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66 it were not true that probably the greater part of the economy was due to the auxiliaries and only in part to the main engines. When it is further considered that a much greater economy could be attained by the use of a good heated draft system, in the light of the stack. temperatures shown, and without sacrificing speed at all, that is to say with saturated steam, it hardly seems worth while to pursue the subject. However for once we may be allowed to go outside for an exactly confirmatory trial quoted in abstract from a Ger- man source.* Trials With And Without Superheat The experiments were made on-a triple engine of which the cylinder dimensions in nearest American fig- ures were 20 x 32 x 52 x 36 inches with two cylindrical boilers, 178 pounds working pressure. Three trials were made, with superheated, semi- superheated and saturated steam. As the degree of superheating is not stated we can only estimate these terms as meaning high or moderate superheat. As the superheaters con- tained nearly 28 per cent of the total heating surface and were of the fire-tube type the degree of super- heat may fairly be assumed to be high. It will be observed that the boiler pressure and the cut-offs were maintained constant and the only variable is the steam temperature. To save space the results with semi-superheated steam are omitted as they fall between the other two. ‘In order to determine the difference in coal consumption for superheated and semi-superheated steam for the same speed the following results were obtained: (a) With superheated steam, rev- olutions 75.2, coal consumption 14.852 tons per day. - (b) With superheated steam: In order to obtain the same revolutions the engine had to be linked out. Revolutions 74.6, coal consumption 16.412 tons per day. With super- heated steam 1.56 tons more coal was required, or 10.5 per cent for approxi- mately the same speed. Saturated steam would show a still greater saving according to my experiments. The advantage of the superheated steam is to be its larger volume compared with saturated steam; the artificially raised temperature is also to retain the pressure of the steam longer during its passage through the engine, thus retarding condensation. If this is true why does the engine Weismann, Hamburger Technische Rund- schau. : MARINE REVIEW make less revolutions with super- heated steam than with saturated steam? Paragraph (1) shows that the high pressure cylinder produces 79 indicated horsepower more than the _ inter- mediate pressure. After the inter- mediate pressure cylinder had been linked in the high pressure cylinder still produced 65 indicated horse- power more than the intermediate pressure. Better results could not be obtained. The considerable drop in the initial pressures of the intermediate pressure and low pressure cylinders should also be noticed. It is evident that the lower initial pressures do not permit the intermediate and low pressure cylinders to produce powers equal to the high cylinder and this fact appears to be the reason for the lower revolutions. Conditions shown in (2) are entirely different. The valve gears can easily be linked up to produce equal power in all cylinders. As noted before superheat’ in- creases the volume of steam with the result that a smaller weight of steam enters the high pressure cylinder. Rapid condensation after leaving the high pressure cylinder does not leave a sufficient volume of steam for equal distribution of power in all cyl- inder'ss.”’* It. will be observed also that the falling off in power figures in (1) occurs in spite of over 2 inches better vacuum, due apparently to about 13 degrees cooler circulating water. The difference in revolutions affected a reduction in speed of about half a knot, therefore tests were made to determine the difference in coal con- sumption at equal speeds. The fig- ures indicate 10% per cent higher consumption with superheated steam. The differences in initial and mean pressures indicate reduced cylinder condensation in the higher-pressure cylinder with superheated steam as is to be expected, but .more rapid con- densation in the receivers and in the intermediate and low-pressure cyl- inders, or to state it another way, apparent higher re-evaporation with saturated steam. Weismann adds “Con- sidering the cost of the superheaters, the greater wear on the cylinder walls, piston rings and valves, the greater consumption of lubricating oil, more frequent cleaning of boilers, leakage of superheater tubes, etc., it is sur- prising that superheaters are still being installed. It must be mentioned that a superheater may be a gain in cases of one-cylinder engines get- ting their steam supply direct from the superheaters.” The foregoing tests may not be February, 1927 conclusive but are an offset to cer- tain other reported tests which show only comparative water rates but ignore the cost of the superheating. Experience so far indicates that the use of more than a very moderate degree of superheat is very largely experimental and hardly justifies any great degree of optimism as to re- sults when weighed against a_ well designed and operated system in which otherwise waste heat is utilized in heating the draft air and so improv- ing fuel economy. It not only makes for much better economy but puts the steam-making under absolute con- trol regardless of weather conditions and very largely independent of grade of fuel. Further it converts the stokehold from an inferno to as com- fortable a place as the engine room and very often more so. The use of heated draft is now so universal that not a carrier has been built without it for many years except in the few recent installations of water- tube boilers and it is to be borne in mind that it is in connection with this type of plant that superheating is most strongly urged. Superheat Reduces Condensation It is admitted that superheating is advantageous in reducing or avoid- ing the condensation in _ cylinders and pipe lines which is the great foe to economy, but it can not be had for nothing and the question is purely one of balancing costs and results. The costs consist of the diverted heat, the equipment itself and its maintenance and added risk, and the results lie in the hoped for improvement in economy. Costs are definite, and in respect of equipment and maintenance are no small item, involving, with more than a very moderate degree of super- heating, special designs and materials in many features. The results are very indefinite, although it is gen- erally. claimed that improvement in engine efficiency should follow. This should be reflected first in reduction in weight of steam for a given power and, if the added heat cost nothing, or was supplied by waste gases, would be further reflected in reduction of fuel. Furthermore the theoretical gain in engine efficiency is necessarily based upon constant power output and constant temperature of the super- heated steam. In a marine engine the first condition obtains probably more nearly than in any other, but the latter is far from the case. Whether with the waste heat or radiant or fire tube type of super- heater the temperature is constantly varying with the condition of the fires