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The Spread of Fire Gases

Created on Monday, August 13, 2018 and posted in Fire Fighting Articles
The Spread of Fire Gases

Fire gases is a mixture of hot air, particles, combustible gases (for example carbon monoxide) and incombustible gases (for example carbon dioxide) that are formed during combustion. The composition and the Spread of Fire Gases is determined among other things by the conditions prevailing at the fire. It can be changed in conjunction with fire ventilation, i.e. during changes in the supply of air to the fire.

Different combustion products are formed depending on the ratio between the two components, the fuel and the air. As a rule the combustion is incomplete, especially during normal fires in buildings. In which case, combustion products are formed that are flammable to a greater or lesser degree, and which therefore can burn under certain conditions. These combustion products follow along with the rest of the fire gases and collects in a hot upper layer.

The fire gases formed during fires consists of two components. The absolutely largest component consists of the air that is mixed with the combustion products from the fire, heated up by the fire, and which is relatively unaffected by the chemical reactions taking place in the fire.

The second component consists of the decomposition and reaction products formed during the fire, including gases such as carbon dioxide, carbon monoxide, water vapour and methane, and particles in solid form (soot) or in liquid form (for example heavier hydrocarbon compounds). This component is very small in terms of both weight and volume. The volume of fire gases formed is therefore basically the same as the volume of the air mixed into the plume of fire, heated up and expanded. The physical properties of fire gases are therefore more or less the same as for air, for which reason the fl ow of fire gases is treated as a fl ow of heated air both during calculations and for tactical assessments of fires. The fl ow conditions will not be affected even if there is a powerful development of fire gases that contains a large number of particles, and which therefore has low transparency.

Reactivity
Substances have different capacities to react chemically with other substances. For example, the metal sodium reacts violently with water during the powerful development of heat and hydrogen
gas. On the other hand sodium does not react with paraffin at all. Substances that react easily with other substances are said to have a high reactivity.

Combustibility
A substance’s combustivity describes how combustible it is, i.e. how easily it sets on fire and burns.

Nevertheless, the chemical properties of fire gases can differ significantly from air, for example in terms of reactivity, combustibility or toxicity. The particles in fi re gases can be very irritating for the eyes, mucous membranes and the respiratory channels. Even during small apartment fires a large amount of fire gases is produced that contains incompletely combusted products, and which are combustible to a greater or lesser degree. Under certain conditions these combustion products can be ignited, for example if the temperature is sufficiently high, if there is an adequate supply of air to the fire gases, or above all if the volume of combustion products exceeds a certain limit. Fire ventilation can change the conditions prevailing during a fire in such a way that incompletely combusted products are ignited. This can lead to a rapid and uncontrolled spread of the fire.

Fire gases can be treated as hot air during tactical assessments. Consideration must, however, be taken to the combustibility and toxicity of the gases. If the fire gases contains a sufficient amount of uncombusted products and if the temperature is sufficiently high, the supply of air that normally takes place during fi re ventilation can lead to the ignition of the fire gases.

Fire gases can be treated as hot air during tactical assessments. Consideration must, however, be taken to the combustibility and toxicity of the gases. If the fire gases contains a sufficient amount of uncombusted products and if the temperature is sufficiently high, the supply of air that normally takes place during fire ventilation can lead to the ignition of the fire gases.

The spread of fire gases

The fl ow of fi re gases always takes place from higher to lower pressure. The magnitude of the difference between the higher and the lower pressure determines the size of the flow, and how
quickly this flow takes place. The magnitude of the pressure difference is in turn determined by the size of the openings between rooms, the wind conditions, the size of the fire and how it develops, and the ventilation system etc. Differences in pressure can cause fire gases and the fire to spread long distances, and in directions that cannot always easily be predicted.

With a knowledge of the different types of pressure differences in buildings and how they arise, the spread of fi re gases can to a certain degree be predicted, and in certain cases also preven
ted. Sometimes it is in fact possible to change the direction of the fire gases, and to steer it through and out from a building. It can, however, be very difficult to produce an overall view of what the pressure differences look like inside buildings.

