QUESTIONS RELATING TO BOILERS, &c. 256. Given the number of feet the bottom of a boiler is below the waterline: to find what pressure of steam is requisite to blow off the brine through the blow-off cock. RULE C. Divide the number of feet and decimals of a foot, if any, which the bottom of the boiler is below the water-line by 2.305;* the quotient is the required pressure, expressed in pounds. EXAMPLE. Ex. What pressure of steam in a boiler will be requisite to blow off the brine through the blow-off cock, the bottom of the boiler being 18 feet 6 inches below the water-line of the ship, allowing a column of water 2.305 feet to equal 1 lb. ? 257. Given the diameter of the tubes, their length and number, also the dimensions of the plates, to find the amount of heating surface. RULE CI. 1°. Find the circumference of tubes (see Rule LXXIV, page 183), which multiply by their length in inches, and the product again by the number of tubes; the result is the heating surface of the tubes. 2°. Find the area of the plate in square inches, by multiplying the length by the breadth, (both in inches). The position of the water level in the boiler often differs from that outside the ship; this would affect considerably the process of blowing out if the boiler be placed low in the ship; hence, calculations of the following nature are at times requisite. It has been noticed that atmospheric pressure at 14'75 lbs. of mercury, sustains a column of water, in a vertical tube, to the height of about 34 feet. We have then this analogy :— whence it appears that 2.305 feet of water is balanced by 1 lb. 3°. Find the area of the mouth of one tube, and multiply it by the number of tubes. NOTE.-There are two plates, but they are not entire, the openings of the tubes (found by No. 3°) must be deducted. 4°. Deduct the area of the tube mouths (No. 3°) from the area of the plate (No. 2°); the remainder is the heating surface of one plate, this multiplied by 2 is the heating surface of both plates. 5°. Add together the heating surface of the two plates (No 4°) and the heating surface of the tubes (No. 1°); the sum divided by 144 is the heating surface, in square feet, of the tube and tube-plates as required. EXAMPLE. Ex. Required the heating surface in the tubes and tube-plates of a boiler, the tubes being 2 inches in diameter, 8 feet 3 inches in length, and 462 in number, and the plates 19 feet 8 inches long, by 8 feet 7 inches wide. Heating surface of tubes = 2.5 inch X 3'1416 X 99 inch X 462. 258. Given the length and width of boiler, the pressure of steam in the boiler, and the height of the water in the boiler, to find the total pressure on the bottom of the boiler. It is also required to find the number of stays of given diameter. RULE CII. 1o. Divide the height of the water in the boiler by 2.305 feet (See Rule C and Note); the result is the pressure of water per square inch in the boiler. 2o. To the pressure of water (No. 1°) add the pressure of steam; this gives the total pressure per square inch on the boiler. 3°. Multiply the length of the bottom of the boiler by its breadth (both in inches), and thus obtain the area of the bottom of the boiler. 4°. Multiply the area by the total pressure per square inch; the product is the total pressure on the bottom of the boiler. 5°. Multiply the square of the diameter of the stays by 7854; the result is the sectional area (see Rule LXXVI, page 184). 6°. Multiply this last result (No. 5o) by the strain that each square inch of section shall bear; the product is the strain each stay bears. 7°. Divide the total pressure (No. 4°) by the strain each stay bears (No. 6°); the quotient is the number of stays required. EXAMPLE. Ex. What is the total pressure on the flat bottom of a boiler 10 feet 6 inches long, 4 feet 3 inches wide, the pressure of steam in the boiler being 35 lbs. per square inch, and the height of the water in the boiler 3 feet 9 inches? Also, the number of stays 2 inches in diameter required, so that each square inch of section of the stays shall bear 4500 lbs. ? Length 10 feet 6 inches = 126 inches, breadth 4 feet 3 inches = 51 inches. The number of stays must evidently be 17, since 16 stays will not completely stay the boiler. Ex. A boiler measures 10 feet long, and 15 feet broad, rectangular; the water is 9 feet high, and one-third of that space is furnaces and tubes, the rest is water. The furnaces consume 10 cwt. of coal per hour, and each pound of coal is equal to heating 12000 pounds of water one degree Fahrenheit. The difference between the temperature of 10 pounds steam and that of 25 pounds steam is 28 degrees: in what time will the pressure rise from the former to the latter pressure, if all the valves are shut and the fires in action ? (In the calculation, neglect the heat absorbed by the steam, and suppose the circulation of the water to be perfect. It should, however, be remembered that practically, the circulation is always imperfect, and especially so under the above circumstances; the pressure will, therefore, in practice, increase more rapidly than will be shown by the answer.) Water space 10'5 feet X 15'5 feet X 9 feet = 1464'75 cubic feet. Deduct for furnaces and tubes 146475÷ 3 = 488·25. ... actual water space = 1464'75 — 488*25 = 976*50. Taking the weight of cubic foot of fresh water at 62.5 lbs., then the Weight of water in boilers 976·50 × 62°5 = =61031.25 lbs. Coal required to heat it 1° 6103125 12000 = 5*086 28°5'086 X 28=142°408 lbs. lbs. of coal consumed per hour 10 X 112 1120 lbs. The difference in temperature of the steam at 10 lbs. and 28 lbs. = 28° Then if one lb. of coal heats 12000 lbs. of water 1o in 1a, it will raise 61031.25 lbs. 1800 in one hour. 20000 1031 1120 lbs. of coal will raise it 1120 X in one hour, that is, 220°3°. Then Again, taking the above example, and supposing it to be salt water, then Taking the weight of a cubic foot of salt water at 64 lbs., the weight of water in boiler equals 62496 lbs., which multiplied by the difference of temperature 28° 1749888 the number of units of heat to raise the water in the boiler to 25 lbs. pressure. The number of units of heat given out by furnaces in one minute is The stays on a flat sided boiler are 12 inches apart, centre to centre; some of these have wasted away to " diameter at the smallest section; if the pressure of steam carried be 30 lbs. per square inch, what strain is there upon the square inch of the section of these stays? (What is the total pressure upon the area supported by the stay? Divide that by the sectional area of the stay.) G G |