Minimum number of heat exchangers

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

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... 

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... 

In summary, six heat exchangers is the minimum for this network when it is required that the hot and cold utilities be minimized as well. As discussed in Section 10.4, the minimum number of heat exchangers for this system is five, which can be achieved either by breaking heat loops, usually at the price of exceeding the MER targets, or by stream splitting. ... 

Before proceeding, it is important to recognize that, as pointed out by Hohmann (1971), the minimum number of heat exchangers in a HEN is... 

In a more general result, Douglas (1988) shows that the minimum number of heat exchangers is also dependent on the number of independent networks, Nnw that is, the number of sub-networks consisting of linked paths between the connected streams ... 

It is required to design a HEN, with a minimum number of heat exchangers that satisfy ATmin = 10°C and a hot utility MER target of 300 kW, for three streams on the hot side of the pinch ... 

Since an MER design implies separate HENs on both sides of the pinch, it is helpful to compute the minimum number of heat exchangers in each HEN. On the hot side of the pinch, only streams HI and Cl exist, and so NHx,min 2, while on the cold side of the pinch, all streams participate, and = 4. Thus, N x%in = A x.min + NJjx,min = 6. The HEN in Figure 10.15a has the minimum number of heat exchangers, while that in Figure 10.15c exceeds the minimum by two heat exchangers. 

Design a HEN for the streams in Exercise 10.4 that meets the MER targets with the minimum number of heat exchangers. 

When Ar in = 10°C, the minimum utilities for heating and cooling are 237 kW and 145 kW, respectively, with pinch temperatures at 110°C and 100°C. Design a HEN that satisfies the MER targets and has the minimum number of heat exchangers,... 

The minimum number of heat exchangers Nux needed is obtained by... 

The minimum number of heat exchangers required for a given composite diagram can be obtained from such a diagram. The number of exchangers required in a section is... 

The minimum number of heat exchangers required for the problem is determined with the aid of Figure 4.14 and Eqn (a). On the graph, vertical lines are drawn to divide the curves into four independent... 

Calculate the minimum number of heat exchangers above and below the pinch. 

Step 4 Calculate the Minimum Number of Heat Exchangers. Once the pinch temperatures have been found from Step 3, it is necessary to find the minimum number of heat exchangers required to carry out the heat transfer for the minimum utility design. From this point on, the heat transfer problem will be split into two and we will consider above and below the pinch as separate systems. 

The final heat-exchanger network is shown in Figure 15.8. The exchangers are represented by single circles, with fluid flowing through both sides. This network has the minimum number of heat exchangers. 

The minimum number of mass exchangers is shown in Figure E15.10(c). The method exacdy parallels that for the minimum number of heat exchangers. In this example, there are three mass-exchange devices... 

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