1、Lesson 13轮机英语Unit 2 Auxiliary MachineryLesson 13 Marine Boilers and Their ConstructionA boiler in one form or another will be found on every type of ship. Where the main machinery is steam powered, one or more large water -tube boilers will be fitted to produce steam at very high temperatures and pr
2、essures. On a diesel main machinery vessel, a smaller (usually fire tube type) boiler will be fitted to provide steam for the various ship services. Even within the two basic design types, water tube and fire tube, a variety of designs and variations exist.A boiler is used to heat feed water in orde
3、r to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of t
4、he fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water.A drum must be provided where steam and water can separate. There must also be a variety of fittings and controls to ensure t
5、hat fuel oil, air and feed water supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler.In the steam generation process the feed water enters the boiler where it is heated and becomes steam. The feed water c
6、irculates from the steam drum to the water drum and is heated in the process. Some of the feed water passes through tubes surrounding the furnace, i.e. water wall and floor tubes, where it is heated and returned to the steam drum. The steam is produced in a steam drum and may be drawn off for use fr
7、om here. It is known as wet or saturated steam in this condition because it will contain small quantities of water. Alternatively the steam may pass to a super heater which is located within the boiler. Here steam is further heated and dried, i.e. all traces of water are converted into steam. This s
8、uperheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. An attemperator, i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature. The type of the marine boiler There are two
9、 distinct types of marine boilers in use on board ship, the fire-tube boiler in which the hot gases from the furnaces pass through the tubes while the water is on the outside, and the water-tube boiler in which the water through the inside of the tubes while the hot furnace gases pass around the out
10、side.Water tube boilerThe water tube boiler is employed for high pressure, high temperature, and high capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Fire tube boiler are used for auxiliary purposes to provide smaller quantities of low pressure
11、steam on diesel engine powered ships.Fire tube boilerThe fire tube boiler is usually chosen for low pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feed water of medium quality may be employed. The name tank boiler is sometimes used for fire tube
12、boiler because of their large water capacity. The terms smoke tube and donkey boiler and also in use.Cochran boilersThe modern vertical Cochran boiler has a fully spherical furnace and is known as the spheroid. The furnace and is known as thespheroid. The furnace is surrounded by water and therefore
13、 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.Spanner boilersThe spanner vertical
14、 fire tube boiler uses a patented design of tube known as Swirlyflo. The special twist of the tube is said to improve heat transfer.Double evaporation boilersA double evaporation boiler uses two independent systems for steam generation and therefore avoids any contamination between the primary and s
15、econdary feed water. The prinary circuit is in effect a conventional water tube boiler which provides steam to the heating coils of a steam-to-steam generator, which is the secondary system. The complete boiler is enclosed in a pressured casing.Auxiliary steam plant systemThe auxiliary steam install
16、ation provided in modern diesel powered tankers usually uses all exhaust gas heat exchanger at the base of tile funnel and one or perhaps two water tube boilers. Saturated or superheated steam may be obtained from the auxiliary boiler. At sea it acts as a steam receiver for the exhaust gas heat exch
17、anger, which is circulated through it. In port it is oil fired in the usual way.Exhaust gas boilersAuxiliary boilers on diesel main propulsion ships, other than tankers, are usually of composite form, enabling steam generation using oil tiring or the exhaust gases from the diesel engine. With this a
18、rrangement the boiler acts as the heat exchanger and raises steam in its own drum.The boiler constructionWater tube boilers, which use small diameter tubes and have small steam drums, enables the generation or production of steam at high temperatures and pressures. The weight of the-boiler is much l
19、ess than an equivalent fire tube boiler and the steam raising process is much quicker. Design arrangements are flexible, efficiency is high and the feed water has a good natural circulation. These are some of the many reasons reasons why the water tube boiler has replaced the fire tube boiler as the
20、 major steam producer.Early water tube 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 h
21、as two drums, an integral furnace and is often referred to as the Dtype because of its shape.The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These water wall tubes are connected either to upper and lower headers or a lower header and the steam drum. Uppe
22、r 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. These provide the main heat transfer surfaces for steam generation. Large bore pipes or down comers are fitted between th
23、e steam and water drum to ensure good natural circulation of the water. In the arrangement shown, the super heater 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 a
24、nd also behind the water walls. 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 controlled to other boiler arrangements being used . The original External Super
25、 heater D(ESD) type of boiler used a primary and secondary super heater located after the main generating tube bank. 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
26、unit( an additional economiser) between the primary and secondary super heaters. Linked dampers directed the hot gases over the control unit or the super heater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low temperature superheating
27、was requred.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 Fig. 13-3, the furnace is fully water-cooled and of monowall construction, which is produces from finned tubes welded
28、together to form a gastight casing. With monowall construction 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 other end to upper and lower waterwall headers. The lower wate
29、rwall headers are connected by external downcomers from the steam drum and the upper waterwall 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 considera
30、ble 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 su
31、perheater 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 boiler are a more recent development, in which the radiant heat of combustion is abso
32、rbed to raise steam, by infrared radiation. This usually requires roof firing and a considerable height in order to function efficiently. 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 openin
33、g 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 superhe
