« ZurückWeiter »
section at the threads, should not exceed 4,300 lbs., to provide against wasting from corrosion. The distance of centres of staybolts is found thus: Multiply the constant number, 4.300, by the area of the staybolt, and divide the product by the working pressure; then take the square root of the quotient, and the answer will be the proper centres. The usual pitch for locomotive fire-box stays is 4 inches centres, irrespective of the thickness of plate. The dished end of a cylindrical shell, such as the top of a dome, should be dished to a radius equal to the diameter of the cylinder, in order to make it equal in strength to the cylinder, a hollow sphere being twice as strong as a cylindrical shell, of the same radius and thickness. Position of Feed Delivery in Boilers.-In Cornish and Lancashire boilers, the feed should be introduced on one side of the front end plate, about 4 inches above the furnace crown, through an internal dispersing pipe, carried inside the boiler to at least one-third of its length, and perforated for the last half of the pipe's length ; and in vertical boilers, the feed should be introduced through a short perforated pipe, so as to deliver just below the water-level, but clear of the fire-box and tubes. When the feed is introduced below the furnace crown, if anything gets into the backpressure valve to prevent its closing, the pressure in the boiler will force the water back through the feedpipe, and the furnace crown will become bare and overheated. Heating Feedwater.—In order to prevent unequal expansion and contraction, by keeping an even temperature in the boiler, and also to save fuel, the feedwater should always be heated. In heating by exhaust steam, the feedwater should not be allowed to come in direct contact with the exhaust steam, but the steam should pass through pipes around which the feedwater should be made to pass. One great advantage of a feedwater heater, is, that it arrests the substances held in suspension by the water, and scale, &c., is deposited in the heater, which would otherwise form in the boiler. The exhaust steam from a non-condensing engine, will heat the feedwater to within a few degrees of the boiling point (212°), and a saving of about 13 per cent. will be effected over cold water. In condensing engines the feedwater is generally taken from the hot well, at about 100°, effecting a saving of about 4% per cent. over cold Water. Nominal Horse-power of Boilers.-The nominal horse-power of a boiler is estimated by its size, and may be found by the following rules deduced from practice :— Nominal horse-power of plain, cylindrical, or egg-ended boilers: Multiply the diameter in feet by the length in feet and divide by 6. Nominal horse-power of Cornish boilers: Add the diameter of the shell, and the diameter of the flue together, in feet, and multiply the sum by the length in feet, and divide the product by 8. Nominal horse-power of Lancashire boilers: Add the diameters of both slues, and the diameter of the shell together, in feet, multiply the sum by the length in feet, and divide the product by 8. Nominal horse-power of vertical cross tube boilers: Add together the diameter of the shell, the diameter of the fire box, the diameters of all the tubes, and the diameter of the uptake tube, all in feet; multiply the sum by the length in feet, and divide by Io. Nominal horse-power of vertical tubular boilers, with vertical tubes: Add together the diameter of the shell, the diameter of the fire box, the diameters of all the tubes, all in feet; multiply the sum by the length in feet and divide by 12. The actual horse-power of a boiler may be estimated by dividing the weight of water evaporated to steam per hour, by the weight of steam consumed per indicated horse-power of the engine per hour. Evaporative Test of a Boiler.—The simplest way of ascertaining the approximate actual evaporation of any boiler is as follows. When the boiler is working satisfactorily, feed the boiler up to the top of the watergauge glass, then shut off the feed, weigh all the coal used after this time, and observe the time occupied in reducing the water from the top to the bottom of the glass; fire carefully, and see that the same quantity of fire is left at the end as at the beginning of the test. Then the evaporative power may be ascertained, from the data obtained in the test, by the following rules:— To find the number of cubic feet of water evaporated per hour: Multiply the number of square feet of water-surface, by the evaporation in inches of gauge-glass, multiply the product by 5, and divide the result by the number of minutes occupied in evaporation. To find the quantity of water in lbs. evaporated per lb. of coal: Multiply the number of cubic feet of water evaporated per hour, by 62.5 and divide the product by the quantity of coal in lbs. consumed per hour. Heating Surface.—The evaporative power of a boiler depends upon the efficiency of its heating surface, the values of which are as follows:— All horizontal surface above the flame # vertical surface - - - . . / Being taken as effec1} the diameter of tubes or round flues . ( tive heating surface. 1; convex surfaces above the flame . Horizontal surfaces beneath the flame, are of no value as heating surfaces. The Dome should be equal to one-half the diameter of the boiler in diameter, and in height, and the hole through the plate should not be larger than a man-hole; the edge of the hole should be strengthened by riveting a strong ring to it; but as a steam-dome weakens the boiler shell, and is apt to leak at the base, it is better to dispense with it, and take the steam through an internal perforated pipe, about 6 feet long, fixed close to the top of the shell.
Cornish and Lancashire Boilers of 5 feet in diameter and upwards, should have the longitudinal seams of the plates either doubie-riveted or treble-riveted according to the working pressure. In fixing Galloway tubes, the welded part should face the back end of the boiler. The Galloway Boiler in its present improved form is an excellent and economical steam producer—an 8 hours test of one with 70 lbs. pressure of steam, was conducted as follows:—steam being raised to 70 lbs., the height of water in the boiler was noted, and the fires drawn: the fires were then re-lighted, all the fuel used was weighed, allowance being made for unconsumed fuel in the fires at the end of the test. Calorimeter observations were taken, a certain weight of steam being condensed in a given quantity of water, the dampness of the steam being determined by the increase of weight and temperature in the water, the feed-water was measured and also weighed. The boiler evaporated 11:72 lbs. of water at 212° F. per lb. of coal, or 2603 lbs. of water per hour, with a heating surface of 973 square feet: the boiler power being 41-64 horse power, at 1 cubic foot of water evaporated per hour, percentage of water in steam 5: coal burnt per hour 283 lbs., or 7°269 lbs. per square foot of grate per hour: temperature of gases leaving the boiler 324°: cubic feet of water space per horse-power, 14 Io: cubic feet of steam space per horse-power, 4'O4. To find the Number of Gallons of Water a Boiler will hold.—If a plain cylinder without tubes, multiply the square of the diameter in feet by the length in feet, and by 4'89; the answer will be in gallons. Or. multiply the square of the diameter in inches by the length in feet, and by ‘oj4. Or multiply the square of the diameter in inches, by the length in inches, and by 'ooz83. To find the Number of Gallons in a Cornish or Lancashire Boiler.—Multiply the sectional area of the boiler shell in inches, by the length of the shell in inches; multiply the combined sectional area of the flues in inches by their length in inches; subtract this product from the first, and divide the remainder by 1728; this will give the number of cubic feet of water the boiler will contain, which multiplied by 6:24 will give the contents in gallons of the boiler when full of water.
The Firebox should be formed of plates of very ductile and superior quality. There should be one mudhole opposite the large end of each cross tube, and mudholes should be placed round the bottom of the boiler. The diameter of firebox given in the table is at the bottom; the top should be less in diameter; the taper should be about 1 inch per foot in height.
The Cross-Tubes should be lower at the small end than at the other, and their seams should be placed away from the fire.