Heat exchangers temperature difference

The ethylbenzene feed needs to be heated and the reactor effluent cooled. Heat exchange between them is feasible and desirable. Setting an approach temperature for a countercurrent exchanger (temperature difference between... 

Fig. 6 Heat exchangers of different size yielding the same heat output with the same mass flows and inlet temperatures. In the heat exchangers i, 2 and 3 the tube diameters (outer/inner) are d

Figure 5.12 (a-c) Axial temperature profiles in microchannel gas-to-gas heat exchanger with different wall heat conductivities [18], (Adapted with permission from Wiley.)... 

These rules are both necessary and sufficient to ensure that the target is achieved, providing the initialization rule is adhered to that no individual heat exchanger should have a temperature difference smaller than... 

Increasing the chosen value of process energy consumption also increases all temperature differences available for heat recovery and hence decreases the necessary heat exchanger surface area (see Fig. 6.6). The network area can be distributed over the targeted number of units or shells to obtain a capital cost using Eq. (7.21). This capital cost can be annualized as detailed in App. A. The annualized capital cost can be traded off against the annual utility cost as shown in Fig. 6.6. The total cost shows a minimum at the optimal energy consumption. 

Having decided that no exchanger should have a temperature difference smaller than ATmi, two rules were deduced. If the energy target set by the composite curves (or the problem table algorithm) is to be achieved, there must be no heat transfer across the pinch by... 

Thus loops, utility paths, and stream splits offer the degrees of freedom for manipulating the network cost. The problem is one of multivariable nonlinear optimization. The constraints are only those of feasible heat transfer positive temperature difference and nonnegative heat duty for each exchanger. Furthermore, if stream splits exist, then positive bremch flow rates are additional constraints. 

Condition (3) applies to Eq. (C.2) when R = 1. Both conditions (1) and (2) are always true for a feasible heat exchange with positive temperature differences. 

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). 

The LMTD, ie, logarithmic mean temperature difference, is an effective overall temperature difference between the two fluids for heat transfer and is a function of the terminal temperature differences at both ends of the heat exchanger. 

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