Fog machines are great for Halloween... but fog tends to rise up into the air, spread out, and disappear. If you do not have a very powerful fog machine, this can ruin the whole effect.

The solution to this is to build a fog chiller. The fog chiller cools the hot fog from the machine, causing it to stay low and billow along the ground.

This is a small fog chiller, which works well and is cheap. It takes an hour or two to build.

Step 1: Design and Theory

About Fog
The principles behind a fog chiller are the ideal gas laws. The warmer air is, the less dense it is, and therefore is pushed upwards by sinking cold air. Simply put, warm air rises, cool air sinks.

Fog machines work by pumping fog liquid onto a very hot "heat exchanger", which flash-vaporizes the liquid. This expansion pushes it out of the nozzle. The fog exiting the machine is very hot, and will rise up.

This device cools the air in which the fog is suspended, therefore causing it to sink to the ground.

Design
I designed this chiller myself, and I think the design is unique.

Fog enters the device at the top. At the top, there is a large expansion chamber, so that the fog can properly mix with the air.

Below this expansion area, there is ice. This cools the fog. Once the fog is cool, it will sink down below the ice into another chamber.

On one end of this chamber, there is a fan to blow the fog. On the other end, there is a hole for the fog to exit. This further mixes the fog with the air, and blows it out.

The theory is that the fog will expand in the first chamber, as well as cool. Once the hot fog has cooled enough to sink below normal air, it drops below the ice and is blown out.

If the amount of incoming fog is too fast for the cooling, then the fog will not cool as much, but will still be forced through the layer of ice, most likely resulting in enough cooling to retain the effect.

The whole device is fairly small, only about twice the size of a fog machine, and can be built with easily obtainable materials for not much money.

Step 2: Materials

Stuff you'll probably have to buy:

Cheap Styrofoam Cooler: $3 at grocery store
1.5" PVC Coupling: Less than $1 at Home Depot
Screen: $5 at Home Depot
Computer fan: $2 at Goldmine, plus shipping

Stuff you probably have:

Ice
A fog machine
9v battery
9v battery clip (optional)
String
Tape

Tools

A small flathead screw driver
Pliers (locking ones recommended)
Stapler (a normal one, spring-loaded Staplers ones won't work well because of the safety)

Step 3: How to Cut Holes in Styrofoam

This is not an actual step, but you will need to use it 3 times during this project.

1. Poke holes through the Styrofoam outlining area you want to cut
2. Loop a piece of string around the screwdriver
3. Push the screwdriver and string through the Styrofoam
4. Pull one end of the string from the inside until it is through the Styrofoam
5. Slowly pull the string back and forth, pushing it in the direction you want to cut
6. Move the string around the outline until you're back where you started. Poke out the piece you cut.

Step 4: Create the Screen

1. Unbend a coat hanger with pliers until it is straight. Cut off the ends if you can't straighten them.
2. Use a tape measure to measure the inside of the cooler. If yours is tapered, like mine is, measure it about half way down.
3. Bend the coat hanger into a rectangle matching your cooler's shape, using the dimensions you just measured.
4. Use pliers to wrap the coat hanger around itself to finish it off. Cut off any extra.
5. Gently push the coat hanger rectangle into the cooler. It should be able to slide about halfway. Don't push it in too hard, because you may break the cooler, or not be able to get it out. Once you're sure it will work, take it out.

Step 5: Create the Screen (cont.)

1. Measure and cut out a piece of screen about 1" bigger on all sides than the coat hanger rectangle
2. Fold the edges over the coat hanger
3. Pull the edges tight, and staple them
4. Put staples all around the edges, as close to the coat hanger as possible
5. You may need to manually bend the staples together
6. Trim off extra screen

Set it aside for later.

Step 6: Make the Lower Chamber

1. Refer to step two to cut a hole near the bottom of the cooler, on one of the short sides. It's a good idea to use the 1.5" PVC coupling as a guide.
2. Cut another hole of the same diameter directly opposite this one (on the other side of the cooler)
3. Place the fan over the hole, so that it blows into the box, and the wires are facing up. You may need to test it with the 9v battery. Poke holes through the Styrofoam where the mounting holes of the fan are.
4. Thread a long piece of string through one of the mounting holes, and into the cooler.
5. Finish mounting the fan by "sewing" it onto the cooler. Tie off the string when finished.
6. If you have a 9v battery clip, wire up the fan directly to the clip. A switch is optional, since it is easy enough to just disconnect the battery.

Step 7: Finish Up

1. Cut a hole above the fan, using the outside of the PVC coupling as a template. The top of the hole should be about 1" (2 cm) from the top of the cooler.
2. Insert the screen you made in step 3-4 into the cooler, pushing it down about half way
3. Push the PVC coupling into the hole. If it's not a good fit, seal it with tape. Make it flush with the inside of the cooler.
4. Poke some holes on the bottom front corners (near the hole with no fan). This will allow the melted ice water to drain.
5. Done! It should look like this.

Step 8: Using the Chiller

1. Fill the top of the cooler with ice, up to the bottom of the PVC coupling
2. Find something (a box, anything) to put the fog machine on. Place it so that the nozzle lines up with the PVC coupling.
3. Push the fog machine and fog chiller together until the PVC coupling completely covers the nozzle
3. Connect a 9v battery to the computer fan
4. Turn on the fog machine
5. Put the cover on the cooler, and weigh it down with something heavy (but don't break it!)

It is a good idea to paint the cooler black to be less conspicuous. If you plan on running it very often, you should also find a more permanent stand for the fog machine.

Step 9: Wrapping It Up

Notes for best preformance:

Make sure no fog can leak out between the PVC coupling and the fog machine
Make sure the cooler cover seals tightly
Be sure to add ice every half hour or so
Angle the whole setup slightly, so that the melted ice water can drain out the front of the machine.
The only place fog should be coming out is the exit hole, opposite the fan. If it is leaking anywhere else, seal it.
Wind kills fog. Running this on a breezy day will yield very poor results.

Here are some pictures of the fog chiller in action:
1. Through chiller, with ice
2. Normal Fog
3. Black Chiller