Natural circulation

Fire-tube  or  Scotch-Marine  type  boilers  depend entirely on natural thermal circulation of water within the boiler shell. This determines its main features.

Water is convected upward between the tubes, usually faster in the rear of the boiler than in the front. Colder water flows downward along the shell,  then  upward  around  the  furnace  tubes  to complete the pattern. The circulation is caused by the difference in density between the water and steam/water mixture. As the steam  bubbles  form,  they  rise  to  the  water  surface and release in the steam disengaging area.

There  is  an  inherent  lag  characteristic  with  this system  which  limits  operational  flexibility  such  as quick  start-up,  rapid  load  change,  and  quick  shut-down. For example, the circulation rate is different at minimum and maximum firing rates. It requires a few moments  after  a firing  rate  change  to  establish  the equilibrium  which  involves  circulation,  tube  wall temperature and other factors. Steam  and  water  storage  capacity  is  large.

Water surface  area  is  also  large  to  permit  steam  release with  limited agitation to control moisture carry-over. Moisture  carryover  is  maximum  at  full  load  (2  to  3 percent)  and  higher  during  sudden  steam  demand. In  some  instances,  an  external  steam/moisture separator  is  required  to  maintain  desirable  steam quality.

Large  water  storage,  at  saturation  temperature, presents an explosion hazard. For this reason, shell and  tube  maintenance  is  critical  and  inspection  of their  condition  must  be  regularly  scheduled.  The necessary  strong  shell  construction  and  required large  water  volume  results  in  great  weight  and physical size.

The uneven tube temperatures which occur in rapid startups  and  sudden  load  changes  cause  high stresses  in  the  fire-tubes,  resulting  in  warped  tube sheets,  rear  door  leakage,  shortened  life  and increased down-time for the unit.

Depending on design, the  fire box, furnace or front and  back  sections  (whichever  is  applicable)  are lined  with refractory cement  which results in added weight and lost heat during start-up.