A state-of-the-art review on thermo fluid performance of brazed plate heat exchanger for HVAC applic

Abstract

Keywords: CFD; Fluid Flow; Friction Factor; Heat Transfer; Plate Heat Exchangers

Introduction

Heat exchangers serve as crucial devices for transferring heat between a solid and a fluid medium or between two fluid mediums. From household refrigerators to thermal power plants, these versatile devices come in various forms, shapes, and sizes, catering to a wide range of applications. For over a century, heat exchangers have been integral components of refrigeration and air conditioning systems, as well as numerous industrial processes requiring heating, cooling, or isothermal operations. Compact heat exchangers are those heat exchangers which have a surface area to

volume ratio of more than 700 m2/m3 for gasses and 300 m2/m3 for liquids They find extensive use in equipment where achieving a close temperature approach is essential. While several design methods, such as the Logarithmic Mean Temperature Difference (LMTD) method or the effectiveness – Number of transfer units (ε-NTU) method, are prevalent in the literature, determining the heat transfer coefficient and pressure drop remains a persistent challenge. Over the years, research efforts have yielded established correlations for many commonly encountered geometries, providing valuable insights into heat exchanger

*Corresponding author. *E-mail address: sachindra106@gmail.com This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

performance. However, ongoing research continues to explore specialized cases, indicating that we may just be scratching the surface of the field’s potential. The plate heat exchanger, credited to Dr. Richard Seligman in 1923, comprises a series of plates stacked atop each other, each featuring embossed corrugations. These corrugations, angled to create channels when the plates are flipped and stacked, facilitate local turbulence even at low Reynolds numbers upon fluid entry. The ends of the exchanger are sealed using either gaskets or brazing, ensuring the separation of hot and cold fluids via a lining of gasket material or brazed metal at the entry and exit ports. When designing for specific applications, existing geometries can be leveraged. Heat transfer rates can be controlled by adjusting plate quantities, altering flow patterns, or modifying plate arrangements, allowing for tailored design without necessitating the creation of an entirely new device. A classification of plate heat exchangers is depicted in Figure 1.

Gasketed plate heat exchangers (GPHEs) stand as the most prevalent type among plate heat exchangers. These devices feature a gasket lining between stacked corrugated plates, ensuring the integrity of the liquid channels by preventing leaks into the surrounding environment and between individual fluid streams, as illustrated in Figure 2. The primary domain of application for GPHEs lies within the food industry, particularly in milk processing plants. Here, they play a pivotal role in cooling fluids like yoghurt and processed milk. Additionally, they find widespread use as chillers across various industries due to their superior effectiveness compared to traditional shell and tube or tubular heat exchangers, all while occupying significantly less space. In BPHEs, gaskets are replaced with brazed joints to enhance plate strength. By brazing the contact points of the corrugations between two plates, the unsupported length of the load-bearing corrugation significantly reduces, thereby improving the pressure limits and overall operating conditions of the exchanger. Moreover, brazing eliminates the

need for certain support structures necessary for Gasketed Plate Heat Exchangers (GPHEs), making BPHEs lighter and more compact for the same heat transfer capacity. This reduction in size and weight is particularly advantageous in applications where space is limited or weight constraints are a concern. This article provides a comprehensive summary of research conducted on BPHEs. While the focus is primarily on BPHEs, certain sections also include references to studies involving gasketed plate heat exchangers (GPHEs) due to their similar flow behavior. The content is organized into distinct sections based on the type of study conducted, such as single-phase flow analysis, numerical simulations, etc. As a result, certain articles may be referenced in multiple sections to ensure thorough coverage of relevant research findings. The following Table 1 contains the layout of the article with a brief description.

Chronology

The plate heat exchanger, invented by Dr. Richard Seligman in 1923 [1], saw significant advancements in its design and construction over the ensuing decades. In 1968, Butt [2] filed a patent outlining methods for constructing and repairing plate heat exchangers. Subsequently, in the early 1970s, Alfa Laval revolutionized the technology by introducing BPHE. By brazing the joints and eliminating gaskets, Alfa Laval enhanced the efficiency and durability of plate heat exchangers. The exploration of plate heat exchangers by researchers initially paved the way for investigations into BPHE. In the 1980s, pioneering works by Thonon [3], Focke et al. [4], Shah [5] etc., were among the pioneers of research in this field, providing insights into the working of plate heat exchangers. These studies yielded correlations for key parameters such as the friction factor (f) and Colburn factor (j), elucidating the effects of varying geometric and flow parameters. Further innovations continued to emerge

Introduction

A brief introduction to the compact heat exchangers and brazed plate heat exchangers

This section gives an overview of the timeline of the research conducted over the years since the invention of the plate heat exchanger.

