Lancashire boiler construction and working

A Lancashire boiler is a horizontal, internally fired, natural-circulation fire tube boiler with two large furnace tubes (flues) running through the shell. It is a stationary boiler for medium pressure.

Construction

• A cylindrical shell about 2 to 3 m in diameter and 7 to 9 m long, set in brickwork and filled with water to well above the furnace tubes.
• Two large-diameter furnace tubes pass lengthwise through the shell. The grate is at the front end of each tube.
• The brickwork forms a bottom flue under the shell and two side flues along its sides.
• Typical rating: pressure up to about 16 bar and steam output up to about 8 t/h.

   Steam stop valve   Safety valves
         \            /
   ______[=====steam space=====]____
  |  front <---- water ---- rear   |
  |  ===== furnace tube 1 =====>   |
  |  ===== furnace tube 2 =====>   |
  |___________  shell  ____________|
   grate        |  bottom flue  |
   ----> hot gas path:           |
   tubes (1st pass) -> bottom flue (2nd pass,
   to front) -> side flues (3rd pass) -> chimney

Working

1. Fuel burns on the grates inside the furnace tubes.
2. Hot gases travel to the rear through the furnace tubes (first pass), heating the water.
3. At the rear they turn downward into the bottom flue and flow forward under the shell (second pass).
4. At the front they divide into the two side flues and flow backward (third pass), then reach the chimney through sliding dampers that control the draught.
5. Steam collects in the steam space and is taken out through the stop valve (an anti-priming pipe or steam separator can be fitted).

Mountings

Water level gauge, pressure gauge, safety valve (dead-weight or spring-loaded), high-steam low-water safety valve, fusible plug (on the furnace tubes), steam stop valve, feed check valve, blow-off cock, manhole and mud hole.

Merits: simple, reliable, large water reserve, easy to operate and clean. Demerits: slow steaming, large floor space, low pressure and capacity.