36

MODERN STEAM PRACTICE.

length of the grate, making an allowance of 75 of a square foot pernominal horse-power, we have—

-■ ~ r p — 6'6 feet in length.

The area over the bridge is generally about 18 square inches pernominal horse-power.

Water and Steam Room .—For boilers with hemispherical ends,the water should fäll the boiler two-thirds of its diameter, thus leav-ing one-third as steam-room. For Cornish arrangement with twofurnaces (otherwise known as the Butterly boiler) the water generallyfills the boiler three-fourths of its diameter, the remainder being thesteam-room. One foot height of water over the furnaces is allowed;when one furnace is adopted the steam-room in the boiler can be in-creased, and it is always advisable to have steam domes fitted tothe top.

RELATIVE VALUE OF PIEATING SURFACE.

Horizontal surface above the flame,,Vertical >, ,,

Horizontal surface below the flame,Tubes and flues...

= 1^ of their diameter.

BOILER FOUNDATIONS.

With the foregoing proportions we may now commence to lay outthe boiler foundations. The boilers are generally ordered in dupli-cate, so that no stoppage may occur in the event of one of themrequiring repairs; indeed, when deposits rapidly form from impuritiesin the water, frequent inspection is necessary, periodical scaling andcleaning out being required. After the ground is excavated, a bedof concrete is laid all over, on which is built the superstructure forcarrying and bedding the boiler thereon. The Cornish or London boiler, with inside furnaces, rests on a mid wall, having cast-ironSupports imbedded in the middle wall, and should be of sufficientheight to leave about 3 feet 4 inches from the stoking-floor to thedead plates on the furnace front. The boiler is surrounded withwhat is technically termed a wheel-flue, that is to say, the flame andthe heated gases pass through the internal furnaces and the backflues contained in the boiler, then wheel round at the end, and returnto the front—along one side, and pass along the other side nearestthe chimney, an opening being left in the mid wall at the bottom