Pressure differences in buildings

It is relatively well known how the build up of pressure takes place and how fire gases spreads in a room that is burning, but when a complete building is affected by the fire the problem becomes somewhat more complex. When a fire is in progress it is seldom, or never possible to make a more extensive analysis of the pressure differences and their causes, but it is important to have a certain understanding of what it is that influences the spread of fire gases in a building and out from the building.

The dynamic pressure differences can be divided up into two categories: normal pressure differences that always exist in a building or between a building and its surroundings, and pressure differences created by the fire.

Normal pressure differences

  • differences in temperature between outdoor and indoor air
  • the effect of the wind
  • comfort ventilation (mechanical ventilation and natural ventilation)

Pressure differences created by the fire

  • inhibited thermal expansion
  • thermal buoyancy force

In what follows these pressure differences will be treated separately. In actual fact several or indeed all of these different types of pressure differences will arise and act simultaneously, for which reason a more simple line of reasoning around the problem will be left to the end of the chapter.

Differences in temperature between outdoor and indoor air

The air indoors is most often warmer than the air outdoors. Air that is heated up expands, takes up more space, and has a lower density than cold air. The pressure inside a building, where the air is warmer than outside, will therefore be higher than outside. This pressure strives towards equilibrium with the surroundings, and therefore the heated air flows out from the building; from the higher pressure inside the building to the lower pressure outside the building. Since a building is seldom or never completely tight, the (heated) air will always be forced out from the building and, at least gradually, be replaced by cold air fl owing in. If the openings are small, or if the pressure difference is large in relation to the size of the openings, this fl ow will take place through different openings. The inflow takes place through one opening, and the outflow through another. If the openings are large, or if the pressure difference is small in relation to the size of the openings, the flow can take place through the same opening.

In addition hot air rises upwards, which normally causes the outflow to take place through openings situated high up and the inflow through openings situated much lower down. This also means that the pressure difference in relation to the surroundings will be highest at the top of the building and lowest at the bottom. In low buildings, or in one and the same floor, these pressure differences are often negligible, while in high buildings they can be very large.

This can be the case in high-rise buildings with several floors, but also applies to warehouse buildings or industrial workshops with high ceilings. An upward moving air flow is created in the vertical shafts etc. of high buildings. If it is warmer outside than inside the conditions can be the reverse, and the flow of air will go downwards instead.

In buildings with openings both at the top and the bottom a so-called neutral level will be formed at the height where the pressure inside the building is the same as outside the building. If the air inside the building is heated up and becomes hotter than the air outside, air will flow out from the building over the neutral level and into the building below the neutral level.

The Spread of Fire Gases

The effect of the wind

All buildings allow in air to a greater or lesser degree, and in many cases the wind can have a very large effect on how fire gases flows inside a building. The wind pressure is proportional to the square of the wind speed. This means that if the wind speed increases from 1 m/s to 10 m/s, the pressure on the wind side will increase from, for example, 0.4 Pa to 40 Pa, or from 0.6 Pa to 60 Pa (typical values).

Vertical surfaces (walls) normally produce positive pressure on the wind side (at right angles to the wind) and negative pressure on the leeward side (opposite/parallel side). The negative pressure on the leeward side is approximately half the pressure on the wind side. The pressure on roof surfaces exposed to wind depends on the angle of the roof. At angles of over 45° a positive pressure is created on the wind side, and a negative pressure on the leeward side. Both the positive pressure and the negative pressure are greatest at the respective bases of the roof, and diminish successively up towards the ridge. At roof angles between 30° and 45° there can, however, also be a negative pressure on the wind side closest to the ridge. At roof angles of less than 30° the entire roof is exposed to negative pressure. The negative pressure is highest on the wind side.

The Spread of Fire Gases

Gable roofs can be exposed to negative pressure along the entire surface, if the wind blows parallel to the ridge, regardless of the angle of the roof. The differences in the pressure conditions that arise as a result of the roof angle can in certain cases be utilised to make the effect of fi re ventilation better than if only the thermal buoyancy force was used. Fire ventilation through windows and doors (vertical outlets) can be better if they are made on the leeward side of the building, while inlets are made on the wind side.

