Steam Engines Machinery's Reference Series, Number 70

Excerpt

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.

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. shows a longitudinal section through an engine of simple design, and illustrates the principal parts and their relation to one another.

Fig. 1. Longitudinal Section through the Ames High-speed Engine

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. The balance wheel or flywheel J carries the crank past the dead centers at the ends of the stroke, and gives a uniform motion to the shaft. The various parts of the engine are carried on a rigid bed K, usually of cast iron, which in turn is bolted to a foundation of brick or concrete. The power developed is taken off by means of a belted pulley attached to the main shaft, or, in certain cases, in the form of electrical energy from a direct-connected dynamo.

When in action, a certain amount of steam (1?4 to 1?3 of the total cylinder volume in simple engines) is admitted to one end of the cylinder, while the other is open to the atmosphere. The steam forces the piston forward a certain distance by its direct action at the boiler pressure. After the supply is shut off, the forward movement of the piston is continued to the end of the stroke by the expansion of the steam. Steam is now admitted to the other end of the cylinder, and the operation repeated on the backward or return stroke.

Fig. 2. Section of Cylinder, showing Slide Valve

An enlarged section of the cylinder showing the action of the valve for admitting and exhausting the steam is shown in . In this case the piston is shown in its extreme backward position, ready for the forward stroke. The steam chest L is filled with steam at boiler pressure, which is being admitted to the narrow space back of the piston through the valve N, as indicated by the arrows. The exhaust port M is in communication with the other end of the cylinder and allows the piston to move forward without resistance, except that due to the piston-rod, which transfers the work done by the expanding steam to the crank-pin. The valve N is operated automatically by a crank or eccentric attached to the main shaft, and opens and closes the supply and exhaust ports at the proper time to secure the results described....