A.—If to condense a cubic inch of water raised into steam 28.9 cubic inches of condensing water are required, then the cold water pump ought to be 28.9 times larger than the feed pump, supposing that its losses were equally great. The feed pump, however, is made sufficiently large to compensate for leaks in the boiler and loss of steam through the safety valve, so that it will be sufficient if the cold water pump be 24 times larger than the feed pump. This ratio is preserved by the following rule:— multiply the capacity of the cylinder in cubic inches by the total pressure of the steam per square inch, or the pressure on the safety valve plus 15, and divide the product by 200. The quotient is the proper capacity of the cold water pump in cubic inches when the engine is double acting, and the pump single acting.
340. Q.—By what considerations do you determine the dimensions of the fly wheel of an engine?
A.—By a reference to the power generated, each half stroke of the engine, and the number of half strokes that are necessary to give to the fly wheel its standard velocity, supposing the whole power devoted to that object. In practice the power resident in the fly varies from 2-1/2 to 6 times that generated each half stroke; and if the weight of the wheel be equal to the pressure on the piston, its velocity must be such as it would acquire by falling through a height equal to from 2-1/2 to 6 times the stroke, according to the purpose for which the engine is intended. If a very equable motion is required, a heavier or swifter fly wheel must be employed.
341. Q.—What is Boulton and Watt’s rule for fly wheels?
A.—Their rule is one which under any given circumstances fixes the sectional area of the fly wheel rim, and it is as follows:—multiply 44,000 times the square of the diameter of the cylinder in inches by the length of the stroke in feet, and divide this product by the product of the square of the number of revolutions of the fly wheel per minute, multiplied by the cube of its diameter in feet. The quotient is the area of section of the fly wheel rim in square inches.
342. Q.—Can you give a rule for telling the proper thickness of the cylinders of steam engines?
A.—In low pressure engines the thickness of metal of the cylinder, in engines of a medium size, should be about 1/40th of the diameter of the cylinder, which, with a pressure of steam of 20 lbs. above the atmosphere, will occasion a strain of only 400 Lbs. per square inch of section of the metal; the thickness of the metal of the trunnion bearings of oscillating engines should be 1/32d of the diameter of the cylinder, and the breadth of the bearing should be about half its diameter. In high pressure engines the thickness