Types of Boilers

Based on the order of Evaporation, the boilers are categorized into:

1) Cast Iron sectional Boilers

These are used for hot water services with a maximum operating pressure of 5 bar and a maximum output in the order of 1500 KW.Site assembly of the unit is necessary and will consist of a bank of cast iron sections. Each section has internal waterways.

The sections are assembled with screwed or taper nipples at top and bottom for water circulation and sealing between the sections to contain the products of combustion. Tie rods compress the sections together.

A standard section may be used to give a range of outputs dependent on the number of sections used. After assembly of the sections, the mountings, insulation and combustion appliance are fitted.

This system makes them suitable for locations where it is impractical to deliver a package unit, eg.basements where inadequate access is available or rooftop plant rooms where sections may be taken up using the elevator shafts. Models available use liquid, gaseous and solid fuel.

2) Steel Boilers

These are similar in rated outputs to the cast iron sectional boiler. Construction is of rolled steel annular drums for the pressure vessel. They may be of either vertical or horizontal configuration, depending upon the manufacturer. In their vertical pattern they may be supplied for steam rising.

3) Electrode Boilers

These are available for steam raising up to 3600kg/h and manufacture is of two designs. The smaller units are element boilers with evaporation less than 500kg/h.In these, an immersed electric element heats the water and a set of water-level probes positioned above the element controls the water level being interconnected to the feed water pump and the element electrical supply.

Larger units are electrode boilers. Normal working pressure would be 10 bar but higher pressures are available. Construction is a vertical pattern pressure shell containing the electrodes. The lengths of the electrodes control the maximum and minimum water level. The electrical resistance of the water allows a current to flow through the water, which in turn, boils and releases steam. Since water has to be present within the electrode system, lack of water cannot burn out the boiler. The main advantage with these units is that they may be located at the point where steam is required and, as no combustion fumes are produced, no chimney is required. Steam may also be raised relatively quickly, as there is little thermal stressing to consider.

4) Steam Generators

This coil type boilers works in the evaporative range up to 3600 kg/h of steam. Because of the steam pressure being contained within the tubular coil, pressures of 35bar and above are available, although the majority is supplied to operate at up to 10 bar. They are suitable for firing with liquid and gaseous fuels, although the use of heavy fuel oil is unusual.

The coiled tube is contained within a pressurized combustion chamber and receives both radiant and convected heat. A control system matches the burner-firing rate proportional to the steam demand. Feed water is pumped through the coil and partially flashed to steam in a separator.

The remaining water is recirculated to a feed water heat exchanger before being run to waste. Because there is no stored water in this type of unit they are lighter in weight and therefore suitable for sitting on mezzanine or upper floors adjacent to the plant requiring steam.

Also, as the water content is minimal, steam raising can be achieved very quickly and can respond to fluctuating demand within the capacity of the generator. It must be noted that close control of suitable water treatment is essential to protect the coil against any build-up of deposits.

5) Vertical Shell Boilers

This is a cylindrical boiler where the shell axis is vertical to the firing floor. Originally it comprised a chamber at the lower end of the shell, which contained the combustion appliance. The gases rose vertically through a flue surrounded by water. Large diameter (100mm) cross tubes were fitted across this flue to help extract heat from the gases which then proceeded to the chimney. Later versions had the vertical flue replaced by one or two banks of small-bore tubes running horizontally before the gases discharged to the chimney. The steam was contained in a hemispherical chamber forming the top of the shell.

The present vertical boiler is generally used for heat recovery from exhaust gases from power generation or marine applications. The gases pass through small-bore vertical tube banks. The same shell may also contain an independently fired section to produce steam at such times, as there is insufficient or no exhaust gas available.

6) Waste Heat Boilers

These may be horizontal or vertical shell boilers or water tube boilers. They would be designed to suit individual applications ranging through gases from furnaces, incinerators,gas turbines and diesel exhausts. The prime requirement is that the waste gases must contain sufficient usable heat to produce steam or hot water at the condition required.

