Boiler types

The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.

Watertube boilers

The construction of watertube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising

Foster Wheeler Boiler Esd
Figure 4.2 Foster Wheeler D-Type boiler

process is much quicker. Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the watertube boiler has replaced the firetube boiler as the major steam producer.

Early watertube boilers used a single drum. Headers were connected to the drum by short, bent pipes with straight tubes between the headers. The hot gases from the furnace passed over the tubes, often in a single pass.

A later development was the bent tube design. This boiler has two drums, an integral furnace and is often referred to as the 'D' type because of its shape (Figure 4.2). The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These waterwall tubes are connected either to upper and lower headers or a lower header and the steam drum. Upper headers are connected by return tubes to the steam drum. Between the steam drum and the smaller water drum below, large numbers of smaller-diameter generating tubes are fitted.

Economiser

First pass superheater Attemper a tor

Cross Drum Boilers

Second pass superheater

Figure 4.3 Foster Wheeler Type ESD I boiler

Second pass superheater

Floor tubes Refractory

Economiser

First pass superheater Attemper a tor

Figure 4.3 Foster Wheeler Type ESD I boiler

These provide the main heat transfer surfaces for steam generation. Large-bore pipes or downcomers are fitted between the steam and water drum to ensure good natural circulation of the water. In the arrangement shown, the superheater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brickwork is used on the furnace floor, the burner wall and also behind the waterwalls. The double casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner.

The need for a wider range of superheated steam temperature control led to other boiler arrangements being used. The original External Superheater 'D' (ESD) type of boiler used a primary and secondary superheater located after the main generating tube bank (Figure 4.3). An attemperator located in the combustion air path was used to control the steam temperature.

The later ESD II type boiler was similar in construction to the ESD I but used a control unit (an additional economiser) between the primary and secondary superheaters. Linked dampers directed the hot gases over the control unit or the superheater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low temperature superheating was required.

In the ESD III boiler the burners are located in the furnace roof, which provides a long flame path and even heat transfer throughout the furnace. In the boiler shown in Figure 4.4, the furnace is fully water-cooled and of monowall construction, which is produced from fmned tubes welded together to form a gastight casing. With monowall c onstruction no refractory material is necessary in the furnace.

The furnace side, floor and roof tubes are welded into the steam and water drums The front and rear walls are connected at either end to upper and lower water-wall headers. The lower water-wall headers are connected by external downcomers from the steam drum and the upper water-wall headers are connected to the steam drum by riser tubes.

The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over the primary superheater and the economiser before passing to exhaust. The dry pipe is located in the steam drum to obtain reasonably dry saturated steam from the boiler. This is then passed to the primary superheater and then to the secondary superheater. Steam temperature control is achieved by the use of an attemperator, located in the steam drum, operating between the primary and secondary superheaters.

Radiant-type boilers are a more recent development, in which the radiant heat of combustion is absorbed to raise steam, being transmitted

Cast iron gilled section__ f

Sootbtower-

Hinged access door

Steel fin sections

Feed water pipe Saturated off-take

Dry pipe

Perforated plates Upper front water wall header furnace side

Furnace risers

Cast iron gilled section__ f

Sootbtower-

Steel fin sections

Dry pipe

Perforated plates Upper front water wall header furnace side

Furnace risers

Hinged access door

Feed water pipe Saturated off-take

Superheater primary inlet header

Superheater primary outlet header Superheater secondary outlet header

Superheater secondary inlet header

Lower rear water wall header superheater side

Downcomers

Superheater Main bank screen tubes

Lower front water wall header superheater side

Figure 4.4 Foster Wheeler Type ESD III monowall boiler

Superheater primary inlet header

Superheater primary outlet header Superheater secondary outlet header

Superheater secondary inlet header

Lower rear water wall header superheater side

Lower front water wail ^ header furnace side

Sootb lower Water drum

Downcomers

Superheater Main bank screen tubes

Lower front water wall header superheater side

Figure 4.4 Foster Wheeler Type ESD III monowall boiler by infra-red radiation. This usually requires roof firing and a considerable height in order to function efficiently. The ESD IV boiler shown in Figure 4.5 is of the radiant type. Both the furnace and the outer chamber are fully watercooled. There is no conventional bank of generating tubes. The hot gases leave the furnace through an opening at the lower end of the screen wall and pass to the outer chamber. The outer chamber contains the convection heating surfaces which include the primary and secondary superheaters. Superheat temperature control is by means of an attemperator in the steam drum. The hot gases, after leaving the primary superheater, pass over a steaming économiser. This is a heat exchanger in which the steam-water mixture

