进程装备与操纵工程专业英语.pdfVIP

Unit 13 Principles of Heat Transfer Practically all the operations that are ca ied out by the chemical engineer involve the production or absorption of energy in the form of heat. The laws governing the transfer of heat and the types of apparatus that have for their main object the control of heat flow are therefore of great importance. 事实上，所有的由化学工程师进行的操作都要涉及热量的产生和吸收。因此，操纵传热的定律和以操纵热流为要紧目的的仪器 类型都是很重要的。 1. Nature of Heat Flow When two objects at different temperatures are brought into thermal contact, heat flows from the object at the higher temperature to that at the lower temperature. The net flow is always in the direction of the temperature decrease. The mechanisms by which the heat may flow are three, conduction, convection, and radiation. 当两种不同温度的物体开始接触后，热流就会从高温物体传给低温物体。净热流老是随着温度降低的方向。传热的机理通常分 三种：热传导，热对流，热辐射。 Conduction If a temperature gradient exists in a continuous substance, heat can flow unaccompanied by any observable motion of matter. Heat flow of this kind is called conduction. In metallic solids, thermal conduction results from the motion of unbound electrons, and there is close co espondence between thermal conductivity and electrical conductivity. In solids which are poor conductors of electricity, and in most liquids, thermal conduction results from the transport of momentum of individual molecules along the temperature gradient. In gases conduction occurs by the random motion of molecules, so that heat is diffused from hotter regions to colder ones. The most common example of conduction is heat flow in opaque solids, as in the brick wall of a furnace or the metal wall of a tube. 若是在物质表面存在温度梯度，那么热量能够不伴随着任何可观看的运动形式而流动。这种热量流动叫做热传导。在金属体中， 热传导是由于自由运动的电子产生的，因此在导热和导电之间存在着相似性。在不导电的固体和大多数的液体中，热传导是由个别 分子沿着温度梯度传递而产生的。在气体中，传热主若是由分子的无规则的运动而产生的，因此热流是由热区域扩散到低温区域。 象炉子的砖壁和管件的金属壁如此的不透明固体中的最多见的传热形式是热传导。 Convection When a cu ent of macroscopic p