Types of Boilers, Fire Tube Boilers and Water Tube Boilers, and there features

The demand for steam for power generation as well as industrial

processes is continuously increasing. Steam boilers now cover a

wide range - from those required to run a laundry to the very

l a rge ones used in electric power generation. Some of the recent

boilers installed in Hong Kong deliver approximately 2,000

tonnes of steam per hour and consume 1.90 million tonnes of

coal per year per unit. Steam pressures and temperatures can be as

high as 163 kg/cm2 (2,400 psi) and 538ºC (1,000ºF) respectively.

In general, fossil-fuel boilers are of two main types:-

(a) The Fire-tube or Shell Boiler; and

(b) The Water-tube Boiler.

Features of Fire - Tube boiler

Fire-tube boilers are potentially dangerous as the steam pressures

are high and the volume of hot water and steam content is larg e .

Features normally include:-

(a) water surrounds the tubes through which hot combustion

gases pass before venting to atmosphere through gas

uptakes.

(b) water converted into high pressure steam by transfer of heat

from the very high temperature combustion gases.

(c) hot combustion gases produced by burning diesel oil, town

gas etc. in the furnace.

(d) designed for automatic control.

(e) essential mountings include safety valves, pressure switches,

pressure gauges, water level gauges, valve fittings, lowwater

level cut-off and alarm devices etc.

There have been various designs and makes of the fire-tube boiler

e.g. Vertical Cross Tube Boiler, Cochran Boiler, Vertical Dry To p

B o i l e r, Scotch Marine Boiler, Horizontal Return Tube Boiler and

Packaged Boilers. Packaged Boilers are completely self-contained

units and are the latest development of the Scotch Marine Boiler.

These boilers are factory assembled, tested and adjusted. Figure 1

illustrates one of these units. Each unit has electrically driven feed

and fuel pumps, forced draught fan, combustion equipment,

operating and safety control equipment for manual or automatic

o p e r a t i o n .

F e a t u res of Wa t e r- Tube Boiler

Wa t e r-tube boilers provide for better water circulation, more

heating surface and reduced explosion hazard. A basic water- t u b e

boiler is illustrated in figure 4. Feature normally include:-

(a) hot combustion gases surround the tubes through which

water/steam circulates.

(b) water is converted into high pressure steam, saturated or

superheated, by transfer of heat from the very high

temperature combustion gases.

(c) hot combustion gases produced by burning coal, oil or gas

in specially designed water cooled furnaces.

(d) designed for automatic control.

(e) essential mountings include safety valves, feedwater

c o n t r o l l e r, combustion control system, water level

c o n t r o l l e r, pressure and temperature controllers, and lowwater

level alarm and cut-off devices etc.

The water-tube boilers advanced considerably through

development of a design connecting the steam generating tubes

directly to a steam separating drum and featuring the use of bent

tubes as against earlier straight tubes. Steam pressure and steam

conditions also increased through adoption of steam reheating,

regenerative feedwater heating and construction of larger boilers

incorporating pulverized coal firing rather than the old fashion

stokers that proved inadequate for modern highly forced boilers.

The higher volumetric combustion rates and unit sizes of

pulverized coal firing units has been achieved through the use of

water cooled furnaces. This technique eliminated the problem of

rapid deterioration of refractory walls due to slag, and reduced

fouling of convection heating surfaces to manageable proportions

by lowering the temperature of the gases leaving the furnace.

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M❤K