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Performance Evaluation Criteria for Two-Phase Heat Exchangers
Published in Ralph L. Webb, Nae-Hyun Kim, Principles of Enhanced Heat Transfer, 2004
A third example is distillation processes, in which heat rejection occurs to cooling water. Heat, in the form of steam, is added to a reboiler and heat is rejected in a water-cooled condenser. Enhanced surfaces may be employed for the same purposes as in the first two examples. Mechanical energy (e.g., compressors or turbines) is not used in the above examples. Performance improvements may be affected by use of enhanced surfaces for three different purposes: Reduced heat transfer surface area for fixed operating temperaturesIncreased heat exchange capacity for fixed amount of heat exchange surface areaReduced LMTD for fixed amount of heat exchange surface area, which will increase the thermodynamic efficiency of the process or cycle
5 Ocean Thermal Energy Harvesting
Published in Alireza Khaligh, Omer C. Onar, Energy Harvesting, 2017
To determine the temperature driving force for heat transfer in a heat exchanger, the LMTD is used. It is the logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. For countercurrent flow, it is expressed as LMTD=(T1-t2)-(T2-t1)ln((T1-t2)/(T2-t1)).
Thermal Principles Relevant to Equipment and Systems
Published in T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl, Heating and Cooling of Buildings, 2016
T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl
The LMTD is the logarithmic average temperature difference between the two fluid streams. In simple heat exchangers without the change of phase, it is given by LMTD=ΔT1−ΔT2ln(ΔT1/ΔT2)
Detailed evaluation of a heat exchanger in terms of effectiveness and second law
Published in Journal of Turbulence, 2022
Two types of problems are encountered in the analysis of heat exchangers. The first type of problem is the calculation of the dimensions of the heat exchangers. It is quite easy to find the dimensions of the heat exchanger with the logarithmic mean temperature difference (LMTD) method when the mass flow rates and the inlet and outlet temperatures of hot and cold fluids are known or can be deduced from the energy balance. The second type of problem is to find the outlet temperatures of hot and cold fluids in the heat exchanger, heat transfer rate, mass flow rates and inlet temperatures are known. The LMTD method can be used to solve this problem, but it is not useful because it requires many iterations. In 1955, Kays and London developed the e-NTU method, which significantly simplifies the solution of such problems [49].
Methods to Evaluate Heat Transfer Enhancement in an Ultrasonic Heat Exchanger
Published in Heat Transfer Engineering, 2020
Mathieu Legay, Odin Bulliard-Sauret, Sébastien Ferrouillat, Primius Boldo, Nicolas Gondrexon
As a first approximation, only inlet and outlet temperatures were known, and straight lines were drawn on the temperatures evolution graphs. In this section, the log mean temperature difference (LMTD) is used to evaluate more precisely the average temperature difference between the two fluids along the heat exchanger length. A first graph in Figure 14 plots the LMTD as a function of the cold water flow rate in the annular space for a constant hot fluid flow rate in the central pipe. The trend shown by this curve is very typical, as was confirmed by drawing similar graphs for all range of hot fluid flow rates.
Passage Arrangement Optimization of Plate-Fin Heat Exchanger under Uncertain Operating Conditions
Published in Heat Transfer Engineering, 2023
Qilong Gao, Congda Lu, Xiang Peng, Jiquan Li, Shaofei Jiang
The basic assumption of the LMTD method is that the fluid properties do not change during the heat transfer process [31]. Under small temperature and flow fluctuations, the heat transfer coefficient h of the fluid changes slightly [9, 37]. We ignore the change of h, which is also a common assumption in the design of the passage arrangement of the heat exchanger [16, 34]. Similarly, U is calculated from h, and we also ignore the changes in U.