Plate heat exchangers offer higher heat transfer rates than traditional shell and tube heat exchangers. This section glances over the studies that observed the heat transfer and flow patterns inside the heat exchangers

This section covers the studies done on the heat exchangers carrying the fluid that doesn’t change its phase throughout the study

Most refrigerants are studied in brazed plate heat exchangers owing to their application in HVAC industries. The majority of the work has been done on twophase flows in a brazed plate heat exchanger. This section deals with the research done over the years in two-phase flow

A small part of the community studied the heat exchangers using simulation software such as ANSYS Fluent. This section deals with the studies that observed the heat exchangers virtually by simulating the flow conditions inside them in a computer

Correlations are bound to be developed based on the experimental and numerical data available to the researchers. This section has the correlations developed over the years based on the data collected by the respective studies

throughout the decade. In 1985, Allison [6] patented a procedure for simplifying maintenance by removing end bars. Additionally, in 1989, Fuerschbach et al. [7] secured a European patent detailing a construction method that incorporated brazed joints to eliminate unnecessary structural components, thereby reducing the weight of the heat exchanger. These developments underscored a concerted effort to enhance the efficiency, maintenance, and structural integrity of plate heat exchangers. As plate heat exchangers gained traction as evaporators and condensers in the refrigeration industry, Yam et al. [8] emerged as pioneers in the late 1990s, providing correlations for the Nusselt number and friction factor. Subsequent years saw a surge in research focusing on the effects of geometry, with scholars delving deeper into these aspects. In the year 2001, William and Glen Hubman [9] a novel construction method for BPHE, employing metal gaskets to enhance structural integrity. The following year, Heil et al. [10] introduced and patented another construction method for plate heat exchangers, further expanding the technological repertoire in the field. Between 2000 and 2010, several CFD studies were conducted on plate heat exchangers, exploring phenomena such as fouling [11] and investigating the impact of corrugations [12] and corrugation angle [13] on pressure drop and heat transfer. Notably, in 2013, Otahal and Hofer [14] patented a plate heat exchanger featuring a turbulence generator, aiming to optimize heat transfer efficiency. A few years later, Krantz [15] from Alfa Laval Corporate patented innovative gasket and plate designs that purported to enhance heat transfer performance in plate heat exchangers. These developments underscored ongoing efforts to refine and innovate within the realm of plate heat exchanger technology. The pertinent literature on BPHE began to emerge around 1995, with Bogeart and Boles [16] conducting a study on the performance of an industrial prototype BPHE. In 2000, Palmer et al. [17] investigated the effects of flammable refrigerants in BPHEs, shedding light on safety considerations in their application. Subsequently, Han et al. [18] explored R410A evaporation in BPHEs in 2003, providing insights into refrigerant behavior within these exchangers. Longo et al., continued this line of research, focusing on the evaporation and condensation of various refrigerants in BPHEs since 2007. Their work has significantly contributed to understanding the thermal performance of BPHEs across different operating conditions. Gullapalli [19] utilized a combination of CFD and experimental data to optimize BPHE designs, culminating in a doctoral thesis in 2013. Further publications [20] and [21] by Gullapalli and others have built upon this research, refining BPHE design methodologies. Due to their compactness and high heat transfer rates, BPHEs have found application in carrying nanofluids and as evaporators in Organic Rankine Cycle (ORC) systems. Barzegarian et al. [22] investigated the heat transfer and pressure drop of TiO2 nanofluids in BPHEs in 2015, while Teng et al. [23] examined the characteristics of

carbon-based nanofluids in BPHEs. Recent years have witnessed contributions from researchers such as Desideri et al. [24], Gullapalli [21], Kim and Kim [25] and Nematollahi et al. [26] towards integrating BPHEs into ORC systems, expanding the scope of their applications in energy conversion processes. Through the analysis of the optical and thermal behavior of particulate media, it is clear that photo-thermal energy conversion is important to not only the solar thermal systems but also to the electric power generation and solar chemical technology. This research investigates the effects of nanoparticle suspensions (water/TiO2 nanoparticle suspension) at different particle concentrations on the radiative properties and radiative transfer phenomena. The effect and contribution of the TiO2 nanoparticles on the radiative properties in the UV-Vis-NIR wavelength ranges are observed, which have a significant impact for the solar thermal applications. Study of Heat Transfer and Flow Friction Phenomena The study of heat transfer and flow friction phenomena in BPHE is crucial due to their compactness and high heat load capacities in small sizes. The intricate channels formed between the plates play a significant role in achieving these high heat transfer rates. While most studies have historically focused on gasketed plate heat exchangers, the similarities in design between gasketed and BPHE allow findings from these studies to be applied to BPHEs. In the early days of BPHE research, limited experimental studies provided valuable insights into the flow patterns responsible for heat transfer and pressure drop characteristics. However, with the advent of computational fluid dynamics (CFD) software such as ANSYS Fluent, visualization of flow patterns has become much more accessible. This technological advancement has enabled researchers to gain a better understanding of how the geometric features of BPHEs impact their performance. Overall, the combination of experimental studies and CFD simulations has greatly contributed to advancing our knowledge of heat transfer and flow friction phenomena in BPHEs, facilitating the design and optimization of these highly efficient heat exchangers. Study of flow patterns and flow friction phenomenon Dović and Švaić [12] investigated flow patterns and flow friction in BPHE to gain deeper insights into their performance characteristics. Employing visual aids, the researchers examined flow patterns within BPHEs utilizing two heat exchangers with chevron angles of 28° and 61°. Transparent plates specifically designed for this study allowed observation of a die inserted into the flow at various Reynolds numbers, enabling detection of any changes in direction due to geometric factors. The flow was characterized into two components: the longitudinal component, which flowed alternately along adjacent plates, and the furrow component, representing flow along the corrugations or furrows.

