Valve Gears.— The Stephenson link mo tion and the Walschaert radial valve gear are the most common valve gears used in this country and have been briefly described tinder the caption Historical. More detailed descrip tion will be found in texts on Valve Gears.
Superheaters.— Few locomotives are now built without superheaters. The Schmidt fire tube superheater was applied successfully as early as 1906, and the resulting economy in fuel and water together with its suitability to the requirements of American railroads has placed the superheater among the important elements of the modern locomotive. To-day there are over 21,000 locomotives equipped with superheaters in the United States and Canada. Superheaters are applied to between 90 and 95 per cent of all the standard gauge steam loco motives built in the United States. The appli cation of superheaters results in an increased boiler capacity of about one-third, and a sav ing in fuel of between 20 and 25 per cent. This increase in economy has made it possible for the average fireman to fire heavier locomotives. Superheated steam effects an economy by rea son of its temperature being above the satura tion temperature of steam at the same pres sure and also by reason of its, increased vol ume. At high temperatures superheated steam behaves like a gas. Considerable heat may be abstracted without producing liquefaction,• whereas the slightest absorption of heat from saturated steam results in condensation. If superheat is high enough to supply not only the heat absorbed by the cylinder walls but also the heat equivalent of the work done during ex pansion, then the steam will be dry and satu rated at release. Greater superheat than this will result in a loss of energy unless the steam is exhausted into . another cylinder. In most cases superheat is only carried so far as to reduce initial condensation. To obtain dry steam at release the steam at cut off must be superheated 100° to 300° F. above satura tion temperature, depending upon the initial condition of the steam and the.number of ex pansions. It is evident that the increased ef ficiency obtained by any superheat in excess of that needed to prevent condensation must be due to increased volume of the steam per unit weight.
A moderate amount of superheat produces a considerable increase in volume, the pres sure remaining constant, and reduces the weight of steam per stroke for a definite amount of work.
The general trend of engineers is toward the use of higher superheats and steam tem perature of 600° F. are not uncommon. There is no doubt that the limit of su perheat is reached when the exposed machine parts are unable to withstand the higher tem peratures.
Several types of superheaters have been ap plied to locomotives but the design of the fire tube superheater fits so well the requirements of American railroads that it is the universal favorite.
Feed-Water Heaters.— The function of the feed-water heater as applied to the locomo tive is that of heating the feed water, while that of the feed-water heater installed in a power plant is often to assist in purification of the feed water. Generally speaking there is a gain of 1 per cent in heat for every 10 de grees that the feed water is heated when the heat which increases the feed water tempera ture would otherwise be wasted. Again, the smaller the difference in temperature between the feed water and the steam, the less would be the strain on the various parts subjected to changes of temperature. In European coun
tries the application of feed-water heaters is rapidly becoming general. Not a great amount of attention however has been given to the methods of heating feed water for locomotive boilers in this country, though many American railways are alive to the possibilities and the increased economy resulting therefrom.
Waste heat from two different sources is available, namely, exhaust steam and gases from the combustion chamber. The thermal efficiency of the average locomotive operating under good condition is about 7 per cent. In other words, 7 per cent of the total heat in the coal fired represents useful work at the draw bar. Losses in the gases from the com bustion chamber total nearly 18 per cent and that lost in the discharged exhaust steam about 65 per cent. Proportional and maintenance difficulties are greater in heaters designed to absorb heat from the furnace gases though the gases are at a higher temperature and greater transmission per unit of area might be ex pected if sufficient quantities of the gases could be brought in contact with the heating surfaces. Attention should be first given to perfecting a feed-water heater to absorb all the heat possible from the exhaust steam and later attempt to use the heat in the hot gases as is done by the economizer in steam-power plants. The exhaust steam does some work in producing draft, and of course cannot be con sidered as a dead loss. However, a small pro portion of this heat could perform the work in producing a satisfactory draft.
A feed-water heater based on the principle of the Lovekin film heater applied in marine practice has been designed and used on high speed passenger locomotives capable of heating feed water from 45° to F. A back pres sure of 10 pounds existed in the cylinder. Two hundred and twenty-five degrees feed water is still about 160° below steam tempera ture and any additional heating could be done by the exhaust gases which frequently have a temperature of 700° F. or more.
From what work has been done it appears that a 10 per cent economy can be expected under reasonable conditions of operation. The logical location of a feed-water heater is on the front deck close to the cylinders, and un der the extension at the front. It is advisable in order to secure higher temperatures in the feed water to use the dosed type of beater with a pump to deliver the water to the boiler through the heater against the boiler pressure. The exhaust steam from the pump should be discharged into the heater.
Locomotive Performance.— The indicated horse power, or horse power developed in the cylinders, of any locomotive may be computed as for any steam engine. The work done in the cylinder equals the product of the average force acting on the piston and the space traversed. Using the units in pounds and feet gives a result in foot pounds which if divided by 33,000 foot pounds considering one minute in time gives horse power.
Let A = Area of piston in square inches.
d = diameter of cylinder in inches.
length of stroke of piston in feet.
N = revolutions per minute.
P—mean effective pressure on piston i in pounds per square inch during one revolution.
Then the horse power developed in one end of the cylinder will equal I HP —