Possibilities of Using Mobile Fans and the Parameters Conditioning the Effectiveness of Tactical Mechanical Ventilation

Aim: The aim of the article is to indicate the possible scope of application of mobile fans during the execution of rescue actions and identification of the parameters conditioning the effectiveness of using this type of units. Method Design: Mobile overpressure fans are a tool used during rescue operations mainly to remove hot gases and smoke, which accumulate in confined spaces covered by fire – primarily within the evacuation routes, but also in the rooms. In literature there are examples of other uses of the considered fans, e.g. to support the liquidation of fire hazards of free-standing objects (i.e. fires of cars or containers) and the rescue of trapped people in inaccessible spaces by supplying fresh air. This paper, which is based on literature review, is devoted to providing an approximation of the above applications. The effective use of mobile fans requires specialized theoretical and practical preparation. Therefore, the study also identified factors that may determine the successful implementation of the adopted tactical intent, which include in particular: the selection of appropriate openings (outlet and inlet), the selection of the gas exchange path and the proper positioning of a mobile fan. Conclusions: Literature analysis of the problem presented in this paper will be a fundamental point of reference for the research work carried out in subsequent stages related to the evaluation of the efficiency of mobile fans. As part of this work, large-scale tests will be carried out using appropriately designed and constructed test benches to evaluate the effectiveness of mobile units in real conditions. The knowledge gained in this way is intended to serve as material for further considerations on the creation of concepts of both subsequent methodologies and test stands required for their implementation, enabling the verification of parameters characterizing the efficiency and reliability of mobile fans. Klapsa


Introduction
The possibility of using mechanical ventilation with the use of mobile fans -especially in a situation where during a fire there is a risk of cutting off the escape routes from a building -is an important tool that can significantly assist in achieving the tactical intent adopted by the manager of the rescue operations.
Using ventilation allows, above all, to remove hot and toxic fire gases that accumulate in the spaces covered by a fire and pose a real threat to the users of such an object, as well as arriving rescue teams on the scene. In this aspect, the primary area of application of mobile fans during rescue operations is smoke removal (ventilation) of horizontal and vertical escape routes. In large rooms, e.g. underground garages, appropriately prepared mobile fans may also considerably facilitate the access to the source of fire (e.g. a burning vehicle), supporting in this respect the smoke removal systems functioning in a given building. In order to achieve the desired results, it is also necessary to have good knowledge of the layout of the internal structure of the rooms in a facility, the fire protection equipment installed in it and the fire scenarios adopted by the designer and fire protection expert. The above examples of mobile fan applications do not exhaust their capabilities. Due to their functionality, these devices can also be used during the implementation of a number of other activities.
Among their potential applications, the literature mentions supporting fire suppression activities for free-standing objects, such as cars or containers for recycling materials. In addition, electric-powered fans are also mentioned for their use during rescue Using tactical mechanical ventilation requires proper theoretical and practical preparation, and -as can be seen from the analysis of the available literature data -so far no universal procedures have been developed to ensure effective aeration of objects and removal of smoke and fire gases from them. There is no doubt that this state of affairs is influenced by many factors, including spatial diversity of buildings, different locations of fires, the functioning of fire-fighting equipment installed in buildings, the SFT VOL. 59 ISSUE 1, 2022, PP. 58-82 variability of atmospheric conditions, etc. Despite a wide range of variables mentioned above, it is possible to identify issues whose appropriate resolution will greatly affect the effectiveness of the implemented activities. These include, in particular, selecting the correct inlet and outlet openings, identifying the gas exchange path, and properly positioning the fan itself.

Tactical ventilation
As mentioned earlier, the activities included in the area of tactical ventilation are a very important element of tactical-operational intentions which are to eliminate fire hazards. Over the past few years, firefighters have developed many methods to move thermal decomposition products out of a building space. Definitions of tactical ventilation, assigned to activities dedicated to firefighting, have been included in many training manuals, scientific publications, or fire industry journals. According to the definition presented by Paul Grimwood [1], tactical ventilation is a set  wietrzaniu. W okresach letnich, gdy temperatura powietrza zewnętrznego jest wyższa niż temperatura panująca w budynku, obecny jest niekorzystny rozkład ciśnienia i ciepłe powietrze może być zasysane do wnętrza ventilation process itself, but is a prelude to introducing fans into the operations. It allows to understand the relationship between the air flowing into a facility, fire gases leaving the combustion zone and the presence and influence on the smoke removal of gas exchange paths and openings in the structure of the facility, affecting the combustion process and rescue as well as firefighting operations.
