(Foam Machine, Snow Machine or Flocker)
Compressed Air Foamer in Action
In recent years the idea of compressed air foam systems (CAFS) have been catching on with fire services around the world, although their use is still not widespread. These systems have significant advantages over other types of fire fighting foam, in particular the distance the foam will project from the nozzle, the small quantity of water required to fight the fire and the ‘clingyness’ of the foam produced. All of these are distinct advantages for the sort of activities that we make use of foam for, so it set me thinking.
CAFS works by injecting a supply of air into the flow of fluid where it exits the pump. This travels along the pipe entrained in the fluid, mixed by the roughness of the pipe. It then expands at the nozzle to form a foam. Expansion ratios of 20:1 to 30:1 are typical, forming a very dense foam with small bubbles, reminiscent of shaving foam.
This project started coming together when I found a standard foam fire extinguisher in the local scrapyard. Many modern extinguishers make use of a Schrader valve to pressurise them making recharging them with a footpump or compressor really easy (compared to older styles that use a small cylinder of carbon dioxide). This style of extinguisher are getting easier to find and lend themselves well to experimenting.
To prove the CAFS concept, I first emptied and cleaned the extinguisher and then re-filled with water and bubble bath. When let off, foam was produced, but was somewhat disappointing- much like a standard extinguisher (as would be expected!). The next stage was to drill a small hole in the dip tube close to where it exits the extinguisher and to remove the foam nozzle from the hose. The results of this were promising, although short lived. A good dense foam was produced, but the pressure dropped very rapidly in the cylinder. It wasn’t giving a burst of much longer than a few seconds. This did prove however that the principal was sound. So I set about making a better system.
Before I go into the description of how I constructed a fully working unit, I shall cover the safety bit:
This unit was built using suitably rated industrial components, it is unlikely you will have access to identical components- select your parts carefully. Systems using compressed air are potentially very dangerous if mishandled, therefore this is not a project suitable for someone new to compressed air projects- please read up, and build up your experience on simpler projects first.
The unit is mounted on a piece of aluminium sheet, folded for about 40mm at the edges to form a cradle. A small piece of angle is fixed at one end to allow a handle to be attached. At the other end of the cradle, two pieces of angle are fitted at the other end to create a base for it to sit on.
The fluid reservoir is a stainless steel keg that had been used for a water softener system. Simlar units are used for dispensing post mix in pubs and simialar places, commonly known as Soda Kegs. This particular one is an unidentified brand, but similar to a Cornelius ‘Corny’ Keg:
http://en.wikipedia.org/wiki/Cornelius_kegs
The keg is attached to the frame using a couple of giant stainless worm drive clamps salvaged from the scrapyard.
All of the pipework, valves, regulators etc were parts I already had in stock, mostly found in the local scrapyard, and in some cases unused parts. Sorting out how to connect the hotch-potch of parts together did take some time, but was worth it because of the savings made. If you were to start from scratch with new parts, this would be much easier as you wouldn’t have to take account of loads of different thread sizes and sexes.
The main air supply comes through a PCL male fitting (Air tool connector).
From the PCL connector, the air supply passes through a regulator. This enables the air to be held at a constant known pressure, and hence give a consistent flow at a known pressure to the foam fluid exiting the tank. This is important in making a consistent foam. Following the regulator there is a tee, one leg going to the feed port on the soda keg. From the other leg of the tee, air is fed through a flow regulator into the water stream. The water comes from the dip tube in the tank and is mixed with the air flow in another tee piece. The output of the tee piece is then fed through a ball valve, and a second PCL fitting, this time a female one.
In practice this incarnation was reached after a number of only partially successful attempts. The original unit had two regulators and one way valves- one to control the pressure in the tank, and the other the feed into the water flow. This however caused the system to pulse heavily, and didn’t give the control over the output I had hoped. The simpler set-up described works better.
The other area that seems to have a somewhat disproportionate affect on the output is the hose size and nozzle. The setup as shown works well, but I had hoped to have a trigger nozzle to turn the output on and off easily. Attaching a air blowgun allowed the control, but completely stopped it foaming. I’m sure that there must be a way to make this work, but haven’t yet worked out the optimum.