Figure 4. Schematic of the plate geometry. Results depicted in Figure 5 illustrated that as the ratio of amplitude to the length of the corrugation increased, the furrow component became more dominant in the flow. At low Reynolds numbers, the longitudinal component prevailed due to insufficient velocity to induce mixing among sub-streams. However, at higher chevron angles and Reynolds numbers, although the longitudinal component remained dominant, increased mixing among streams enhanced heat transfer efficiency. Furthermore, the study highlighted the influence of chevron angle and aspect ratio on heat transfer performance. Lower amplitude-to-length ratios were favored at lower chevron angles, where the longitudinal component played a more significant role, leading to improved performance. Conversely, higher aspect ratios

proved beneficial at higher angles, promoting enhanced heat transfer efficiency. These findings underscored the importance of geometric parameters in optimizing heat transfer in BPHEs. Study of heat transfer characteristics While research on flow patterns was limited, investigations into the impact of corrugations and associated geometry on heat transfer were more prevalent. Focke et al. [4] delved into the effect of chevron angle on pressure drop and heat transfer coefficient. Ten machined plates with angles ranging from 0° to 90° were stacked for the study. It was observed that at a 90° angle, flow separation occurred even at low Reynolds numbers (Re), leading to reduced pressure

Figure 5. Longitudinal and furrow components [From Dović and Švaić [12] with permission from Author].

drop and heat transfer. Additionally, the study highlighted that the ends of plates contributed minimally to pressure drop and heat transfer calculations. Kandlikar and Shah [27] developed effectiveness formulae for various flow patterns and configurations of multi-pass plate heat exchangers (PHEs), using a computer program to calculate temperature effectiveness. With increasing number of plates, the influence of end plates diminished gradually, stabilizing after 40 plates without significant changes in temperature effectiveness. Galeazzo et al. [28], a four-channel PHE was modeled using CFD. They compared the results with experimental data and a 1D plug flow model. Parallel and counter-flow arrangements were simulated and experimented with in this study. Due to the omission of ambient heat losses in CFD, the values of heat load were consistently higher than actual experimental values. The 1D model proved to be unreliable because of the requirement of numerous experimental runs for adjusting the correlations. The correlations were accurate with the experimental data. Muley and Manglik [29] conducted experimental studies on turbulent flow heat transfer and pressure drop in plate heat exchangers, noting discrepancies in previous studies regarding the use of power law curve fit equations and variations in error percentages. Their experiments utilized plate combinations with chevron angles of 30° and 30°, 30° and 60°, and 60° and 60°. Iterative multiple linear regression analysis was employed to formulate correlations for the waterside, serving as calibration equations for the experimental setup. Results indicated that Nusselt number (Nu) increased with chevron angle, suggesting intensified swirl flows. Despite some disagreements with previous studies, it was concluded that flow and thermal patterns were primarily influenced by geometric differences like amplitude and pitch. Additionally, it was noted that Focke et al. [4] had not considered correction factors for viscosity variations in their Nusselt number equations. The area enlargement factor, reflecting the ratio of actual surface area to projected area, significantly impacted heat transfer, with deeper corrugations promoting swirl flow and effective area enlargement. Ultimately, heat transfer enhancement of up to 2.8 times that of a flat plate channel was achieved, with correlations for Nusselt number and friction factor presented based on the study’s data. Study of Single-Phase Flow in Brazed Plate Heat Exchanger Expanding upon the foundation established by single-phase flow studies in gasketed plate heat exchangers, research in BPHE has evolved. Correlations of f and Nu from Kumar [30], Focke et al. [4], and others have been instrumental in validating and refining experimental results. While brazed plate heat exchangers are typically utilized as evaporators or condensers, scenarios exist where both sides experience single-phase flow. Consequently, ongoing research in single-phase flow aims to establish correlations

