Lesson 16

1. Lesson 16 • Marine boilers Jimei university

2. There are two distinct types of marine boilers in use on board ship, Jimei university

3. 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, Jimei university

4. 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. Jimei university

5. The water-tube boiler is employed forhigh pressure, high temperature, high capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump(货油泵) turbines. Jimei university

6. Fire-tube boilers are used for auxiliary purposes to provide smaller quantities of low pressure steam on diesel engine powered ships. Jimei university

7. 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. Jimei university

8. 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. Jimei university

9. Water tube boilers have distinct advantages over firetube boilers. Jimei university

10. (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 Jimei university

11. without the danger experienced in firetube boilers having larger temperature differences betweentop and bottom parts of the boiler causing unequal expansion resulting inmechanical straining(机械变形). Jimei university

12. (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. Jimei university

13. (iii) Less mass of water is carried in the boiler, hencea saving in weight and less space is taken upfor the same output. Jimei university

14. 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 costlythan the firetube boiler. Jimei university

15. A later development of water-tube boilers is the benttube design. Jimei university

16. This boiler has two drums, an integral furnace and is often referred to as the ‘D’ type because of its shape. Jimei university

17. Jimei university

18. The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. Jimei university

19. These waterwall tubes are connectedeither to upper and lowerheaders(联箱)or a lower header and the steam drum. Jimei university

20. Between the steam drum and the smaller water drum below, large numbers of smaller diameter generating tubes(蒸发管束) are fitted. Jimei university

21. These provide the main heat transfer surfaces for steam generation. Jimei university

22. Large bore pipes or downcomers(下降管) are fitted between the steam and water drums to ensure good natural circulation of the water. Jimei university

23. A donkey boiler(辅助锅炉) is an auxiliary boiler sometimes included in a steamship’s installationfor supplying steam to the auxiliaries in port, Jimei university

24. and in motorshipswith steam auxiliariesfor supplying steam at sea and in port. Jimei university

25. In motor ships, an exhaust gas boiler (sometimes called a waste-heat boiler) is often used to recover some of the heat carried in the exhaust gases from the main engines. Jimei university

26. Composite boilers(组合式锅炉) are often fitted wherein the generation of steam can be maintained by oil firing when the exhaust gas temperature falls due to slow running of the engines, or cease when the engines are stopped. Jimei university

27. Boiler mountings(附件) • Certain fittings(附件) are necessary on a boiler to ensure its safe operation. Jimei university

28. They are usually referred to as boiler mountings. The mountings usually found on a boiler are: Jimei university

29. Safety valves • These are mounted in pairs to protect the boiler against(from) overpressure. Jimei university

30. Once the valve lifting pressure is set in the presence of a Surveyor(验船师) it is locked and cannot be changed. Jimei university

31. The valve is arranged to open automatically at the pre-set blow off pressure. Jimei university

32. Main steam stop valve • This valve is fitted in the main steam supply line and is usually of the non-return type. Jimei university

33. Auxiliary steam stop valve • This is a smaller valve fitted in the auxiliary steam supply line, and is usually of the non-return type. Jimei university

34. Feed check or control valve(给水止回阀或控制阀) • A pair of valves are fitted: one is the main valve, the other the auxiliary or standby. They are non-return valves and must give an indication of their open and closed position. Jimei university

35. Water level gauge • Water level gauges or ‘gauge glasses’ are fitted in pairs, at opposite ends of the boiler, the construction of the level gauge depends on the boiler pressure. Jimei university

36. Air release cock • These are fitted in the headers, boiler drum, etc., to release air when filling the boiler or initially raisingsteam. Jimei university

37. Sampling connection • A water outlet cock and cooling arrangement is provided for the sampling and analysis of feed water. Jimei university

38. Blow down valve(排污阀) • This valve enables water to be blow down or emptied from the boiler. It may be used when partially or completely emptying the boiler. Jimei university

39. Scum valve(上排污阀) • A shallow dish positioned at the normal water level is connected to the scumvalve. Jimei university

40. This enables the blowing down or removal of scum and impurities from the water surface. Jimei university

41. Automatic feed water regulator • Fitted in the feed line prior to the main check valve, this device is essential to ensure the correct water level in the boiler during all load conditions. Jimei university

42. Sootblowers(吹灰器) • Operated by steam or compressed air, they are act to blow away soot and the products of combustion from the tube surfaces. Jimei university

43. Waste heat recovery system (W. H. R) • Generation of electric power from a steam turbo-generator and the supply of saturated steam(饱合蒸汽) to heating services by exhaust gas heat recovery systems is effected in many ships. Jimei university

44. The type of W. H. R. unit mainly fitted nowadays for medium to large outputs is made of horizontal extended(水平分布) surface steel tubes, Jimei university

45. force-circulated with water taken from an oil fired boiler and returned as water/vapour emulsion. Jimei university

46. The boiler is fired whenever the demand for steam exceeds that available from exhaust gas heating. Jimei university

47. Additional heat extraction from(提取) the exhaust gases can be effected by adding an economizer(省煤器，经济器) sectionto the unit in plants where the arrangement permits the supply of cooled circulating water or feed water at the economizer inlet. Jimei university

48. Similar results, but with some disadvantages, can be obtained by producing low-pressure steam from a separate heat exchange unit at the tail end of the gas stream. Jimei university

49. Circulation of the W. H. R. plant(equipment) must be maintained at all times when the main engine is on load. Jimei university

50. Also it must be continued for at least two hours after the engine has been stoppedto ensure that Jimei university