two-plunger pump in the fuel-pump block. Lubricating oil is used for a pressure medium. Fuel System.-Fuel is supplied to the gravity tanks under the pressure of the circulating water system, circulating water being used to displace the fuel oil and thereby also compensate for the weight of fuel used. The fuel found is very light, about the consistency of kerosene, and smells like kerosene. Indicator Gear.-Indicator gear is provided for taking power cards. The indicator cock connection is attached to the inboard side of each cylinder head. The stroke motion is obtained from the piston-cooling return link motion by means of a small spindle which passes up through an oiltight sleeve in the housing. Air-compressor cylinders are also provided for taking cards. Workmanship. The workmanship on the engines is uniformly excellent. The material itself is uniformly fine in structure and shows utmost skill in manufacture. All working parts are machine finished and accurately fitted. The engines are apparently designed in every detail to produce a finished product, each detail contributing to the final result. No makeshift jobs or evidences of afterthought were visible, as is often the case in less scientific machines. Practically all nuts on the engine are wrench-tight on their bolts or studs the full length of the thread, and all are secured by means of pins, wires, or locking washers. Handwheels on high-pressure air lines are large and fitted with a quarter to a half turn of lost motion on their stems to give a slight striking force for opening or closing. Pipe flanges which require frequent disassembly are made up with portable fastenings. The flanges are slotted in place of drilled bolt holes and the bolts are hinged under the lower flange. Air bottles are of steel, two spray flasks and two starting bottles being supplied for two 1,200-horsepower engines. The combined capacity of the starting bottles is about 35 cubic feet. Due to rigid construction and fine balance, the engines operate with extremely little vibration and are comparatively quiet. The engine-room air is free of smoke and gases and the exhaust is perfectly clear and well muffled. CURRENT-CARRYING CAPACITY OF ELECTRIC CABLES, U. S. NAVY. CALCULATIONS OF WIRE AND FUSE SIZES. BY LIEUT.-COMDR. ALEXANDER M. CHARLTON, U. S. NAVY, MEMBER. When electricity was first introduced in the service the current-carrying capacity of electric cable was specified as not over 1,000 ampères per square inch cross section of conductor. This was equivalent to 1,273 circular mils per ampère of current carried. Later the specifications were changed to call for 1,000 circular mils per ampère for continuous loading and 500 circular mils per ampère for intermittent loading. Both of the above specifications were arbitrary assumptions considered to give a factor of safety which would eliminate the possibility of overheating the cables. No consideration was given to initial temperature or to temperature rises. As these specifications were coupled with a percentage of voltage drop which in most cases was the limiting factor, no serious difficulty arose, as the minimum drop allowed usually required a larger cable than was necessary to carry the current. These arbitrary specifications based on experience with comparatively small cables had no scientific basis and as the size of cables. increased aboard ship occasional cases of overheated cables were reported. As will be shown later, we were underloading our cables below the 200,000 circular mil size and overloading them above this size for temperature conditions now considered as safe. In 1914 the Bureau of Steam Engineering requested the Navy Yard, New York, to conduct tests to determine the current-carrying capacity of Navy standard cables. In test Number 175, dated December 16, 1915, the Navy Yard reported tests on a number of leaded and armored cables taken from stock, the following sizes being used: Single Conductor. Twin Conductor. 9,030 circular mils. 75,850 circular mils. 98,820 circular mils. 198,860 circular mils. 521,970 circular mils. 4,494 circular mils. 9,030 circular mils. 14,350 circular mils. 22,820 circular mils. The cables tested were suspended in a horizontal position four inches from the floor, exposed on all sides to the air. The cables, which were from twenty to thirty feet long, with copper terminals soldered to the ends, were tested one at a time, the ends being connected directly to a motor generator set. The tests were made for a temperature rise of 30 degrees C. commencing with a room temperature of 25 degrees C. (77 degrees F. to 131 degrees F.). The following results were obtained: Plotting these results we get a curve which varies considerably from the Navy specification of 1,000 circular mils per ampère. As will be seen later, the initial temperature used in this test was considered by the Bureau too low, and the allowed temperature rise too great for conditions aboard ship. Also, no grouping and location factor was allowed for. The values obtained by New York, however, agree closely with those in the standard table when reduced to the same limits. The current which a cable will carry without reaching a temperature dangerous to the copper or insulation depends upon the establishment of a balance between the heat generated by the flow of current through the conductor and the heat conveyed from the conductor to the insulation and thence radiated to the atmosphere. The area over which the dissipation of heat must take place depends upon the circumference of the cable, that is, it is a function of the diameter. The heat generated varies as the square of the current, and inversely as the square of the diameter of the cable. the current allowed for a certain temperature rise would vary as the diameter to the 3/2 power. In consequence the ampèrage per circular mil of cross-section must be less for the large size conductors than for the small ones in order to maintain. the same temperature rise. The formula worked out by the New York Navy Yard for this particular test is I d = x = diameter of copper in mils thickness of insulation including tape and .015 constant depending on temperature condi tions, conductivity of insulation, etc. |