Attaining High Energy Efficiency with Less Materials Using
Smaller-Diameter, Inner-Grooved Copper Tubes
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REAL INSIGHTS IN THE AGE OF SIMULATIONS |
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Accuracy of Modeling of Heat Exchangers is Uncanny |
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Practically every field of science and engineering now uses simulations. As a case in point, the Nobel Prize in Physics 2021 was awarded "for groundbreaking contributions to our understanding of complex systems" [1]. One half was awarded jointly to Syukuro Manabe and Klaus Hasselmann "for the physical modelling of Earth's climate, quantifying variability and reliably predicting global warming" and the other half to Giorgio Parisi "for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales."
The use of simulations has become commonplace in practically every field of engineering design, including heat exchangers. The mathematics has been available for decades or even for centuries and now computing power has caught up with the mathematics, making powerful simulation software widely available to product design engineers. Simulation tools for design engineers have also become relatively easy to use, with menu driven interfaces and 3D visualizations of results.
One does not need to be an expert in the mathematics of complex systems to run a simulation of a heat exchanger design. That is fortunate because there is a need to bring new technologies to bear on new product designs. There is an urgent demand for heat pumps to facilitate electrification. A new generation of refrigeration and AC appliances and systems is needed for F-gas reduction and improved energy efficiency [2]. Refrigerant formulations, heat-exchanger technology and compressor designs are rapidly changing even as the times-to-market are accelerating in a once stodgy industry.
What follows is an overview of several important simulation software tools available to design engineers today. Well-known programs are the Heat Pump Design Model (HPDM) from Oak Ridge National Laboratory (ORNL); CoilDesigner® from Optimized Thermal Systems (OTS) and the University of Maryland; and HXSim from Shanghai Jiao Tong University and the ICA.
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The Heat Pump Design Model (HPDM) from ORNL
HPDM has a long history. A FORTRAN version was thoroughly documented in a 1983 report from Oak Ridge National Laboratory (ORNL). According to that report, the starting point for HPDM was a model developed at MIT in 1976 [3]. This ORNL report available online today is interesting for comparisons between yesterday with today [4].
In 1983, HPDM allowed for the following inputs and outputs:
Inputs
- System operating conditions
- Compressor characteristics
- Refrigerant flow control devices,
- Fin-and-tube heat exchanger parameters
- Fan and indoor duct characteristics, and
- Any of ten refrigerants
Outputs
- System capacity and COP (or EER),
- Compressor and fan motor power consumptions,
- Coil outlet air dry- and wet-bulb temperatures,
- Air- and refrigerant-side pressure drops,
- A summary of the refrigerant-side states throughout the cycle, and
- Overall compressor efficiencies and heat exchanger effectiveness.
See the original report for a detailed description of input. According to the report:
“The philosophy of the model development has been to base the program on underlying physical principles and generalized correlations to the greatest extent possible, so as to avoid the limitations of empirical correlations derived from manufacturers' literature.”
This seminal model was shared with industry and academics for years to follow and an extensive literature grew around the HPDM software as archived on an ORNL website [5a]. Forty years later, HPDM is available as an HTML program that runs online. The latest HPDM (Mark VIII) is in the public domain and is available as a downloadable desktop version.
The software allows for separately simulating heat exchanger performance and for simulating the complete system. There are quite a few choices available in HPDM. Application modeling included not only heat pumps but also heat pump water heaters and water-tank heaters. HPDM can simulate tube-fin heat exchanger (TFHX) with diameters as small as five millimeters with units in English (inches).
The ORNL Heat Pump Design Model remains a popular tool and has been used and adopted by several major US manufacturers as their in-house product design tool.
The program has an intuitive interface that allows users to set parameters for all the components of the heat pump, including both the evaporator and condenser. The software continues to be supported by the ORNL with ongoing work to develop correlations for alternative refrigerants and refrigerant blends. It can be used to simulate individual components such as the evaporator alone or the condenser alone. Yet it can also simulate the complete systems including compressors.
Clearly the United States Department of Energy (DoE) research on heat pumps is aligned with the basic purposes of the DoE. If one is developing heat pumps then it is wise to become familiar with the HPDM.
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CoilDesigner® from the University of Maryland
CoilDesigner® software is being developed by the Center for Environmental Energy Engineering (CEEE) at the University of Maryland (UMD) since 2001.
CoilDesigner is a proprietary software that is typically licensed to OEMs and component suppliers involved in the design and manufacture of heat exchangers for residential or commercial applications. CoilDesigner allows designers to simulate and optimize the performance of heat exchangers, ultimately aiding in shortening the product development timeline and associated cost. It is a highly customizable software tool and can be used for tube-fin, microchannel, and coaxial heat exchanges. Simulation can be conducted with any refrigerant available in the NIST REFPROP database with the ability to also add a custom fluid.
While CoilDesigner is used for simulation of air-to-refrigerant heat exchangers, its companion simulation tool VapCyc® is used for system simulation. Heat exchangers designed with CoilDesigner can be imported into VapCyc to evaluate system capacity, COP, and EER. VapCyc is a component-based simulation tool for vapor compression heat pump simulation and optimization.
Besides licensing CoilDesigner software, many OEMs sponsor the CEEE and participate in regular workshops. Active participants can have a role in developing new directions for HVAC&R research.
