Boiler Capacity. Importance. In the early days of the locomotive very little attention was given to the size of the boiler. If the cylinders were large enough to pull a train of reasonable size up the maximum grade and the driving wheels were loaded sufficiently to prevent slipping, the results secured were generally considered satisfactory. Today, however, conditions are changed. Now the capacity of the locomotive boiler for the generation of steam is looked upon as the most important feature in connection with the design of a locomotive and, as a rule, the boiler is made as large as possible, consistent with total weight desired. Wherever possible the weight of parts is reduced in order to favor the boiler. It is now known that no locomotive was ever impaired in any way by having a boiler that steamed too freely, for the greater the boiler capacity the greater the speed that can be maintained. As the demand for speed and the loads hauled increased, it was soon discovered that the speed of a train of a given length and weight was limited by the capacity of the boiler. Complaints were made of the boiler "not steaming", and, although the insufficient supply of steam might have been attributed to an inferior grade of fuel, improper firing, bad adjustment, "front end" arrangement, flues in bad condition, or negligence in the manipulation of the engine, it soon became recognized that, with all boiler conditions in perfect 6rder and the locomotive operated by experienced men, it was impossible to make a small boiler supply a sufficient amount of steam for large cylinders operating at high rates of speed. As a result the boiler gradually grew in size, and with it a desire to arrive at a rational proportioning of its various parts, such as heating surface, grate area, length of tubes, etc., necessary to maintain a definite tractive effort at a definite speed.
Effect of Area of Heating Surface. All the various dimensions of the different parts of the boiler are more or less important in their relation to the question of steam generation. Perhaps the most important of these are the dimensions of the heating surface. The area of the grate surface limits the amount of coal that can be burned in a given time, but the amount of coal burned per unit of heating surface governs, to a great extent, the rate of evaporation. Concerning the rate of combustion per square foot of heating surface, it is found that the same condition exists as in stationary boiler practice, namely, that the lower the rate of combustion the greater the evaporation per pound of coal.
Effect of Tube Length. The capacity of the boiler is also affected to a certain extent by the length of the tubes. It was found in a series of extensive experiments conducted in Europe a number of years ago that the most economical length of tubes was 14 feet. This length was found with a draft in the fire-box of 3 inches of water. In the United States a much higher draft is employed and for this reason much longer tubes can be used. Tubes over 20 feet in length are now quite common. As long as the temperature of the gases in the smoke-box is above that corresponding to the pressure of steam in the boiler there will be heat transferred from the front end of the tubes to the water in the boiler. Increasing the length of the tubes will, of course, reduce the draft in the fire-box and, as a result, the amount of coal burned will be reduced. For this reason the tubes should be of a definite length for maximum efficiency.
Effect of Scale. The transmission of heat through the tubes and fire-box sheets is dependent to a large extent on the condition of the inner surfaces. If they are covered with a thin layer of scale, the heat transmitted will be materially reduced. Experiments conducted in 1898 on the Illinois Central Railroad gave some very interesting results on the effect of scale on the steaming capacity of a locomotive boiler. Tests were first made on a locomotive which had been in service 21 months. After the test the engine was sent to the shops and received new tubes and a thorough cleaning. The total weight of scale removed from the boiler was 485 pounds and it had an average thickness on the principal heating surfaces of 3/64 inch. After the engine had received the cleaning and new tubes, a second test was conducted in which the same coal per square foot of heating surface was burned as in the first test. The result of the second test showed the steam-making capacity of the boiler to have been increased 13 per cent.
Effect of Radiation. The loss of heat from the outer surface of a locomotive boiler by radiation and the ultimate effect on its capacity are items worthy of consideration. The heat lost in this manner is so great with an unprotected boiler shell that it is necessary to use some form of insulating material to minimize the loss. Covering a boiler with insulating material is more necessary with high pressure than with low pressure because of the greater temperature difference. Results of tests of boiler covering reported to the Master Mechanics' Association in 1898 show that a loss of 0.34 B.t.u. per square foot of radiating surface per hour per degree difference in temperature was obtained by the use of mineral wool, while under the same conditions with a lagging of wood and sheet iron the loss was increased to 1.10 heat units. In both cases the temperature difference was reckoned between the temperature of the steam in the boiler and that of the surrounding air. The results show a saving of 0.76 B.t.u. in favor of the mineral wool lagging. Let us consider a boiler carrying steam at 200 pounds per square inch gage pressure, which represents a temperature of 388° F. Assuming the temperature of the atmosphere to be 32° F., this represents a temperature difference of 356 degrees. Assume further a locomotive boiler having an outside surface of 600 square feet. The heat of vaporization per pound of steam at 200 pounds per square inch gage pressure is 838 B.t.u. The pounds of steam condensed in the boiler per hour due to radiation in case a wood lagging is used, in excess of the amount that would be condensed if mineral wool were used, is equal to
Assuming that the steam consumption per i.h.p.hr. is 20 pounds, the above figure represents 9.6 horsepower.
The foregoing figures represent results obtained in still air. The radiation losses are increased very much when the locomotive is in service. This fact is demonstrated by the results of tests conducted on the Chicago and Northwestern Railway in 1899. The locomotive employed had 219 square feet of covered boiler surface and 139 square feet uncovered. Assuming, for this type of engine, the steam consumption per i.h.p.hr. to be 26 pounds, the results of the tests showed a condensation representing a horsepower of 4.5 when at rest and 9 when being pushed at a rate of 28 miles per hour.
Boiler Horsepower. In the foregoing we have considered the determination of the greatest amount of steam which a locomotive boiler can produce and it is evident that the boiler capacity limits the work that can be performed by the engine. Under some circumstances it is more convenient to express the boiler capacity in terms of an evaporative unit. The term "boiler horsepower" is such a unit, but the use of this expression is sometimes misleading in speaking of the capacity of a locomotive, for a given boiler will produce a greater horsepower with a compound than with a simple engine and with an early and economical cut-off than with a later and more wasteful one.
A boiler horsepower, as defined by the American Society of Mechanical Engineers, is the production of 30 pounds of steam per hour at a gage pressure of 70 pounds per square inch evaporated from a feed-water temperature of 100° F. This is considered equivalent to the evaporation of 341-1/2 pounds of water per hour from a temperature of 212° F. into steam at the same temperature.
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