were put under positive pressure; this tactic made it more difficult for the smoke gases to spread to other buildings in the area. This approach was combined with the use of seven cold cutters. The smoke gases at the edge of the burning compartment were cooled in this way. This combined approach of cold cutters and positive pressure ventilation ensured that the fire was confined to the compartment of origin. The insurance company investigated and concluded that the fire brigade’s actions had prevented $20 million in damages.
You can apply the general principles of fluid mechanics to smoke extraction. Ventilators can be regarded as air pumps. By positioning them in strategic places, they can extract smoke from underground parking garages or basements having various underground building layers. Small electrical ventilators fitted on the cage of laddered fire trucks during normal use are ideal for this. Consumption of electricity by such ventilators is extremely limited. It is often possible to connect the ventilator to the wall socket in the basement. A ventilator is positioned to send the airflow in the right direction in places where the direction of the airflow needs to be changed. The same is done in places where the air has lost its speed because of the friction of the walls. In such cases, the ventilator works like an air booster pump. In such a configuration, you must constantly adjust the configuration. As a matter of fact, in due course, the smoke will disappear at the farthest point. Ventilators can be moved to other points when they are no longer needed. It is a time-consuming method, but there are really not many alternatives for underground buildings for which no smoke and heat extraction systems have been constructed.
This article is an attempt to share the knowledge I gained in writing my thesis. It deals only with a limited part of the subject of ventilation and is incomplete. My aim is to encourage increased knowledge and a better understanding of ventilation.
1. Karel Lambert, “Experimentele studie van het gebruik van overdrukventilatie in een traphal bij een brandweerinterventie” (“Experimental study of the use of positive pressure ventilation in staircases in an intervention by fire-fighters”), Master thesis for postgraduate studies in fire safety engineering, Ghent University, 2012.
2. Mark Yates, “The wind of change,” Brigade command dissertation, Fire Service College, 2002.
3. Kriss Garcia, “Positive Pressure Attack for Ventilation & Firefighting,” Reinhard Kauffmann & Ray Schelbe, Fire Engineering, 2006.
4. Martin Thomas, “The Use of Positive Pressure Ventilation in Firefighting Operations,” not dated.
5. Lougheed, Mcbride & Carpenter, “Positive pressure ventilation for high-rise buildings,” National Research Council Canada, August 2002.
6. Christian Emrich, “Flow characteristics of the different fan technologies.” 2009.
7. Koen Desmet, presentation Ventilatie bij branden (“Ventilation in the Case of Fire”), 2007.
8. New South Wales Fire Brigades, Tactical Ventilation Level 1 course, 2004.
9. Various presentations and practical sessions on the course in 3D-Firefighting, given in Germany in October 2009. Peter Mcbride, “Wind-driven fire; Tactics & techniques of vertical ventilation; and Smoke movement & control in high rises; and Shan Raffel, PPV Siting & Safe zoning.
10. Södra Älvsborg Fire & Rescue Service (SERF) with SP Technical Institute of Sweden, “Cutting Extinguishing Concept –practical and operational use,” 2010.
● KAREL LAMBERT works as a battalion chief with the Brussels Fire Department in Belgium and is a volunteer firefighter in his hometown. He has a master’s degree in civil engineering and in occupational safety and health. He is co-author of a Dutch book about firefighting and wrote several articles in Dutch and French for various European firefighter magazines.
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