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84                         METALLURGY   OF   CAST   IRON.
or worn-out pipes. For this reason much care is exercised to prevent the temperature rising above 1,100 degrees in the oven.
There is a plan used in iron stoves of suspending the iron pipes from the top of the oven instead of letting them rest with their weight on the "bed pipe," as shown in Fig. 15. This plan prevents the iron pipes from *' buckling '' or bending from their own weight when they get red hot.
The usual plan adopted for heating cold air to make " hot blast " in the iron stove will be readily understood by a study of the design illustrated in Fig. 15. The arrow seen at A, Fig. 15, is the point at which the cold air enters the iron pipes in the hot blast oven. As soon as the cold air enters the first "bed pipe" E, it takes the direction shown by the arrow in the pipe B; passing from this to the ubed pipe" F. then travel-
.                                 FIG.   17.
ing up the pipe D and down into the bed pipe H, continuing such a line of travel through four to six more pipes, according to the length of an oven, until the blast reaches the outlet at K on the right, from which it then enters the blast furnace as " hot blast." The action of gases is next to be considered. A point to be understood is that of the means employed for heating the oven or iron pipes to create " hot blast." This is accomplished through the use of waste gases, which escape at the top of a furnace, and are passed down through the '' down-comer,'' seen on the right, to a flue N N, and then rising into the ovens through the openings M and P, until they reach the combustion chamber R, where they ignite as soon as they reach the point S, by reason of the gas being metreplaced by the brick stoves.ding " or "slips" from the use of fine ores, etc. It may also beency                :|,