This boiler and its setting are both clearly shown in Fig. 67. This boiler is of an entirely different type from both the two preceding ones in two important particulars. First, there are no headers but three circular drums above and one below. From each of these three a set of tubes run to the lower one. Second, as the tubes all enter the drums radially they are necessarily curved at the ends to a greater or less extent.

This construction eliminates the header altogether, but introduces the curved tube, and also cuts off access to the tube with a bar or cleaner of any type except a flexible one.

This for many years was the most serious obstacle to the continued successful operation of these boilers. They were furnished with scrapers mounted on chains with a weight on the end, this weight was dropped through each tube in turn and the scraper then pulled through after it. It will be seen that this was an exceedingly laborious operation, particularly as the drums are small, only 3 to 4 feet in diameter, and men could consequently only work lying down or crouched very low. The air in these drums is generally not of the best and the slightest leak of hot water from the steam connections above, or the blow-off below, makes them an almost intolerable place in which to do any work, especially the violent physical labor involved in hand-scraping.

Fig. 67. Stirling boiler in setting.

These conditions have been very much changed by the introduction of power cleaners, which we will describe briefly at the end of this chapter.

The baffling in these boilers is inclined, following the line of the batteries of the tubes running to the different drums. The course of the gases is up, down, and then up through the stack connection to the draft opening just back of the rear drum. Three ordinary cleaning doors are provided above and one below, through which a steam jet can be introduced into the setting and the dust blown from the tubes, but owing to the steep angle at which the latter stand dust has not ordinarily the same tendency to settle upon them that it does on tubes that are approximately horizontal.

The bottom drum serves as a mud drum, and the sediment concentrated by the evaporation of the water gathers there, at least in part, and is blown off through the blow-off valve.

The feed takes place in the rear upper drum, which is entirely filled with water, and from there the feed water passes forward to the second and finally to the forward drum, there being a set of water tubes below the water line, and also steam tubes above it, connecting the front and middle drums, which serve as steam drums, particularly the middle one.

The setting of this boiler is regularly provided with a partial combustion chamber, and therefore furnace gases may be burnt under it by introducing the burner immediately above the fire-door, but for best results with furnace gases the setting should be modified by throwing its lower part forward so that the combustion chamber is made longer.

As between the Stirling and the Babcock & Wilcox boiler some prefer the Babcock on account of its straight tubes and ease of cleaning, while others prefer the Stirling, claiming that it gives drier steam, and also because it does not have a multiplicity of hand-hole plates to put on, take off, and keep tight.