Unit 7 Heat transfer

Unit7 CAD/CAM /CAPPCAD/CAM is a term which means computer-aided design and compuer-aided manufacturing. It isCAD/CAM这个词条的意思是计算机辅助设计和计算机辅助制造。

它是在设计和制造中运用the technology concerned which the use of digital computers to perform certain functions in数字化计算机执行某些职能的技术。

这项技术正在向设计制造更高的一体化发展。

design and production. This technology is moving in the direction of greater integration of design这两个在生产企业中被看作是截然不同的，相互分离的。

最终，cad/cam将会提供给未来的计算机集成化工厂技术基础。

and manufacturing, two activities which have traditionally been treated as distinct and separate functions in a production firm. Ultimately, CAD/CAM will provide the technology base for the computer-intgrated factory of the future.Computer-aided design(CAD) can be defined as the use of computer systems to assist in the计算机辅助设计可以定义为使用计算机系统来协助一个设计方案的形成，修改，分析及优化creation, modification, analysis, or optimization of a design. The computer systems consist of the计算机系统由硬件和软件组成来进行公司特定用户需要的专门设计功能。

Unit 13 Principles of heat transferPractically all the operations that are carried 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.Nature of heat flowWhen 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.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 correspondence between thermal conductivity and electrical conductivity. Insolids 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.Convection When a current of macroscopic particle of fluid crosses a specific surface such as the boundary of a control volume, it carries with it a definite quantity of enthalpy. Such a flow of enthalpy is called a connective flow of heat or simply convection. Since convection is a macroscopic phenomenon, it can occur only when forces act on the particle or stream of fluid and maintain its motion against the force of friction. Convection is closely associated with fluid mechanics. In fact, thermodynamically, convection is not considered as heat flow but as flux of enthalpy. The identification of convection with heat flow is a matter of convenience,because in practice it is difficult to separate convection form true conduction when both are lumped together under the name convection. Examples of convection are the transfer of enthalpy by the eddies of turbulent flow and by the current of warm air flowing across and away from an ordinary radiator.Natural and forced convection The forces used to create convection currents in fluids are of two types. If the currents are the result of buoyancy forces generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called nature convection. The flow of air across a heated radiator is an example of natural convection. If the currents are set in motion by the action of a mechanical divide such as a pump or agitator, the flow is independent of density gradients and is called forced convection. Heat flow to a fluid pumped through a heated pipe is an example of forced convection. The two kinds of force may be active simultaneously in the same fluid, and natural andforced convection then occur together.Radiation Radiation is a term given to the transfer of energy through space by electromagnetic waves. If radiation is passing through empty space, it is not transformed into heat or any other form of energy nor is it diverted from its path. If, however, matter appears in its path, the radiation will be transmitted, reflected, or absorbed. It is only the absorbed energy that appears as heat, and this transformation is quantitative. For example, fused quartz transmits practically all the radiation that strikes it；a polished opaque surface will absorb most of the radiation received by it and will transform such absorbed energy quantitatively into heat.Monatomic and diatomic gases are transparent to thermal radiation, and it is quite common to find that heat is flowing through masses of such gases both by radiation and by conduction-convection. Examples are the loss of heat from a radiator or unlagged steam pipe to the ambient air of the room and heat transfer in furnaces and other high-temperature gas-heating equipment. The two mechanisms are mutuallyindependent and occur in parallel, so that one type of heat flow can be controlled or varied independently of the other. Conduction-convection and radiation can be studied separately and their separate effects added together in cases where both are important. In very general terms radiation becomes important at high temperature and is independent of the circumstances of the flow of the fluid. Conduction-convection is sensitive to flow conditions and is relatively unaffected by temperature level.2Rate of heat transferHeat flux Heat-transfer calculations are based on the area of the heating surface and are expressed in Btu per hour per square foot( or watts per square meter) of surface through which the heat flow. The rate of heat transfer per unit area is called the heat flux. In many types of heat-transfer equipment the transfer surface are constructed from tubes or pipes. Heat flux may then be based either on the inside area or the outside area of the tubes. Although the choice is arbitrary, it must be clearly stated, because the numerical magnitude of the heat fluxes will not be thesame for both.Average temperature of fluid stream When a fluid is being heated or cooled, the temperature will vary throughout the cross section of the stream. If the fluid is being heated, the temperature of the fluid is a maximum at the wall of the heating surface and decreases toward the center of the stream. If the fluid is being cooled, the temperature is a minimum at the wall and increase toward the center. Because of these temperature gradients throughout the cross section of the stream, it is necessary, for definiteness, to state what is meant by the temperature of the stream. It is agreed that it is the temperature that would be attained if the entire fluid stream flowing across the section in question were withdrawn and mixed adiabatically to a uniform temperature. The temperature so defined is called the average or mixing-up stream temperature.。

Unit 17 Heat Transfer Process传热过程The formation of polyvinyl chloride is an exothermic reaction. 形成聚氯乙烯是放热反应。

One of the functions of the reactor vessel is to provide the cooling surface for heat removal . 一个职能的反应堆容器是提供冷却散热The reactor size is limited by the ratioof reactor volume to cooling surface and the agitation system needed to maintain the monomer-water mixture at the proper droplet size .反应器的尺寸是有限的比例，反应器体积的冷却和搅拌系统需要保持水混合在适当的雾滴尺寸。

Commercial reactors of 25000-to 50000-gal capacity are standard for suspension polymerization industry .商用反应堆25000到50000gal能力是悬浮聚合行业标准。

The reactor productivity will depend on the heat transfer capacity of the vessel. 反应堆生产力将取决于传热能力的体现The higher the heat transfer capacity, the more polymer that can br made per unit of time.较高的传热能力，更聚合物，可使每单位时间。

Reflux condenser utilization 1回流冷凝器的利用。