However, this sets stringent requirements that the openings are selected with care, and that the direction of the wind can be determined reliably. This is not always possible. Because of friction with the ground surface the wind speed also varies, and thereby also the pressure, with the height. The friction varies in relation to the type of surface. The wind can behave completely different in and around built-up areas than it does in open terrain. It can be pressed together along streets so that wind speed is higher than expected, and in combination with the turbulence often created in built-up areas the wind can also take a completely different direction. Very complex wind conditions can also arise on open areas (squares and parks) where several streets converge.

Comfort ventilation

Comfort ventilation refers to the ventilation often used in buildings to let in fresh air and to ventilate out heat, residual products from our breathing (carbon dioxide), moisture and the smell of cooking. There are two types of comfort ventilation systems:

  • Natural ventilation
  • Mechanical ventilation systems

Comfort ventilation systems in buildings are normally designed so that the spread of fire and fire gases to adjacent fire compartments is limited. This is normally achieved by insulating the ventilation ducts with incombustible insulation, or by providing the ducts with dampers that automatically close if a fire occurs. The ventilation system should in principle have similar protection from the spread of fire and fire gases as the rest of the building.

Natural ventilation

Natural ventilation functions by means of the temperature differences that naturally occur in buildings, in the shafts in the buildings, and in the ducts. This type of system is normally an exhaust air system, i.e. the ducts are normally only intended for exhaust air. Wood stoves and tiled stoves were often used as exhaust air ducts in older houses. The supply air was taken in through natural gaps in the building. From the 1920s, separate ducts were often installed for both supply air and exhaust air. Natural ventilation can now almost only be found in summer houses. Natural ventilation does not normally contribute to the spread of fire gases between fire compartments, since the ventilation takes place in separate ducts.

Mechanical ventilation systems

Mechanical ventilation systems can designed as:

  • Supply air systems. Fans supply air through the ventilation ducts. The exhaust air is pressed out through gaps in the room or to adjoining rooms. The system creates positive pressure in the fire compartments.
  • Exhaust air systems. Fans suck out air through the ventilation ducts. The supply air is drawn in through gaps in the room or from adjoining rooms. The system creates negative pressure in the fi re compartments.
  • Closed supply and exhaust air systems. Fans connected to the rooms, supply them with both supply air and exhaust air through ventilation ducts.
  • Open supply and exhaust air systems. Fans connected to the rooms, supply them with both supply air and exhaust air through ducts. The exhaust air is also allowed to flow through gaps both to the surroundings and to adjoining rooms.

Normally the spread of fire gases to adjoining rooms in the building would be on a small scale, as long as the mechanical ventilation systems are in operation. If all, or parts of the mechanical ventilation systems stop working, this can strongly contribute to the spread of fire gases in buildings. This can be a problem, especially in the case of open and closed supply and exhaust air systems, since the different fire compartments are often linked through ventilation ducts. As a rule the spread of fire gases (and also smells from cooking and bad/old air etc.) is prevented by the pressure difference over the ventilation device (the hole in the wall). If the ventilation system stops working this pressure difference will be lost, and fire gases can easily be spread via the ventilation ducts between the different fire compartments/rooms. With functioning mechanical ventilation systems there is little risk of spreading fire gases through the ventilation system. If the ventilation system stops working this can contribute to the spread of fire gases between fire compartments in buildings.

A modern ventilation system can be an integrated part of the protection in the building, in which case the design and function of the system would then be based on a specific dimensioned fire pressure. By ensuring that the ventilation system works even during a fire, and as long as this dimensioned fire pressure is not exceeded, the system can often manage to prevent the spread of fire and fire gases for quite a long time. In such cases it can be important to make
sure that the ventilation system maintains its function by, for example, the fi re and rescue services putting in additional measures to protect the system.

Thermal expansion

When there is a fire in a completely closed room there will be a build up of pressure as a result of the heating and expansion of the air. For small or moderate differences in temperature this pressure will be small, i.e. when it is not burning, but in the case of fires where the temperature can reach many hundred degrees this pressure difference can have a substantial effect, especially if the fire develops quickly. If the size of the fire remains constant the pressure will increase linearly, i.e. the pressure increases constantly in time. Normally there is a certain leakage of air in the fire room, for example in the form of comfort ventilation or gaps at windows and doors. Since a fire in a room normally grows in size, the pressure will be subsequently equalised as a result of gaps and leakage. The increase in pressure will therefore as a rule only be in the magnitude of ten or several tens Pascal.

