INTRODUCTION

The purpose of this chapter is to outline the thermal considerations responsible for preparing a thermal design specification sufficient that selection and mechanical design can be completed. The object is not, therefore, to prepare a detailed description of the thermal design process. Such description is available in a number of excellent texts, to which the reader should refer. However, it is necessary to provide a brief explanation of some of the more important aspects of design and design choice, so that any differences between the decisions made by turbine builders can be appreciated. It is hoped this allows a more informed evaluation of alternate offerings that are made to the buyer at those times he is evaluating a number of bids and having to choose between them to award contracts.

The steam turbine is a heat engine. It is an engine designed specifically to convert the thermal potential energy of steam to rotational kinetic energy, which can then be utilized to drive a generator or other machine and undertake mechanical work. With current and future anticipated fuel costs, it is essential this energy conversion process is undertaken as efficiently as possible, and for this reason there is considerable pressure applied to current manufacturers to produce units at acceptable costs to undertake this conversion as efficiently and reliably as possible. It can be shown that relatively small changes in element design, arrangement or the incorporation of small changes in cycle detail can, if they introduce an increase in the efficiency...