Brick Arches. A brick arch is an arrangement placed in the fire-box to effect a better combustion and to secure a more even distribution of the hot gases in their passage through the tubes. Fig. 33 illustrates a longitudinal section of the fire-box fitted with a brick arch A. Its method of action is very simple. It acts as a mixer of the products of combustion with the air and as a reflector of the radiant heat of the fire and the escaping gases. It is maintained at a very high temperature and in this condition meets the air and gases as they come in contact with it and turns them back to the narrow opening above. By this action it maintains a temperature sufficiently high to burn with the smallest possible quantity of air all the carbonic oxide and the hydrocarbons that arise from the coal. It thus effects a very considerable saving in the cost of running, does away to a great extent with the production of smoke, and develops a high calorific power in comparatively small fire-boxes. This is a valuable property since it is possible for the boiler to utilize the heat value of the coal to the greatest possible extent. The bricks are usually about 4 or 5 inches thick and are ordinarily supported either by water tubes, as shown in Fig. 33 and Fig. 45, or by brackets in the form of angleirons riveted to the side sheets. The disadvantage accruing from the use of the brick arch is that it is somewhat expensive to maintain because of the rapid deterioration and burning away of the material.
Smoke-Box and Front End Arrangement. By the term front end is meant all that portion of the boiler beyond the front tube sheet and includes the cylindrical shell of the boiler and all the parts contained therein such as the steam or branch pipes, exhaust nozzle, netting, diaphragm, and draft or petticoat pipes. These parts referred to above are illustrated in the sectional view shown in Fig. 32.
The Steam or Branch Pipes. These pipes, 33, follow closely the contour of the shell and connect the T-head, 34, with the steam passage leading to the cylinder and conduct the steam from the dry-pipe to both the right and the left cylinders.
Exhaust Nozzle. The exhaust nozzle is the passage through which the steam escapes from the cylinders to the stack.
Netting. The netting, 26, is a coarse wire gauze placed in the front end which prevents large cinders from being thrown out by the action of the exhaust and thereby reduces the chances for fires being started along the right of way.
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