A natural circulation water tube package boiler having a plurality of self-contained steam generating units in tandem, which package boiler comprises
- (a) a first SGU having
- (i) an enclosure formed by bottom, top, and opposed side walls joined horizontally along their longest dimension to form four sides of a rectangular parallelepiped having a horizontal longitudinal axis and the general proportions of a boxcar;
- (ii) a cylindrical mud drum located within the enclosure near the bottom wall and having its longitudinal axis parallel to the longitudinal axis of the enclosure;
- (iii) a cylindrical steam drum located within the enclosure near the top wall and having its longitudinal axis parallel to the longitudinal axis of the enclosure, which steam drum contains equipment for separating steam from water;
- (iv) riser tubes connecting the mud drum to the steam drum so as to permit water, saturated steam, and mixtures thereof to flow from the mud drum to the steam drum, which riser tubes are radiant tubes;
- (v) an entrance opening for fuel and air;
- (vi) a combustion chamber located generally between the drums and defined by the radiant tubes which surround it and are connected to each other by fins to form membrane walls extending along, joined to, and supporting the side walls in a hollow parallelepiped array;
- (vii) a large rear opening for combustion gases to exit, which opening is perpendicular to said horizontal longitudinal axis;
- and
- (viii) no convection tubes;
- (b) a second SGU having
- (i) an enclosure formed by bottom, top, and opposed side walls joined horizontally along their longest dimension to form four sides of a rectangular parallelepiped having a horizontal longitudinal axis and the general proportions of a boxcar;
- (ii) a cylindrical mud drum located within the enclosure near the bottom wall and having its longitudinal axis parallel to the longitudinal axis of the enclosure;
- (iii) a cylindrical steam drum located within the enclosure near the top wall and having its longitudinal axis parallel to the longitudinal axis of the enclosure, which steam drum contains equipment for separating steam from water;
- (iv) riser tubes connecting the mud drum to the steam drum so as to permit water, saturated steam, and mixtures thereof to flow from the mud drum to the steam drum, which riser tubes are convection tubes in a forest-like array throughout the second SGU, with the outermost convection tubes connected to each other by fins to form membrane walls extending along, joined to, and supporting the side walls;
- (v) a large front opening for combustion gases to enter, which opening is perpendicular to said horizontal longitudinal axis;
- (vi) a large rear opening for combustion gases to exit;
- and
- (vii) no combustion chamber;
- (c) a large SGU-connecting duct connecting the rear of the first SGU to the front of the second SGU so that the two SGUs are in tandem;
- (d) first inter-SGU piping connecting said mud drums for transferring water heated by the mud drum of the second SGU to the mud drum of the first SGU;
- (e) second inter-SGU piping connecting said steam drums for transferring water from the lower portion of the steam drum of the second SGU to the lower portion of the steam drum of the first SGU;
- (f) third inter-SGU piping connecting said steam drums for transferring steam from the upper portion of the steam drum of the first SGU to the upper portion of the steam drum of the second SGU;
- and
- (g) fourth inter-SGU piping connecting the rear, lower portion of the steam drum of the second SGU and the mud drum of the first SGU, for transferring water from that steam drum downwardly to that mud drum;
- wherein the membrane walls of the SGUs and the SGU-connecting duct define a flow path for combustion gases which extends from the combustion chamber of the first SGU within the perimeter of the radiant tubes, through the SGU-connecting duct, and through the second SGU around the convection tubes in the forest-like array, and then through the exit opening, without making a U-turn within either SGU.