OK, so I haven't written an 'ible for over a year due to health issues. Now I'm back on my feet here's a little project for your Halloween fun, or a party, or in my case a new SFX addition to my studio.
I wanted to create that low lying fog / mist effect to compliment some new backdrops I had on order. I bought a cheap 400W smoke machine, they are around £35.00 in the UK, but I think you can pick them up for around $30.00 USD. Mine came with a long lead with a control pad. It was pre-filled with fluid and on initial testing it threw out a fair amount of 'smoke'. However, the smoke just rose up and dissipated rapidly leaving the entire studio in mist. Not the effect I wanted at all. I soon realised that the smoke coming from the machine was warm and the studio temperature is deliberately kept cool due to the nature of the work I do. The smoke was too warm and was rising and dispersing due to thermal dynamics.
I tried some different fluids (there are lots of different ones out there) and although the density of the smoke altered, the dispersal problem remained. So I searched around for a solution and found that lots of people were having the same problem. I needed a fog chiller!
There are lots of projects and tutorials on the net for building a chiller, most use an old Hoover hose coiled in a cool box with some freezer blocks. I tried this approach but the amount of smoke was greatly reduced and it still wasn't great as an effect. The tube was acting as a condenser and killing a lot of the smoke before it got through the hose. So in desperation I turned to a contact I had when I was doing commercial photography. He worked for an effects company and used to bring in a dry ice (frozen CO2) machine. This is prohibitively expensive but I asked him for ideas. He brought over a commercial chiller by Vortex, and this design is based on theirs.
Step 1: The Tech Bit
I like to know why and how stuff works in case I want to use similar systems for other stuff. The fog machine works by heating up the fluid, then atomising it and blowing it out of the nozzle. The microscopic droplets then react with the free air around them to create the smoke. The machine needs the ambient air to work correctly. This poses a problem if we stick a tube over the nozzle because there's not enough free air for the machine to work correctly. Furthermore I wanted a more efficient cooling system than random freezer blocks, whilst creating as much dense 'fog' as possible.
The Vortex commercial system is simple and ingenious. It uses two chambers in a box with an inlet and an exhaust port. The first chamber allows the fog to interact with free air, and causes the gas to swirl in a vortex as it expands generating plenty of fog. The fog is then forced into the second chamber through a thick layer of ice cubes, which rapidly cool the gas. However it cannot fall back into the first chamber (which would be normal thermo dynamics) because the first chamber is still filling with warm smoke. It's only escape route is down through a chimney and out of the exhaust port. This only leaves us with the problem of blowback out of the inlet pipe. To get around this problem we will use a 'venturi'. Put simply, a fast moving column of gas causes a pressure drop at it's edges. By using a vented pipe we can create a vacuum effect at the inlet which effectively 'sucks' the fog and more free air into the inlet so it doesn't blow back or simply go everywhere.
The parts required to make this seemingly elaborate system are quite simple. A plastic insulated picnic cool box with a well fitting lid, a length of 50mm (2 inch) waste pipe, a 90 degree bend to suit the pipe, and a perforated sheet of plastic to separate the two chambers and support the ice cubes (I used a pound store plastic chopping board).
Step 2: Creating the Ports
Begin by cutting the inlet and exhaust pipes. For the inlet pipe allow at least 30mm - 40mm (1 1/2") on the inside of the cool box and 50mm (2") on the outside. Mark up and drill a series of holes on the part that will be OUTSIDE the box. This will create the venturi.
For the exhaust, mark up about 25mm (an inch) on the outside of the box and allow enough inside the box that when the 90 degree angle is fitted the inner 'chimney' will clear the side of the cool box by about 25mm (1"). It does not have to be massively accurate. Cut the chimney so that once fitted there is a gap of at least 25mm (1") between the top of the chimney and the lid of the box.
Step 3: Machining the Box
You will need a hole saw to suit your pipe. Use the nozzle of the smoke machine as a guide to the height required on the inlet pipe. Drill a hole for the inlet pipe on one side of the cooler box about 30mm (1 1/4") from the end.
Drill the hole for the exhaust pipe on the side at right angles to the inlet at the far end and centralised. Again drill this as low as you can allowing for the elbow fitting and insulation clearance.
Glue the inlet pipe into place using hot glue or something similar. You need a good seal without damaging the internal insulation. Glue the outside and the inside to ensure a decent seal. The vent holes for the venturi need to be on the OUTSIDE of the box.
Glue the exhaust pipe into place such that the internal 'chimney is perpendicular and at least an inch from the sidewall. It will make life easier for you if the distance from the box side wall to the chimney is a known value when you come to fit the ice tray. Again glue both outside and inside to get a decent seal.