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Attemperator Design

PART SECTIONAL PLAN

Figure 4.5 Foster Wheeler radiant-type boiler

PART SECTIONAL PLAN

Figure 4.5 Foster Wheeler radiant-type boiler is flowing parallel to the gas. The furnace gases finally pass over a conventional economiser on their way to the funnel.

Reheat boilers are used with reheat arranged turbine systems. Steam after expansion in the high-pressure turbine is returned to a reheater in the boiler. Here the steam energy content is raised before it is supplied to the low-pressure turbine. Reheat boilers are based on boiler designs such as the 'D' type or the radiant type.

Furnace wall construction

The problems associated with furnace refractory materials, particularly on vertical walls, have resulted in two water-wall arrangements without exposed refractory. These are known as 'tangent tube' and 'monowall' or "membrane wall'.

In the tangent tube arrangement closely pitched tubes are backed by refractory, insulation and the boiler casing (Figure 4.6(a)). In the monowall or membrane wall arrangement the tubes have a steel strip welded between them to form a completely gas-tight enclosure (Figure 4.6(b)). Only a layer of insulation and cleading is required on the outside of this construction.

(b) Monowall arrangement Figure 4.6 Furnace wall construction

The monowall construction eliminates the problems of refractory and expanded joints. However, in the event of tube failure, a welded repair must be carried out. Alternatively the tube can be plugged at either end, but refractory material must be placed over the failed tube to protect the insulation behind it. With tangent tube construction a failed tube can be plugged and the boiler operated normally without further attention.

(a) Tangent tube arrangement

(b) Monowall arrangement Figure 4.6 Furnace wall construction yy' •+— Insulation

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Cleading

Firetube boilers

The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed. The name 'tank boiler' is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler' are also in use.

Package boilers

Most firetube boilers are now supplied as a completely packaged unit. T his will include the oil burner, fuel pump, forced-draught fan, feed pumps and automatic controls for the system. The boiler will be fitted with all the appropriate boiler mountings.

A single-furnace three-pass design is shown in Figure 4.7. The first pass is through the partly corrugated furnace and into the cylindrical wetback combustion chamber. The second pass is back over the furnace through small-bore smoke tubes and then the flow divides at the front central smoke box. The third pass is through outer smoke tubes to the gas exit at the back of the boiler. There is no combustion chamber refractory lining other than a lining

Figure 4.7 Package boiler

to the combustion chamber access door and the primary and secondary quart.

Fully automatic controls are provided and located in a control panel at the side of the boiler.

Cochran boilers

The modern vertical Cochran boiler has a fully spherical furnace and is known as the 'spheroid' (Figure 4.8). The furnace is surrounded by water and therefore requires no refractory lining. The hot gases make a single pass through the horizontal tube bank before passing away to exhaust. The use of small-bore tubes fitted with retarders ensures better heat transfer and cleaner tubes as a result of the turbulent gas flow.

Composite boilers

A composite boiler arrangement permits steam generation either by oil firing when necessary or by using the engine exhaust gases when the ship is at sea. Composite boilers are based on firetube boiler designs. The Cochran boiler, for example, would have a section of the tube bank separately arranged for the engine exhaust gases to pass through and exit via their own exhaust duct.

Manhole access

Cochran Spheroids Boiler Diagram

Figure 4.8 Cochran spheroid boiler

Manhole access

Refractory

Figure 4.8 Cochran spheroid boiler

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