Filters can be used in ventilation systems to remove species. The phenomenon has been implemented in CFAST to remove trace species and soot. It is implemented by modifying the source terms which describe gas flow. Mass that is filtered remains on the filter and is removed from the air stream. Both the resulting species density and total species removed can be analyzed.
High efficiency filters will remove 99% of the material in an airstream.
An example of the use and effect of filtering is on a common room arrangement in the Department of Energy community. A laboratory used for processing nuclear material usually consists of a compartment that can be entered through an airlock from a long hallway. Often there is a window between the process room and the corridor, but no direct access to the corridor. A schematic is shown in Figure 1. This scenario matches the example in CFAST Computer Code Application Guidance for Documented Safety Analysis. The assembly compartment contains a glove box and polyethelyne trash (containing radioactive waste) which is the presumed fire source. It is connected to an airlock which has doors to the corridor. In addition, an observation window connects the assembly compartment to the corridor. For this simulation, the whole building is surrounded by a container which is used as surrogate for the outside world in order to measure the trace species which escapes the facility. So long as the outside compartment is large (in volume) compared to the simulated environment, and the openings are large enough that there is not a significant pressure increase, there will not be an effect on the flow.
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Figure 1. Schematic of glovebox/disassembly facility |
For the first scenario (no fan/filter), the airlock doors are opened 60 s after ignition to allow personnel to escape. This is followed by the observation window breaking at 120 s when the fire reaches a critical size. To show the effect of filtering, we remove the window breakage, add a fan and duct system from the assembly compartment to the outside, and turn filtering on. The changes are
!!Fan and duct from assembly to the pseudo outside
MVENT,1,4,1,H,2.44,0.37,H,1.52,0.37,1.142,0,200,1
!!Filtering turned on at the beginning of the fire
EVENT,F,1,5,1,0,0.99,1
The results in terms of species mass are shown in figure 2a (original) and 2b (modified by a fan/duct/filter). The heat release rate is the same for both cases. Flow through the window has only a small effect on the species concentration in the corridor.
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Figure 2 - Mass in each compartment | |
| Figure 2a - no filtering | Figure 2b - with filtering |
An alternative way to report the amount of material that has escape from the source compartment is to show this same information as a fraction of the pyrolyzed material.
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Figure 3 - Mass in each compartment as a fraction of the amount pyrolyzed. | |
| Figure 3a - no filtering | Figure 3b - with filtering |
Since the filter also removes soot, so there will be an indirect effect on the temperature distribution , since soot affects the radiation balance. This effect is not illustrated since we have not shown the upper/lower layer temperature distribution.
The effects of aerosol dispersal can also be seen from the graph (and report) of species concentration. Figures 4 and 5 shows how the filtering system can protect those working around such fires as well as the wider environment. In the first case, the amount of escaped material will continue to climb. In the second, case, the amount that escapes is reduced and declines over time. The progression of the fire and movement of contaminants can also be seen in figure 5. At the time of ignition, all of the fire and radioactive aerosols are in the glove-box assembly room.
As the fire grows, the contaminant moves from the assembly to the airlock to the corridor and finally to the outside. It should be noted that though there is flow from the assembly compartment to the outside through leaks, no fire gases escape through this path. When the fan/filter system is turned on, there is not even this small leakage.
