Đang chuẩn bị liên kết để tải về tài liệu:
Encyclopedia of Global Resources part 125

Encyclopedia of Global Resources part 125 provides a wide variety of perspectives on both traditional and more recent views of Earth's resources. It serves as a bridge connecting the domains of resource exploitation, environmentalism, geology, and biology, and it explains their interrelationships in terms that students and other nonspecialists can understand. The articles in this set are extremely diverse, with articles covering soil, fisheries, forests, aluminum, the Industrial Revolution, the U.S. Department of the Interior, the hydrologic cycle, glass, and placer mineral deposits. . | 1146 Steam engine Global Resources gines operate at much higher pressure than Watt s they are similar in design. Principles of Operation Early steam engines would be considered upsidedown by modern standards. The piston was connected to a rod that emerged from the top of the engine and steam was fed into the cylinder below the piston. A chain connected the piston rod to one end of a pivoted beam suspended above the engine and the other end of the beam was connected to a pump that drew water up from the bottom of a mine. The weight of the pump rod was sufficient to pull the pump end of the pivoted beam downward which caused the other end of the beam to rise and lift the piston upward. As the piston rose steam at just above atmospheric pressure flowed from the boiler into the growing space below the piston. When the piston reached the top of its stroke the valve between boiler and cylinder closed and in Newcomen s engine water was sprayed into the cylinder. As the water absorbed heat from the steam the steam condensed which created a partial vacuum. This vacuum combined with atmospheric pressure acting on the upper side of the piston caused the piston to move downward. When the piston reached the bottom of its stroke the steam valve opened again. The steam pressure balanced the atmospheric pressure on the other side of the piston and the weight of the pump rod again raised the piston to the top of its stroke. Watt recognized that spraying cold water directly into the cylinder not only condensed the steam but also cooled off the cylinder itself. On the next stroke some incoming steam was wasted in reheating the cylinder. The separate condenser in Watt s engine condensed the steam without chilling the cylinder. This resulted in a dramatic improvement in fuel consumption. Watt also closed off the upper side of the piston and provided a hole just big enough for the piston rod to pass out. Constant-pressure steam was admitted to the space above the piston and this steam .