All About Engines

284 All About Engines second and third nozzles, water is drawn in through the slots in the upper portions, and the jet from the first is reinforced. In his “ Manual of Marine Engineering ” Mr. A. E. Seaton says: “ Without this instrument the bottom of a large boiler remains cold long after steam is raised ; with it the temperature at the bottom differs very little from that at the top. Steam can in this way be safely raised in a shorter time than usual, and at no extra cost, and the endurance of the boiler is very consider- ably increased. There are many other ways of pro- moting the circulation when steam is up, but none do this so efficiently during the time of raising steam as the hydrokineter.” Marine engines, for two reasons, are always of the compound condensing type. In the first place, there is the need for economy of fuel to which reference was made on page 272 ; and in the second there is need for economy of water. Every particle of steam that can be trapped and condensed after leaving the engine must be returned to the boiler. A little waste is unavoidable, and this has to be made up by the evaporation from sea water, because space is far too valuable for cargo to be used in carrying fresh water to supply the boilers. The air pumps and condensers must be highly efficient, therefore, not only to extract the last unit of energy from the steam, but also in order to retain as much as possible of that steam for returning to the boilers. Moreover, if reciprocating engines are used the steam must be passed through filters to remove oil, to avoid trouble in the boilers.