for friction factor and Nusselt number, facilitating broader applicability across various studies in the field. Blomerius and Mitra [31] conducted numerical investigations on convective heat transfer and pressure drop in wavy ducts angled at 45° and 90°, considering the influence of unsteadiness. Grijspeerdt et al. [11] employed CFD modelling to analyze the hydrodynamics of plate heat exchangers (PHEs) with a 56° chevron angle, primarily focusing on applications in the milk processing industry due to fouling concerns. Similarly, Galeazzo et al. [28] modelled a four-channel PHE using CFD, comparing results with experimental data and a 1D plug flow model. Jain et al. [13] simulated small-sized PHEs, studying flow maldistribution effects and comparing results with experimental data. Kanaris et al. [32] utilized optimization techniques to design a PHE with V-shaped corrugations, achieving optimal performance by minimizing the obtuseness of corrugations and reducing channel width. They highlighted the dominance of secondary flows in corrugations, leading to increased heat transfer augmentation through flow separation and reattachment. Han et al. [33] conducted numerical simulations and provided optimized results for single-phase turbulent flow in a plate heat exchanger with sinusoidal corrugations, utilizing a different optimization technique from Kanaris et al. [32]. They also presented a formula for the area enlargement factor, similar to Shah’s textbook approach. Simulations covered chevron angles of 30° and 60° across a wide range of Reynolds numbers. Fernandes et al. [34] observed low Reynolds number flow in chevron plates for highly viscous fluids, noting increased flow tortuosity and resistance with reduced corrugation angles. They introduced a coefficient ‘K’, derived from the shape factor and tortuosity coefficient, equivalent to the product of the Fanning friction factor and Reynolds number, suitable for laminar flow analysis using the Hagen-Poiseuille formula. Khan et al. [35] investigated single-phase heat transfer in plate configurations with varying chevron angles (30°/30°, 60°/60°, 30°/60°) and Reynolds numbers ranging from 500 to 2500. Results indicated a Nusselt number increase of 4 to 9 times compared to flat plate Nu, depending on the corrugation angle configuration, at a given Reynolds number. Manigandan et al. [36] experimentally observed the counter and parallel flow characteristics in a brazed plate heat exchanger. Different flow rates of water in the hot and cold sections were used along with various inlet temperatures. No significant correlations were used or derived from the data and only the outlet temperatures and heat loads were plotted for parallel and counter flow. Gurel et al. [37] introduced a lung-patterned design onto plates, mimicking the efficient heat transfer capabilities of the human pulmonary system. This intricate design aimed to leverage the rapid heat exchange properties of human lungs, with flow velocity highest at pattern ends. Increasing pattern branches stabilized pressure drop while enhancing heat transfer efficiency. Compared to a referenced plate heat

exchanger (PHE), the lung-patterned design demonstrated a significant 71% increase in heat transfer and a notable 67.8% reduction in pressure drop. Suggestions included rounding branch ends for further pressure drop reduction and energy savings. Fernández-Seara et al. [38] explored the use of offset strip fins in BPHE, analyzing flow and heat transfer characteristics for water-ethylene glycol solutions. Cremasachi et al. [39] explored fouling performance in BPHEs used in cooling towers and air conditioning condensers, using four heat exchangers with 30° and 63° angles. Lower angles exhibited higher fouling and pressure drop due to settling calcium carbonate, with pressure drop nearly 200% higher. Kim and Park [40] compared two BPHEs with similar chevron angles but different hydraulic diameters, finding similar performance parameters despite Nusselt number variations, suggesting excess heat transfer increased pressure drop. Jin and Hrnjak [41] investigated the impact of end plates in frame plate and BPHE, noting their significance, especially with fewer plates and improved contact points. Li and Hrnjak [42] continued the study on end plate effects, conducting a theoretical analysis and developing an iterative model to compensate for their impact on heat transfer, incorporating insights from literature, and numerical, and analytical models. Wu and Ju [43] replaced plate-fin heat exchangers with BPHE in four LNG processes and optimized cases to assess process flow and economic performance parameters. Zhong et al. [44] examined BPHE performance at low Reynolds numbers (5 to 50) through experimental and numerical analyses, establishing correlations for f. Observations revealed that increased angle and spacing correlated with higher friction factors. Jafari et al. [45] simulated three BPHE geometries, focusing on corrugation profile modifications and brazing joint presence. While the last geometry proved most accurate, its results lacked validation above 60°, with existing correlations unable to predict f and Nu effectively. Ham et al., [46] explored LiBr solution usage in BPHEs, proposing new Nu and f correlations from experiments conducted in a Vapor Absorption Refrigeration System, with one side handling a weak solution and the other a strong solution. Nanofluids, containing nanoparticles in a liquid, exhibit enhanced heat transfer compared to the base fluid. Barzegarian et al. [22] investigated TiO2-water nanofluid in a BPHE, noting an increase in convective heat transfer coefficient with nanoparticle concentration. Teng et al. [23] explored carbon-based nanofluids in laminar flow BPHE, finding higher heat exchange capacity and lower pumping power for lower-concentration nanofluids. Fazeli et al. [47] optimized MWCNT-CuO nanofluid heat transfer in BPHE, noting a 39.27% thermal conductivity increase. Mehrarad et al. [48] reported a 94% Nusselt number increase and a 12.87% friction factor increase for the same nanofluid. Additionally, they emphasized the potential of nanofluids for enhancing heat transfer efficiency in BPHEs, especially in applications where compactness and high heat transfer

