Construction and Working of Steam Boilers (With Diagram) | Thermodynamics

In this article we will discuss about the construction and working of various boilers with diagram.

Construction of Lancashire Boilers:

The modern Lancashire boilers are dish ended and welded pattern at least for higher pressures, instead of the older flat ended riveted type. The dish ended construction reduces the tendency of bulging of the end plates and therefore it does not require stays.

The flat ended riveted construction of the Lancashire boiler is made of several courses. Each course is constructed from a single mild steel plate, bent into a cylindrical form. The meeting edges of the plate are commonly connected by a treble riveted butt joint.

The alternate course of the shell is made smaller in diameter than the others by an amount equal to twice the thickness of the plates. Therefore, circumferential joints can be prepared as lap joints.

They are mostly double riveted lap joints. The longitudinal joints are arranged near the top of the boiler above the brickwork and placed out of line so as to avoid continuous rows of rivets.

The front end plate is connected to the shell by means of outside angled ring as shown in fig. 4-3. It is riveted to the end plate and also to the shell. The back end plate is flanged into the shell and is riveted to it as shown in fig. 4-4. The front and back end plates being flat ones cannot retain their shape without bulging, unless they are stayed. They are stayed to the cylindrical shell by means of gusset stays. Fig. 4-5 shows the gusset stay staying the back end plate with the boiler shell.

The internal flue tubes are composed of a number of rings, of which two at the rearmost end are sufficiently reduced in diameter to permit access between the two tubes. The expansion and contraction of tubes are accommodated by Adamson’s flanged ring joint as shown in fig. 4-6.

In this joint the end of each ring are flanged outwards and the rings are connected together, end to end, by rivets, passing through flanges. A welded expansion ring is placed between the flanges. It facilitates caulking which gives steam tight joint.

In the case of boilers with unstayed dished end-plates, a length of circumferential corrugated tube is incorporated as a provision to accommodate the expansion and contraction of the tubes. Thus, stresses on the end plates are prevented. The front end rings enclosing the furnaces are constructed with plain plates.

This has two advantages over corrugated construction. Firstly it reduces the risk of overheating due to scales and secondly it absorbs the whole of the radiation from the fire instantly. Also beyond the furnace where conduction and convection are the means of heat transfer, the hot gases only come in contact with the projecting portion of the rings, in case of corrugated construction, so leaving stagnant pockets of gas in recesses.

Therefore, the corrugated construction does not give increased efficiency due to increased heating surface but it does help in accommodating expansion and contraction of tubes.

The external flues of a Lancashire boiler are lined with fire- bricks. Air spaces are kept at the back of the fire-bricks to prevent the cracking of the brickwork due to expansion of the walls next to the flues.

Air spaces also reduce loss of heat through walls. The boiler rests on fireclay seating blocks.

Construction of Locomotive Boiler:

The fire-box consists of inside, front, rear and side sheets riveted together and also, to the fire-box wrapper sheet, which forms the top of the fire-box. The outside sheets are separated from the inside ones by a space, forming the water legs, the lower end of which is closed by a steel ring called the foundation ring.

The front outside sheet is riveted to the rear course of the shell. Handhole openings on a level with the crown sheet and clean out plugs above the mud ring are provided for cleaning purposes.

Fire-box roof stays are used to prevent failure of the fire-box wrapper plate when under pressure. Its connections with the top plate and the wrapper plate are shown in fig. 4-9. Stay bolts are used to brace the flat surfaces forming water legs. A stay bolt is shown in detail in fig. 4-10. A hole 5 mm in diameter is often drilled in the outer end of each stay bolt to a depth slightly beyond the inside of the plate, as shown in fig. 4-10 in order to detect a broken stay bolt. Flexible stay bolts may be used to reduce the number of broken stay bolts caused by unequal expansion of inner and outer plates of the fire-box.

All the flue-tubes are of steel and are solid drawn. They are reduced in diameter at the fire-box end and enlarged at the smoke-box and. They support the tube plates of the fire-box, as well as that of the smoke-box. The unsupported portion of the smoke-box, tube plate and the fire-box-outer front plate are supported by bar stays.

The connection of the bar stay with the plate is shown in fig. 4-10. The ends of the bar are screwed to receive two nuts between which the end plate is locked. At one end, the end plate is stiffened by a large thick washer riveted to it. The bar stays are generally made of steel.

Another design of the bar stay is shown in fig. 4-12 and has a nut of extra depth on the outside. The screw-threads shown in fig. 4-12 are fine threads. Generally the smoke-box ring which has screens to prevent the escape of large cinders is riveted to the front course of the shell.

Construction of Cochran Boiler:

The shell is made of mild steel plates. The furnace is hemispherical in shape and is hydraulically pressed from one plate without weld or seam. Hemispherical furnace is the strongest structure under compression.

As there are no seams in the furnace, this source of furnace trouble is removed in this boiler. The combustion chamber is dry-backed and is lined with fire-bricks. The horizontal tubes are all straight and of equal length and are arranged in a group with wide spaces between them and the shell so as to help convection currents.

The ends of these tubes are fitted in the smoke-box tube plate and combustion chamber tube plates and then expanded at the ends to make steam tight joints.

