Abstract: A steam generator which may be mobile or stationary, which includes a fire box the outlet of which is connected to a cyclone section; the outlet of the cyclone section opens into a water tube steam boiler, with the connection between the cyclone section and the water tube steam boiler being such that the cyclone section substantially isolates the water tube steam boiler from the radiant heat of a fire in the fire box; a structural shell contains the fire box, cyclone section and water tube steam boiler and is arranged to both provide a containment vessel if the contained components fail in use and also to bear any of the structural loads applied to the generator in use.