rates are crucial, such as in refrigeration and HVAC systems. Gungor [49] compared energy consumption between H-type and L-type BPHEs, finding superior heat transfer and pressure drop for the H-type, especially at higher flow rates. This research underscores the importance of nanofluids in optimizing heat transfer performance and energy efficiency in BPHE applications, with potential implications for various industries and systems requiring efficient heat exchange. In addition to the existing research, Pulagam et al. [50] investigated the impact of Al2O3-water nanofluid in a brazed plate heat exchanger. Utilizing CFD modelling, they assessed the nanofluid’s influence on heat transfer and friction factor across a variety of BPHE designs. Their findings indicated minimal changes in friction factor and Nusselt number, a crucial observation considering factors such as economic feasibility and fluid preparation complexities. Jalili et al. [51] investigated the use of a curved rectangular fin in a double-pipe heat exchanger carrying nanofluid. An improvement of 81% to 85% in efficiency was observed. Another work by Jalili et al. [52] investigated the thermo-hydraulic performance of a non-continuous baffle with different helix angles and hybrid nanoparticles. It was observed that increasing the helix angle decreases the heat transfer coefficient by almost 50%. Both of the works were influential in the usage of nanofluids in heat exchangers. Kaplan et al. [53], Bousri et al. [54], Aouanouk et al. [55], Zhu et al. [56] have also made significant contributions. The study of single-phase flow in plate heat exchangers has laid a solid foundation for understanding fluid behavior and optimizing heat transfer performance. Various studies have investigated the effects of geometric parameters, such as corrugation angle and amplitude, on friction factor and Nusselt number correlations. These findings have been crucial in designing efficient heat exchangers for diverse applications. Moving forward, the focus shifts to exploring two-phase flow phenomena, including boiling and condensation, which are essential for applications like refrigeration, HVAC systems, and power generation. By building upon the insights gained from single-phase flow studies, researchers aim to further enhance the performance and efficiency of plate heat exchangers in handling multiphase fluid flows. Study of Single-Phase Flow in Brazed Plate Heat Exchanger The increasing use of BPHE as evaporators/condensers has led to extensive research on the behavior of various refrigerants within these systems. While some researchers focused on specific refrigerants like R134a [8] or R410A [18], Giovanni A. Longo stands out for his comprehensive exploration of numerous refrigerants since the CFC ban. Longo’s research, spanning multiple refrigerants, has been pivotal in developing correlations for evaporation and condensation within BPHE. His meticulous documentation and extensive data collection have provided valuable insights into refrigerant behavior in these systems, shaping

our understanding of their performance and efficiency. Subsequent sections will delve into Longo’s work, highlighting the range of refrigerants studied and the comparative analyses conducted Yan et al. studied R-134a evaporation [8] and condensation [57] in a plate heat exchanger. They found that R-134a, being a potential CFC replacement, was explored due to its ban. Unlike prior studies focusing on smooth pipes, this was the first examination of R-134a in a plate heat exchanger. Using a 60° corrugation angle heat exchanger, they observed linear relationships between vapor quality and condensation heat transfer coefficient. Despite minimal impact from mass flux variations, condensation heat transfer was notably higher in plate heat exchangers compared to circular pipes. Pressure drop increased with heat flux but showed little change beyond a certain threshold. For evaporation, the heat transfer coefficient increased exponentially with vapor quality and mass flux, with pressure drop rising accordingly. Overall, these findings shed light on the unique behavior of R-134a in plate heat exchangers. Hsieh and Lin [58] studied R410A evaporation, while Kuo et al. [59] focused on its condensation, addressing the lack of two-phase heat transfer data in plate heat exchangers. Their experiments revealed increasing heat transfer coefficients with vapor quality and mass flux, with notable effects in condensation. System pressure minimally affected heat transfer, contrasting with R-134a. The studies offered insights into R410A behavior, essential for its adoption as a refrigerant replacement. Longo (and Gasparella) examined R410A’s vaporization [60] and condensation [61] in a brazed plate heat exchanger, revealing a significant sensitivity to heat flux and

