Number of heat exchangers

Example A ventilation system (Fig. 9.64) handling 20 mVs of air needs to heat the supply air from 10 °C to 20 C. Doubling the number of heat exchangers from one to two increases the heat-recovery efficiency from 50% to 75% and introduces an extra pressure drop of 300 Pa. As we can see from Table 9.19, this is probably a cost-efficient measure. 

Zinin Minimum number of heat exchangers in network —... 

As a second example (category 4), a typical plant contains large numbers of heat exchangers used to transfer heat from one process stream to another. It is important to continue to use the heat in the streams efficiently throughout the process. Incoming crude oil is heated against various product and reflux streams... 

The hydrogenated gaseous stream enters the cold section, where it is passed through a number of heat exchangers and separators. A portion of the liquid stream from unit 10 is used as the recycle stream. Hydrogen is separated as a gaseous stream in unit 12. 

Step (i) was proved in Appendix A of Floudas et al. (1986), and hence the minimum number of matches predicted by the MILP transshipment model can always be represented with an equal number of heat exchanger units in a feasible HEN network. 

Each stream superstructure has one input, one output, a number of heat exchangers equal to the matches that involve the particular stream, and all possible connections from the input to the units, between each pair of units, and from the units to the output. To illustrate such a representation, we will consider the following example that has... 

Note also that the set of constraints (A)-(G) exhibit the nice feature of linearity in the continuous and binary variables. Furthermore, the flow rates do not participate in the formulation at all and hence there is a reduction in the number of continuous variables by the number of flowrates. The penalty that we pay for this desirable feature is threefold (i) we introduce more binary variables since we have possible process matches that are equal to the number of stages times the actual number of potential matches, (ii) we deal with a simplified set of alternatives that excludes a number of desirable structures, and (iii) we need to solve an NLP suboptimization problem to determine the flow rates of the split streams and possible reduce the number of heat exchangers if the resulting network exhibits splitting of the streams. 

MEUmax maximum number of heat-exchange units... 

One can easily predict the likely least number of heat exchangers required in this network. 

A number of heat-exchanger surfaces do not fall into the categories discussed in the foregoing sections. Most notable are the compact exchangers which achieve a very high surface area per unit volume. These exchangers are most... 

The term Exc is the exergy consumed by the overall processes. We may combine the pinch analysis with the exergy analysis to reduce the number of unit operations and thermodynamic parameters, leading to an optimum operation. The minimum number of heat exchangers needed is obtained by... 

Do not transfer heat across the pinch point. For example, any process stream heat that is transferred from one side of the pinch point to the other side only increases the requirements for both utilities. The optimal network uses the least number of heat exchangers. 

The minimum number of heat exchangers required for the problem is determined with the aid of Figure 4.47 and Eq. (4.234). On the graph, vertical lines are drawn to divide the curves into four independent exchange sections. The cold utility section shows three streams and requires two exchangers. The process section below the pinch requires two exchangers since there are three streams. The process section above the pinch requires three exchangers... 

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