Smoke detection

Early detection in rooms with ventilation and/or high ceilings is a difficult task.

A rough estimate of how severe a fire must be in order to take control of the airflow pattern in the room is given in Thomas and Andersson1 . This estimate results in fires in the order of a MegaWatts (MW). A one MW fire causes severe damage in many cases and therefore smoke detectors should be placed at locations where the smoke reaches the detector at an early stage in the fire. In order to calculate when a detector will be activated one should know the smoke production from the fire. Further, we need to know how the smoke is transported and the sensitivity of the detector.

Smoke production from fires in electrical equipment and packaging materials

Fires in electrical equipment and packaging materials are relatively common in industries. The smoke production from these are usually not known, especially when in the beginning of the fire with smouldering combustion. There is data available in the literature on smoke production from mainly pure fuels and usually from flaming combustion.

Testing of smoke detectors

Smoke detectors are tested according to EN54. In these test the detectors are tested against certain test fires and the smoke density together with an ionisation current is measured during the test. Publicly available test results on smoke detectors against other fires are very scarce.

Usually when doing smoke spread calculations a uniform temperature profile in the room is assumed together with no airflow before the fire starts. This is not the case in the reality, in rooms with high ceiling there is usually a significant temperature gradient and the air flow in the room can be rather large due to the ventilation system. There is also the problem of other heating sources in the room that causes airflows. These problems cannot be modelled in, for instance, two zone models. In general they require CFD type of simulations. Only a very limited number of simulationsii of this kind have been published.

Performed tests

In a BRANDFORSK financed project the smoke production from some packaging materials and electrical equipment was measured in the cone calorimeter. In addition tests were performed with some of these fires in a EN54 test room. A CFD simulation was made of the experiments. Different types of ventilation systems were also studied in this project. The technical note from this project "Smoke Production and Detection" by Andersson and Ingason can be downloaded.

The work continued in another BRANDFORSK financed project "Tidig detektion i lokaler med hög takhöjd" Brandforsk nr 628-011. In this project full-scale experiments were conducted at two different industrial sites using two different smoke generators and some of the fires from which the smoke production had been measured. Normal production was maintained at the sites during the experiments. The industrial building used had different types of ventilation systems, i.e. one total mixing and one displacement system.

Before the experiments, parametric studies were conducted by means of CFD simulations to study the influence from different temperature gradients and ventilation system on the smoke movement. These simulations are presented in CFD simulations of smoke detection in rooms with high ceilings by Frédéric Conte

Simulations of full-scale experiments

The full-scale experiments were simulated and the results compared. The experiments showed that the smoke movement varied very much between identical tests, a feature that the simulations cannot capture. In addition, the simulations resulted in a more traditional smoke layer than the experiments. This is probably due to that the simulations only took account of the major disturbances such as the temperature gradient in one case and the air inlets in the other case, and not the local velocities etc.

Full details of the projects are available in SP report 2003:33

References

Thomas, P. H. "The role of the Initial Conditions in the detection of fires" in Minutes from EUSAS-BRANDFORSK WORKSHOP on smoke Detection, September 26-27, 1994, LUTVDG/TVBB 3081, Lund.
Andersson, P. "Evaluation and Mitigation of Industrial Fire Hazards" LUTVDG/(TVBB-1015) Lund 1997.
Tewardson, A. "Generation of Heat and Chemical Compounds in Fires" in The SFPE Handbook of Fire Protection Engineering, First ed.1988, pp.1.179-1.199.

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