The diagram below shows how a typical handheld unit works.
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The backpack contains three cylinder tanks. The two outside tanks hold a flammable, oil-based liquid fuel, similar to the material used to make Greek fire. The tanks have screw-on caps, so they can be refilled easily. The middle tank holds a flammable, compressed gas (such as butane). This tank feeds gas through a pressure regulator to two connected tubes.
One tube leads to the ignition system in the gun, which we'll discuss later on. The other tube leads to the two side fuel tanks, letting the compressed gas into the open area above the flammable liquid. The compressed gas applies a great deal of downward pressure on the fuel, driving it out of the tanks, through a connected hose, into a reservoir in the gun.
The gun housing has a long rod running through it, with a valve plug on the end. A spring at the back of the gun pushes the rod forward, pressing the plug into a valve seat. This keeps the fuel from flowing out through the gun nozzle when the trigger lever is released. When the operator squeezes the trigger lever, it pulls the rod (and the attached plug) backward. With the valve open, the pressurized fuel can flow through the nozzle. A flamethrower like this one can shoot a fuel stream as far as 50 yards (46 meters).
As it exits the nozzle, the fuel flows past the ignition system. Over the years, there have been a variety of ignition systems used in flamethrowers. One of the simpler systems was a coil of high-resistance wire. When electrical current passed through these wires, they released a lot of heat, warming the fuel to the combustion point. The gun in the above diagram has a slightly more elaborate system.
When the ignition valve is open, compressed flammable gas from the middle cylinder tank on the backpack flows through a long length of hose to the end of the gun. Here it is mixed with air and released through several small holes into the chamber in front of the nozzle. The gun also has two spark plugs positioned in front of the nozzle, which are powered by a portable battery. To prepare the gun, the operator opens the ignition valve and presses a button that activates the spark plug. This creates a small flame in front of the nozzle, which ignites the flowing fuel, creating the fire stream.