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Superheat Means Fuel Economy Study of Advantages 1 in Reducing Operating Costs Held by Superheat with Record of Results Obtained N the ultimate analysis, two major considerations really govern the adoption of any method proposed for fuel economy on board ship—first, that economy actually results from the use of the method; second, that the initial and upkeep costs are such that a liberal return on the investment is possible. Before any method can even be considered, the shipowner, through his technical advisor must be shown that real economy will be ob- tained, and also the true limits of such economy. Further than this, it must be demonstrated that the practical ap- plication is entirely feasible, into which enter the question of first cost, effect of the installation on the existing plant, the cost of upkeep, probable length of life, any additions or changes necessary for the installation and not covered in the first cost, ease and sim- plicity of operation, and whether any increase in personnel is required. That superheat applied to steam meets the first of the two conditions stated above has been recognized as a fact for many years. Records show that superheat was first applied in connection with marine boilers about 86 years ago. The unfortunate ex- periences encountered with these early installations, due principally to. the rapid deterioration of the elements of the superheater and the deleterious effect on the valves and cylinders of the engine, with consequent excessive repair costs, have had their influence up to the present time in creating a condition of extreme caution on the part of engineers and shipowners in accepting the use of superheat. These cifficulties have been overcome by ap- plying the experience of the past, by making full use of the advance in the processes of manufacture of the mater- ials necessary, by improving the de- sign and methods of installation and by providing for proper lubrication. Reducing Condensation Thermal efficiency is increased by the use of superheat. Condensation is a direct and serious loss in all.steam engines. This has been recognized by the care which is taken to minimize the radiation of heat by lagging boil- ers, steam pipes and cviinders with the best insulating material known. Sat- urated steam at a given pressure has a corresponding temperature. With the pressure constant, additional keat applied to the boiler will generate more steam but the temperature re- mains the same. Any cooling or use- ful work done during expansion of saturated steam means condensation of a part of the steam. The water so formed is not only useless as a working fluid, but in sufficient quan- tities and under certain conditions is an actual menace due to what is known as water hammer, By adding heat to the steam in the initial stage when no longer in con- tact with the water in the boiler, its temperature may be raised above that due to its pressure and loss of heat through radiation may be experienced ir proportion to the heat added without causing condensation. The steam con- sequently will remain as a pure dry simple gas and act accordingly in the cylinders. Actual serious loss results when the water of condensation froin saturated steam is carried over in multiple expansion engines and amounts to between 20 to 25 per cent of the steam used, varying with the cut off: and speed. Forcing the Boiler When a boiler is forced .under con- ditions of saturated steam, the steam becomes wet and the quantity of water carried over is increased with a re- sulting lower economy. On the other hand, forcing a boiler using super- heat will give a higher degree of superheat. Water in the cylinders will in itself tend to absorb some of the heat of the incoming steam and sv reduce the temperatures of the cylin- der walls. A dry gas, such as super- heated steam, is not a good conductor of heat and: there is consequently less heat transfer between it and the cylin- ders with correspondingly less con- densation. By eliminating the water in the cylinders, the danger of water hammer and_ leaky joints will be avoided. Furthermore, less loss of effective heat will occur in steam pass- ing through long lines of pipe. In triple expansion engines with a steam -temperature of 580 to 620 de- grees Fahr., an économy of 15 per cent in coal consumption over that possible with saturated steam may be expected. The percentages of saving for quadruple expansion may be some- what less, while the saving in the case of compound engines is_ greater. In the turbine, the saving possible 133 by the use of superheat is correspond- ingly great, though for different rea- sons. The increase in thermodynamic efficiency is small but a distinct gain: in steam economy results from. the elimination of water held in suspen- sion by the steam and the correspond- ing reduction in friction. The economy in steam consumption may. be stated as lying between 7 and 16 per cent. with a range of superheat from 100 to 300 degrees. In a turbine, water will form under saturated steam conditions, due to ex- pansion of the steam while doing work, and increase gradually from point of admission to point of exhaust. The fluid friction set up under these con- ditions must be overcome by the use of a correspondingly greater amount, of steam. The losses due to water in suspension, using saturated steam in. turbines, are consequently serious and amount to from 16 to 20 per cent. But further than this, moisture will cause erosion of blading due to the high velocity at which the water particles in suspension impinge upon the edges of the blading. There is also the danger of damage to the blading: due to the separation of the water, causing stripping of blades. Consequently. for both. reciprocating engines and_ tur- bines, distinct advantages accrue in the use of superheated steam at high tem- peratures. In ships where conditions have necessitated the forcing of the boilers, superheaters may even show a greater effectiveness as their: installa- tion will be equivalent to increasing the size of boiler. Initial and Upkeep Costs It is generally admitted, and it can not be denied, from a thermal point of view, that superheat will effect direct and important economy in steam con- sumption and consequently a _ saving in fuel. The next important consid- eration above stated for the adoption of any method to increase economy, that is, that the initial and upkeep costs should be of such as to allow a liberal return on the investment, is naturally a matter of much greater difference of opinion among engineers and shipowners. The principal reason for this goes back to the early installation of super- heaters on board ships. Without ques- tion, great trouble was experienced in deterioration of the superheater ele-