Recently public attention has been once more turned in the direction of the use of superheated steam in our engines. The agitation of this question is by no means new. Along about the middle of the past century superheated steam was used to a small extent in several European countries, but was shortly given up. This was because of mechanical difficulties in the way of successful superheating, and not because the advantages in economical running were unnoticed.

Europeans, too, have been more forward in their attempts to utilize superheated steam than have Americans. On this side of the ocean the main object has been to employ saturated steam, obtain such a range of expansion as would yield the best results. To this end compounding was resorted to, and few attempts were made to increase the efficiency in any other way.

The steam which is generated in our boilers is, under favorable conditions, saturated steam. But with it it always carries a certain amount of excess moisture. This moisture exists even before the steam leaves the boiler. When it enters the steam main, no matter how excellent the covering, a certain amount of heat will be radiated, and this loss of heat can only result in the condensation of some of the steam. Consequently the amount of moisture is increased. Upon entering the engine cylinder, the walls of which are comparatively cool, there is a further loss by radiation, resulting in still further increase of moisture.

The use of superheated steam will not obviate this loss of heat, but it will reduce it to a great extent. Saturated steam is always of the temperature and pressure of the water from which it is generated, and its heat cannot be added to, while in contact with that water, without increase of pressure and temperature. But if it be led away from the boiler and passed through a series of coils, over surfaces which are heated to a higher temperature than the steam itself possesses, it will receive an addition of heat, without a change of pressure. This is the characteristic of superheated steam. It can be of any pressure and can be raised to any temperature within the limits of the superheating apparatus. In this it differs widely from saturated steam, which follows a certain fixed law in the relation of its temperature and pressure.

Let us follow the course of superheated steam sent from superheater to engine. If the degree of superheat is great, there will be no condensation whatever in its passage through the pipes. There will be a loss of heat by radiation, it is true, but unless this loss of heat reduces the temperature to or lower than the temperature corresponding to saturated steam of the same pressure, there will be no condensation. The same is true when the steam enters the cylinder. So that if the amount of heat added by superheating be equal to or greater than that lost in radiation, and so on, the interior walls of steam pipes, steam chest and cylinder will all be perfectly dry as far as moisture is concerned.

This is an additional advantage. Superheated steam, as the amount of superheat grows higher, approaches more nearly a perfect gas in nature and properties. Owing to this absolute dryness, it is less efficient in conducting heat than is the moisture-laden steam and the wet pipes and cylinder. So that the radiation loss when using superheated steam is much less than with saturated or wet steam.

The superheater consists of a coil or series of coils of pipe placed in the path of the flue gases, through which the steam is forced to pass as it travels from the boiler to the engine. Or the superheater may have an independently fired furnace. The main object, or rather the essential condition, being that the steam to be superheated shall pass through pipes surrounded by gases several hundred degrees hotter than the steam itself.

The reason for requiring such a heat for superheating must be evident. The steam is constantly in motion toward the engine, and so remains in the tubes but a short time. Yet in that time it must reach a certain temperature. This requires a high degree of temperature outside the tubes. The Practical Engineer.