1. General

There are two basic types of mechanical draft cooling towers; forced draft and induced draft. A drive train for a typical induced draft tower is depicted schematically in Fig. A. The electric motor is usually located outside the fan stack away from the air flow. This practice makes the drive shaft length nearly equal to one half the fan diameter. The purpose of the drive shaft in mechanical draft cooling towers is to transmit mechanical power usually from an electric motor to a gear reducer. The coupling must also accommodate parallel, angular, and axial misalignment between the motor shaft and gear reducer shaft. The use of larger and more efficient mechanically induced draft cooling tower fans has given rise to the use of long, single-piece fan gear box input shafts. This type of shaft eliminates the maintenance nuisance of intermediate shaft bearings with a potential thermal expansion problem. This paper discusses the advantage of using the single-piece coupling shaft, the design and application considerations necessary to successfully select the proper shaft design, and reviews one electric utility's experience including the potential thermal expansion problem.

2. Introduction

Over the past three decades, mechanically drafted cooling towers increased in cell size due to economies of scale. The size of induced draft fans used on such towers trended from around 14 feet in diameter to diameters in the 30-to 40- foot range. Traditionally, these fans were mounted in right angle, speed reducing gear boxes which were driven via solid or small diameter tubular drive or coupling shafts by horizontal motors mounted outside the fan stacks. Larger fans required longer shafts which led to the use of intermediate drive shaft support bearings, and frequently, intermediate flexible couplings (see below figure A). Several cooling tower designers selected long, single-piece, hollow, large diameter flexible coupling drive shafts see below figure B) as a more reliable alternative to this evolution of the more traditional slender shafts. Early installation with large fans utilized drive shaft systems similar to that depicted in figure A. These installations were plagued with intermediate support bearing failures along with occasional shaft or intermediate coupling failures and associated fan damage. Fan damage caused by flying sections of shafting was reduced by the addition of shaft guards. Refinements in intermediate bearing lubrication systems reduced but did not eliminate bearing failures. In short, the maintenance headaches associated with these installations were causing unnecessary cell down time. With respect to cooling towers that rely on mechanical draft cooling towers, reductions in tower capability at the peak ambient temperature time translated directly into costly curtailment of plant operation. This was and is particularly a problem in some cooling towers where many of cooling tower with summer peak loads. The use of larger diameter single-piece drive shafts circumvented the above problems by deriving their support only from the motor and the gear box without requiring additional bearings. The Design and Application section below elaborates on these and other advantages. However, single-piece drive shafts are not without their own idiosyncrasies. The problem is dependent on tower type (cross flow or counter flow), and types of fill used, and the height of the tower. All are related to the thermal expansion.