outlet conditions. Confirming nucleate boiling dominance using the Lockhart-Martineli parameter used by Palm and Claesson [62], the observation was validated. Frictional pressure drop was correlated to kinetic energy per unit volume with a mean deviation of around 8.5%. Condensation displayed weak sensitivity to saturation temperature, with mass flux doubling enhancing the heat transfer coefficient by up to 30%. A fitted correlation for frictional pressure drop yielded a mean deviation of approximately 22%. Longo (and Gasparella) explored R134a’s vaporization [63] and condensation [64] in the same brazed plate heat exchanger used in prior studies, echoing trends seen with R410A, albeit with slight deviations. Heat transfer coefficients were compared using previous correlations [54] and [55], with a mean deviation of around 8.2% and 12.3% respectively. The Webb model predicted heat transfer coefficients during condensation’s forced convection. They further investigated R236fa’s vaporization [65] and condensation [66], noting similar trends to R410A and R134a. R410A displayed higher coefficients than R134a and R236fa during vaporization but similar ones during condensation, with significantly lower frictional pressure drop. Vaporization of R236fa was dominated by convective boiling, while the other two were by nucleate boiling. Longo et al. [67] conducted a comparative study, akin to [68], assessing heat transfer coefficients and pressure drops in a heat exchanger for R600a, R290, and R1270 as HFC refrigerant alternatives. Notably, R290 demonstrated similar performance to R22 in heat transfer with a 50% lower pressure drop. R1270 exhibited higher heat transfer coefficients than R600a and R290, with lower pressure drops. Further studies on superheating during condensation

Figure 6. Experimental setup used for BPHE testing [From Longo and Gasperalla [60], with permission from Elsevier.]

[69], [70] revealed increased heat transfer coefficients but unchanged pressure drops. These findings indicate a consistent trend in heat transfer coefficient dependencies on saturation pressure, outlet conditions, mass flux, and heat flux, consistent with Longo’s earlier work. Additionally, nucleate boiling was confirmed using previously employed equations, with IR thermography analysis shedding light on heat transfer surface areas affected by vapor superheating. R1234yf [71], studied as a potential R134a replacement, exhibited slightly lower heat transfer coefficients (6%-10%) and pressure drops (10%-18%) compared to R134a. Longo et al. investigated R1234ze(E) condensation [72] and vaporization [73], demonstrating trends consistent with their previous studies. Condensation exhibited a heat transfer coefficient 4%-6% lower and a 10% higher pressure drop than R134a. Vaporization comparisons with R1234yf and R134a revealed similar heat transfer coefficients but slightly higher pressure drops. Longo et al. [74, 75] developed computational models for boiling and condensation in BPHE using data collected over the years. These models categorized refrigerants into HFC and HC, HFO types. Nucleate boiling was dominant in some, while convective boiling prevailed in others. Correlations for heat transfer coefficients in nucleate and convective boiling were provided, along with a model for superheating. For condensation, models were developed for laminar and forced convection regimes, with correlations showing mean absolute deviations of around 20% and less than 16%, respectively, compared with experimental data. Longo et al. [76] examined R152a for condensation, noting its potential as a replacement for R134a in large chillers. Results mirrored those of previous refrigerants tested, with a transition to forced convection condensation at a mass flux of 15 kg/m2s and a notable increase in heat transfer coefficient with doubled mass flux. R152a exhibited higher heat transfer coefficients than R290, R134a, and R1234ze(E), making it suitable for large chiller applications due to its low GWP. Additionally, the vaporization of R32 [77] was studied, revealing similarities in trends with R410A [60] but with lower heat transfer coefficients (20%-30%) and higher pressure drops (30%-40%). This was attributed to R32’s lower reduced pressure compared to R410A, highlighting the importance of limiting superheat to maintain stable operation. Longo et al. [78] explored the vaporization and condensation behaviors of R404A, contrasting its performance with potential substitutes R290 and R1270. With R404A facing replacement due to its high GWP, comparisons were made under similar experimental conditions. Vaporization data showed a maximum 15% mean deviation when compared with existing models. R290 and R1270 demonstrated higher condensation heat transfer coefficients (25%-45%) with similar pressure drops, transitioning to forced condensation around 20 kg/m2s mass flux. IR thermography analysis revealed no dry-out during vaporization, suggesting R290 and R1270 as promising alternatives for R404A in refrigeration. In recent years,

