Modern Machine-Shop Practice, Volumes I and II

by Joshua Rose

The entering steam strikes the face of the partition plate against which the water collects, and from which it drops to the bottom of the box, while the steam passes under the partition and out at the other side to the engine.

The draught of a boiler is caused by the heat expanding the air and lightening it, thus causing it to ascend. It can be checked by stopping the exit of heated air up the funnel by means of a damper, by checking the flow of cold air into the furnace, by closing the dampers, by opening the furnace doors and letting cold air in the furnaces above the fires.[71]

[71] See [page 368], Vol. II.

A blast pipe is a small pipe leading from the superheater to the funnel, and provided with a stop cock.

It is used for letting a jet of steam up the funnel to promote the draught.

Flame seen at the top of the funnel is caused by the combustion of gases that would have been consumed in the furnace had there been sufficient air or sufficient room for complete combustion. It may be caused in a variety of ways, as insufficient openings between the fire bars, too narrow a space between the bridge wall and the boiler, or too deep a fire upon the bars. It is detrimental, because it obviously wastes fuel.

Dampers are used to regulate the draught in the furnace; they are fitted to the ash-pits or to the funnel, and should be fitted to both, because closing a damper in the funnel sets up a certain amount of pressure in the furnace by holding the heat, whereas dampers at the ash pit doors and none in the funnel lets the heat out and prevents cold air from getting in to promote combustion.

When there are no dampers the furnace doors are open instead, to check the draught; this is, however, highly injurious to the boilers.

The most rapid wasting of the plates of a marine boiler occurs alongside the fire bars, on the furnace tops, at the back of the flame box or combustion chamber, and in those plates generally that receive the most intense heat, and especially when they are heavily coated with scale and are not covered with water.

The scale that forms on the face of the tube sheet keeps the water away from contact with the plate, which, with an undue thickness of scale, will crack between the tube holes.