[Sidenote:  Main Boiler Room]

The main boiler room is designed to receive ultimately seventy-two safety water tube three drum boilers, each having 6,008 square feet of effective heating surface, by which the aggregate heating surface of the boiler room will be 432,576 square feet.

There are fifty-two boilers erected in pairs, or batteries, and between each battery is a passageway five feet wide.  The boilers are designed for a working steam pressure of 225 pounds per square inch and for a hydraulic test pressure of 300 pounds per square inch.  Each boiler is provided with twenty-one vertical water tube sections, and each section is fourteen tubes high.  The tubes are of lap welded, charcoal iron, 4 inches in diameter and 18 feet long.  The drums are 42 inches in diameter and 23 feet and 10 inches long.  All parts are of open-hearth steel; the shell plates are 9/16 of an inch thick and the drum head plates 11/16 inch, and in this respect the thickness of material employed is slightly in excess of standard practice.  Another advance on standard practice is in the riveting of the circular seams, these being lap-jointed and double riveted.  All longitudinal seams are butt-strapped, inside and outside, and secured by six rows of rivets.  Manholes are only provided for the front heads, and each front head is provided with a special heavy bronze pad, for making connection to the stop and check feed water valve.

[Illustration:  OPERATING ROOM SHOWING CONDENSERS—­POWER HOUSE]

The setting of the boiler embodies several special features which are new in boiler erection.  The boilers are set higher up from the floor than in standard practice, the center of the drums being 19 feet above the floor line.  This feature provides a higher combustion chamber, for either hand-fired grates or automatic stokers; and for inclined grate stokers the fire is carried well up above the supporting girders under the side walls, so that these girders will not be heated by proximity to the fire.

As regards the masonry setting, practically the entire inside surface exposed to the hot gases is lined with a high grade of fire brick.  The back of the setting, where the rear cleaning is done, is provided with a sliding floor plate, which is used when the upper tubes are being cleaned.  There is also a door at the floor line and another at a higher level for light and ventilation when cleaning.  Over the tubes arrangements have been made for the reception of superheating apparatus without changing the brickwork.  Where the brick walls are constructed, at each side of the building columns at the front, cast-iron plates are erected to a height of 8 feet on each side of the column.  An air space is provided between each cast-iron plate and the column, which is accessible for cleaning from the boiler front; the object of the plates and air space being to prevent the transmission of heat to the steel columns.

An additional feature of the boiler setting consists in the employment of a soot hopper, back of each bridge wall, by which the soot can be discharged into ash cars in the basement.  The main ash hoppers are constructed of 1/2-inch steel plate, the design being a double inverted pyramid with an ash gate at each inverted apex.  The hoppers are well provided with stiffening angles and tees, and the capacity of each is about 80 cubic feet.