Longo et al. [79] explored the boiling characteristics of low GWP refrigerants R1234ze(Z) and R1233zd(E) under similar experimental setups. R1234ze(Z) exhibited higher heat transfer coefficients (17%-22%) and lower frictional pressure drops (7%-32%) compared to R1233zd(E), attributed to its superior thermophysical properties. IR thermography confirmed convective boiling dominance for both refrigerants, with outlet vapor superheating degrading average boiling heat transfer coefficients. Experimental data validated author-proposed models [68]. Additionally, Longo et al. provided local heat transfer coefficients for R32 and R410A [77], [80], showing fair agreement with Longo et al.’s correlations [74], [75]. They also presented local boiling and condensation heat transfer coefficients for R290 and R1270, concluding their performance to be intermediate between R404A and R410A. Han et al. [18] conducted experiments to investigate the evaporation characteristics of R410A and R22 in various geometric configurations of BPHE, varying chevron angles and pitches. Results showed that as mass flux increased, so did the heat transfer coefficient, with a corresponding decrease in chevron angle. Decreased chevron angles led to longer flow times, while the average heat transfer coefficient decreased with rising evaporation temperature. Convective heat transfer prevailed, particularly in high-vapor-quality regions. Pressure drop increased with decreasing evaporation temperature, notably in lower chevron angle heat exchangers, and differed among the BPHEs with increasing vapor quality. Compared to R22, R410A exhibited 10%30% lower pressure drop and 0%-15% higher heat transfer coefficients, suggesting its suitability as a replacement. In another study, Claesson [81] aimed to rectify potential errors in estimating the heat transfer coefficient in BPHEs based on previous work. Through theoretical analysis and comparison with experimental data, Claesson proposed a correction factor to enhance the accuracy of predicting heat transfer coefficients. Assumptions like negligible pressure drop and nucleate boiling were employed to simplify the derivation, yielding a correction factor deemed insignificant for moderate to high logarithmic mean temperature differences. The study affirmed the reliability of the logarithmic mean temperature difference method in predicting heat transfer coefficients in BPHEs. Hayes et al. [82] explored carbon dioxide (CO2) as a potential refrigerant for BPHE due to its high-pressure limits. Using custom BPHEs, they investigated single-phase and two-phase (condensation) flow of CO2, maintaining superheat and sub-cool under 5°C. Pressure loss considerations included gravity effects, estimating a total loss of about 1.5 times the inlet velocity head. Friction factor correlations were provided for both single-phase and two-phase flow, with uncertainties below 3% and 10%, respectively. Muthuraman [83] analyzed BPHE performance with three chevron angle configurations (20°, 35°, and 45°), observing decreasing heat transfer coefficients with increasing chevron angle. Higher vapor quality led to increased heat

transfer coefficients due to elevated turbulence levels, with frictional pressure drop rising with mass flux and vapor quality but decreasing with condensation temperature and chevron angle. A comparison of R410A and R22 showed heat transfer coefficient ratios ranging from 0.8 to 1.1 for different cases. Mancin et al. [84] examined partial condensation of R32 while Rossato et al. [85] investigated flow boiling of R32 in BPHEs. For partial condensation, various inlet superheating levels were tested, revealing heat transfer coefficient ratios independent of mean vapor quality at 15 kg/m2s, increasing with superheating at 20 kg/m2s, and consistent trends at 40 kg/m2s. In flow boiling, heat transfer coefficients increased with mass flux and heat flux but decreased with outlet vapor quality, influenced by nucleate and convective boiling at high heat fluxes. Pang et al. [86] assessed BPHE performance in a Joule-Thomson cooler with mixed refrigerant, highlighting the impact of the recuperative heat exchanger. Despite mass transfer resistance decreasing with heat flux during convective condensation, its fraction in overall heat transfer resistance increased. Forced convective boiling exhibited similar trends. The cooler took 100 minutes to reach 127 K from an ambient temperature of 294.4 K, with overall heat transfer coefficients ranging from 38.3 to 362.5 W/m2K. Exergy efficiency varied from 54.2% to 85.7%. Raveendran and Sekhar [87] investigated the performance of a water-cooled BPHE as a condenser in a domestic refrigeration system, comparing it with typical air-cooled condensers. Results showed significant improvements in energy efficiency, with the water-cooled BPHE reducing compressor work by 24%-28% and increasing coefficient of performance by 52%–68%. Energy consumption was also reduced by 21%-27%, with R290/R600a consuming 8.6% less energy than R134a. Palmer et al. [17] explored the heat transfer performance of flammable refrigerants in BPHEs, developing a common correlation. Taboas et al. [88] studied ammonia water mixture evaporation in a BPHE, finding that heat transfer coefficients varied with mean vapor quality and mass flux. Mutumba et al. [89] proposed new correlations for heat transfer in water to water and water to R1233zd(E) BPHEs across a range of Reynolds numbers. Li and Hrnjak [90] utilized infrared (IR) cameras to examine two-phase regions and refrigerant maldistribution within BPHEs, subsequently validating their findings with experimental data. Their follow-up research [91] highlighted the detrimental impact of maldistribution on heat exchanger performance, attributed to discrepancies in heat transfer coefficients between superheated and two-phase regions. Li [92] further explored this phenomenon, investigating its implications for system control. They also visually analyzed two-phase refrigerants in inlet headers, while Navarro-Peris et al. [93] conducted similar maldistribution studies. Li and Hrnjak [94] extended their investigation to single-phase flow, developing numerical and mechanistic

