It's a common problem -
You want fog to appear from a specific spot, but there is no physical way to get a fog machine with all of its necessary fluid, power and control needs hidden into that area of the set. The solution is ducting the fog from an area off-set to the spot you want the fog to come out.
In the video below, Rosco's Eric Tishman illustrates some helpful techniques for ducting fog to its intended location.
Many of the topics Eric covers in this video are also laid out in the 'How Fog Machines Work' page of our website.
Let's say you're doing a production of Pirates of Penzance and you want to have smoke come out of the prop cannons you have built into your set. Unless your cannons are comically large, there probably isn't room inside for you to mount a theatrical fog machine into the barrel of the cannon. What you want to do is duct the smoke from the fog machine, which is located in an easy to access area hidden from the audience, directly to the mouth of the cannon.
The cannon illustration below highlights some common mistakes made when ducting fog:
#2 - Introduce forward air movement. As Eric's video illustrates, adding a fan to introduce positive pressure down the duct-work helps jettison the fog further, which will help enhance the explosiveness of this cannon effect. There is room in the back of the cannon to add a small fan that you could Y-Connect to the existing duct-work of the design.
#3 - Try to avoid 90 degree bends. When smoke hits a 'wall' created by a right angle of duct-work, it can cause condensation issues. If your situation forces you into 90 degree bends, try to make those the areas of introducing positive air movement with fans to keep the fog moving and the area dry.
