Multiple pass flexible water tube boiler and method of using same

A multiple-pass flexible water tube boiler comprising:

• an enclosure formed by a first side wall, a second side wall, a front wall, a rear wall, a top surface and a bottom surface;
• a cylindrical upper drum having its longitudinal axis proximate to either said first side wall or said second side wall;
• a cylindrical lower drum located at a position substantially vertically aligned with said upper drum;
• boiler tubes of essentially equal length connecting, and allowing communication between, said lower drum and said upper drum wherein said tubes comprise:
• (i) a first group of two water tubes being in the same plane and having a first tube exiting said lower drum extending laterally in an essentially horizontal run, crossing the centerline of the boiler, toward said second wall then upward in a vertical run then turning to execute one or more additional horizontal runs by extending horizontally along a longitudinal center line toward a side wall, crossing the centerline of the boiler, and then turning and running horizontally again and repeating until said first tube terminates at an entrance in said upper drum, and a second tube exiting said lower drum and extending upward in a vertical run along said first side wall, then turning to execute one or more additional horizontal runs by extending horizontally along a longitudinal center line toward a side wall, crossing the centerline of the boiler, and then turning and running horizontally again and repeating until said second tube terminates at an entrance to said upper drum;
• said first tube and said second tube not being of identical shape;
• and,
• (ii) a second group of two water tubes being in the same plane and having a third tube exiting said lower drum, extending laterally in an essentially horizontal run, crossing the centerline of the boiler, toward said second wall then upward in a vertical run then turning to execute one or more additional horizontal runs by extending horizontally along a longitudinal center line toward a side wall, crossing the centerline of the boiler, and then turning and running horizontally again and repeating until said third tube terminates at an entrance in the upper drum, and a fourth tube exiting said lower drum and extending upward in a vertical run along said first side wall, then turning to execute one or more additional horizontal runs by extending horizontally along a longitudinal center line toward a side wall, crossing the centerline of the boiler, and then turning and running horizontally again and repeating until said fourth tube terminates at an entrance to said upper drum, said third tube and said fourth tube not being of identical shape;
• wherein said longitudinal center lines of said fourth tube during horizontal runs are essentially parallel to said longitudinal center lines of said second tube during horizontal runs but are offset in an upward direction;
• a combustion chamber defined by a portion of said boiler tubes;
• and,
• a convection section defined by a portion of said boiler tubes.