(99a) Retrofit Strategy for Heat Exchanger Networks

Numerous heat exchanger network (HEN) retrofit design methods have been proposed over the years, either based on topology modifications, heat transfer enhancement or a combination of both methods. However, selecting the most cost-effective retrofit strategy for a given network remains a great challenge.

In this work, the right balance between complexity of the retrofit solution, cost and benefit regarding energy recovery is sought. As such, the proposed retrofit strategy considers the application of heat transfer enhancement first, which is a low-cost retrofit option. Then structural modifications such as resequencing, stream splitting, adding a new match to create a loop or a path are explored in a step-wise approach. Finally, the third option (once enhancement and structural modifications are explored), combines the benefits of enhancement and modifications to the network to attain high energy recovery at reduced capital investment.

With heat transfer enhancement, a systematic retrofit methodology will be presented. The methodology can evaluate the feasibility of applying heat transfer enhancement techniques for a given HEN, identify the best location to apply enhancement, deal with downstream effects from the application of enhancement, and consider the effects of enhancement on pressure drop.

In general, a limitation with the application of enhancement is its effect on pressure drop together with the constraint in the degree of enhancement a heat exchanger can provide. These two factors influence the energy recovery that can be achieved. Therefore, in cases where high energy recovery is required, the next best retrofit strategy is to explore performing structural modifications. This work presents a new step-by-step retrofit approach (Pinch Retrofit Method) for performing structural modifications. This new approach is based on analysing the key features of an existing network as such providing insights into identifying the best series of structural modifications for a given network. This ensures that fewer modifications are required to achieve a degree of energy saving, and as a result is more cost-effective.

The next retrofit strategy considered is one that combines the benefits of heat transfer enhancement and modifications to the network. This can be achieved either by performing structural modifications and using heat transfer enhancement as a way of reducing the cost required (in terms of additional area required) or, replacing existing exchangers with plate exchangers (e.g. Compabloc). The use of plate heat exchangers is explored in this work as it has the benefits of improved energy efficiency, reduced fouling tendency and minimal plot space compared to shell and tube exchangers.

The methods discussed for retrofit are applied to an industrial case study. Pre-assessment on the case study identified a potential of reducing the furnace energy consumption by ~18%. The network minimum approach temperature is ~34°C, but the network is constrained with an exchanger minimum approach temperature of ~3°C. Applying the retrofit strategy proposed in this work, the best retrofit option was identified, which provided the right balance between complexity, cost and benefit. The results from analysis showed that significant energy recovery can be obtained with a payback of ~2 years.