Thermal buoyancy force

Hot gases are produced when a building is burning. These hot gases have a lower density than the unaffected ambient air and therefore rises upwards, which in this connection we refer to as thermal buoyancy force. In the ideal case we would have a room with an upper layer of hot gases and a lower layer mainly consisting of unaffected air. If we look at the entire building, the hot gases will flow upwards in the building, for example in a stairway.

As long as the fire gases has a higher temperature than the ambient air, and therefore a lower density, it will rise upwards. The buoyancy force, in combination with the thermal expansion, causes the fire gases to be forced out through openings situated high up. This can often be clearly seen in the openings to the fire room, where fresh air flows in through the lower part of openings and hot gases flows out through the upper part. The fire gases cools down as it rises upwards. This means that in high buildings the gases might not reach the roof, but will stop and may in fact subside. In the same way the fire gases can subside to the floor when it flows into a long corridor, or a tunnel, and is cooled by the ceiling and walls.

Pressure conditions during fires in buildings and during fire ventilation

Fire ventilation means that the pressure conditions in and around a building are changed or utilised in such a way that fi re gases is made to flow out from the building. The gases
fl ows from high pressure to low pressure. It is therefore the size of the pressure difference that determines whether the fi re gases will flow at all, and how much and how quickly it
will flow. Large pressure differences produce large flows, or cause the gases to flow at high speed.

The pressure conditions in and around buildings are often extremely complex. The acting pressures, as described above, are primarily produced as a result of:

Normal pressure differences
a. differences in temperature between outdoor and indoor air
b. the effect of the wind
c. comfort ventilation (mechanical ventilation and natural ventilation)

Pressure differences created by the fire
a. inhibited thermal expansion
b. thermal buoyancy force

All these pressures are of the same magnitude, which means that even very small changes in one pressure can influence the fl ow of fire gases significantly.

During different types of fires in different types of buildings, with different conditions, one or more of the type of pressure differences described above can dominate to a greater or lesser degree. A commanding officer about to make a decision about fire ventilation, must therefore picture for himself which of the pressure differences are the most dominant.

In very high buildings with shafts the differences in temperature between the outside and inside air will cause large pressure differences. These pressure differences can be considerable, especially during cold or very warm weather.

The wind can have a very large effect on the spread of fire gases, both inside buildings and to other buildings, especially in certain geographic locations or at high elevtions (high buildings).

Comfort ventilation can in certain types of buildings, above all those with open or closed supply and exhaust air systems, cause problems with the spread of fire gases, especially if the ventilation system stops working.

Fires in buildings create pressure differences as a result of inhibited thermal expansion, i.e. the hot gases expands, and also as a result of the thermal buoyancy force, i.e. hot air rises upwards. In the case of fires where the intensity develops rapidly, or fires that spread quickly, these pressure differences taken together can be considerable. If the boundary of a fire compartment is defective, or if the structure is weakened as a result for example of the
impact of the fire, the pressure differences that arise can further weaken the boundary of the fire compartment or the structure, especially during very intense or rapid fire scenarios or at high temperatures.

Which pressure dominates and causes the most effect will vary from case to case. If there is a strong wind, it can be the wind that causes the largest pressure difference and thereby causes the greatest effect. If the fire is very intense and the temperature in the fire room and even other adjoining rooms is very high, the pressure differences resulting from the thermal buoyancy force or the thermal expansion can be the most dominant. The dominating pressure difference can also vary during the course of the fire. In the initial stage of a fire, or during small fires, it can be the comfort ventilation that has the greatest effect on the spread of fire gases, especially the spread of fire gases that takes place in the fire compartment as a result of the movement of air produced by the comfort ventilation. At a later stage in the fire scenario, especially if the temperature is high or if there is a fully developed fire, the comfort ventilation can also contribute to the spread of fire gases to other fire compartments.

The decision as to what the rescue services should do in these different cases is extremely complex, and an assessment of the tactical problems must be made in each individual situation. When using fire ventilation, however, an attempt should be made to get the different pressure differences to coordinate in order to achieve the desired result.


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