Ventilation systems can either have a benign or disastrous effect on the spread of smoke and fire. Rainer Will of Belimo Systems examines the engineering principles that need to be considered in the context of German regulations.
The zoning of specific areas in a building with fire-proof or fire-resistant walls and ceilings has proved to be an effective, structural means of fire protection. However, as the ducting of ventilation systems has to pass through these structural elements, care has to be taken to ensure that their effectiveness is not seriously compromised.
Fire dampers in ducting systems have been shown to be an ideal means of preventing the spread of fire and smoke. However, the true danger of smoke spread has only begun to be properly recognised in the last few years and, in drawing up suitable concepts for fire protection, the question has constantly arisen: What should be the functions of ventilation systems in the event of fire?
The zoning principle
The absolute minimum requirement is that fire dampers should be closed in good time to prevent a fire spreading. German Federal Building Regulations already include a requirement for the prevention of smoke spread, and a thermal-trip fire damper will actually close sufficiently tightly to prevent hot combustion gases from a fire spreading. However, additional measures will usually be needed in order to prevent the spread of smoke when the temperature is less than 72 degrees C (Figure 1).
In the case of supply-air ventilation systems, the official guidelines (M-LUAR, March 2000) specify that the external air intakes should be placed in a position where it is impossible for smoke to be drawn in. If this is not possible, the spread of smoke mixed with external air must be prevented by means of shut-off devices incorporating smoke trips (fire or smoke dampers), which means smoke trip devices in the flow of supply air are needed. The danger of smoke spread is especially great in the case of recirculating types of ventilation systems (Figure 2). In this case, the smoke trips can be fitted in the exhaust-air, recirculating-air or supply-air ducting and the M-LUAR guidelines also require that the supply-air fans be switched off when the smoke trips operate.
But switching-off the supply-air fans can actually be counter-productive if, for example, some areas of the building receive their supply of air solely from the ventilation system, or where the supply-air fans are needed to maintain specific pressure conditions (for example, a positive air pressure for escape routes and clean rooms). So in the event of a fire in the building, this type of ventilation system can continue operating, provided there are fire dampers with smoke trips in the zone of the building affected by the fire.
The M-LUAR guidelines do not specify whether the exhaust-air fans should also be shut down. Should the exhaust-air system remain operating after the supply-air system has been shut down, there are two particular factors to be taken into account:
– There will be a negative pressure in all areas connected to the system. The spread of smoke can be exacerbated especially if, in the zone affected by the fire, the fire dampers have been operated by thermal trips and a positive pressure has built up there. Then, although the smoke no longer spreads through the ventilation ducting it could possibly travel along the escape and rescue routes.
– Due to the uncontrolled negative pressure, the resulting pressure differentials could cause the maximum permitted door closing force of 100 N to be exceeded, so it would be impossible to open doors to escape routes and rescue routes.
Consequently, the normal course of action in the event of a fire is to shut down the ventilation system completely.
Stopping the ventilation system
When smoke is detected in a building, even though the whole ventilation system is shut down, the fire zones will still be open to each other through the ducting.
As the intensity of the fire increases, so too does the positive pressure in the area of the fire, which pushes towards the adjacent fire zones and spreads smoke in that direction too. In the process, the combustion gases in the ducting cool down so much that the temperature at which the thermal trips operate (normally assumed to be 100 degrees C in practice) is reached either very late or sometimes not at all. So the best advice when a ventilation system is shut down, is to close all the fire dampers too, but this normally assumes, of course, that the fire dampers are motorised.
The guidelines do not mention the special danger that arises from overflow fire dampers – due to the absence of the duct connection, smoke inevitably spreads. If, for example, supply air is introduced into a fire zone while the exhaust air is being removed through an overflow damper with no duct connection at the back, the smoke will be able to flow into the adjacent fire zone. This type of ventilation is often used in school classrooms, with the overflow vents also being designed to discharge into the corridor (the escape route). Consequently, it is absolutely essential for overflow dampers to be fitted with smoke trips (Figure 4).
The M-LUAR guidelines refer to it being useful and not dangerous to leave the ventilation system running in the event of a fire, although, naturally, the spread of smoke via the recirculating air must be prevented. They also comment that while the system is still running – although smoke can be drawn in through the exhaust-air outlets – it cannot then spread into the adjacent zones because there is a negative pressure in the exhaust-air ducting. Due to the positive pressure, no smoke can spread into the supply-air ducting. If the tripping temperature were to be reached, however, the closing of the fire dampers would also eliminate any fear of smoke spread.
This supposedly logical approach, however, does not take into account that the supply-air and exhaust-air fire dampers are tripped at different times. Since the fusible link of the supply-air fire damper is ‘cooled’ by fresh air, past experience has shown the exhaust-air fire damper closes long before the supply-air fire damper. If there are no limit switches available to pass on the signal, or the ventilation system does not shut down due to some other reason, in addition to the combustion gases building up a positive pressure the fire zone will also be ‘pumped up’ by the supply air. The smoke and combustion gases will then be forced through the door apertures, or any other unsealed points, into the adjacent fire zones, corridors and escape routes, so endangering the rescue or escape of occupants.
Shutting down the ventilation system when the exhaust-air fire dampers have been tripped also cannot prevent the spread of smoke. The positive pressure in the area of the fire causes smoke spread until the supply-air fire damper closes.
The sensible course of action is to close the supply-air fire damper immediately after the exhaust-air fire damper, which makes an automatic closing device essential. So motorised fire dampers are usually essential if the ventilation system is to continue running when there is a fire.
