I'm working with a very talented modeller on a couple of articles on Thresh steam disinfectors mounted on the back of Foden Steam trucks to treat uniforms and bedding of troops to kill lice and fleas. WW1 was the first major war in which deaths from disease were lower than combat casualties due to advances in medical and public health sciences in the late 19th century.
The British Army used Foden steam trucks with two Thresh disinfectors mounted on the tray. In some of the images of the trucks they are fitted with a chimney extension. I know about zero of the black art of steam and wondered what the extension was for. Was it simply to direct the fumes from the boiler away from a stationary truck or was it to increase the draft through the truck's boiler?
I'm no expert either, but the first option looks more likely, to keep the exhaust away both from people working the disinfectors and the uniforms and bedding being deloused. The extensions look far too rickety to stay on if the unit tried to move anywhere. Draught would probably be reduced to some extent rather than improved, but that would not be an issue if the Foden was only producing steam for the disinfectors.
I agree with Peter, if you look at your picture of the two Foden together the second one, which does not have an extended chimney, has the canvas pulled down over the front of the cab. the chimney extension looks like a home made one of tin rolled into a pipe shape. The extension would have moved the smoke above the cab and whilst the steam chambers were being opened, loaded/unloaded and closed, away from those working on the chambers.
I would also note that the troops actually steaming the clothes look like Australians as there are both soldiers with Hat KFF brim up and brim down at both vehicles. Iwould think that the Foden Threshers were part of RASC visiting sub-units that went from Division to Division in each Army area and the picture was taken when in just happened to visit an Australian Division.
