STEAM ENGINES 193 



The piston speeds used in modern practice are about as 



Small stationary engines ...... ........ 300 to 600 



Large stationary engines .............. 600 to 1,000 



Corliss engines ....................... 400 to 750 ' 



Marine engines ....................... 200 to 1,200 



Allowance for Area of Piston Rod. It is generally consid- 

 ered sufficiently accurate to take the total area of one side of 

 the piston as the area to be used in calculating the horsepower 

 of an engine. The effective area of one side of the piston is, 

 however, reduced by the sectional area of the piston rod, and 

 if it is important that the power be calculated with the greatest 

 practical degree of accuracy, an allowance for the area of 

 the piston rod must be made. This is done by taking as the 

 piston area one-half the sum of the areas exposed to steam 

 pressure on the two sides of the piston. Thus, if a piston is 

 30 in. in diameter with a 6-in. piston rod, the average area is 



302 x .7854 + (302 X .7854 - 62 X .7854) 



- - = 692.72 sq. in. If the 



piston rod is continued past the piston so as to pass through the 

 head-end cylinder head, that is, if the piston has a tailrod, 

 allowance must be made for the tailrod. Thus, with a piston 

 30 in. in diameter, a piston rod 6 in. in diameter, and a tailrod 

 5 in. in diameter, the average area is 



(302 x .7854 - 52 X .7854) + (3Q2 X .7854 - 6* X .78.54) 



- - - =682.9sq.in. 



Stating Sizes of Engines. The size of a simple engine, that 

 is, an engine having but one cylinder, is commonly stated by 

 giving the diameter of the cylinder, followed by the length of 

 the stroke, both in inches. Thus, a simple engine having a 

 cylinder 12 in. in diameter and a stroke of 24 in. would be 

 referred to as a 12"X24" engine, the multiplication sign in 

 this case serving merely to separate the two numbers. The 

 sizes of compound and multiple-expansion engines are desig- 

 nated in a similar fashion. Thus, a compound engine with a 

 high-pressure cylinder 11 in. in diameter, a low-pressure 



