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David C. Silverman |
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The ability to effectively transfer energy in the form of heat is fundamental to any process in which the temperature is to be maintained at other than that found under ambient conditions. The science that quantifies and predicts the transfer of energy between bodies or fluids that results from a temperature difference has two purposes, (1) to explain how energy is transferred and (2) to predict the rate at which energy is transferred under the defined conditions. Three types of heat transfer exist.
The need for transferring energy can be found in a wide range of systems such as electronic, biological, chemical, and petrochemical systems. Since materials must be specified for the equipment that transfers such energy in the form of heat, virtually all corrosion practitioners are faced at one time or another with considering the impact of heat transfer on corrosion. This energy transfer which usually involves a difference in temperature between the material being specified and the environment can complicate such specification. For example, in the case of a heat exchanger, the temperature at the wall-fluid interface is different from the temperature of the fluid into which or from which energy is being transferred. The bulk temperature is easily measured. But the wall temperature which dictates or at least strongly influences the corrosion rate cannot be measured easily. This temperature and its effect on corrosion have to be predicted. The science of heat transfer provides the theory through which the corrosion practitioner may make this prediction. Unfortunately, the teaching of heat transfer has in recent years been relegated to reduced content in many undergraduate engineering curricula. In addition, many corrosion practitioners have never been exposed to this science. This knowledge is needed to understand the limitations of laboratory simulation. The purpose of this tutorial is to help fill that knowledge gap for the corrosion practitioner.. This tutorial summarizes some important heat transfer fundamentals in systems in which the fluid is moving. The focus is on two types of convection, forced convection and natural convection. The term "forced convection" means that the fluid motion involved in the process under consideration is caused by an externally applied force. The force can be created by a blower, wind, vehicle motion, pump, etc. The term natural convection means that the fluid motion involved in the process is caused by external force fields such as gravity acting on density gradients in the fluid which were caused by the transport process itself. In addition, heat transfer effects have sometimes been ignored in the design of laboratory corrosion tests. Testing at elevated temperatures under isothermal conditions has at times been used to approximate the effect of a hot wall. Routine laboratory tests for reliable corrosion prediction in the presence of heat transfer do not exist. This tutorial provides a brief critique of several of the laboratory tools that have appeared in the literature to evaluate corrosion under heat transfer conditions. The topics covered in this tutorial are:
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David C. Silverman, Ph.D. - Primary Consultant |