Simulation and performance analysis of a plate fin heat exchanger in Unisim Design

. This paper presents a simulation of a plate fin heat exchanger with the aim of choosing optimized geometry and design parameters among given set of parameters using Unisim Design software. The simulation includes an analysis of the effects of varying exchanger parameters on the total annual cost of the plate fin heat exchanger. Results from the simulation indicate that Unisim Design can be used to design different variants of plate-fin heat exchanger with further optimization on cost, heat load or any other parameters. This work provides useful insights into the simulation of plate fin heat exchangers in Unisim Design and can be used to develop more efficient designs.


Introduction
Oxygen, nitrogen, and argon are extensively utilized gases in the steel and metal industry. In steelmaking, oxygen plays a crucial role in the basic oxygen process (BOP), which occurs within a basic oxygen furnace (BOF). This method entails injecting high purity oxygen into a furnace containing molten iron and other scraps to convert them into steel.
Nitrogen finds common application in the metalworking process known as annealing. Nitrogen annealing involves subjecting metals to heat treatment to enhance ductility and reduce hardness.
Argon, in conjunction with oxygen, is employed in the steelmaking procedure called argon oxygen decarburization (AOD). This process is carried out subsequent to melting the metals into a liquid state. Throughout the metallurgy industry, these gases find utility in various stages of metal production, spanning from initial processing to the final stages (https://air-source.com).
Mentioned gases are core components of ambient air and are separated in Air Separation Units (ASU). Nowadays mostly cryogenic air separation technology is used in this industry which includes compression and expansion of air during which cryogenic temperatures are reached and air components are liquified. One of the most important parts of ASU is multistream heat exchanger, often it is plate-fin heat exchanger (PFHE) with big heat exchange areas for each of the streams. General arrangement of mentioned PFHE is given in Figure 1. A plate-fin heat exchanger is a specific design of heat exchanger that employs plates and finned chambers to facilitate heat transfer between fluids, typically gases. This type of heat exchanger is recognized for its compactness, emphasizing the relatively large surface area available for heat transfer in proportion to its volume. It finds widespread use across various industries, particularly in cryogenics, where its ability to facilitate efficient heat transfer even with small temperature differences is highly valuable.
Specifically, aluminum alloy plate fin heat exchangers, also known as Brazed Aluminum Heat Exchangers, were introduced to the cryogenic air separation industry during the second World War. Soon after, they were adopted in cryogenic processes within chemical plants, such as those involved in Natural Gas Processing (https://en.wikipedia.org/wiki/Plate-fin_heat_exchanger).
Modern simulation software makes it possible to design or at least check some design parameters of PFHE with decent accuracy. One of the most recent and leading simulators in industry today is UniSim Design environment, development of Honeywell Company [1][2][3][4]. The Unisim Design Suite, developed by Honeywell, serves as a simulation module for technological objects and processes. This comprehensive software package enables users to simulate various production and field oil and gas processes, as well as refining and petrochemical processes [5][6][7]. Using before mentioned software we can simulate Plate-fin heat exchangers (PFHE) also. There are two options of PFHE simulation using Unisim Design: a) Use the special Unisim Plate fin exchanger modeler software; b) Use the PFE Engine inside Unisim Design. For current work the latter one was chosen. Embedded PFE engine also allows to simulate the PFE in conjunction with field equipment and instrumentation like valves, manometers, thermocouples, vessels, other heat exchangers, etc.

Methodology
In current work will be used Unisim Design R480 edition. Set up for research is as follows: we used LNG Exchanger unit operation inside Unisim Design software and Plate Fin Exchanger (PFE) engine inside it, our heat exchanger has two streams (Cold and Hot), streams inlet conditions are reflected in Table 1.
This paper aims to give some basic insights on implementation of PFHE in Unisim Design, its parameters and tabs. We are going to set up PFHE with sample parameters shown in Table 3 (Base case), change some of them (Cases from A to F in Table 4), get results after changing, make cost calculations and compare results of different cases.
In Unisim Design we created a case with Peng-Robinson fluid package and components: Nitrogen, Oxygen, Argon as we used Air as heat exchanging streams in PFHE. Set up worksheet as in Figure 2: LNG heat exchanger, two inlet and two outlet streams.

