The next task was to figure out how to have the smoke exit the building. I first thought I could simply make some jagged cuts in the acrylic “glass” which covered the skylight on top of the building. This worked fine but the water from the humidifier would condense on the remaining plastic and droplets of water would drip back into the humidifier. This overwhelmed the ultrasonic disk which produces the water vapor stream, causing the water vapor stream to stop. It would clear in about 20 seconds and start working again. However, the 20 seconds on, 20 seconds off cycle was not very realistic. Also, the stream of water vapor was unrealistically straight and strong when it was running.
I removed the plastic skylight that came with the model and fashioned a new, larger one out of black styrene. I inserted a series of black plastic bars which ran down the angle of the skylight. These bars serve two purposes – they provide a smooth surface on which the water condenses so it runs down the bars to their lowest end without dripping; they serve as a good surface onto which the shards of clear acrylic “glass” can be fastened. I made a trough out of styrene that runs around the perimeter of the inside of the plastic skylight to collect the water droplets that run down the angled plastic bars. I then drilled a hole in one end of this trough and fastened a styrene tube to the underside of the hole. The water droplets collect in the trough and are returned to the reservoir via the styrene tube.
This approach worked fine except there were still two issues I wanted to correct. The stream of water vapor was still too straight and strong. Also, when someone looked in the top of the building when the water vapor was coming out (95% of people do) they could see the white plastic top of the humidifier – this spoiled the effect.
I fashioned a black plastic baffle which runs across the width of the opening from which the water vapor flows from the building. The baffle serves the purpose of disrupting the flow of water vapor, slowing it down and causing it to ‘waft’ somewhat randomly as much as it flows. The baffle, being black, also prevents viewers from being able to see the humidifier when they look in the top of the building. The baffle collects water droplets in addition to those collected by the bars. Because the baffle is angled in such a way that the water droplets fall into the water trough, they do not fall directly into the opening in the top of the humidifier.
When the reservoir is filled with distilled water (to avoid any contaminant build up in the humidifier) and the battery is hooked up (housed in a slot in the interior of the building) the water vapor flows out of the skylight quite realistically. The water that accumulates in the trough inside the skylight returns nicely to the water reservoir.
I installed two emergency vehicles with flashing lights in the street in front of the building and added a few fire fighter figures in various poses. This burning building gets a great deal of interest and positive reaction from members of the public as well as other modelers at train shows.
This approach could be easily adapted for a smokestack on an industrial building, a forest fire scene, a steel mill, etc. I suspect the project might be a lot simpler if the structure from which the “smoke” was to come were larger than my little building as it could accommodate a larger humidifier that would require less modification.
One final thought. If you are considering scratch-building a structure where this technique is employed, I suggest that styrene be used in all locations where there is any chance of exposure to water or water vapor. Otherwise your scratch-built structure will quickly turn into a soggy mess.