Heat Exchange Area Targets

The problem with Equation 17.5 is that the overall heat transfer coefficient is not constant throughout the process. Is there some way to extend this model to deal with the individual heat transfer coefficients  

The effect of individual stream film transfer coefficients can be included using the following expression, which is derived in Appendix F3,4  

J = total number of cold streams in enthalpy interval k 

If film transfer coefficients vary significantly from stream to stream, the true minimum area must be predicted using linear programming5,6. However, Equation 17.6 is still a 

Providing film coefficients vary by less than one order of magnitude, then Equation 17.6 has been found to predict network area to within 10% of the actual minimum6. 

Rgura 7.2 To target the number of units for pinched problems, the streams above and below the pinch must be counted separately, with the appropriate utilities included. 

To obtain the network area, Eq. (7.4) can be applied to all enthalpy intervals intervals/f 

I = total numher of hot streams in enthalpy interval k J = total numher of cold streams in enthalpy interval k K = total number of enthalpy intervals 

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Calculate the heat exchange area target for the network. 

Kravanja and Glavic (1997) proposed a match-dependent heat-exchanger area targeting method followed by an NLP optimisation of the structure meeting the area targets. This method fixes the utilities before estimating the heat exchanger area. 

Targets also can be set for total heat exchange area, number of units, and number of shells for 1-2 shell-and-tube heat exchangers. These can be combined to... 

Design targets for the Heat Exchangers Network (HEN), as the total heat exchange area and the number of units for achieving MER ... 

After the input of data the next step is targeting, which consists of finding the optimal as a trade-off between energy and capital costs. On this basis targets for MER can be determined, as well as the overall heat exchange area and the number of units. If the economic data are not reliable, selecting a practical is recommended. 

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. 

Townsend, D. W., and Linnhoff, B., Surface Area Targets for Heat Exchanger Networks, IChemE Annual Research Meeting, Bath, U.K., 1984. 

Saboo, A. K., Morari, M., and Colberg, R. D., RESHEX An Interactive Software Package for the Synthesis and Analysis of Resilient Heat Exchanger Networks II. Discussion of Area Targeting and Network Synthesis Algorithms, Computers Chem. Eng., 10 591, 1986. 

The detailed allocation of fluids to tube-side or shell-side can only be made later in the heat exchanger network design. Also, the area targeting formula does not recognize fluids to be allocated to the tube-side or shell-side. Area targeting only recognizes the individual film heat transfer coefficients. All that can be done in network area targeting... 

One significant advantage of the simplified superstructure in Figure 18.29 is that each exchanger can be modeled by a linear equation if the supply and target temperatures of the streams are fixed. The area for each heat exchanger is... 

Zhu XX, O Neill BK, Roach JR and Wood RM (1995) New Method for Heat Exchanger Network Synthesis Using Area Targeting Procedures, Comp Chem Eng, 19 197. 

Algorithm for the Heat Exchanger Network Area Target... 

R. D. Colberg and M. Morari. Area and capital cost targets for heat exchanger network synthesis with constrained matches and unequal heat transfer coefficients. Comp. Chem. Eng., 14 1,1990. 

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