Additional Boiler types
A fire-tube or Scotch boiler is a boiler in which the water is outside the tube and the hot gases pass through the tube.
Fire-tube boilers generally consist of a cylindrical shell, are internally fired, and are fitted with corrugated furnaces and return-tube fire tubes.
These boilers are both single- and double-end type.
They usually have two or more furnaces in each end.
Furnaces are cylindrical shells with circumferential corrugations to afford strength against collapse and to provide for expansion.
With oil fuel, burners are applied by means of a suitable furnace behind which refractory is placed.
If coal fuel is used, grates are placed approximately on the horizontal center of the furnace.
To direct air through the fuel bed a wall is erected at the rear of the grates.
The purpose of the combustion chamber is to provide combustion to distribute the flue gases to the tubes.
The advantages of the fire-tube boiler are:
ease of maintenance and repair,
ability to use feed water of doubtful quality,
adaptability to practically any fuel.
The disadvantages of the fire-tube boiler are:
they require large quantities of boiler water,
they are heavy and bulky,
they cannot raise steam quickly,
they are extremely hazardous if damaged due to the large quantities of water and steam contained in the shells.
In water-tube boilers the water circulates through the boiler tubes.
Water-tube boilers may be classed according to different parameters:
according to the arrangement of the water tubes there are straight-tube, bent-tube or coil-tube types;
with respect to the number of their water and steam drums or headers there are single-drum, two-drum, three-drum, etc.,
with respect to the arrangement of the drums or headers there are cross or lateral-drum or longitudinal-drum types.
The strong features of the water-tube boiler, as compared with the fire-tube types, are:
saving in weight and space,
the possibility of using high pressure and temperatures,
greater mechanical flexibility,
rapid steam raising,
safety in the event of serious rupture, due to smaller water contents.
The disadvantages of the water-tube boilers are:
their operation requires a greater degree of skill and engineering knowledge,
they require pure, fresh water only.
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