models [95] to estimate these effects, with computational fluid dynamics (CFD) models proving more accurate. Will et al. [96] conducted a thermal analysis using infrared thermography on BPHE, while Zendehboudi et al. [97] explored CO2 refrigerants at supercritical states, observing Nusselt number increases with buoyancy force. Sadeghianjahromi et al. [98] studied chevron angle effects on heat transfer and pressure drop, employing the k-ε realizable model for turbulent simulations. Salman et al. [99] examined saturation flow boiling of R290 in BPHEs with offset strip fins, yielding findings akin to Longo et al. [66] but with numerical differences due to plate configurations. Gullapalli [100] developed a simulation program for Organic Rankine Cycle systems utilizing BPHE, offering design and performance modes for cost minimization and efficiency optimization, respectively. The program integrates thermal, hydraulic correlations, and refrigerant properties to ensure robust simulations. Meanwhile, Desideri et al. [24] investigated flow boiling and pressure drop in such systems using R245fa and R1233zd. They found that heat transfer coefficients decreased at 0.5–0.6 vapor quality due to local dry-out, with nucleate boiling dominating. Pressure drop decreased at higher saturation temperatures but increased with vapor quality, validated by previous studies and correlated well with existing models. Kim and Kim [25] investigated the efficacy of a brazed plate heat exchanger (BPHE) with a metal foam insert in an Organic Rankine cycle (ORC) setup. The modified heat exchanger demonstrated up to 2.3 times higher overall heat transfer coefficients compared to conventional BPHEs, despite increased pressure drop. Operating with R245fa, the brazed foam plate heat exchanger (BFPHE) exhibited lower outlet qualities and superheat temperatures than BPHEs. Despite its smaller size, BFPHE showed remarkable performance, with heat transfer rates ranging from 75 kW to 108 kW, 82% to 198% higher overall heat transfer coefficients, and higher frictional pressure drops compared to BPHEs. Nematollahi et al. [26] explored the application of brazed metal-foam plate heat exchangers (BMPHEs) in compact ORC systems. Utilizing water as the working fluid, BMPHEs demonstrated increased pressure drops and power densities compared to BPHEs, with efficiency gains of at least 1% and slightly enhanced power output, suggesting their suitability for compact ORC setups. Imran et al. [101] investigated the flow boiling characteristics of R245fa in a brazed plate heat exchanger (BPHE) for Organic Rankine Cycle systems. Testing different chevron angles, mass fluxes, vapor qualities, and heat fluxes, they observed that the heat transfer coefficient peaked at a vapor quality of

Conclusion

In conclusion, the extensive research conducted on BPHE has provided valuable insights into their performance across various operating conditions and

applications. Studies on single-phase flows have shed light on the influence of geometric parameters, such as chevron angles, on heat transfer and pressure drop characteristics. Additionally, investigations into two-phase flows, particularly involving refrigerant evaporation and condensation, have resulted in the formulation of correlations for friction factor and Nusselt number, aiding in the design and optimization of BPHEs. Moreover, numerical studies using CFD have allowed for detailed analyses of flow and heat transfer patterns, albeit at a high computational cost. These studies have collectively contributed to a deeper understanding of BPHEs, their potential applications in systems like Organic Rankine cycles, and the challenges associated with their design and optimization. Overall, the research efforts in single-phase, two-phase, and numerical studies have significantly advanced the field of brazed plate heat exchangers, paving the way for further innovations and improvements in their performance and efficiency. The following conclusions were the findings on the advancement in the research of brazed plate heat exchangers. • Flow and heat transfer patterns have been primarily investigated in gasketed plate heat exchangers, but their findings are applicable to brazed plate heat exchangers due to their similar geometry. • Studies on single-phase flows are comparatively limited, with a focus on variations in chevron angles as the most common investigation. • Two-phase flow studies predominantly involve refrigerant evaporation and condensation, leading to the formulation of correlations for friction factor (f) and Nusselt number (Nu) based on extensive databases. However, differences in operating conditions and correlation techniques have resulted in multiple correlations for similar refrigerants. • BPHEs have found applications in Organic Rankine cycle systems as evaporators and with nanofluid applications. However, the impact of nanofluids on heat transfer in BPHEs appears less significant compared to other types of heat exchangers. Computational fluid dynamics (CFD) studies remain essential but computationally expensive due to the complex geometries of BPHEs, necessitating the use of a large number of elements and computational resources.

Nomenclature

Boiling Number Diameter Friction factor Mass flux Acceleration due to gravity Heat transfer coefficient Enthalpy of vaporization Length Nusselt number Prandtl number

pressure Critical pressure Heat flux Reynolds number Roughness Average mean roughness Temperature difference Average velocity Vapor quality Martinelli Parameter

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