in a furnace and the "principles of combustion as applied to the heating of retort furnaces."

The fuel with which the furnace is supplied is the coke which remains in the retort after all the gas has been driven off; and coke consists principally of carbon, as it usually contains about 88 per cent. of the last-mentioned element. Now when carbon is burnt to CO2 a very great amount of heat is given out (1 lb. of carbon when burnt to CO, yielding 14,500 heat units), and it is the resulting combustion of the coke in the furnace with the air which is drawn into the latter

which gives the necessary heat to the retorts.

Furnaces are divided into three classes: the ordinary, or directfired, the generator, and the regenerative, or recuperative. In the ordinary open or direct-fired furnace the fuel is burnt direct to CO2, while in the modern generator, or regenerative furnaces, the combustion takes place in two stages, as described later on.

FIG. 5.

The great defect of the directfired furnace is the difficulty of controlling the air supply, which results in a greatly increased consumption of fuel, together with irregular heating of the setting. It is quite impossible to regulate the air supply to furnaces of this class with any degree of exactness; the speed at which the air must necessarily pass through the furnace to develop the proper amount of heat, and also the impossibility of the whole of the oxygen of the air coming into contact with the carbon to be taken up, compels the admission through the grate bars of an ordinary or direct-fired furnace, of about double the quantity of air theoretically necessary,

and consequently double the quantity of nitrogen, which has the effect of retarding the combustion and reducing the heat; on the other hand, if this excess be not admitted, then there will be a waste of fuel, caused by the combustible gas carbonic oxide, leaving the setting unconsumed, by reason of not being supplied with the oxygen in the form of air necessary for its conversion into CO2.

In any form of furnace the aim should be to admit sufficient air to convert the fuel into CO2, any excess or deficiency being detrimental to the heating of the setting.

In furnaces constructed on the "generator" principle, instead of constructing the furnace so that immediate and complete combustion of the fuel shall take place, it is so designed, that the primary supply of air entering at the bottom of the furnace, shall be sufficient only for the combustion of a portion of the fuel lying nearest to the point of ingress of the air, and that the CO, gas generated by the combustion of this portion of the fuel, shall, in passing through a further considerable stratum of coke, yield up a portion of its oxygen to this mass of fuel, and become thereby reduced to carbonic oxide (CO), thus, CO2 + C = 2 CO. The oxygen absorbed by the stratum of fuel through which the carbonic acid gas as the product of the initial combustion passes, maintains the whole mass of fuel in a state of slow and partial combustion, and the hot gases passing away (consisting chiefly of carbonic oxide, which is a combustible gas) are capable of being rekindled and finally consumed to carbonic acid, with the evolution of a considerable amount of heat. This is effected by introducing at the proper place a separate supply of air, known as the secondary air.

This form of furnace does not require a forced draught, its needs being supplied by not much more than one half the quantity of air necessary for complete combustion, and the fact that the remainder of the air is admitted at the

point of combustion tends still further to reduce the draught. The reason for this is that the preliminary object to be attained is the production of carbonic oxide (CO) and not carbonic acid (CO2); the former gas requiring for its production one half the quantity of oxygen to combine with an equal quantity of carbon that is required for the complete combustion of the same to CO2 The other half of the oxygen is not introduced until the fuel is in a gaseous form, when a much more thorough mixing of the fuel and oxygen is brought about than can be realized when the fuel exists in the solid form as coke.

The method of heating retorts just described, is known as that of gaseous firing, and the furnace in which the combustible gas is generated is known as the producer, or generator. Figures 6 and 7 show details of one form of the same, and their relative position in the setting.

Generators vary in size according to the requirements of the setting, the chief point to be attended to being to allow a good depth of fuel of comparatively even thickness (from 4 to 6 feet deep); they run from 4 to 6 feet in length, and from 2 to 3 feet in width. The arches at the top are generally formed of two rings of 4-inch arch bricks, struck to the required radius, and provided at intervals with nostril holes. The sides are usually made 9 inches in thickness, The air which is admitted to the generator is known as the primary air, and care must be taken to seal the bottom of the producer air-tight, so as to prevent any air entering except through the primary air inlet. The latter must be arranged to admit of easy adjustment, as only just sufficient air is required to produce carbonic oxide, any excess only increasing the temperature at the base of the generator, resulting in local heating.

In order to obtain good results with this class of furnace it is necessary to conduct the combustible gas (CO) under the whole length of the setting, by arranging a number of

ports from which it can issue, the secondary air being admitted alongside of it by means of corresponding ports; this obviates the intense local heating and imperfect distri bution of the heat which occurs when the combustible gas issues in one or two places in a large volume.

Another and most important advantage which attends the use of this form of furnace is, that the waste heat, which in the ordinary furnace goes away to the chimney, is utilized in heating the secondary air, with the result of a considerable gain in fuel. When such heat is utilized, the furnace is said to be a regenerative, or recuperative, furnace, and when the secondary air is not heated, the furnace is termed a generator furnace.

The heating of the secondary air is effected by causing the heating gases, which have done their work in heating the retorts, in place of passing away direct to the chimney, to enter a brickwork chamber, termed a regenerator, placed on either side of the producer; the regenerator is arranged in such a manner that the spent gases pass in a downward direction, through a series of flues, to the main flue, which in this type of furnace is placed underneath the setting, while the incoming secondary air passes by a separate channel in an upward and opposite direction to meet the combustible gas. The details of the construction of regenerators vary somewhat, but the aim in every case is to cause the secondary air to take up as much heat as possible, while the waste gas loses heat in a similar proportion.

A simple form of regenerator consists in dividing the latter into two chambers, one for the waste gas, and another for the secondary air, so that the waste gas travels from back to front downwards, while the secondary air in the adjoining chamber passes from front to back upwards. Another method allows the waste gas to travel vertically downwards while the secondary air travels in a zig-zag direction upwards, and also from side to side, so as to

encircle the waste gas chamber. By this arrangement the secondary air is raised to a temperature of about 1800° F.

From fig. 6, which has some of the details omitted, the student should now be able to follow the arrangement of a regenerative setting.

As shown, on each side of the producer there is a regenerator; the secondary air enters at the bottom, and travels vertically upwards through the hot walls of the secondary air chamber, as indicated by the arrows, and becomes gradually warmer until it emerges by a nostril hole where

it meets the combustible gas CO, the point where combustion takes place being known as the combustion chamber. The products of combustion pass over the retorts, as shown, and then enter the waste gas chamber of the regenerator, by means of a pocket, on their way to the main flue. In travelling through the chamber the waste gases communicate their heat to the brickwork, and the secondary air in the adjoining chamber takes up heat in consequence.

Fig. 7 shows a method of regeneration more in detail.

When generator furnaces are employed, only one fur