Thermal problems in the mechanical design of steam turbines

Abstract

Progress in the design of steam turbines has brought about marked decreases in thermal consumption. The internal efficiency of the turbine has been increased by improvements in the steam path and by reduction in mechanical losses through attention to detail. The rearrangement of the flow path, involving a study of fluid dynamics is promising and will undoubtedly receive emphasis in the future. Considerable attention has also been given to an improvement external to the turbine design proper, by employing the regenerative and the reheating cycles, as well as by increasing the operating pressures and temperatures. Increasing the operating pressures, without a corresponding temperature rise introduces complications from moisture in the exhaust. Notwithstanding, pressures have risen so that large machines generally fall into either the 4.50~-600 lb./in2 or the 1,250 lb./in.2 design ranges. Moisture difficulties have been overcome by resuperheating, by adding moisture drains and by shielding the low pressure blades with materials that will resist erosion. Definite increase in operating temperatures has been in abeyance through lack of knowledge of our engineering materials. Thermodynamically, however, this field possesses possibilities, for with a rise of only 60” F. in temperature, a decrease in heat consumption of about 2 per cent. is possible on a central station turbine. Design at present is centered about throttle temperatures of 850” F. with projected designs for as high as 1,000~ F. With the more severe operating conditions have come increased sizes and speeds. Whereas five years ago single cylinder units of 6,000 kw. were the limit at 3,600 r.p.m., it is not uncommon to design 10,000 kw. single cylinder units