I've not done a technical article for a while, and not so long ago I found a document that went into flamethrowers in excruciating detail. The data is held in a British report gathered from the MI10 intelligence branch, which dealt with enemy equipment.
By 1941 the Germans were using a updated version of the 1935 model flamethrower, named unsurprisingly the Flammenwerfer 41. It had a nitrogen compressed gas tank above a fuel tank, with a wire braided pipe running to a hand unit. Above the hand unit was a 20 inch hydrogen tank used for ignition. This was held in place by what were described as clips like those used to attach a bicycle pump to its frame. However later a new improved version appeared. The British identified it as a Model 42, although the Germans don't seem to have had this designation. The main change was the type of ignition from hydrogen battery type to a cartridge type. The cartridges were 9mm rimless with a length of 22mm. They were angled about 27.5 degrees and aimed at the fuel jet which was 10mm away. When the trigger was pulled it would fire a cartridge and allow the fuel to flow. The new shorter hand unit held ten cartridges, and these were reloaded when the trigger was released. The spent round was ejected forward. Both the loading feed and the ejection system was described as "ingenious, but highly complicated" by the British intelligence assessment.
"Model 41" you can see the Hydrogen tube being used as a handle in this picture.
As well as German flamethrowers the document considered American designs and vehicle based designs and went into quite some detail on them. The US started the war with the M1A1, which weighed in at 63 pounds fully charged with its 4.5 gallons of fuel. The US then developed the E2 version which had a slightly improved flame gun and ignition system. One change was to the tank that allowed the fuel flow to be activated by the operator, on the earlier M1A1 the fuel flow had to be initialised by a second man.
M1A1, if you're never sure which version you're looking at, the M1A1 doesn't have a fore grip
Another change to the tank was the construction material. The M1A1's tank shattered under tests with a .30 cal armour piercing bullet. The E2's tank didn't. It wasn't judged an issue however as any round that hit the tank was almost certainly going to have hit the operator first, or so the theory went.
Now we come to the old story about flamethrowers exploding when hit. It was found that when compressed air was used instead of nitrogen the air mixed with the fuel vapours to make a flammable mix, however nitrogen was a lot harder to use logistically. Where as a E2 could just be wired up to a compressor the British and German models needed a separate source of compressed nitrogen. This would possibly explain why a German training manual stated:
"In order to give the men a greater sense of security attention should be drawn to the fact that should the weapon be struck by an infantry bullet or shell splinter it will not explode."
As British flamethrowers used the same methods of propulsion as the German ones, compressed nitrogen gas, then it's almost certain that the same would apply.
The US suffered another bout of exploding flamethrowers when they tried to develop a single shot flamethrower. The Germans had developed the Einstoss. This was a single shot flamethrower, a tube four inches in diameter and twenty four inches long. A cartridge when fired provided propulsion for the jet of fuel and ignition at the same time.
German Einstoss
The British developed a similar idea in the form of the snapshot flamethrower. Both of these used normal liquid fuel. The Americans however decided to use a self igniting powder called EWP. It appears to have been comprised of phosphorus as the ignition source with phosphorus sesquisulfide to provide the fuel. The latter is a component of match heads. The self detonating of the US single shot flame thrower came from the use of a cartridge with slow burning cordite to propel a piston that created the pressure and fired the powder out the end of the gun. Not all the powder was ejected, this lead to some of the explosions. Additionally the fuel ejection was found to be non uniform or consistent. The pressure was built by at first using a stopper in the barrel of the US machine which would blow out at a certain point ejecting the flaming material. This gave inconsistent results however.
With all this in mind the weapon was redesigned. The EWP was contained in a collapsible tube with a frangible diaphragm at the muzzle, the piston powered by the charge now squeezed the package, and caused pressure to build that ruptured the diaphragm. The upshot of this design was the whole device could be re-loaded easily.
In the end the problems involved with creating the EWP and transporting it (a soft squidgy tube that if ruptured would burst into flames) seem to have killed off the project.
Lifebuoy Mk.II
The British at the time had three designs of flamethrowers, these were the Lifebuoy, the Ack-Pack and the Snapshot. The latter we've already discussed and it never seems to have gotten past a experimental stage, or if it did it seems to have been forgotten about by history. The Lifebuoy was the standard issue flamethrower used throughout the war by British forces. It was so named, or so the story goes, because it resembled a bar of lifebuoy soap. Of course it might be because it resembled an actual lifebuoy.
Lifebuoy soap in the 1940's... very slight resemblance.
The Ack-Pack was also known as the Para-Pack. As the name suggests it was a modified Lifebuoy that could be loaded into containers for dropping as part of airborne operations.

I've done some comparison tables for you:
The little data about the single shot weapons

Part two next week will cover tank mounted flamethrowers, and some rather horrific discoveries. It'll also tackle the question of how effective were flamethrowers.

Image credits:
www.militaryfactory.com, www.ww2incolor.com, www.canadiansoldiers.com and www.tgrantphoto.com