above the 34 once thru 34 load it is understood that several variations may be made in the foregoing without departing from the scope of the invention for example although in the example set forth above the roof 28 is located immediately downstream of the separator 26 a upper furnace steam cooled enclosure wall can be interposed between the outlet of the separator 26 and the roof thus the wet steam from the separator 26 would be fed to the latter enclosure wall prior to passing to the roof 28 in this case the upper furnace enclosure wall would utilize two distinct passes a two phase pass which is a continuation of the lower furnace pass and a wet steam cooled pass further it is understood that the present invention is not limited to the use of vertical waterwall tubes and the particular operating conditions set forth above including the specific ranges set forth in the table for example the waterwalls can be formed by spiral wound tubes as disclosed in u s pat no 4 191 133 and no 4 344 388 both of which are assigned to the assignee of the present invention and both of which are hereby incorporated by reference according to this arrangement the pressure in the steam generator 10 is held constant during relative low loads.