FACTORIES AND MACHINERY (STEAM BOILER AND
UNFIRED PRESSURE VESSEL)
Regulation 41. Heating surface.
(1) For the purpose of determining the area of the safety valve and the inspection fee the heating
surface of a steam boiler shall be the total surface of all plates and tubes exposed to heat on one
side and in contact with water on the other, measured on the water or fire side, whichever is the
(2) The heating surface of an economiser other than a steaming economiser and of a superheater
shall not be included in the steam boiler heating surface for the purpose of determining the
inspection fee, but shall be considered when calculating the capacity of the steam boiler safety
(3) For a Lancashire or Cornish type steam boiler the total heating surface shall include the
wetted surface of the furnace between the end plates, the fire surface of the cross tubes where
fitted and that part of the external shell below the side flue covers. In estimating areas, any
furnace shall be considered as a plain cylinder and the area of its wetted surface shall be taken
as its mean external circumference multiplied by the length of the steam boiler between the end
plates. For the shell, the width of the part of the circumference below the flue covers shall be
taken as equivalent to twice the mean diameter of the shell and this width multiplied by the length
between the end plates shall be taken as the area of the shell heating surface. Any part of the
back end plate exposed to heat shall be ignored.
(4) For a water-tube steam boiler the heating surface of a steam and water drum shall be taken
as half its external mean circumference multiplied by the clear length between the outer brick
walls or centres of cross boxes, as the case may be. The heating surface of a tube shall be taken
as its external surface between tube plates or headers. The surface of any header shall be
(5) For a marine steam boiler of the fire-tube type the heating surface shall include the wetted
surface of any furnace between the tube plates (considered as for a Lancashire steam boiler), the
wetted surface of any combustion chamber (less the area of the tube holes) and the wetted
surface of any tubes between the tube plates. Any part of the front tube plate exposed to heat
shall be ignored.
(6) For a traction (loco-portable) or a vertical steam boiler the heating surface shall include the
wetted surface of the fire-box above the foundation ring (less the area of the fire-hole and ring
and the tube holes) and the surface of any cross or other tube and uptake below a point half way
up the gauge glass.
(7) For a locomotive type steam boiler the heating surface shall include the wetted surface of the
inner fire-box plates ( excluding the surface covered by the foundation and fire-hole rings, the
area of the tube holes and the area within the junction of the plates in the case of a flange firehole)
and the wetted surface of the tubes between the
tube plates. The smoke-box plate shall be
(8) No deduction shall be made for the area of any stay in calculating the heating surface.
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