to one side of which are fitted 35 pipes, C, each 21⁄2 inches in diameter. These pipes are uniformly distributed and serve for carrying off the gas and vapor. They all lead to a condenser, which may also receive the pipes of another distilling apparatus.

The stills are frequently heated with crude oil, or the residuum. The arrangements for this purpose will be described in detail in speaking of the use of petroleum as fuel.

In some refineries superheated steam is used as fuel, but the expense of operation is thereby much increased, since the quantity of coal required for the generation of steam and its superheating is considerably greater than that needed for direct heating. Although with the use of steam decomposition of the crude oil does not readily take place, the products of distillation are of less value than those obtained by direct heating, because the directly acting superheated steam carries along particles of heavy oil and even of crude oil, and thus contaminates the distillates. In the production of lubricating oils, superheated steam, however, plays an important part, and in speaking of them, its nature and generation will be discussed. Where crude oil is comparatively dear, and other fuel is not cheaper, coal is used for direct firing.

No general rule can be laid down for the choice of a fuel; that which is cheapest and most readily obtainable will of course be used. A comparison of the different varieties of coal shows, however, that the quality of the fuel deserves due consideration, since the consumption of fuel is less and the operation can be more readily regulated by the use of the best quality of coal.

The gas dome or head forms an important part of the still. Drops of oil carried along with the gases are retained in it, and it is, therefore, advisable to insert in it nets to prevent the oil from spurting over. In many refineries, chiefly at Baku, a special dephlegmating contrivance is placed between the dome. and the still to retain the heavy oils which are mechanically carried along; they either run back into the still or are directly drawn off. This arrangement is of advantage only in working crude oils poor in illuminating oil.

The head or gas dome is of various forms and its location on the still differs in different constructions. The old bomb-shape with goose-neck has been abandoned, at least in refineries working on a large scale. Its present form is, as a rule, that of a cylinder with a circular or an elliptical cross section. For more rapid cooling it is generally constructed of wrought iron, and seldom of cast iron. Its proportions should be in accordance with the capacity of the still. With too small a dome condensation proceeds badly, since the still evolves proportionally more gases than the dome can hold, which, in case they cannot escape from the still, produce a tension in the latter and cause disturbances in the distillation. With too large a dome condensation is also defective on account of the heavy parts. being readily carried along. For the distillation of crude oil a larger dome is used than for the distillation of the residuum, because in the first case stress is laid upon a partial condensation, while in the latter case provision has to be made for the rapid conveyance of the gases.

The dome is generally placed on the highest point of the still, though on very large stills it is occasionally found on the side. It should always be on the side of the still nearest to the condensing apparatus, so as to make the connection with the latter as short as possible. There are, however, many stills with the dome on the opposite side or in the centre of the still. The connection of the dome with the condensing apparatus is effected by means of pipes, the number and shape of which vary very much. Frequently a single conical pipe, which starts from the side of the dome and becomes gradually narrower, forms the connection with the condensing apparatus. But the use of several pipes is advisable. They consist of 3 or 4-inch gas pipes which rise either from the lid or from the side of the dome. The inclination of the condensing pipes is also of importance, because as a part of the condensation-namely, that of the heaviest oils-takes place in the dome and in the pipes by air-cooling, it will be readily understood that with inclined pipes a portion of the heavy oils will flow back into the

still and be decomposed on its superheated sides. This is the case if the condensing pipes are but slightly inclined. In arranging the distilling apparatus, it is, therefore, recommended to take the composition of the oil into consideration. If oils poor in illuminating oil are to be worked, the dome should have a more elliptical cross section and the pipes leading to the cooling apparatus should be slightly inclined. By this means the heavy oils carried along condense sooner, become decomposed and increase the yield of illuminating oil. On the other hand, when working oil rich in illuminating oil, or when the residuum is to be utilized, horizontal pipes or pipes with a slight downward slant are used.

Condensing Apparatus.

The condensing apparatus serves for condensing the vapors of the crude oil evolved in distillation. The portion of the apparatus intended for the absorption of the latent heat of the vapors is generally constructed of copper or iron, these metals being good conductors of heat. Successful condensation depends on the size of the cooling surface and the temperature of the surroundings; the lower the latter the smaller the cooling surface may be, and vice versa. Cooling is, as a rule, effected by means of water, though in exceptional cases air is used for the purpose. Crude oil or residuum is occasionally used for cooling, for instance, in winter when the water in the condensers freezes; this effects at the same time a saving in fuel in consequence of the preparatory heating of the oil or residuum. In the practice condensers with a much greater surface than necesssary, according to calculation, for the mere condensation of the vapors to a fluid are used, the purpose being to cool the latter to such a degree that even the lightest portions cannot volatilize. In many refineries at Baku and also in the United States the distillates leave the condensing apparatus warm and even hot, the cooling water being not changed and even allowed to become boiling hot. It is claimed that by this means distillates with a high burning point are obtained. The disad

vantage of such an arrangement will be readily understood when it is considered that the advantages claimed are effected at the expense of quality and yield.

The cooling of the vapors is effected on the sides of the pipes, no actual condensation taking place in the interior. Hence the condensing surface is augmented by an extended pipe system with a narrow cross section, the relative cooling surface being thereby increased. It must, however, be taken into consideration that with the use of too narrow pipes, distillation and condensation may be disturbed by friction in the pipes. For pipe condensers the counter-current system is decidedly the best; the cold water on entering the condensing apparatus meets the completely condensed oil and only cools it off. On passing through the pipes the water becomes gradually warm and meets hotter, though condensed products of distillation, so that at the point where it leaves the apparatus it is just capable of condensing the oil vapors.

The use of air for condensing purposes is not practicable, since in consequence of its slight capacity for heat and its bad conducting power the apparatus would be required to be much larger (nearly two hundred times as large) to produce the same condensing effect as water. Air can only be used with advantage in winter, but it is absolutely necessary in the production of machine oils, etc., which will be considered later on. In case of a lack of water and great differences in temperature between the seasons of the year, a form of condenser in which water condensation is supplemented by an arrangement for aërial condensation is employed.

Originally condensers were of copper coils, which were soon supplanted by coils of iron pipe in an iron tank. Coils of pipes are still sometimes used, but the general plan for condensers is to have straight pipes of convenient lengths laid one above another and connected together by return bends or manifolds in a rectangular box of iron or wood. The arrangement is shown in Fig. 85. The worm S consists of straight pipes connected with each other by knees. The arrangement