Structure and Area Optimization of Flexible Heat Exchanger Networks

Abstract

A novel method is outlined for flexible heat exchanger network synthesis including nonconvex problems. The presented method is sequentially implemented by two main steps: structure synthesis and area optimization; nevertheless, the optimization of heat exchanger areas can still react on the structure to gain the global optimal solution. The structure is initially synthesized at the nominal operating point and renewed by the topological union with the structure of the critical point and the improved heat transfer loops disconnection strategy. For area optimization, an iterative approach with strong robustness is proposed based on the influences of heat exchanger areas on flexibility index and total annual cost, respectively. The direction matrix method is employed to provide the operational flexibility of the network and the critical operating point. Two examples with nonconvex feasible regions have been studied, and the results well demonstrate the effectiveness of the proposed approach.

A Multi-Period Synthesis Approach to Designing Flexible Heat- Exchanger Networks

“…The methods that have been used for the synthesis of flexible heat-exchanger networks have been both sequential [1][2][3] and simultaneous in nature [4][5][6][7][8][9][10][11][12][13][14][15][16]. Some of the sequential methods are an automated multi-period version of the mixed integer linear programme (MILP) transshipment model [17] and the non-linear programme (NLP) minimum investment network cost model [18].…”

“…Furthermore, additional costs and complex controllability issues will be incurred due to excessive piping and associated instrumentations. Hence, some other synthesis methods that result in networks that have a greater degree of flexibility have been presented in the literature [10][11][12][13][14][15]. One of the methods entails [10] a three-step approach where a network is designed based on a finite set of operating points in the first step.…”

“…Another method [13] in the area of flexible HEN designs used a two-stage strategy which is based on the SWS model for the synthesis of flexible and controllable networks. Li et al [14] developed a two-step approach for flexible HENS. The first step entails the synthesis of the network structure using the nominal set of operating conditions.…”

“…In the second step, the areas of the heat exchangers obtained in the first step are further optimised considering flexibility and total annual cost (TAC). It should be known that in the method of Li et al [14], the area optimisation is only done after obtaining structures that qualify from the flexibility step. This implies that a true simultaneous optimisation may somewhat be omitted.…”

“…A further criterion that needs to be considered while designing the representative candidate multi-period network, which has erstwhile been ignored by existing flexible HENS methods, is the length of periods for each of the critical operating points used to generate the representative candidate multi-period network in the first step. The existing methods [5,[9][10][11][12][13][14] used the average costs of utility usage by each period of operation present in the first-step candidate multi-period network to determine its minimum TAC and associated network structure as shown in Eqs. (1) and (2) …”