The ports and passages must be large enough to allow the steam to follow up the advancing piston without loss of pressure. The maximum allowable velocity of the steam in the passages, when they are short, is about 160 feet per second. But, with the ordinary ratio between the length of connecting rod and length of crank, the average velocity is about 5% of the maximum. Hence, the allowable average velocities are 100 to 125 feet per second for long and short passages, respectively. S = average piston speed in feet per second. v = average velocity of steam in feet per second. Then, area of port X velocity of steam = area of piston X velocity of piston. Take v Take v = or 1 b v AS, AS whence 1 b = —. V 100 for long indirect passages. The constant 100 may be used for v, when designing plain slide valve engines of the ordinary type which cut off late in the stroke, and 125 may be used for high speed engines with early cut-off, and for the Corliss type. The area of the exhaust port or ports may be from 1% to 2% times the area of a steam port. The area of the cross-section of the steam pipe is approximately equal to the area of the steam port; likewise, the area of the exhaust pipe should be equal to that of the exhaust port. The length of the port may be .6D to .9 D for slide valve engines, and about .9 D to D for the Corliss type. The height w, Fig. 1, of the valve seat must be such that the area of the most contracted part of the exhaust port is not less than 75% of the area of the steam port. THE STEAM CHEST. Fig. 2 shows a steam chest for the cylinder illustrated in Fig. 1. The principal dimensions are to be determined by the follow ing proportions, which are based upon the thickness of the cylinder walls, and upon the travel and dimensions of the valve : a = length of valve + travel of valve + twice the clearance between the valve and the steam chest at ends of valve travel. b = breadth of valve + twice the clearance between one end of valve and steam chest. C = .75 i. FIG. 2. d = 2.75 o, where o is the nominal diameter of the steam chest bolts, as in Fig. 1. e = .04 A+.125" for all areas above 100 sq. in. A' = area f = 1.3 e. t = .85 1. = 2.5 i. NOTE.-When the area of the steam chest cover is less than 100 square inches, its thickness e may be equal to i. When the area of the steam chest cover exceeds 600 square inches, the height of the ribs should be 3.5 i, and their number should be increased. Fig. 3 shows a design for a steam chest cover when the steam pipe flange is on one side of the steam chest. Determine the thickness e by the same formula and rules as for the cover in Fig. 2. The other dimensions are found as follows: p should never exceed the distance in inches given by the SOLUTION.-Substituting in the formula for p, we have Fig. 4 shows a Corliss engine cylinder which may be designed according to the following proportions: |