2. Gravity ventilation -the action of taking advantage of the difference in density of the fire gases resulting from the temperature difference between the combustion zone and the external environment. The principle of its operation is based on the phenomenon of convection, i.e. the rising of hot smoke and fire gases (the so-called chimney effect). In this case, free convection involves air movement caused by a temperature difference and, therefore, a difference in air density. Through heat exchange (initiated, for example, by opening a door to the fire room), air density and static pressure change, causing heated air of lower density to rise and air of higher density to fall. The more intense the heat exchange, the faster the air movement, which contributes to increased smoke removal efficiency. When using this type of ventilation, it must be remembered to provide compensating openings through which fresh air will flow, ensuring that the proportion of received and supplied gases is maintained. According to the manual of Main Headquarters of the State Fire Service (KG PSP) [2], to ensure adequate effectiveness of the indicated tactics, the area of inlet openings should be larger than the area of outlet openings -the amount of supplied air should be larger than the amount of discharged air. Moreover, inlet openings should be located lower than outlet openings to take advantage of thermal buoyancy, which is the basis of heat transfer (convection). When considering issues related to tactical gravity ventilation, it should be pointed out that this type of action involves the supply of oxygen with the incoming air to the fire focus, which consequently may lead to an increase in the dynamics of combustion. It is also worth mentioning that the use of gravity ventilation will not always be effective, especially in the case of smoke extraction of spaces where the smoke has already cooled, the combustion process has ended, and the action is only ventilation.
During summer periods, when the outside air temperature is higher than the temperature inside a building, an unfavourable pressure distribution is present and warm air can be drawn into the building through the openings (windows or doors). This type of phenomenon is called reverse chimney draft. In practice, this phenomenon has a very unfavourable effect in the formation of smoke traps in the building, which may have a negative impact on the evacuation conditions or even make it impossible to carry it out in a safe manner. It should also be mentioned that a flagship example of SFT VOL. 59 ISSUE 1, 2022, PP. 58-82 such a situation are the activities related to the removal of toxic gases, vapours of hazardous substances or very often occurring on the burned area carbon monoxide.
3. Hydraulic ventilation -this technique is based on the phenomenon related to water droplets delivered from a nozzle, which -dispersed into an area where fire gases of lower density are present -displace them from their flight path, causing the movement of gases [2]. When analysing the trajectory of a water droplet, it should be pointed out that just in front of the droplet, a positive pressure is created due to the momentum and dynamic pressure of the fluid set in motion, and just behind the droplet, a negative pressure is created. As the length of the distance increases, the dispersed water droplets decrease the value of the momentum force and at the same time set in motion the gas (including combustion products) in the area of influence of the water dispersion source, due to the resulting differences in the pressure values. The source in question mentions tactical solutions to direct the flow of the fire gases using short-circuit currents. They consist of applying a dense current of water, directed at the areas of the discharge opening. Another variant of hydraulic ventilation is a technique involving the application of diffused current from inside a smoky room through a window to the outside of the building. As a result of the created pressure difference, the smoke from inside the room will be sucked outside together with the water current.
This technique is mostly used by emergency responders working on a fire scene to quickly improve visibility in a room.    Another way to use a mobile fan is for negative pressure ventilation. The operation is analogous to positive pressure ventilation with the difference that the fan unit is positioned in the outlet opening in such a way that it sucks the accumulated fire gases from the inside of the building and blows them outside.
When applying this type of ventilation, it is necessary to determine the inlet and outlet openings, select the gas exchange path, and properly mount the fan unit. In order to achieve the desired effects of negative pressure ventilation, it is possible to locate the unit in the upper area of the outlet opening frame. This solution allows the hot fire gases accumulated in the ceiling space to be discharged efficiently. A graphical illustration of vacuum ventilation solutions is shown in Figures 2 and 3.   According to the recommendations presented by K. Garcia [7], mobile fans can also be used to fight the threats of free-standing objects, e.g. fires of cars or garbage containers.
When referring to the issue of motor vehicle fires, one should be aware that due to the presence of large amounts of plastic materials in their construction, in the event of a fire, highly toxic products of thermal decomposition are intensively generated.
Due to the use of a mobile ventilator, the rescuers can properly direct the air stream to the surface of the vehicle, which will reduce the inflow of toxic fire gases into the areas of firefighting activities (easier access), or where there may be bystanders who do not have respiratory protection. In addition, the use of PPV ventilation during a vehicle fire can improve visibility so that the source of the fire can be located more quickly. When using this technique, it is important to remember that the fan, whenever possible and reasonable to achieve the accepted tactical intent, should be positioned on the windward side so that the air stream is directed in the same direction as the wind. This will increase the efficiency of the used ventilation.
