________________________________________________________________________ | | | | | | Characteristics | Class A | Class B | Class C | |_____________________|________________|________________|________________| | | | | | | Fuse Point, Degrees | Safe at Degrees| Safe at Degrees| Safe at Degrees| | Fahrenheit | 3200-3300 | 2900-3200 | 2900-3000 | | | | | | | Compression Pounds | 6500-7500 | 7500-11,000 | 8500-15,000 | | | | | | | Hardness Relative | 1-2 | 2-4 | 4-6 | | | | | | | Size of Nodules | Medium | Medium to |Medium to Large | | | | Medium Large | | | | | | | | Ratio of Nodules | High | Medium to High | Medium Low | | | | | to Medium | |_____________________|________________|________________|________________| An approximate determination of the quality of a fire brick may be made from the appearance of a fracture. Where such a fracture is open, clean, white and flinty, the brick in all probability is of a good quality. If this fracture has the fine uniform texture of bread, the brick is probably poor. In considering the heavy duty of brick in boiler furnaces, experience shows that arches are the only part that ordinarily give trouble. These fail from the following causes: Bad workmanship in laying up of brick. This feature is treated below. The tendency of a brick to become plastic at a temperature below the fusing point. The limits of allowable plastic temperature have already been pointed out. Spalling. This action occurs on the inner ends of combustion arches where they are swept by gases at a high velocity at the full furnace temperature. The most troublesome spalling arises through cold air striking the heated brickwork. Failure from this cause is becoming rare, due to the large increase in number of stoker installations in which rapid temperature changes are to a great degree eliminated. Furthermore, there are a number of brick on the market practically free from such defects and where a new brick is considered, it can be tried out and if the defect exists, can be readily detected and the brick discarded. Failures of arches from the expansive power of brick are also rare, due to the fact that there are a number of brick in which the expansion is well within the allowable limits and the ease with which such defects may be determined before a brick is used.