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THE KIND OF BOILER THAT CAN BE TAKEN APART TO CLEAN. An English patent compound superheated steam semi-stationary condensing engine.
GIVEN a chance to save a million dollars in one lump any . American would acquit himself creditably, if not brilliantly. But no free-born American citizen would so demean himself as to save a paltry hundred dollars. As for saving a nickel the man who would even hint at such a thing would disgrace himself and his family to the third generation. But in countries where the unit of the monetary system is the franc or the lire, which are but the fifth part of a dollar, or the mark or the shilling, which are a shade less than a fourth of a dollar, things are different. In Europe it is so long between francs
that before a man gets hold of the next one he has plenty of time to figure out that it only takes a hundred centimes to make a franc; and that when he gets enough francs he will have a million. In these few sentences is contained the essence of all the reasons why those fellows on the other side of the Atlantic can beat us so badly in building small motors that we do not deserve to be included "Among those Present" in the competition.
It was in Germany that the first practical gas engine was evolved. It was a German, too, who invented and developed the Diesel oil engine, which seems to have landed on the solar plexus of the steam engine, not merely for use on land, but for the propulsion of ships. Also, it was the Germans who recently developed the small steam engine to such a marvelous degree that the gas engine and the oil engine have almost been counted out. As a result of all this there has developed across the water about the loveliest three cornered contest for supremacy between these various types of prime movers that one could wish to see. First the gas engine forges ahead, then the oil engine gets a pfennig or two the best of the gas engine, and then the steam engine downs them both.
At the present time the honors appear to be with the steam engine, but by so narrow a margin that only an expert accountant could measure it. At all events the small steam engine has been developed in Europe to an extent that is worth talking about.
In England the new type of steam engine is spoken of, at least among engineers, as an "Overtype superheat steam engine." The Germans, economical of breath as of other things, name it the "lokomobile." while the French call it the "Demi-fixe" and let it go at that. Whatever the name, the new engines are identical in principle and nearly so in details of construction. They are portable, or semi-portable steam engines equipped with superheaters and other improvements.
Lest those foreigners should become too conceited about their achievements it may be well to say, first of all, that we know just as much about engines as they do. If some of those gentlemen from Germany who spend all their lives building small engines were to come over here they would be speechless with awe upon seeing some of our enormous power plants. Both in size of plants and in the degree of economy realized in these large plants we lead the world. Only in smaller power plants, say up to a thousand horse power, do European builders excel.
The superheater is the essential feature of the lokomobile. Superheated steam is defined as "Steam having a temperature higher than that due to its pressure." In superheating steam expands to a slight degree if the pressure is constant; but if the volume is constant su
perheating will increase the pressure. In ordinary practice superheating increases the volume so slightly that the rise in pressure is not appreciable. Steam at 200 pounds pressure when superheated 100 degrees will have a temperature of 487 degrees and still remain at 200 pounds pressure. Superheated steam partakes of the nature of a perfect gas. Like a gas it is a poor conductor of heat. At 170 pounds pressure with 200 degrees of superheat it takes 3.27 cubic feet of steam to weigh one pound, while only 2.47 pounds of saturated steam at the same pressure will weigh one pound.
The object in superheating is chiefly to eliminate wasteful condensation and consequent loss of heat in the cylinders. This can only be done by admitting the steam at such a temperature that it can give up enough heat to raise the temperature of the cylinder walls and still not lower its own temperature below the point of evaporation. Opinion differs regarding the most economical degree of superheat, though it is well understood that a small amount is not worth while. In practice the amount of superheat varies from 100 to 200 degrees. The highest superheat yet attempted is 250 degrees, which means an actual temperature of nearly 650 degrees. If the temperature was carried any higher it would result in the rapid reduction of the strength of the steel.
The apparatus by which this wonderful improvement in the quality of steam is effected is nothing more than a series of small pipes in the smokebox. The arrangement of the coils of pipe varies, and so does the style of the header in which all the coils terminate. But all alike are so arranged as to expose the whole surface of pipes and header to the hot gases from the furnace. In an internally fired boiler the heating surface of the superheater is usually about thirty times the grate area; and in a watertube boiler about ten times the grate area. To make a different comparison, the superheater's heating surface ranges all the way from half that of the boiler to as large an area as the boiler has. After being generated in the boiler the steam is conducted to the superheater where it lies in wait for the wasted heat rushing through the flues and captures it before it can seize an opportunity to escape up the smoke stack.
Superheated steam can travel twice as fast as saturated steam. It is possible to transmit superheated steam a distance of six to ten thousand feet without a loss of more than ten per cent in pressure; and by the use of separators to get rid of what condensation there may be, to deliver the steam in every way satisfactory for use. But no such formidable journey is ahead of the steam in a lokomobile; and this brings up the second important feature which makes the new type of engine so superior to the American home-made kind.
The great problem before the steam engineer is to prevent waste of the heat after it has been put into the steam. As one means of preventing this loss the engine is mounted on the boiler of the lokomobile. This eliminates the necessity of providing outside steampipes with their enormous possibilities for waste of heat by radiation. American engines of small size are commonly mounted on the boiler, to be sure; but the European
builders have left us far behind by the simple expedient of casting the cylinder and the steam dome all in one piece, with the former inside the latter, so that the walls of the cylinder are always surrounded by steam at boiler temperature. Only the heads are outside, where they are as readily accessible as those of any other type of engine.
It is well known that a compound engine under proper conditions is more economical than a simple engine. So the lokomobile usually has a compound engine. After expanding the steam in the high pressure cylinder until its temperature is reduced to about 235 degrees it is returned to a second edition of the superheater where its temperature is raised to about 360 degrees before being admitted to the low pressure cylinder. From the low pressure cylinder the steam is exhausted into a condenser where it gets down into water again. In some factories using lokomobiles of the larger size the exhaust steam is used for heating the building before It goes to the condenser. From the condenser the late