tailieunhanh - ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 1 Part 2

Tham khảo tài liệu 'arnold, k. (1999). design of gas-handling systems and facilities (2nd ed.) episode 1 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Heat Transfer Theory 11 Multiple Transfer Mechanisms Most heat transfer processes used in production facilities involve combinations of conduction and convection transfer processes. For example in heat exchangers the transfer of heat energy from the hot fluid to the cold fluid involves three steps. First the heat energy is transferred from the hot fluid to the exchanger tube then through the exchanger tube wall and finally from the tube wall to the cold fluid. The first and third steps are convection transfer processes while the second step is conduction process. lb calculate the rate of heat transfer in each of the steps the individual temperature difference would have to be known. It is difficult to measure accurately the temperatures at each boundary such as at the surface of the heat exchanger tube. Therefore in practice the heat transfer calculations are based on the overall temperature difference such as the difference between the hot and cold fluid temperatures. The heat transfer rale is expressed by the following equation similar to the conductive convec tive transfer process q UA AT 2-3 where q overall heat transfer rate Btu hr Ư - overall heat transfer coefficient Btu hr-ft2-c F A - heat transfer area ft2 AT - overall temperature difference F Examples of overall heat transfer coefficient and overall temperature difference calculations are discussed in the following sections. Overall Temperature Difference The temperature difference may not remain constant throughout the flow path. Plots of temperature vs. pipe length for a system of two concentric pipes in which the annular fluid is cooled and the pipe fluid heated are shown in Figures 2-2 and 2-3. When the two fluids travel in opposite directions as in Figure 2-2 they are in countercurrent flow. When the fluids travel in the same direction as in Figure 2-3 they are in co-current flow. The temperature of the inner pipe fluid in either case varies according to one curve as it proceeds along the length of .