Step 4: Creating the Ice Tray
I've used a heap plastic cutting board for this but any perforated sheet capable of supporting the ice will do. Obviously don't use hardboard or MDF or anything that won't like getting wet. First of all cut the edges so that the sheet will fit in the box. The gaps at the edges must not be more than 6mm (1/4") or the smoke will simply bypass the ice and not get cooled. Cut a hole so that the chimney will pass through, this is where your known value and the central position of the chimney will be useful. If it's cut accurately enough then the ice tray will sit happily on the hip of the elbow fitting. If not you will need supports to hold the ice tray at between 1/3 and 1/2 the depth of the box to create the lower chamber.
Since I used a solid sheet of plastic I had to mark a grid and then drill out all the perforations by hand. I added two support dowels near the inlet end so that the ice tray sits level and does not fall too far down the box.
Step 5: Final Assembly
Now drop the ice tray down over the chimney. Don't glue it in place, it only needs to support the ice and you will need to remove it to make any corrections or to clean the box out. We will carry out a couple of tests in the next step and alterations may be required later.
For the construction phase the chiller is now complete.
Step 6: Flow Testing
Initially test the chiller WITHOUT ice added. This will tell you if everything is working correctly. Start with the smoke machine nozzle about 25mm (1") away from the inlet pipe pointing directly into the chiller. When you fire up the smoke machine first check that all the smoke is being sucked into the inlet pipe. If smoke seems to be coming back out of the pipe when the machine is running, check the seal around the inlet pipe first, try moving the machine a little closer or a little further away. If it's still blowing back try enlarging the venturi holes or adding more.
Smoke will often leak from the lid, don't worry we'll correct this in the final adjustments stage. If it's only a small amount don't worry too much.
Check the output at the exhaust pipe. You should see a good, dense flow of fog. It should also react quickly when you pulse the smoke machine. If it does not then check the ice tray holes are allowing the smoke to pass through freely. If not enlarge the holes or add more. Once you are happy that the smoke is passing through correctly without ice added it's time to make final adjustments and then test with ice.
Step 7: Final Refinements
First I needed to try and stem the leaks from the lid. This is mostly due to the poor design of this particular box. This is especially true around the hinge. If your box has a fully removable lid you may have less trouble. In any event create some gaskets using 'funky foam' and impact adhesive (barge cement). It took me a good couple of hours to try and plug all the leaks and in the end I found that it was impossible to make the lid of this box a gas tight seal. I'll just have to live with some minor leakage.
The box also still needs a drain plug since the melting ice cubes will soon start to fill the lower part of the box and drip back out of the inlet pipe. The easiest way to do this is to locate the drain point on the side with the exhaust port as low down in one corner (or underneath)as you can. This will allow you to simply open the drain, tilt the box and pour the excess water out into a bucket. The only reason I haven't done this yet is that I haven't found a suitable fitting.
*I have edited the ible with two extra steps now that I have found something that will do the job*.
Once all the seals are in place add ice cubes. I bought a bag for 99p (about a buck) and it just fills the upper chamber nicely.
Step 8: Fitting a Drain Part 1
So I cast around for a suitable drain system, it needed to be cheap and simple. I came up with the tap on a 'box of wine'. In fact they are a bag of wine inside a box that come fitted with a simple spring loaded diaphragm valve tap.
The bags have a soft white plastic (possibly nylon or PTFE or similar) insert that is an integral part of the bag. The wine is filled using this port. Then a harder plastic pipe with the tap is inserted into the white nylon to seal the bag and contents.
By carefully slicing through the nylon (it cuts very easily), I was able to separate the white nylon collar from the black ABS tap. This leaves us with a tap connected to about 25mm (1") of black ABS plastic with a large flange which should give a good seal.
Step 9: Fitting a Drain Part 2
You will need a hole saw to suit your tap. The next problem is that we need the drain to be as low as possible, but there is a fair thickness of insulation within the walls of the box so trying to judge the distances on the outside is too difficult especially with the curved bottom.
The easiest way I could think of was to lay the hole saw directly in the cooler box and hand rotate it to make a small indent into the plastic material. I marked that with a Sharpie, then used the flex shaft on my Dremel to drill a small pilot hole (3 mm) through the wall of the cooler box from the inside.
Then from the outside I used the pilot hole to guide the hole saw through the wall of the box. I test fitted the tap to check that it would all work and was level with the bottom on the inside. I then used hot glue to cement it in place. I then used hot glue again to ensure a good seal on both the outside and the inside.
Step 10: In Use
Here's a ten second blast of the fog machine through the chiller filled with ice cubes. This is exactly the effect I was looking for. The smoke is dense and full bodied and it cascades downwards leaving the upper part of the room clear. Low lying for effects without the need for dry ice and without the fog machine actually being in contact with the chiller unit.