a way that the panels 54 that are transverse relative to the furnace are 1 5 to 2 times wider than the corresponding longitudinal panels 56 thereby the material flows coming from the front and rear walls of the furnace in other words from the long outer walls thereof or for example the flames of the start up burners may be arranged in such a way that they do not directly hit the longitudinal water tube panels 56 especially when the width of the panel parts in the evaporator surface units is a significant portion of the corresponding dimension of the furnace an opening 58 is or openings are formed in the panels especially to the lower parts thereof to enable as free a flow of the solid material in the furnace as possible fig3 illustrates in more detail the inlets of the water tube panels 62 64 in an evaporator surface unit 60 of the shape of a symmetric cross through the furnace ceiling 20 by means of a bellows box 66 and the connecting of water tubes of the panels 62 64 to the water cycle boiler the vapor formed in an evaporator surface unit 60 is preferably gathered to two outlet headers 36 38 parallel to the water tube panels 62 64 thereby the extensions of the water tubes required for connecting the water tubes of the water tube panels 62 64 to different sides of the outlet headers 36 38 and especially the tube bends 68 thereof can be formed in a simple manner in a compact space the vapor gathered in outlet headers 36 38 is guided to the steam separator by means of connecting tubes 70 72 connected to outlet headers 36 38 for balancing the vapor.