tailieunhanh - Heat Transfer Handbook part 105

Heat Transfer Handbook part 105. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | INTRODUCTION 1037 It may be noted that objectives 1 2 and 4 yield savings in operating or energy costs and objective 3 lends to material savings and reduced capital costs. These objective functions and constraints have been described by many different performance evaluation criteria PEC in the literature Bergles et al. 1974a b Webb 1981 1994 Bergles 1998 for exmmple . It is instructive to consider some of these CEC for the more common case of applying enhancement techniques to the conventional shell-and-tube heat eechanger. The heat eechanger performance is represented by two dependent variables heat transfer rate Q and pressure drop Ap or pumping power P which can be eepressed as Q UA ATm Ap 2fL and P Ap di p p Here the primary independent operating variables are the approach temperature difference A Ti A Tm A Ti and the mass flow rate m and in the case of the tubular geometry the design variables heat transfer surface area A or exchanger iee are the diameter di and length L of tubes and number ff Hilees N per pass. PEC arc estah-lished for the process stream of interest by selecting one of the operational variables for the performance objective and applying the design constraints on the remaining variables. For single-phase flow heat transfer inside enhanced and smooth tubes of the same envelope diameter PEC for 12 different cases H in i by Bergles 1998 and Webb 1994 are listed in Table . They represent criteria for comparing the TABLE Performance Evaluation Criteria for Single-Phase Forced Convection in Enhanced Tubes of Same Envelope Diameter di as the Smooth Tube Case Geometry Fixed Objective m P Q ATi FG-1a N L X X Q t FG-1b N L X X ATi 4 FG-2a N L X X Qt FG-2b N L X X ATi 4 FG-3 N L X X P 4 FN-1 N X X X L 4 FN-2 N X X X L4 FN-3 N X X X P4 VG-1 X X X X NL a 4 VG-2a NL a X X X Qt VG-2b NL a X X X ATi 4 VG-3 NL a X X X P4 The product of N and L. 1038 HEAT TRANSFER ENHANCEMENT enhanced performance on the basis of the following three broad