I have owned a fog machine for many years now, but I had always wanted to make a fog chiller to get a crawling fog effect. However, due to the wind around my house in the fall, it makes it usually pointless for halloween, so until now I had held off on spending any money on it. But when low lying fog was needed for a youth play I was helping setup and run lighting for, I finally had the excuse I needed.
So before any details, a bit of physics… A simple fog machine works by vaporizing a fluid (usually made of mineral oil, glycol, or glycol and water mixture) into a heat exchanger, where the fluid is quickly vaporized. This means that fog is usually somewhat warm, as it does cool down quickly as it expands, similar to compressed air. However it is still warm enough that it always rises as it is released. To remedy this, a fog chiller is used to cool down the fog faster so it lays low and clings to the ground. Some simple examples can be found on youtube that use a metal pipe or plate and ice to cool down the fog, which to my surprise does not need a lot of ice to achieve the task. Though watching expensive professional fog machines run is rather impressive. [Check it out]
Still I wanted to do something more than a simple metal tube, so after purchasing the cheapest plastic bin I could find ($5) I gathered up some PVC tube, mesh window screen, a 120mm computer fan and a plastic sandwich bag.
As you can see from my pictures the mesh is bent and woven into a wave shape for a clear path the smoke can move through, this ensures maximum exposure to the ice is possible and allows the fog to fill the container. Next the mesh smoke path is placed between the PVC inlet and the outlet fan using the lip in the container to to hold it up. Ice is filled all around the mesh as well as in between the gaps between the smoke path. A variable voltage transformer and a remote controlled outlet is used to control power and the speed of the fan. It is pointless to leave the fan running all the time as it will melt the ice faster. Finally a plastic bag with a stiff edge is used as a simple laminar to help smooth and direct the fog to the ground for a gentle rolling effect. While I did not get a chance to take a video of it in action I have included a video of another person's fog chiller which produced the same effect. (AKA, It is not my home, and I do not own such a sketchy rug...)
I was quite happy with the results, the ice lasted and with plenty to spare, while sitting in-between uses for a good hour. And produced a good layer of fog for the two scenes that needed it. Interestingly enough a chiller seems to works better in cold air than warm, due to it keeping the fog cool longer, which is the opposite of what I assumed, as I thought it would be better to have warm ambient air to keep the fog as the cooler sinking air. Also the slower the fog is, the lower it will stay, so if you have a container to pump it into first before cooling it will help slow it, but I will save that for the v2. As you can see from my results, if you own or want a standard fog machine (quite cheap now a-days) you need to make a fog chiller, also don't buy a combined model as they only produce a slow steady output of fog, which is only is useful for an small indoor room.
