Steam Engines

by Anonymous

These quantities vary in different types of engines, but in the absence of more specific data the values in [Table III] will be found useful. The back pressure may be taken as 17 pounds per square inch, absolute, for non-condensing engines, and as 3 pounds for condensing engines as previously stated.

TABLE III. PRESSURE, CLEARANCE, CUT-OFF AND PISTONSPEED OF STEAM ENGINES
Type of Engine	Initial Pressure, (Gage)			Clearance, Per Cent			Cut-off, Proportion of Stroke			Piston Speed, Feet per Minute
Simple high-speed	80	to	90	4	to	10	1⁄4	to	1⁄3	600	to	800
Simple medium-speed	80	to	90	3	to	8	1⁄4	to	1⁄3	500	to	700
Simple Corliss	80	to	90	1.5	to	3.5	1⁄4	to	1⁄3	400	to	600
Compound high-speed	130	to	140	4	to	10	1⁄10	to	1⁄8	600	to	800
Compound medium-speed	130	to	140	3	to	8	1⁄10	to	1⁄8	500	to	700
Compound Corliss	130	to	140	1.5	to	3.5	1⁄10	to	1⁄8	400	to	600

The first step in proportioning the cylinder is to compute the approximate mean effective pressure from the assumed initial pressure, clearance, and cut-off, by the method already explained. Next assume the piston speed for the type of engine to be designed, and determine the piston area by the following formula:

	33,000 H. P.
A =	——————————.
	M. E. P. × piston speed

This formula usually gives the diameter of the piston in inches and fractions of an inch, while it is desirable to make this dimension an even number of inches. This may be done by taking as the diameter the nearest whole number, and changing the piston speed to correspond. This is done by the use of the following equation.

First piston speed × first piston area
——————————————	= new piston speed.
new piston area

In calculating the effective piston area, the area of the piston rod upon one side must be allowed for. The effective or average piston area will then be (2A - a)⁄2, in which A = area of piston, a = area of piston rod. This latter area must be assumed. After assuming a new piston diameter of even inches, its effective or average area must be used in determining the new piston speed. The length of stroke is commonly proportioned to the diameter of cylinder, and the piston speed divided by this will give the number of strokes per minute.

Example:—Find the diameter of cylinder, length of stroke, and revolutions per minute for a simple high-speed non-condensing engine of 200 I. H. P., with the following assumptions: Initial pressure, 90 pounds gage; clearance, 7 per cent; cut-off, 1⁄4; piston speed, 700 feet per minute; length of stroke, 1.5 times cylinder diameter.

By using the rules and formulas in the foregoing, we have:

M. E. P. = (90 + 15) × 0.63 - 17 = 49 pounds.