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Smoke tube Boilers

Tank (scotch) Boiler

These were the most common form of boiler design before the introduction of water tube designs. See Comparisons of water tube and Smoke tube boilers.

This style of boiler still see active service were low quantities of low quality steam are required, such as for cargo and fuel tank heating when in port.
This style of boiler is relatively cheap, supplied as a packaged unit and requires less stringent feed water conditioning and level control.

Design

Consists of a shell wrapper plate to which is welded ( or for later designs riveted, end plates.. Pressure is naturally container in the shell plate due to is cylindrical design. The flat end plates, however, must be 'stayed' to prevent buckling and distortion. The combustion chamber is of similar section and is also 'stayed'.
The boiler shown above is a single furnace, two pass design. Larger boilers may have multiple furnaces and have multiple passes by replacing the exhaust stack with a return chamber and fitting another bank of tubes.
The smoke tubes may be plain or threaded to act as stays. There are one stay tube for every three plain tubes approx.

To aid circulation the tubes are arranged in vertical rows to offer minimum resistance.

Fuel is combusted in the corrugated watercooled furnace. The corrugations increase the surface area and allow a degree of flexibility to allow for expansion and contraction.

The hot gas passes to the water cooled combustion space though to the smoketubes. The upper portion of the combustion chamber lies close to the water level and is therefore liable to distortion due to in correct water level maintenance.
Access to the boiler is via a manhole door on the upper shell plate. In addition a smaller door may be fitted below the furnace to allow inspection and scale/sludge removal.

Modern (packaged) boiler

This style of boiler may be fitted to the vessel as a complete unit with its own fuel and water delivery systems, control and safety equipment mounted directly on the unit. Alarms and shut downs may be are given at the local control panel which may be interfaced with ships alarms system to allow UMS operation.. The design is simialr to the scothch boiler other than the combustion chamber wich requires no stays. This design is a three pass design

Operation

Although the maintenance of the water level is not so critical as with water tube designs, it should not be allowed to fall too much as overheating of furnace and combustion spaces leads to catastrophic failure due to component collapse. The content of the boiler is then expelled via the furnace door. Similarly , although water treatment is not so critical scale must not be allowed to build up which can lead to overheating of material

Authors Note

Although package boilers of this design are fairly robust it should not be forgotten the potential for danger a poorly maintained unit can be. The author carried out a supposed routine inspection on one such unit. Opening the upper manhole revealed that the unit has been 'wet layed' with water left at normal working level rather than being pressed up . Severe pitting was present at and just below the water level.
The lower man hole door beneath the furnace was opened after draining the water. Heavy wastage could be seen later measured at 60 % of the shell plating with pitting on top of this. Evidently no oxygen scavenger, such as Sodium Sulphite, had been added before laying up.
In this condition, operation at full load would almost certainly have led to catastrophic failure.