largely upon the fuel, anthracite coal taking more surface than soft coal or wood.

Q. How is the fire removed from the fire-box?

A. In soft-coal engines, by a drop door held up by arms controlled by a lever outside the fire-box. When this lever is turned, the arms which hold up the drop door are removed, and the weight brings down the door so that the coals may be taken out by a suitable opening, and, by raising the ash-pan damper, may be raked out. (See Figs. 76 and 77.)

Q. What material is usually employed for ordinary grate-bars?

A. Cast iron.

Q. What for water-grates?

A. Wrought iron.

Q. Why are grates sometimes made slanting?

A. (1) To get the fire low down; (2) to get all the heating surface possible, if the fire-box front comes behind the main axle; (3) in shallow fire-boxes, to protect the tubes from the flame. In addition to pitching them forward, thereby increasing the protecting distance between the fire and the tubes, a brick arch is often used to hold the flame further away from the tubes and throw it toward the upper back corner, where needed.

Q. What is the advantage of deep fire-boxes?

A. They heat the air before it enters the fire, and are more durable, by reason of the cooling action of the air.

Q. Are water-tube grates always made with the tubes in one horizontal plane?

A. No. In some cases some of them-say every fourth one- -are raised above the rest.

Q. What is the objection to the method of putting water-tubes in from the front end?

A. They are more difficult to get at for cleaning.

5

6

6. Drop-plate Handle

Figs. 76. and 77. Plain Grate for Soft Coal.

5. Drop-plate Handle. 3. End-holder. 4. Drop-plate. Support. 7. Drop-plate Shaft. 8. Drop-plate Shaft Bearing.

1. Bar.

2. Dead-plate.

Q. What is the use of rocking grates?

A. To clear the fire where there is used bituminous coal containing material which causes it to clinker, or otherwise interfere with free combustion. The shaking or rocking grate breaks up the clinkers or other foreign or residuary matters that may collect on the grate, and which tend to choke the draft between the bars; causes such matter to work down between the bars into the ashpan; and serves to distribute the fuel evenly over the grate.

Q. What is the advantage of putting drop grates midway between the flue sheet and the fire-door?

A. The drop grate at the front end is considered by many as doing harm to the flues. Another disadvantage of having this grate at the front end is in cleaning the fire out of the box. A man doing this work, while poking cinders through drop grates, is at a great disadvantage, because he is working over a bed of hot coals and the slash bar gets very hot. He also becomes overheated, especially with long or large fire-boxes having badly clinkered or dirty fires.

Q. What about putting the drop grate at the back of the box?

A. There are some large engines on which the grates do not rock, but tip; and a drop grate is at the back end of box. In cleaning fire, the clinker-pit man pulls the grate lever back its full length. This puts the grates straight up and a great portion of the fire drops into the pan. With this arrangement fires are cleaned very quickly, the door is not opened, nor the blower on so long as where the grates rock, in which case large pieces of clinker must be broken and pulled back to the drop grate. There seem to be no bad results to the back sheet by having the drop grate at the back end of the fire-box. Q. Is it possible to lower the grates in a fire-box; if so, how?

A. The grates should be as low as possible in order to

get all possible heating surface on the sides and ends of the fire-box. Their location, however, is limited by the mud-ring; consequently they cannot be lowered farther. Q. What can you say of the relation between grate opening and fire thickness?

A. The larger the grate openings the thicker the fire can and should be carried.

Q. How much grate area should there be for each square foot of heating surface with a soft-coal burner, and how much with a culm burner?

A. With a soft-coal burner, as a general rule, the grate area is about 1/70 of the heating surface; with culm burners about 1/30. In this, as in other things, circumstances alter cases, but roughly speaking these proportions are fairly accurate. A study of various engines illustrated in Railway and Locomotive Engineering shows: Soft coal burners, B. & O., 2-8-0, 49.1 square feet of heating surface to 1 square foot of grate area; Wabash, 4-6-0, 83.4 heating to 1 of grate; D. & W., 2-8-0, 60.1; Monon, 4-6-2, 66 to 1; Frisco, 4-6-0, 61.9 to 1; M., K. & T., 4-6-0, 92.8 to 1; B. & M., 2-8-0, 61.5 to 1; N. C. & St. L., 4-6-0, 78.5 to 1; O. S. L., 4-6-2, 61.5 to 1; N. & W., 4-6-2, 76.1 to 1; Southern, 4-6-2, 71.4 to 1; culm burners, C. P. R., 4-6-0, 30.4 to 1; D., L. & W., 4-4-0, 24.4 to 1. The average of these soft coal engines is 69.3 to 1; of the culm burners, 27.4 to 1.

CHAPTER XIII

THE COMBUSTION CHAMBER

Q. What is a combustion chamber, and what are its uses?

A. A compartment of space in a boiler between the fire-box and the back tube-sheet to promote combustion and secure additional heat from the combustion gases before they enter the tubes. It is not yet commonly used. The back tube-sheet is in this case from 8 to 20 inches or more ahead of the fire-box throat, because with wide, shallow fire-boxes, the tubes give trouble by leaking unless kept away from the direct heat of the burning coal, where they are subjected to sudden and decided changes of temperature, as by putting on a strong blast, or by leaving open the fire door.

Q. Have combustion chambers been tried with locomotive boilers?

A. Yes, and especially recently with success. They tend not only to favor combustion but to keep the flues from the action of the flames.

Q. Is there any difficulty with combustion chambers filling up?

A. There is some accumulation of fine ash, but not enough to make trouble.

Q. What is the disadvantage of a combustion chamber?

A. With a given length of boiler, there is less actual heating surface.

Q. Then where is the principal advantage?

A. In lessening the repair bill and improving the combustion.

Q. How is the combustion improved?

A. Time is given to the unconsumed solid particles of fuel, and carbonic monoxide to combine with the oxygen of any free particles of air, the whirling action aiding.