There is undoubtedly an increasing trend for ventilation systems to assume the task of smoke extraction systems, mainly to achieve economies. Legitimising this course of action, however, is a controversial matter. In Germany, section16 (2) of the commercial premises regulations of September 1995 states: “In the event of a fire in commercial premises with sprinkler systems, it must be possible to operate ventilation systems in sales areas and shopping aisles so that they only extract air, provided the rules governing the use of shut-off devices for the prevention of fire spread allow.”
In practice, this has been interpreted to mean that smoke extraction systems can be dispensed with entirely in commercial premises with sprinkler systems, with the ventilation system undertaking extraction of the ‘cold smoke’. In actual fact ‘cold smoke’ means smoke at a temperature below that of thermal trips (i.e. fusible links). VDI 3819/2 defines cold smoke as combustion gases that are only slightly different in temperature from the ambient air and cannot be removed without an additional flow of air.
Even clearer is section 5.6.3 of the industrial premises regulations of March 2000 for areas with automatic fire extinguishing systems: “Instead of smoke extraction systems it is possible to use ventilation systems with control systems which, when there is a fire, only allow air to be exhausted. The fans of these ventilation systems do not have to be designed for fire and in other respects they must satisfy the requirements of the regulations for fire protection equipment in ventilation systems.”
Although the elimination of a smoke extraction system in industrial buildings might possibly appear to be justified, for commercial premises with large numbers of untrained people inside it is frequently irresponsible. In addition, the various floor levels are usually of open design (up to 3000 square metres over three floors is allowed) which also helps to promote the spread of smoke. The principle arguments against smoke extraction through the ventilation system are:
– Ventilation systems are designed for comfort. The number of air changes they perform is usually neither suitable for significantly slowing down the increase in the layer of smoke produced by a fire, nor for achieving any noticeable reduction in room temperature even when, as required, the supply-air system provides additional extraction.
– The cooling effect from the sprinkler system will not be able to safely prevent failure of the ventilation system, due to the generation of heat as the fire continues. Therefore, there is no reliable foundation from which to plan any suitable evacuation scenarios.
– Without the proper extraction of smoke, triggering the sprinkler system can make it more difficult to rescue people close to the affected area, because as the volume of combustion gas increases, the smoke cools down and is forced downwards.
– If the ventilation system is to be switched over completely to air exhaust, suitable apertures would be needed to allow pulses of ‘topping-up air’ to be introduced. This is almost never the case in actual practice and so the effectiveness of the cold smoke extraction will be reduced even more.
– l The idea of cold smoke extraction is leading to the formal use of building products in ways other than those approved by the relevant general building regulations. This is especially true of fire dampers.
The removal of smoke by means of the ventilation system must not be continued after the fire dampers dividing the fire zones have been triggered. A sprinkler system too will not be able to safely prevent the triggering temperature being exceeded. In order, nevertheless, to allow the ventilation system to continue operating, an attempt is often made to re-open motorised fire dampers that have been triggered thermally by short-circuiting the thermal contacts. This is also called the ‘fire service switch’ but it conceals some major risks. Firstly the thermal triggering of a fire damper must be permanent and irreversible, so such a switch would no longer receive general building regulations approval – individual approval from the overriding building authority (not the fire service!) is needed. Secondly, the triggering temperature of a fire damper would have to be chosen, so that the spread of fire through the ventilation system could be safely prevented. The re-opening of a triggered fire damper represents a safety risk that it is impossible to calculate in terms of fire spread, so the use of the ventilation system for smoke removal is only possible within very close limits.
Smoke removal by scavenging
The classic mechanical means of smoke extraction (by drawing in the combustion gases released by the fire and exhausting them to the outside) produces specific layers of little smoke and also help to cool the seat of the fire. Pressurising systems for smoke protection are based on the principle of preventing the penetration of smoke, especially into escape and rescue routes. For this purpose, suitable fans are installed in the rescue routes aimed at producing a positive pressure of between 15 and 50 Pa. The ventilation system is also suitable, in principle, with the aid of an actively controlled build-up of pressure differences to counter the spread of smoke and provide support for the action of smoke extraction or pressurised smoke protection systems.
In large buildings of open construction in particular, ventilation systems keep thousands of cubic metres of air flowing continuously in specific directions. In the event of a fire, it is frequently necessary for those flow conditions to be reversed, and it is here that the ventilation system can make a valuable contribution (Figure 5). In the areas not affected by the fire, a positive pressure can be produced by closing the exhaust-air fire dampers, which will act against the spread of smoke. In an ideal case, supply-air fire dampers in the zones affected by the fire would be closed in good time by automatic smoke tripping devices and other apertures (such as windows, open exhaust-air fire dampers) would be opened to allow air to flow through the building in the direction of the seat of the fire. The effectiveness of this must be verified by suitable trials, and it is particularly important not to exceed the maximum permitted door closing forces from the pressure conditions that have been created.
So it will usually only be possible to prevent the spread of smoke if smoke tripping devices can detect and pinpoint the fire quickly, and operate the fire dampers automatically by remote control. The requirement for such devices was first introduced into the building regulations in June 2003 in North Rhine Westphalia, which began setting the standards for all the other German Lands.
It has also become clear that motorising fire dampers creates the opportunity to respond flexibly to different dangerous situations. In order to be able to implement intelligent scenarios, fire and smoke dampers are increasingly being controlled through bus systems by higher level instrumentation and control systems. In future, these automation systems will need to be the subject of specific requirements regarding functionality and security of data.
