COAL-TAR AND WATER-GAS TAR CREOSOTES. 13 



composition of tar is the relation of the equipment at the gas plant 

 to the demand for gas. The most efficient manner of operating any 

 plant is not always the most expedient. During the war, when it 

 was hard or impossible to obtain machinery, and at the same time 

 the demand for the manufactured products was great, by shortening 

 the time of coking and not attempting to remove all the gas, it was 

 then found possible to increase the output of a given equipment to a 

 considerable extent. Under such conditions the tar would be pro- 

 duced at a lower average temperature and hence the paraffin content 

 would be higher. 



Inclined-retort benches. — In the inclined-retort system the retorts 

 are set at an angle which is as near as possible to the angle of recline 

 of coal. This is in the neighborhood of 30 degrees. Coal is charged 

 by gravity at the upper end of the retort, and under ideal conditions 

 fills the retort to an even depth for its full length. This condition is 

 not always realized in practice, and not infrequently a considerable 

 amount of unburned coal is found near the lower door, while the upper 

 part of the retort is nearly bare. This results in the tar containing 

 high amounts of paraffin. At the present time the use of this system 

 is not extending in this country. 



Vertical-retort benches. — The fact that much space in textbooks is 

 being devoted to the vertical retort would seem to indicate that in 

 time this type of retort may replace the horizontal for gas-making 

 purposes. The retorts are oval in cross-section and slightly larger 

 at the bottom than at the top. The coal is charged and the coke 

 extracted by gravity. For this reason the coal completely fills the 

 cross-section of the retort. The escaping gases are, therefore, 

 forced to travel chiefly through the cool core of uncarbonized coal. 

 The resulting tars contain considerable amounts of paraffin bodies. 

 In general, the specific gravity of tars from vertical retorts is less than 

 that of tars from horizontal rerorts. 



By-product coJce ovens. — There are several types of by-product 

 coke ovens in this country, but by far the greater proportion of them 

 belongs to one or another of three systems. These systems differ 

 from each other in several v/ays, as in methods of heating, in methods 

 of regeneration of heat, and in methods of recovery of by-products, 

 but the general principle underlying the manufacture of coke is the 

 same as that used in retort practice. 



The three principal systems are the Otto system, with its various 

 modifications, the Semet-Solvay system, and the Koppers system. 

 Figures 4, 5, 6, 7, and 8 show views of these three types of oven. 

 By-product coke ovens are, in general, long, rectangular ovens which 

 may vary in height from 3 to 9 feet and may be as long as 35 feet. 

 The width of the oven, however, varies within narrow limits, being 

 17 to 19 inches. Inasmuch as coke ovens are generally erected in 



