The Titanic was built as a “triple-screw steamer,” which meant that she had three propellors, one mounted on the centerline of the ship, and two “wing screws,” one mounted each to port and starboard. Turning these screws would be a distinctive powerplant arrangement of reciprocating engines turning the wing propellors and an unusual low-pressure turbine powering the center one. But for the turbine to spin and the reciprocating engines to turn, steam was needed in greater quantities than any ship had ever before generated. To create this giant head of steam, twenty-nine boilers were installed: twenty-four double-ended (that is with fireboxes at each end) and five single-ended. 
Each end had three fireboxes, making a total of 162 furnaces that had to be stoked with coal, a shovelful at a time. Nearly 600 tons of coal a day were needed to maintain a speed of 22 knots. Two hundred grimy, sweating stokers, firemen, and trimmers, who would move the coal from the bunkers, shovel it into the fireboxes, and keep the fires burning evenly across the firegrates, would be needed to feed the insatiable maws of these boilers, which stood fully two stories tall, twenty-one feet in diameter.
In addition to supplying steam for the engines, the boilers also provided the steam which turned the electric dynamos, which generated electricity for the ship, and the steam that powered the Titanic’s steering motor, which turned her rudder. The ship’s refrigeration plant was also steam powered, as were the winches which raised and lowered the anchors, along with the winches that worked the mooring lines. Finally, steam was used to heat the ship’s public rooms, as well as the passenger cabins in Second and Third Class.
The working pressure of the boilers was 215 psi, and to keep the steam up, the fires had to be constantly fed with coal.
The task began with the trimmers, who used wheelbarrows to deliver great lumps of coal measuring as much as twenty inches in length and eight inches thick, from the bunkers to the foot of the fireboxes. There the firemen took over, first breaking the large lumps of coal brought by the trimmers into something more manageable: using his shovel and slice bar, a fireman would reduce the larger pieces into fragments roughly the size of a man’s fist. Next, timing his movements to the roll and pitch of the ship, he would swing open the door to a firebox and quickly thrust home his slice bar along the fire-grate, working it back and forth four times, once for each track of the grate, to improve the draft across the burning coals by breaking ashes and clinkers loose. These were quickly raked into a pit below the firebox and the fire-door swung closed again. On double-ended boilers the stokers worked in tandem so that doors at the opposite ends were never open at the same time, preventing back-drafts that could blow the fire out into the firemen’s faces. The fire-door would be opened again, and the fireman would shovel a layer of coal across the grate–a skilled fireman would usually feed in no more than four shovelfuls of coal, spreading them over the grate at a uniform depth of four inches. At the same time, other crewmen known as water tenders would keep a close eye on the water gauges, careful to keep a level of two inches in the boiler, a combination that maximized the amount of steam each fire-grate could produce.
The whole routine moved to the inexorable ringing of Kilroy’s Patent Stoking Indicator, a mechanical timer that could be set for intervals between eight and thirty minutes. The higher the speed of the ship, the lower the interval between rings on the Indicator; the amount of time for which the Indicator was set was the total allotted to performing the entire cycle of breaking coal, slicing, clinkering and stoking. The usual settings would be between eight and ten minutes for a complete cycle. At the end of each four-hour watch the firemen would finish by raking the ashes and clinkers out of the pits, hosing them down to cool them, then shoveling them into hoppers that mixed them with seawater and then ejected them out scuttles near the ship’s waterline.