Document KJb01VdwgNjEgyeG1be5Kg25o

100 CHAPTER 4 1950 Guide [I] Fluid Meters, Their Selection and Installation (American Society of Mechanical Engineers, 1933). [J] The Orifice Meter for Measurement of Flow of Gases and Liquids, by Allen D. MacLean (Pittsburgh Equitable Meter Co., 1938). {K] Flow Measurement (American Society of Mechanical Engineers, 1940). [L] Pitot Tube Practice, by Edward S. Cole (A.S.M.E. Transactions 57,1935, 281294; Discussion, idem. 58, 1936, 146-156). [M] Pitot Tubes in Large Pipes, by Edward S. Cole and E. Shaw Cole (A.S.M.E. Transactions, 61,1939, 465-473; Discussion, idem. 61,1939, 473-475). [N] Investigation of Errors of Pitot Tubes, by C. W. Hubbard (A.S.MJ2. Trans actions, 61, 477-497; Discussion, idem. 61, 1939, 497-506). [O] Piping Arrangements for Acceptable Flowmeter Accuracy, by R. E. Sprenkle (A.S.MJJ. Transactions, 67, 345-357, 1945; Discussion, idem. 67, 357-360, 1945). CHAPTER 5 HEAT TRANSFER duction Convection, Radiation; Equations for Conduction, Convection, RadiLn ation and Combined Convection and Radiation; Heat-Flow Resistance, in Series and Parallel; Practical Heat Transfer Problems HEAT is that form of energy that is transferred by virtue of an existing temperature difference. The temperature difference is the potential which causes the transfer, the latter in turn being resisted by the thermal properties of the material combined in a single term known as the resis tance Energy exchange associated with evaporation, condensation, etc., is treated elsewhere such as in the section on cooling tower design in Chapter 34. The objectives of this chapter are to: 1 Describe the mechanisms and present the rate equations for the different modes of heat transfer. 2 Illustrate the application of the basic concepts to steady-state problems (tem perature independent of time or acyclic variable thereof) by means of several typical solutions of heat transfer systems. Further applications to specific systems will be found throughout The Guide. CONDUCTION, CONVECTION AND RADIATION Thermal conduction is the term applied to the mechanism of heat trans fer whereby the molecules of higher kinetic energy transmit part of their energy to adjacent molecules of lower kinetic energy by direct molecular action. Since the temperature is proportional to the average kinetic energy of the molecules, thermal transfer will occur in the direction of decreasing temperature. The motion of the molecules is random; there is no net material flow associated with the conduction mechanism. In the case of flowing fluids, thermal conduction is significant in the region very close to a solid boundary or wall, for in this region the flow is laminar, parallel with the wall surface, and there are practically no cross currents in the direction of the heat transfer across the solid fluid boundary. In solid bodies the significant mechanism of heat transfer is always thermal conduction. Contrasted to the thermal conduction mechanism, thermal convection involves energy transfer by eddy mixing and diffusion1 in addition to conduction. This is shown schematically in Fig. 1 which exhibits transfer from a pipe wall at surface temperature U to a colder fluid at a bulk tem perature tu (Bulk temperature is that which would be attained if the fluid stream were drawn off at a certain section and mixed. It is therefore slightly higher than the lowest temperature in the stream). In the laminar sublayer, immediately adjacent to the wall, the heat transfer occurs by thermal conduction; in the transition region, which is called the buffer layer, eddy mixing as well as conduction effects are significant; in the eddy or turbulent region the major fraction of the transfer occurs by eddy mixing. In most commercial equipment the main body of the fluid is in turbu lent flow, and the laminar film exists at the solid walls only, as shown in 101