Another possibility, mentioned earlier, for the use of mobile fans is fires involving waste containers. According to the recommendations presented by K. Garcia et al. [7], the principle of implementing actions in this group of objects is similar to that of vehicles. One of the problems that can occur during a container fire is the inability to identify the burning material (and fire experience shows that very often materials that produce highly toxic smoke and fire gases are involved, i.e. plastics, industrial waste, etc.). Unfortunately, it happens that dishonest entrepreneurs carry out a kind of (for them "no-cost") disposal of substances that are very dangerous to human health and life, as well as to the environment, intentionally causing a fire in such containers. In this context, the possibility of reducing the impact of the effects of combustion of these substances on the rescuers appears to be an extremely important factor, reducing the risk of loss of protective properties (contamination) by clothing and other personal protective equipment used during the operations.
These devices can be used not only to protect the rescuers and bystanders, but also to protect objects directly adjacent to the place of a fire. Appropriate positioning of the fan makes it possible to limit the inflow of hot fire gases towards these objects and their impact, for example, on the surface of the building's facade. A diagram illustrating techniques to assist the elimination of fire hazards in waste containers is shown in Figure 5.   The next issue that needs to be addressed in terms of creating conditions for effective ventilation is the provision of an outlet opening of adequate size. Such an opening should be selected or constructed as close as possible to the source of the fire so that the path of transport of toxic combustion products is as short as possible. As for the size ratio of the inlet and outlet opening, as recommended in [8], it should be between 1:¾ and 1:1¾, respectively (see Figure 7). If the smoke discharge opening is too small, the migration of combustion products will be slower and some will move to other areas of the facility. According to U. Cimolino et al. [9] the size of the outlet opening should at least match the size of the inlet opening. However, It should be pointed out that when the discharge opening is too large, other hazards arise, such as failure to provide sufficient positive pressure in the path of gas movement and greater susceptibility to weather conditions. Random objects, such as a trashcan, can also be used to block the openings (see Figure 8), but their use does not provide the same assurance as using dedicated equipment.  Badania pod kątem zwiększenia skuteczności zastosowania kurtyny dymowej, potwierdzone analizami (symulacjami) The described parameters are the geometrical indications at which the fan, when tested based on a dedicated test [10], obtained the best result in terms of aerodynamic efficiency.
In case when a given fan available on the market has not been tested based on a dedicated testing standard (such situations occur in reality), one can be supported by the results of empirical studies characterized in the literature [11][12] and recommenda- The study Tactical Ventilation -Applied Ventilation Systems [9] indicates that for conventional fans, the distance from the supply opening should be approximately equal to the height of the opening.
In the available publications [12][13][14][15][16][17]  Due to the fact that the jet produced by the turbo fan has a shape similar to a cylinder, and not a cone, in case of using smaller turbo fans (e.g. with a rotor diameter less than 450 mm) it becomes practically impossible to obtain the effect of a "tight cone". However, the losses associated with this are compensated for in the effect of obtaining better momentum performance from the air stream. Provision may also be made for an additional smoke curtain, mounted in the upper area of the door, to increase the efficiency of this type of unit (see Figure 9).
Research for increasing the effectiveness of a smoke curtain, confirmed by numerical analyses (simulations), is presented in the article by P. Panindre et al. [11]. In this study, it was shown that the use of a smoke curtain when implementing smoke control in a 7-story building improved the flow rate by up to 30%.
It is worth noting that there are fan units available on the market that have graphics to facilitate their proper use, located on the impeller housing, showing recommendations for geometric parameters of fan positioning in front of the inlet opening. An example of such a graphic is presented in Figure 10.
numerycznymi, zostały przedstawione w artykule autorstwa P. Panindre i in. [11]. W tym opracowaniu wykazano, że zastosowanie kurtyny dymowej podczas realizacji oddymiania w 7-kondygnacyjnym obiekcie budowlanym poprawiło wielkość przepływu nawet o 30%. Figure 9. Cooperation of the mobile high-pressure fan with a smoke curtain Rycina 9. Współpraca mobilnego wentylatora nadciśnieniowego z kurtyną dymową Source: Own elaboration based on [11]. Źródło: Opracowanie własne na podstawie [11]. placed in front of the door in one line (axis). The first fan should be placed at a distance of 0.6-0.9 m from the door opening, and the second fan should be placed at a distance of 2.4-3.0 m from the door threshold. In the system described above, the first fan is responsible for pumping a stream of air into the interior of an object, while the second fan delivers another portion of air and seals the inlet opening, preventing backflow of products of combustion. A diagram of the location of the fans in the configuration described above is shown in Figures 11 and 12.   Figure 11. Use of mobile fans in a line perpendicular to the door opening (one behind the other configuration) Rycina 11. Zastosowanie mobilnych wentylatorów w linii prostopadłej do otworu drzwiowego (konfiguracja jeden za drugim) Source: Own elaboration based on [7]. Źródło: Opracowanie własne na podstawie [7].  [7] showed that adding another fan in the line will increase the air inflow volume by about 30%.
