E-Book, Englisch, 280 Seiten, Format (B × H): 191 mm x 235 mm
A Practical Guide for Planning, Selecting and Designing of Shell and Tube Exchangers
E-Book, Englisch, 280 Seiten, Format (B × H): 191 mm x 235 mm
ISBN: 978-0-12-803822-2
Verlag: Academic Press
Format: EPUB
Kopierschutz: 6 - ePub Watermark
Users will learn how to calculate heat transfer coefficients for convective heat transfer, condensing, and evaporating using simple equations. Dew and bubble points and lines are covered, with all calculations supported with examples.
This practical guide is designed to help engineers solve typical problems they might encounter in their day-to-day work, and will also serve as a useful reference for students learning about the field. The book is extensively illustrated with figures in support of the text and includes calculation examples to ensure users are fully equipped to select, design, and operate heat exchangers.
- Covers design method and practical correlations needed to design practical heat exchangers for process application
- Includes geometrical calculations for the tube and shell side, also covering boiling and condensation heat transfer
- Explores heat transfer coefficients and temperature differences
- Designed to help engineers solve typical problems they might encounter in their day-to-day work, but also ideal as a useful reference for students learning about the field
Zielgruppe
<p>Chemical engineers, mechanical engineers, process engineers, chemical and mechanical engineering students</p>
Autoren/Hrsg.
Fachgebiete
Weitere Infos & Material
1.0 Heat exchanger design
2.0 Calculation of the temperature differences LMTD and CMTD
3.0 Calculation of the heat transfer coefficients and pressure losses in convective heat transfer
4.0 Geometrical heat exchanger calculations
5.0 Dimensionsless characterisation numbers for heat transfer
6.0 Overall heat transfer coefficient and temperature profile of a heat exchanger
7.0 Process engineering calculations
8.0 Design of condensers
9.0 Design of evaporators
10.0 Design of thermosiphon evaporators
11.0 Double tube-, plain tube- and cross-flow heat exchanger
12.0 Finned-tube heat exchanger
Chapter 2 Calculations of the Temperature Differences LMTD and CMTD
Abstract
First the logarithmic mean temperature difference (LMTD) for equipments with ideal countercurrent stream is derived. In multipass heat exchangers with nonideal countercurrent stream, the LMTD must be converted to the corrected effective mean temperature difference (CMTD) with the temperature efficiency factor F. It is shown how the temperature efficiency factor F can be graphically determined or how the CMTD can be calculated. Also, the effect of the bypass streams on the temperature difference LMTD and the determination of the mean weighted temperature difference for curved condensation curves are explained. Finally, the heat exchanger outlet temperature for a given heat exchanger is determined. Keywords
Bypass stream; Corrected effective mean temperature difference (CMTD); Logarithmic mean temperature difference (LMTD); Mean weighted temperature difference (WMTD); Outlet temperatures; Temperature efficiency factor (F) Contents 2.1 Logarithmic Mean Temperature Difference for Ideal Countercurrent Flow 21 2.2 Corrected Temperature Difference for Multipass Heat Exchanger 22 2.3 Influence of Bypass Streams on LMTD 29 2.4 Mean Weighted Temperature Difference 30 2.5 Determination of the Heat Exchanger Outlet Temperatures 32 2.5.1 Calculation of the outlet temperatures in a multipass heat exchanger under consideration of the temperature efficiency factor F for nonideal countercurrent flow 33 2.5.1.1 Calculation of the outlet temperature Tc2 of the cold medium 33 2.5.2 Calculation of the outlet temperature th2 for ideal countercurrent without F 33 References and Further Reading 35 2.1. Logarithmic Mean Temperature Difference for Ideal Countercurrent Flow
The logarithmic mean temperature difference (LMTD) for ideal countercurrent flow is determined from the two temperature differences ?t1 and ?t2. =?t1-?t2ln?t1?t2?t1=T1-t2?t2=T2-t1 T1 = shell-side inlet temperature (°C) T2 = shell-side outlet temperature (°C) t1 = tube-side inlet temperature (°C) t2 = tube-side outlet temperature (°C) Example 1: Calculation of the logarithmic mean temperature difference
Figure 2.1 Heat exchanger TEMA—type E with two tube passes. 2.2. Corrected Temperature Difference for Multipass Heat Exchanger
In Chapter 1, it was already pointed out that with a multipass heat exchanger, there is no ideal countercurrent but a mixture of co- and countercurrent flow. This makes the effective temperature gradient worse. In Figures 2.2 and 2.3, temperature efficiency diagrams are shown, it is clear that the driving temperature difference in the one-pass equipment with ideal countercurrent is better than in the two-pass heat exchanger. Procedure for the determination of the corrected effective mean temperature difference (CMTD) for heat exchangers with nonideal countercurrent flow: First, the LMTD is calculated from the inlet and outlet temperatures on the tube and shell side. With nonideal countercurrent, the LMTD must be corrected with a temperature efficiency factor F. =F×LMTD F = temperature efficiency factor LMTD = logarithmic mean temperature difference (°C) for countercurrent flow CMTD = corrected effective mean temperature difference (°C) for multipass heat exchangers
Figure 2.2 Temperature profile as function of the heat load in a one-pass heat exchanger with ideal countercurrent flow. The temperature efficiency factor F can be calculated for a heat exchanger shell type E according to TEMA using the following equation: =(R2+1R-1)ln[(1-Pz)/(1-RPz)]ln[(2/Pz)-1-R+R2+1(2/Pz)-1-R-R2+1]Pz=1-(RP-1P-1)1/NR-(RP-1P-1)1/N
Figure 2.3 Temperature profile as function of the heat load in a two-pass heat exchanger with co- and countercurrent flow. =t2-t1T1-t1R=T1-T2t2-t1 N = number of heat exchangers in series The following table shows how large the temperature approach or a temperature cross deteriorates the CMTD, and that one can improve the temperature efficiency factor with multiple heat exchangers in series. 58 42 15 35.1 1 24.9 0.906 22.6 58 42 20 40.1 1 19.9 0.844 16.8 58 42 25 45.1 1 14.9 0.670 10 Table Continued 58 42 25 45.1 2 14.9 0.936 13.9 58 42 30 50.1 2 9.8 0.840 8.2 58 42 35 55.1 4 4.7 0.818 3.8 58 42 37 57.1 6 2.4 0.631 1.5 58 42 37 57.1 7 2.4 0.764 1.8 Corollary: 1. With large approach of temperatures at the tube and shell side, the temperature efficiency factor F and hence the CMTD deteriorates. 2. Through the arrangement of multiple heat exchangers one after the other, one can improve the temperature efficiency factor F because with this arrangement one approaches the countercurrent flow. 3. A large temperature cross is only possible with multiple equipments in series. The temperature efficiency factor F can also be determined graphically using the diagrams in Figure 2.4 with the calculation variables P and R. Alternatively, the CMTD in multipass heat exchangers can be determined with the calculation variables O and M. =MlnO+MO-M(°C)O=(T1-t2)+(T2-t1)M=(T1-T2)2+(t2-t1)2 Example 2: Calculation of LMTD and CMTD for the heat exchangers in Figures 2.2 and 2.3 Shell inlet temperature T1 = 80 °C Shell outlet temperature T2 = 50 °C Tube inlet temperature t1 = 20 °C Tube outlet temperature...
