202 APPENDIX VII I 3600 (11.8 ft.) It is well known that when patching bottom it is necessary to keep the doors closed as much as possible in order to avoid chills, which are likely to develop into cracks and leaks in the bottom. Many beautiful octahedral steel crystals have been discovered in tearing out old furnace bottoms. When the pressure in the furnace gets higher than the atmos- phere a sharp sting of flame is developed and fuel must be burned to maintain it. Some m e 11 e r s and many heaters keep the doors of their furnaces deco- rated with a halo of flame. In some cases it is necessary to main- tain this sting owing to the defective design of 3200 (10.50 ft.) I , 1 /. rni the furnace. There is no question whatever that this method of working will prevent cold air being drawn into the furnace. It is te easiest way "; the company pays for the excess fuel, and the melter or heater has plenty of time to sit down and "watch 'er burn." In foreign plants the technical control of the furnaces is more closely maintained than in this country because fuel is expensive. In order to sinter the bottom of the furnace in place it is necessary to direct the jet of flame so that it will lick the hearth. In order to do this the air and gas ports must be given a suitable inclination toward the hearth and the velocity of the flame must be sufficient to carry it down to the bottom. The velocity which will be impressed upon the jet of air entering the furnace will be fixed by the height from the bottom of the regenerators to the port and by the area of the port and the flues leading to it. In the case of the gas a slightly greater head is available as it enters the Scrap Process /^///////////^ FIG. 142.—Sections Showing Profile of Hearth Slope According to Professor Groume- Grjimailo.