In the course of providing air to objects with larger cubic capacity, having openings of dimensions larger than standard (0.9 x 2.03 m), e.g. garage door opening, in accordance with the recommendations described in Positive Pressure Attack for Ventilation and Firefighting [7], it is recommended to place fans in a configuration parallel to the opening (one next to the other).
Still another way of positioning the fans is the so-called "V" configuration, where the fans can be placed at a 45° angle to the door opening [7]. In this method of positioning, the conical airflows generated by the mobile fans are directed at the door surface so that they intersect while covering the entire door opening (see Figure 14). In this technique it is possible to control the setting of the inlet angle of the impeller surface so that one fan covers the lower area of the inlet opening with the air stream and the other fan covers the upper area with the air stream. With regard to the "V 45°" configuration, it should be pointed out that, according to the results of a study by K. Garcia et al. [7], the described positioning of fans allows to inject 10% more air than in the case of perpendicular or parallel positioning using two fans.
Kerber et al. [14], in a study published in the National Institute of Standard and Technology (USA) reports, showed that V-shaped positioning of fans allows more air to be forced into the object and creates more positive pressure than a series configuration.
Moreover, the authors of this study indicate that thanks to the specific fan arrangement, the described configuration provides greater access to a building, thus it does not impede other activities performed by the firefighters (e.g. hose line deployment).
Using a larger number of units to ensure effective ventilation should be considered in particular for buildings with large volumes (e.g. warehouse halls) and a proportionally larger ventilation opening. In such cases, the tactic of placing the fans in two lines, parallel to the inlet opening surface, can be used as such equipment becomes available. A schematic of such a solution is shown in Figure 15.
When approaching the problem of smoke removal from large-volume facilities, the results of a study by Lambert et al. [15] should also be cited. They evaluated the feasibility of using mobile  In order to lower the temperature and reduce the intensity of heat transfer inside the stairwells, devices to reduce the effect of wind on the fire environment are recommended. According to the recommendations presented in the study on smoke ventilation techniques [9], it has been shown that wind pressure (acting at a speed of 25 km/h) will significantly reduce the effectiveness of ventilation. However, in light of the information presented in the article [8], positive pressure ventilation may be effectively carried out against the wind provided that its speed does not exceed 40 km/h. The study also notes that at lower wind speeds, ventilation efficiency will decrease if the wind acts directly on the surface of the smoke opening.
At the University of Greenwich, the effect of wind on the efficiency of smoke extraction using mobile fans was determined from CFD studies and simulations [16]. In the referenced study, the authors showed that there is a critical wind speed (corresponding to the average discharge velocity generated by a mobile fan in a smoke-filled volume) at which airflow is attenuated. On the other hand, when considering the issue of differences in the sensitivity of buildings to wind impact depending on their construction and location, it should be taken into account that it will be greatest for free-standing buildings located in open spaces.
The impact of the considered factor is also significant in relation to multi-storey buildings -the higher the storey, the greater the impact. In 2017 P. Panindre et al. [11] evaluated the application of PPV in fires of high-rise buildings including the effect of wind. It was shown in this study that for fires where wind action is present, the effectiveness of PPV decreases as wind speed increases.
Analysing the problem of wind influence on the efficiency of the rescue operations with the use of mobile fans, one should be aware that this type of factor is not always possible to eliminate. However, this effect should be limited, e.g. by reducing the smoke extraction opening in order to increase the overpressure inside a building (generated by the air flow of the fan). This overpressure value can also be increased by using a portable curtain screen equipped with 4 ropes for optimal adjustment at the door opening (see Figure 17). This tool is manufactured using non-flammable materials, so it can be used even on the surface of a window, whose glass has degraded as a result of thermal exposure. Research confirming the effectiveness of the indicated solution was conducted in the United States by the National Institute of Standards and Technology (NIST). The current state of art presented in this paper and its conclusions will be an essential point of reference in the implementation of further research work, allowing to explore techniques to increase the efficiency of mobile positive pressure fans. In the next stage, field tests (large scale) will be carried out to evaluate the effectiveness of the fan units on a test stand in the form of a staircase, allowing to change its actual volume and number of floors (in the range of 1-7). The fan units currently available on the market (with different impeller sizes, efficiencies, generating different jet shapes) will be comprehensively examined for effectiveness of their use, including different configurations and ventilation techniques characterized in this paper. The knowledge acquired in this way will allow the development of further concepts of methodologies and test stands, enabling the study of the parameters characterizing the efficiency and reliability of mobile fans. The authors hope that the results of the work mentioned above will also allow to propose better tactical solutions, whose improvement will be possible during training courses on techniques for fighting internal fires or smoke removal of objects in which mechanical ventilation is necessary for the implemen-