Supplementary firing equipment may also be included if a standby heat load is to be met and the waste-gas source is intermittent. Waste-heat boilers may be designed to us either radiant or convected heat sources.

In some cases, problems may arise due to the source of waste heat, and due consideration must be taken of this, with examples being plastic content in waste being burned in incinerators, carry-over from some type of furnaces causing strongly bonded deposits and carbon from heavy oil fired engines.

Some may be dealt with by maintaining gas-exit temperatures at a predetermined level to prevent dew point being reached and others by soot blowing. Currently, there is a strong interest in small combined heat and power (CHP) stations, and these will normally incorporate a waste-heat boiler.

7) Fluid-bed Boilers

The name derives from the fire bed produced by containing a mixture of silica sand and ash through which air is blown to maintain the particles in suspension.

The beds are in three categories

i) Shallow bed
ii) Deep bed and
iii) Recirculating bed

Shallow beds are mostly used and are about 150-250mm in depth in their slumped condition and around twice that when fluidized. Heat is applied to this bed to raise its temperature to around 600C by auxiliary oil or gas burners. At this temperature coal and/or waste is fed into the bed, which is controlled to operate at 800-900C.Water-cooling surfaces are incorporated into this bed connected to the water system of the boiler.

The deep bed, as its name implies, is similar to the shallow bed but in this case may be up to 3m deep in its fluidized state, making it suitable only for large boilers. Similarly, the recirculating fluid bed is only applicable to large water tube boilers.

Several applications of the shallow-bed system are available for industrial boilers; the two most used being the open-bottom shell boiler and the composite boiler. With the open-bottom shell the combustor is sited below the shell and the gases then pass through two banks of horizontal tubes.

In the composite boiler the combustion space a water tube chamber directly connected to a single-pass shell boiler forms housing the fluid bed. In order to fluidize the bed the fan power required will be greater than that with other forms of firing equipment.

To its advantage, the fluid bed may utilize fuels with high ash contents, which affect the availability of other systems. It is also possible to control the acid emissions by additions to the bed during combustion. They are also less selective in fuels and can cope with a wide range of solid-fuel characteristics.


Auxiliary boilers on Navy ships may be divided into two groups: FIRE-TUBE BOILERS and WATER-TUBE BOILERS.

Fire-Tube Boilers

Fire-tube boilers are generally similar to Scotch marine or locomotive boilers. In this type of boiler, the gases of combustion pass through tubes that are surrounded by water. There are a number of auxiliary boilers of the fire-tube type in use in diesel-driven ships. Figure illustrates a cutaway view of the fire-tube boiler shown in figure.

Water-Tube, Natural-Circulation Boilers

Water-tube, natural-circulation boilers consist basically of a steam drum and a water drum connected by a bank of generating tubes. The two drums are also connected by a row of water tubes, which forms a water-cooled sidewall opposite the tube bank. The water-wall tubes pass beneath the refractory furnace floor before they enter the water drum. In natural-circulation boilers, the steam and water

Figure Natural-circulation, water-tube boiler.

drums are connected by several tubes of larger diameter, called DOWNCOMERS or WATER TUBES (not shown). These tubes are positioned away from the flow of hot gases of combustion. Refractory is also used to protect these downcomers from contact with the combustion gases.

The operating principle of a natural-circulation boiler is quite simple. It relies on the difference in density (weight) between the cooler (heavier) water in the water tubes (or downcomers) and the hot, less dense (lighter) water in the steam-generating tubes. This is the force that causes the hot water and steam mixture to rise in the tubes in the generating bank, from the water drum to the steam drum, where the steam is separated from the water and rises to the top of the steam drum. The flow of water up the tubes of the steam-generating bank must be maintained; otherwise, the tubes would quickly melt. A constant flow of water and steam up the tubes is required to carry away heat at the proper rate. If the flow from natural circulation is allowed to stop, such as when the water level in the steam drum falls below the openings of the bank of tubes for the water wall, the tubes of the generating bank will be severely damaged and the boiler will need major repairs. (Replacing boiler tubes is an expensive operation.)

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