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Steam Engines Machinery's Reference Series, Number 70   By:

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First Page:

Machinery's Reference Series

Each Number Is a Unit in a Series on Electrical and Steam Engineering Drawing and Machine Design and Shop Practice

NUMBER 70

STEAM ENGINES

CONTENTS

Action of Steam Engines 3 Rating and General Proportions of Steam Engines 11 Steam Engine Details 15 Steam Engine Economy 30 Types of Steam Engines 36 Steam Engine Testing 41

Copyright, 1911, The Industrial Press, Publishers of MACHINERY, 49 55 Lafayette Street, New York City.

CHAPTER I

ACTION OF STEAM ENGINES

A steam engine is a device by means of which heat is transformed into work . Work may be defined as the result produced by a force acting through space, and is commonly measured in foot pounds; a foot pound represents the work done in raising 1 pound 1 foot in height. The rate of doing work is called power . It has been found by experiment that there is a definite relation between heat and work, in the ratio of 1 thermal unit to 778 foot pounds of work. The number 778 is commonly called the heat equivalent of work or the mechanical equivalent of heat.

Heat may be transformed into mechanical work through the medium of steam, by confining a given amount in a closed chamber, and then allowing it to expand by means of a movable wall (piston) fitted into one side of the chamber. Heat is given up in the process of expansion, as shown by the lowered pressure and temperature of the steam, and work has been done in moving the wall (piston) of the closed chamber against a resisting force or pressure. When the expansion of steam takes place without the loss of heat by radiation or conduction, the relation between the pressure and volume is practically constant; that is, if a given quantity of steam expands to twice its volume in a closed chamber of the kind above described, its final pressure will be one half that of the initial pressure before expansion took place. A pound of steam at an absolute pressure of 20 pounds per square inch has a volume of practically 20 cubic feet, and a temperature of 228 degrees. If now it be expanded so that its volume is doubled (40 cubic feet), the pressure will drop to approximately 10 pounds per square inch and the temperature will be only about 190 degrees. The drop in temperature is due to the loss of heat which has been transformed into work in the process of expansion and in moving the wall (piston) of the chamber against a resisting force, as already noted.

Principle of the Steam Engine

The steam engine makes use of a closed chamber with a movable wall in transforming the heat of steam into mechanical work in the manner just described. Fig. 1 shows a longitudinal section through an engine of simple design, and illustrates the principal parts and their relation to one another.

The cylinder A is the closed chamber in which expansion takes place, and the piston B , the movable wall. The cylinder is of cast iron, accurately bored and finished to a circular cross section. The piston is carefully fitted to slide easily in the cylinder, being made practically steam tight by means of packing rings. The work generated in moving the piston is transferred to the crank pin H by means of the piston rod C , and the connecting rod F . The piston rod passes out of the cylinder through a stuffing box, which prevents the leakage of steam around it. The cross head D serves to guide the piston rod in a straight line, and also contains the wrist pin E which joins the piston rod and connecting rod. The cross head slides upon the guide plate G , which causes it to move in an accurate line, and at the same time takes the downward thrust from the connecting rod.

The crank pin is connected with the main shaft I by means of a crank arm, which in this case is made in the form of a disk in order to give a better balance... Continue reading book >>




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