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1、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 pressures. On a
2、 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 order to produce
3、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 the fuel to ta
4、ke 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 that fuel oil,
5、 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 circulates fro
6、m 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 from here. It i
7、s 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 superheated st
8、eam 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 distinct typ
9、es 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 outside.Water tu
10、be 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 steam on dies
11、el 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 boiler becaus
12、e 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 requires no
13、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 fire tube bo
14、iler 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 secondary feed
15、 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 installation provide
16、d 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 exchanger, which is circulated through it. In port it is oil fired in the usual way.Exhaust gas boilersAuxiliary boilers on d
