Toc: Content: Marine Engineering Series, Page iiFront Matter, Page iiiCopyright, Page ivPreface, Pages v-viiAcknowledgements, Page xi1 - Historical development of the marine boiler, Pages 1-92 - Theoretical development of the marine boiler, Pages 10-173 - Tank type boilers, Pages 18-584 - Water tube boilers, Pages 59-1535 - Dual-fired boilers for oil and liquified natural gas, Pages 154-1646 - Composite boilers and exhaust-gas heat exchangers, Pages 165-2097 - Forced circulation boilers, Pages 210-2168 - Low-pressure steam generators, Pages 217-2329 - Superheaters and economisers, Pages 233-26410 - Materials used in construction, Pages 265-27711 - Boiler construction, Pages 278-33012 - Refractories and insulation, Pages 331-33513 - Boiler mountings, Pages 336-38114 - Boiler controls, Pages 382-40915 - Treatment of boiler water and feed water, Pages 410-43216 - Steam generation and boiler operation, Pages 433-46917 - Fire-fighting appliances, Pages 470-47918 - Water tube boiler surveys and repairs, Pages 480-52319 - Tank type boiler surveys defects and repairs, Pages 524-56420 - Certificates of competency, Pages 565-574Appendix, Pages 575-580Index, Pages 581,583-591
Early boilers used seawater directly, but this gave problems with the build-up of brine and scale. For efficiency, as well as conserving feedwater, marine engines have usually been condensing engines. By 1865, the use of an improved surface condenser permitted the use of fresh water feed, as the additional feedwater now required was only the small amount required to make up for losses, rather than the total passed through the boiler. Despite this, fresh water makeup to the feedwater system of a large warship under full power could still require up to 100 tons per day. Attention was also paid to de-aereating feedwater, to further reduce boiler corrosion.
Operation of an evaporator represents a costly consumption of main boiler steam, thus fuel. Evaporators for a warship must also be adequate to supply the boilers at continuous full power when required, even though this is rarely required. Varying the vacuum under which the evaporator works, and thus the boiling point of the feedwater, may optimise production for either maximum output, or better efficiency, depending on which is needed at the time. Greatest output is achieved when the evaporator operates at near atmospheric pressure and a high temperature (for saturated steam this will be at a limit of 100 °C), which may then have an efficiency of 0.87 kg of feedwater produced for each kg of steam supplied.
The unevaporated seawater in an evaporator gradually becomes a concentrated brine and, like the early steam boilers with seawater feed, this brine must be intermittently blown down every six to eight hours and dumped overboard. Early evaporators were simply mounted high-up and dumped their brine by gravity. As the increasing complexity of surface condensers demanded better feedwater quality, a pump became part of the evaporator equipment. This pump had three combined functions as a seawater feed pump, a fresh water delivery pump and a brine extraction pump, each of progressively smaller capacity. The brine salinity was an important factor in evaporator efficiency: too dense encouraged scale formation, but too little represented a waste of heated seawater. The optimum operating salinity was thus fixed at three times that of seawater, and so the brine pump had to remove at least one third of the total feedwater supply rate. These pumps resembled the steam-powered reciprocating feedwater pumps already in service. They were usually produced by the well-known makers, such as G & J Weir. Vertical and horizontal pumps were used, although horizontal pumps were favoured as they encouraged the de-aeration of feedwater. Electrically powered rotary centrifugal pumps were later adopted, as more efficient and more reliable. There were initial concerns whether these would be capable of pumping brine against the vacuum of the evaporator and so there was also a transitional type where a worm gear-driven plunger pump for brine was driven from the rotary shaft.
Diesel-powered motorships do not use steam boilers as part of their main propulsion system and so may not have steam supplies available to drive evaporators. Some do, as they use auxiliary boilers for non-propulsion tasks such as this. Such boilers may even be heat-recovery boilers that are heated by the engine exhaust. 2b1af7f3a8