The smoke-box is built of steel plates. It is fitted with a hinged door which gives easy access to smoke tubes for cleaning and inspection. The stack is placed on the top of the smoke-box.

The grate for hand fired boilers consists of straight cast iron fire-bars which form a square in the centre, the area then left being filled in with pieces of design similar to the bars.

The ashpit is formed by the extension of the boiler shell and is fitted with a portable cover.

Construction of Scotch Marine Fire-Tube Boiler:

The boiler shell is made of two courses being constructed mostly of two plates with butt joints so as to give a perfect cylinder. The circumferential joints are lap riveted. The heads are made of two plates riveted together and flanged for riveting to the shell; however heads upto 4-5 metre in diameter are made from single plates. The heads are stayed, where unsupported, by longitudinal bar stays.

The furnaces are corrugated and are made of steel. They are about 1000 mm to 1200 mm in diameter. A thin ash plate rests below the grate on the bottom of the ashpit to facilitate the removal of ashes which owing to the corrugations would otherwise be difficult to remove. For hand fired boilers the grate is 1.8 metre long and is made in two lengths of fire bars resting on a dead plate, on centre bearers and on the bridge supports.

The rear wall of the combustion chamber is flat but slopes slightly forward in order to allow the steam bubbles forming on the plate to disengage themselves instead of creeping up the plate. This wall is supported from the rear head by stay bolts. A crown sheet forms the top of the combustion chamber and is supported by girder stays or crown bars.

The combustion chamber bottoms are strengthened by means of T’s riveted to the outside of the chambers. The bottom of the combustion chamber front plate is not necessary to be supported as they are supported by furnaces and top part of it is stayed by means of stay tubes which are screwed at both ends. The holes in the front end of the shell and in the tube plates of the combustion chambers are screwed to receive them. The flue-tubes are simply pushed into plate and then expanded at the tube plates to make steam tight joints.

Access to the boiler is obtained by three manholes placed on the front end; one near the top and one between the centre and each side furnace.

Scotch boiler was developed primarily for marine service and stationary practice. This boiler is further developed called “dry back” type boiler in which the stay bolted rear combustion chamber of marine Scotch boiler is replaced by a brickwork. This stationary boiler is known as Scotch dry back boiler.

Construction of Babcock and Wilcox Type Boiler:

The drum of the boiler is made of a single course joined at either one or two places by a longitudinal butt’ strap as required by the diameter of the boiler. The heads of the drum are forged by hydraulic press and are dished to a radius equal to the diameter of the shell.

When so, the heads are seldom stayed. The cross box is riveted to the end courses of each drum and is connected to headers. The unit of the tubes is called a section. These sections may be arranged single deck or double deck or triple deck. The capacity of the boiler depends upon the number of sections and tubes used. These tubes are inclined horizontally at an angle or 22°.

A manhole is flanged in at the centre of each dished head and the flanged edges are machined to fit manhole opening and are held in place by forged steel yoke and bolts. Flat surfaces are provided in the front of the head for water column and feed water connections.

The headers are made of cast iron or steel depending on the pressure of the boiler. The tubes are expanded in the headers and the ends are flared and not beaded. The handholes and their covers for tube cleaning have their joints made steam tight either by having the surfaces milled and the joints made of metal or by placing thin gaskets between the surfaces.

The forged steel mud box is 185 mm square and is attached to the rear headers by means of wrought iron nipples expanded into the mud drum and the headers. Babcock and Wilcox boilers are mostly fitted with chain grate stokers.

Settings for the Babcock and Wilcox Boiler:

The settings for this boiler are higher because the horizontal drum has to be hung at a height suitable for the grade of fuel to be burnt. Ordinarily, the distance from the grate to the lowest row of tubes should be at least 1.8 metre. The side and front walls of the setting, together with the arch or bridge wall, enclose the furnace space.

The combustion chamber is separated into compartments, called passes, by baffle plates or walls that force the gases to take a definite path from the first pass to the flue connection in the rear wall. The baffle walls may be constructed either of cast iron baffle plates lined with special fire-brick and held in position by clamps or of fire-brick tile, separated by a positive expansion joint.

There are usually three passes for the gas flow provided by inclined baffles; though sometimes a four pass design is used; but the disadvantage of this type is that the bottom of the second and third passes may become filled with soot and ash thrown out as the gases make the turn. The draught is then impaired and a greater draught loss results from the large number of passes.

The volumetric size of the passes decreases from furnace to gas exit in order that equal gas velocity may be maintained throughout the travel. Cooling gases contract and would tend to slow up under equal draught and cross sectional area of the passes, High gas velocity conduces to efficient heat transfer to the tubes; for, a slow gas travel would tend to allow a stagnant insulating film of gas to form around the tube surfaces.

The front baffle wall is supported by the tubes at the rear wall of the furnace and the rear baffle wall is attached to a curtain wall which is supported by a special cross girder.

Cleaning doors are provided to give access to the tubes for cleaning purposes. Small dusting doors are located in the side of the setting to permit the cleaning of all parts of the heating surface and sufficient spaces is allowed between settings for this purpose. The front of the boiler is enclosed by a cast iron front.