PRESSURE DROP OPTIMISATION IN DESIGN OF MULTI-STREAM PLATE-FIN HEAT EXCHANGERS,CONSIDERING VARIABLE PHYSICAL PROPERTIES Nassim Tahouni, 1 * Samira Miryahyaie, 1 Fatemeh Joda, 1 Hamid Reza Fallahi 2 and Mohammad Hassan Panjeshahi 1 1. School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran 2. Petro Hitech Co., Tehran, Iran A modified method for the design of multi-stream plate-fin heat exchangers that considers variable physical properties is proposed in this paper. The new method, based on Pinch Technology, exploits the dependency of physical properties (heat capacity, viscosity, density and thermal conductivity) on temperature variations. A set of temperature correction factors based on variable physical properties is derived for the hot and cold streams of a multi-stream heat exchanger. This allows calculation of effective stream pressure drops, which can lead to a valid trade-off between operating and capital cost in the targeting stage. Accordingly, composite curves are constructed; based on the enthalpy intervals, the multi-stream heat exchanger is subdivided into a number of block sections. A plate-fin heat exchanger is then designed for each section by maximising the allowable effective pressure drops. Next, using a Genetic Algorithm, the method is completed in order to optimise the pressure drop of streams. Therefore, fin types for each individual stream are considered as optimising variables. By taking the variable physical properties of each stream into account and using the best fin selection, one can achieve accurate results in the design stage. Keywords: multi-stream plate-fin heat exchangers, pinch technology, genetic algorithm, pressure drop optimisation, variable physical properties INTRODUCTION A considerable portion of total plant cost is consumed by heat exchangers, which has motivated designers to find a method to optimally transfer heat. Because of their remarkable ability to exchange heat between more than two streams simultaneously, in addition to certain environmental and investment considerations, multi-stream plate-fin heat exchang- ers have been the focus of several publications. The large heat transfer areas, which are needed on both fluid sides of the com- pact heat exchanger, are satisfied by adding secondary surfaces. Examples of secondary surfaces are plain fins, strip fins and lou- vered fins. The fins extend the heat transfer surfaces and promote turbulence. [1] Because heat exchangers cover an extremely wide range of applications, many design methods have been suggested over the years. Pinch Technology is a conceptual method commonly adopted for the analysis of heat exchanger networks. Picon- Nunez et al. [2] extended this technique for multi-stream plate-fin heat exchangers using the temperature versus enthalpy diagrams known as composite curves. In their approach, maximising the allowable pressure drop of the critical streams in each interval is observed as a design objective; however, under this specification the optimal results are not necessarily obtained. Additionally, according to some sensitivity analysis carried out in different experimental works, [3,4] it is important to make the results more realistic when considering the variation of physical properties and their effects on pressure drop. Estimation of phys- ical properties in small temperature intervals using MATLAB and HYSYS software is a method used by Ghannadi et al., [5] but cal- culation is difficult. In contrast, Fallahi [6] proposed a new method in the targeting and design of heat exchanger networks that led to reliable and accurate results in a short time with the introduction of temperature correction factors for both sides of shell-and-tube heat exchangers. In this paper, by extending Fallahi’s method, a new temperature correction factor () for plate-fin heat exchangers is derived for use in the design of multi-stream heat exchangers. The application of this new method is then investigated in a case study. In recent years, many methods have been proposed for the opti- misation of heat exchangers; in these methods, various parameters are evaluated to achieve optimal results. Combining the Simulated Annealing algorithm and Pinch Technology principles, Panjeshahi et al. [7] presented a new method for designing multi-stream heat exchangers based on optimum pressure drops. Using a Neural Network, Peng and Ling [8] described hydraulic and thermal spec- ifications for five types of fins that could be used to find the minimum weight and total annual cost of a multi-stream heat exchanger. A methodology was developed by Ghosh et al. [9] to determine the optimal staking pattern using a Genetic Algorithm; in this methodology, two methods of area splitting and succes- sive partitioning were used. However, ignoring the variation of physical properties leads to wrong results. Searching through 52 fin types to adopt the appropriate one for each stream is the focus of this paper. To achieve optimum pressure drops, a modi- fied method for determining the best fin type for each stream is developed based on Pinch Technology principles using the Genetic Algorithm. Because of the offsets that variations in physical prop- erties have upon pressure drop and the heat transfer coefficient of the streams, it is crucial to consider the physical properties of the streams as variable parameters. In this effort, the optimi- sation algorithm considers the temperature correction factors for ∗ Author to whom correspondence may be addressed. E-mail address: ntahuni@ut.ac.ir Can. J. Chem. Eng. 9999:1–10, 2013 © 2013 Canadian Society for Chemical Engineering DOI 10.1002/cjce.21781 Published online in Wiley Online Library (wileyonlinelibrary.com). | VOLUME 9999, 2013 | | THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING | 1 |