TABLE SHOWING THE EFFLUENT VELOCITY WITH WHICH STEAM, AT DIFFERENT PRESSURES, WILL FLOW INTO THE ATMOSPHERE, OR INTO STEAM AT A Rule for Finding the Amount of Gain derived from Work ing Steam Expansively. Divide the length of the stroke in feet by the cut-off, 1, 1, 1, as the case may be; then find on the table on page 68 the hyperbolic logarithm nearest to that of the quotient, to which add 1. This sum will give the ratio of gain. Example. Suppose 50 pounds per square inch to be the initial pressure; length of stroke, 10 feet; cut-off, ; find the mean pressure. 10÷2·5=4. The hyperbolic logarithm of 4 is 1.38629, which, with 1 added, becomes 2.38629, which is the ratio of gain. 4:238629:50 age pressure. 2.38629 × 50 29.82862 lbs. mean or aver If a given quantity of Steam, the expansive power of which, at full pressure, is represented by 1, be admitted to a cylinder of a certain size, and cut off when the piston travels through of the stroke, its effect will be raised by expansion to 1.69; if cut off at , the effect will be 2:10; at 1, 2:39; at 4, 261; at t, 279; at 4, 2-95; at 1, 308; but the expansion cannot be carried beneficially as far as in all classes of engines. TABLE OF HYPERBOLIC LOGARITHMS TO BE USED IN CONNECTION WITH THE Rule for finding the mean or average pressure in the cylinder of a steam-engine. Divide the length of the stroke in inches (including the clearance) by the distance that the steam follows the piston before being cut off; the quotient will be the expansion the steam undergoes. Then find in the expansion column, in the following table, the number corresponding to it; take the multiplier opposite, and multiply the full pressure of the steam per square inch, as it enters the cylinder, by it. The product will be the average pressure. Example. Suppose the initial pressure be 70 lbs. per sq. inch and cut-off at half-stroke, the stroke being 3 ft. Then 3 ft. = 36 in. +0.5 for clearance Stroke = 18 in. + 0.5 66 36.5. = = 18.5. Then 365185197, the relative expansion between 1.9 and 2. By referring to the table, the multiplier for 19 will be found to be 0.864, and the difference between that and the multiplier for 2 is 0.017. Hence, by multiplying 0·017 by 07, and subtracting the product 0011, the remainder, 0-86281, is the multiplier for 197. Therefore, 0-86281 × 70=60-3967 lbs. per sq. inch, the mean effective pressure on the piston. TABLE OF MULTIPLIERS BY WHICH TO FIND THE MEAN PRESSURE OF STEAM AT VARIOUS POINTS OF CUT-OFF. OF CONSTANT NUMBERS, BY WHICH TO ASCERTAIN THE AVERAGE PRESSURE OF THE STEAM AGAINST THE PISTON FOR DIFFERENT PRESSURES AND POINTS OF CUT-OFF, FROM TO OF THE STROKE. Multiply the pressure in pounds, as shown by the gauge, by the constant number opposite the point of cut-off in the left column. The product is the average pressure. TABLE OF CONSTANT NUMBERS FOR FINDING THE REQUIRED “LAP" FOR SLIDEVALVES, WHEN THE TRAVEL OF THE VALVE IS KNOWN. Multiplier. 354 •323 •289 •250 •204 177 •144 Multiply the valve-stroke by the decimals opposite each point of cut-off. There are two methods of applying the power of steam to the cylinders of steam-engines, one being to allow it to flow from the boiler to the cylinder through the whole length of the stroke, the other to cut off the supply when the piston has travelled a certain distance. The advantage of the latter over the former consists in the saving of fuel; which may be explained as follows: If steam be applied the full length of the stroke, the average pressure will be as the pressure per square inch on the piston; but if the steam be cut off at half stroke,- suppose the pressure to be 65 lbs. per square inch, when the pressure of the atmosphere is added, there will be a mean equivalent, or average pressure, throughout the stroke of about 55 lbs. per square inch, being only 10 lbs. less than full pressure, or 16 per cent. of a loss in power, though only half the former quantity of steam has been used, Steam-ports. A term applied to the passages through which the steam enters the cylinder; they are generally the area of the piston, but vary for different boiler pressures and piston speeds, those of locomotives being about, which have the largest ports of any class of engines. The area of the exhaust-port should be from to that of the cylinder. As a rule, the exhaust, when passing out of the cylinder after the first rush is over, should not have to travel faster than 100 feet per second; but with some designs of engines the velocity of the steam may be greater, without creating injurious back pressure. The form of the ports is immaterial, providing they are large enough to give admission to the amount of steam requisite to keep the pressure up to its initial point until it is cut off by the valve, and give free egress for its escape. TABLE SHOWING THE AVERAGE PRESSURE OF THE STEAM UPON THE PISTON THROUGHOUT THE STROKE, WHEN CUT OFF IN THE CYLINDER FROM TO, COMMENCING WITH 10 POUNDS AND ADVANCING IN 5 POUNDS UP TO 55 POUNDS PRESSURE. Steam cut off in the Cylinder. Pressure in Pounds at the Commencement of the Stroke. 10 15 20 25 30 35 40 45 50 55 Average Pressure in Pounds upon the Piston. |