Heat exchangers analysis

Values of thermal conductivity are temperature-dependent and vary widely for different materials. Table 1 summarizes the thermal conductivity values of a few materials relevant to heat-exchanger analysis (1,2). 

Kays and London [3] have presented effectiveness ratios for various heat-exchanger arrangements, and some of the results of their analyses are available in chart form in Figs. 10-12 to 10-17. Examples 10-9 to 10-14 illustrate the use of the effectiveness-NTU method in heat-exchanger analysis. 

The log-mean-temperature-difference and effectiveness approaches are presented in heat-exchanger analysis since both are in wide use and each offers its own advantages to the designer. A brief introduction to diffusion and mass transfer is presented in order to acquaint the reader with these processes and to establish more firmly the important analogies between heat, mass, and momentum transfer. 

In heat exchanger analysis, it is often convenient to combine the product of the mass flow rale and the specific heat of a fluid into a single quantity. This quantity is called the heat capacity rate and is defined for the hot and cold fluid streams as... 

The log mean temperature difference (I.MTD) method discu.sscd in Section 11-4 is easy to use in heat exchanger analysis when the inlet and the outlet temperatures of the hot and cold fluids are known or can be determined from an energy balance. Once Ar,T, the mass flow rates, and the overall heat... 

A second kind of problem encountered in heat exchanger analysis is the determination of the heal transfer rale and the outlet temperatures of the hot and cold fluids for prescribed fluid mass flow rales and inlet temperatures when the type and size of Ihe heat exchanger are specified. The heat transfer surface area of the heat exchanger in this case is known, but Ihe outlet temperatures are not. Here the task is to determine the heat transfer performance of a specified heat exchanger or to determine if a heat exchanger available in storage will do the job. 

The LMTD method could still be used for this alternative problem, but the procedure would require tedious iterations, and Ihus it is not practical. In an attempt to eliminate the iterations from the solution of such problems, Kays and London came up with a method in 1955 called the cffecUveness-NTU method, which greatly simplified heat exchanger analysis. 

In heat exchanger analysis, it is also convenient to define another dimensionless quantity called the capacity ratio c as... 

C Under what conditions is the effecliveness-NTU method definitely preferred over the LMTD method in heat exchanger analysis ... 

Heat Exchanger Analysis The Effect veness-NTU Method... 

Our objective is to develop relationships between the heat transfer rate q exchanged between two fluids, heat transfer surface area A, heat capacity rates C of individual fluid streams, overall heat transfer coefficient U, and fluid terminal temperatures. In this section, starting with idealizations for heat exchanger analysis and the thermal circuit associated with a two-fluid exchanger, e-NTU, P-NTU, and MTD methods used for an exchanger analysis are presented,... 

Idealizations for Heat Exchanger Analysis. The energy balances, the rate equations, and the subsequent analyses are subject to the following idealizations. 

Thermodynamic Vent System Heat Exchanger Analysis. 238... 

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