Also find this idea interesting. But would liquid nitrogen be a typical thing for the era? To stay true to it it would have to be something else but what? Since liquid nitrogen is fairly modern.
A chemspray of sorts? Then it would need large pressurised tanks to the gun.
Gun would be like a firefighting platform but used as a weapon with short range.
Kinda like what they have on ships at sea.
Effect would be slowly degrading health like moonshine to engines or fire.
What would counter it? I believe if a unit gets freezed and is on fire it would nullify the fire and slowly break the component. If freezed and attacked with fire it should stop or reverse the damage slightly then take a short time before it sets the unit on fire. I don't believe they should be able to get used simultaneously. That could affect balance.
Woudl it be a light or medium weapon?
The visual effect on component something like the heatsink but more white?
The arc would have to act like a fluid. as long as pressure fairly low arc but then falls down quickly at end with maybe a light spray so the closer the better?
The weapon itself to withstand the pressurised cold would have to be more like the heavy flak or echidna than the flamethrower. Short spray of water with slight damage on weapon itself while firing due to the cold, then it would regenerate it back while reloading?
Hot items like engines, guns and balloon would have more slow damage but hull would break faster. It could be a hullbreaker for sure.
What would it be named? Permafrost?