lesson 16　 Marine boilers 轮机英语课件.ppt

Lesson 16 Marine boilers There are two distinct types of marine boilers in use on board ship, the fire-tube boiler(烟管锅炉、火管锅炉) in which the hot gases from the furnaces pass through the tubes while the water is on the outside, and the water-tube boiler(水管锅炉) in which the water flows through the inside of the tubes while the hot furnace gases pass around the outside. The water-tube boiler is employed for high pressure, high temperature, high capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump(货油泵) turbines. Fire-tube boilers are used for auxiliary purposes to provide smaller quantities of low pressure steam on diesel engine powered ships. Firetube boilers were commonly used with steam reciprocating engines on board ship, but because of their comparative poor efficiency and low power/weight ratio(功率重量比), these systems are no longer installed for propulsion plant. There are many types of watertube boilers in general use on board ship and each manufacturer has a variety of designs to offer depending upon requirements. Water tube boilers have distinct advantages over firetube boilers. (i) Circulation of the water is natural and immediate on lighting up, thus steam can be raised from cold water in a shorter period of time without without the danger experienced in firetube boilers having larger temperature differences between top and bottom parts of the boiler causing unequal expansion resulting in mechanical straining(机械变形). (ii) The steam and water drums(锅筒) are small in diameter compared with the large shell(锅壳) of the firetube boilers, therefore they are stronger and suitable for much higher steam pressure. (iii) Less mass of water is carried in the boiler, hence a saving in weight and less space is taken up for the same output. The watertube boiler does however require greater skill in operating and maintaining，especially with regard to the purity of the feed water, and its upkeep (维修费) is more costly than the firetube