Tube arrangement in a once-through horizontal evaporator

A method comprising:

• discharging a working fluid through a once-through evaporator;
• where the once-through evaporator comprises:
• an inlet manifold;
• one or more inlet headers in fluid communication with the inlet manifold;
• one or more tube stacks, where each tube stack comprises one or more inclined evaporator tubes;
• the one or more tube stacks being in fluid communication with the one or more inlet headers;
• where the inclined tubes are inclined at an angle of less than 90 degrees or greater than 90 degrees to a vertical;
• where each tube stack comprises a plurality of tubes arranged in a plurality of columns and a plurality of rows;
• where a plurality of tubes in a first column are offset from a plurality of tubes in a second column by a distance d2 and where a plurality of tubes in a first row are offset from a plurality of tubes in a second row by a distance d1;
• where d1 varies from 0.1d2 to 1000d2;
• one or more outlet headers in fluid communication with one or more tube stacks;
• and
• an outlet manifold in fluid communication with the one or more outlet headers;
• discharging a hot gas from a furnace or boiler through the once-through evaporator;
• and
• transferring heat from the hot gas to the working fluid.