Unisim PFE settings
In LNG heat exchanger unit operation we used Unisim PFE tab. In further steps each tab inside Unisim PFE is studied with regards to which of them are changed in this work. In Exchanger tab we choose inlet streams, set flow direction for them inside PFE: in our case we have counter flow exchanger, set number of layers for each stream: 10 in Base case, distance to start of main fin: 50mm for all cases, orientation: 1 for all cases, exchangers in parallel: 1 for all cases, effective width: 500mm for Base case, exchanger metal: aluminium for all cases.
Process tab lets us to set estimated values mainly for pressure and temperature, it is recommended to do in some complicated PFE cases when we want to "help" Unisim to start calculations with values that are near to ones that should be. In our case none of parameters inside are changed.
Distributors tab is designed to set inlet, outlet and intermediate distributors parameters, types and dimensions which makes Unisim PFE model as near to real PFE as possible. None of parameters in this tab is set in current work.
In Layer patterns tab we can set different non-uniform layer patterns for PFE. It is especially important during design of heat exchangers with many layers. None of parameters in this tab is set in current work.
In Fins tab fins geometry such as fin height, thickness and frequency can be set, we also can choose their types from the list of Plain, Perforated, Serrated and Wavy fins. For base case fin height is 50mm, thickness is 0.5mm and fin frequency is 500 1/m. Next tab to be changed is Stream Details, where for the Base case we choose Fin number (1) for each stream in heat exchanger and set the length of first Main Fin (500mm). Other parameters are not changed in current work.
Methods tab we are leaving as it is.
In Full results and Profiles tabs we can check summarized report on process parameters inside PFE and view the temperature profile as a plot. It is very convenient when comparing different PFE designs.

Thermal-economic calculations
For thermal-economic calculations we the heat exchanger's total annual cost (TAC). The TAC is the summation of investment cost (Ccp) and operating cost (C op ). C cp includes the unit area's construction cost, heat exchanger surface area, and annual coefficient factor, where C op contains the compressor electricity price as defined below [3]: where C A , e, ζ, τ, and η presents the cost per unit surface area, exponent of nonlinear increase with area, electricity price of the compressor, operation time, and compressor effectivity in order. A cf indicates the annual coefficient factor as below: The heat exchanger surface area for both hot and cold sides, A (= A h + A c ), can be calculated in equations: L h , L c -fins length of hot and cold sides of exchanger respectively N h , N c -Number of fin layers of hot and cold sides of exchanger respectively H, n, t t -fin height, fin frequency and exchanger thickness respectively

Results
After process streams are set up as per Table 1 and PFE is set up as per Base case parameters in Table 3 we calculated Total Annual Cost (TAC) for Base case. After that we started changing PFE parameters as per Table 3.  Table 4 also included TAC for every subcase.

Conclusion
As we can see from the results of our work each PFE design parameter affects the total annual cost of the exchanger. TAC was chosen as a parameter for comparing all subcases and finding the minimum among them. All PFE parameters are abstract ones and economical constants for calculating TAC are taken from [8]. TAC is calculated with regards to pressure drop through PFE thus we are choosing the most energy saving case and outlet temperatures of streams are not taken into account. Among all cases covered in the current work subcase 3 of Case B has minimal TAC of 1096 $/year. In current work, as it was stated in the beginning, were given only basic insights of implementation of Plate-fin heat exchanger in Unisim Design, was used two-stream set up and Total Annual Cost was calculated for different cases. This work can be expanded in several ways: more than 2 streams can be used, PFE parameters can be more complicated (more layers, different layer patterns, mixing of streams, etc.), optimization can be done not only by calculating TAC but also considering heat load, outlet temperature or any other target parameter using different optimization algorithms.