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New Feature: modelling the effect of fog on air density

Martin Griffith

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A question that comes up from users sometimes is: what is the effect of fog density on the mine ventilation system? Ventsim Heat Simulation already predicts fog, but it only does it really for the purpose of modelling visibility. Usually the effect of fog on air density, and then by extension the mine ventilation system and fan performance, is not significant. In Ventsim, fog concentration has a maximum value of 1200 mg/m3; if condensation of water out of the air results in the fog exceeding this limit, then this extra condensation is simply removed from the system, with an assumption that it falls out of the air or onto the airway walls and floors as droplets of water. 1200 mg/m3 is equal to 0.0012 kg/m3, which is a 0.1 % variation on standard air density, so not something we are likely to notice in our fan performance.

However, there are some situations where this fog limit may not be valid. In a long exhaust shaft with a high air velocity and a large decompression, fog formation is likely. Dynamic water suspension is possible for shafts speeds roughly in the range of 7 -12 m/s. This is where condensing water builds up and is suspended and carried up the shaft; but not carried fast enough, with the amount of condensation building up to a sufficient weight that is falls down the shaft. The process can then repeat, putting a detrimental varying load on the ventilation fan. Ventsim already has a feature to warn the user when shaft speeds occur in this range (Settings | Simulation | Airflow | Water Suspension Checking)

Another possibility at velocities greater than 12 m/s is that the condensation is carried all the way out of the shaft. In this case the density increases over the length of the shaft, putting an extra load on the fan. To model this we've introduced 2 new settings: 

Settings | Simulation | Airflow | Water Suspension Carry Condensate 

During Heat Simulation, this setting will enable the carrying of condensate as fog (condensate = 0) in vertical (>80 degrees) shafts, meaning it is passed from one airway to the next and builds up as it goes. But it will only happen in high velocity rising shafts, that is, at velocities greater than the Settings | Simulation | Airflow | Water Suspension Upper Limit

 Settings | Simulation | Airflow | Water Suspension Adjust For Condensate

This setting will take the fog and add its density to the air density in every airway in the model during an Air Simulation. In a rising air column (and in tandem with Settings | Simulation | Airflow | Water Suspension Carry Condensate), this could be sufficient to have an effect on fan performance. 

If this feature interests you or you have any questions about it, please get in touch. 

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  • Martin Griffith changed the title to New Feature: modelling the effect of fog on air density

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