A. As a rule, more coal is burned along the sides and in the corners than in the middle of the grates; the fire should consequently be kept somewhat heavier along the sides and corners than in the middle.
45. Q. How is the fire affected by and what causes clinkers?
A. A clinker shuts off area of grate surface according to its size, and thereby shutting off that much of the air supply and interfering with proper combustion. Clinkers are caused by firing too heavy in spots, which prevents sufficient air passing up through these spots and allows the coal to run together, melting the ash, and sand; running a hoe or slash bar through the fire will bring the points of melted sand together, thereby causing a clinker.
46. Q. How can you best avoid their formation and dispose of them?
A. Light firing and occasionally moving the grates lightly is the best preventive. When once formed, they should be removed if possible by firing around and burning them out.
47. Q. How can you explain the slower burning of the coke and how understand the proper manner of supplying fresh coal?
A. The gases of coal are lighter than air and will pass away whether consumed or not. The slow burning of the coke is due to the fact that it burns from the outside only. When a fire reaches a white or incandescent heat it indicates that the gases are burned and a fresh supply of coal should be added; this is to be done as light as the service performed by the engine will permit.
48. Q. When and for what purpose is the use of a rake on the fire bed allowable?
A. The rake should be used on the fire very seldom, because raking the fire bed tends to form clinkers, especially when the rake is plunged down through the fire to the grate. It may be used when necessary to rake the fire lightly when on the road for the purpose of breaking the crust, which may be found as a consequence of too heavy firing.
49. Q. Within what limits may steam pressure be allowed to vary, and why?