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OTS-ICA Collaboration
Optimized Thermal Systems Inc., Beltsville, Maryland (OTS) has an exclusive license agreement with the UMD to provide customized versions and assist in the integration of of CoilDesigner software. OTS actively works with various HVAC&R designers around the globe to optimize heat exchanger geometry.
ICA has worked with OTS to develop MicroGroove correlations, which have been implemented in the CoilDesigner® software. These correlations include airside (fin) and refrigerant side (tube) correlations.
In partnership with ICA, Daniel Bacellar and Dennis Nasuta from OTS delivered a webinar that explains the use of these new correlations in the CoilDesigner software. This webinar and the corresponding slideshow can be accessed via the OTS-ICA landing page on the microgroove.net website [9]. For those interested in learning the “nuts and bolts” of how CoilDesigner works with fin and tube correlations, the webinar titled “The Advantages of Small Diameter Copper Tube-Fin Heat Exchangers” is recommended. It is quite technically detailed and is the first in a series of six webinars promoted by ICA and OTS with the Copper Development Association (CDA). There are also several academic papers from OTS on the development of the MicroGroove correlations. There is a special landing page dedicated to the ICA-OTS partnership on the microgroove.net website.
An early paper on CoilDesigner® was published in 2002 [10]. Other key papers from CEEE were published in 2006, 2008 and 2009 [11-13].
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Genetic Algorithms
OTS has its own staff of researchers and the company works with OEMs in training their staff on the CoilDesigner software. OTS staff also offers their expertise in optimizing heat exchanger design through the use of Multiple Objective Genetic Algorithms (MOGA). This optimization approach allows for evaluation of a very large design space while managing computation time and power.
MOGA allows for a systematic exploration of the design space, allowing for the gradual evolution of an optimal design. This approach is akin to the “natural selection” of inherited traits that aid the survival of living organisms.
Genetic algorithms were originally developed by Holland (1975) and have been thoroughly reviewed in textbooks by Goldberg (1989) and Engelbrecht (2007) [14-16]. Evolutionary algorithms and specifically genetic algorithms (GA) are widely used in many fields. These algorithms are based on ideas about natural selection and natural genetics as originally developed in the 1960s and 1970s by John H. Holland at the University of Michigan. Interest in this multidisciplinary subject has only increased in recent years as computer simulations have improved in accuracy.
MOGA is an especially powerful approach toward optimization of designs in which there are multiple constraints and objectives. A description of the application of MOGA towards the optimization of heat exchangers has been presented by Aute et al. (2004) [17].
“We have been successful in identifying improved designs using CoilDesigner® and MOGA optimization for numerous applications,” says Cara Martin, COO at OTS. ICA sponsored several MOGA studies with OEMs. Select results have been summarized in a paper delivered at the most recent ICR held in Montreal in 2019 [18].
Recent research at CEEE has expanded beyond simple simulations and optimization to include complex heat exchanger circuitry with splits and merges. Zhenning Li from ORNL and Vikrant Aute from University of Maryland outlined their algorithm for the optimization of circuitry in evaporators in a paper titled “Enhanced Integer Permutation based Genetic Algorithm for Optimization of Tube-Fin Heat Exchanger Circuitry with Splits and Merges” (Purdue 202One, Paper 2574) [19]. According to their case studies, optimal designs obtained using their algorithm exhibit higher capacity, lower pressure drop and better manufacturability compared to baseline designs.
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HXSim from SJTU
As a reader of the MicroGroove Update, you may already be acquainted with HXSim simulation software.
Have you downloaded this powerful simulation software?
The software has been made available at no charge to qualified users under the auspices of the International Copper Association. Once you have registered the software (by providing your name and email to ICA) then you will have a full working version. The registration will need to be renewed on an annual basis, to encourage users to download the latest version.
The HXSim software originated at the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University (SJTU) in Shanghai, China. The software was developed with the support of a consortium of manufacturers, universities and associations, including the International Copper Association. An early version of the software was described in 2004 by Liu et al. [21]. A paper titled “Simulation-Based Design Methods for Room Air Conditioners with Smaller Diameter Copper Tubes” presented at the International Congress of Refrigeration in Prague (IIR ICR in 2011) describes the inner working of the software including the physical equations that it solves [22].
Heat exchanger simulation software at SJTU continued to be developed alongside laboratory measurements of correlations for smaller diameter copper tubes and fin designs in cooperation with members of the consortium. More and more features were added as well as a user-friendly menu-driven interface and the software. By 2018, the software was made available at no charge to qualified users by contacting the ICA. More recently, It was also made available to download at no charge along with a user guide translated into English [23].
In the current version of HXSim, the user enters design parameters such as coil block type, tube spacing, fin spacing and so on through a graphical user interface. Operating conditions such as refrigerant and air mass flow are entered. The software then calculates the behavior of the coil and displays the results in a table, chart or as 3D visualizations.
“The user guide gives a detailed description of how to build and simulate the performance of a heat exchanger,” says Yoram Shabtay of Heat Transfer Technologies. “The learning curve is quite gentle. A person knowledgeable of heat exchangers could master the software interface within a few hours.”
According to Shabtay, the menu driven interface makes it easy to enter key geometrical parameters. The software has a rich database of correlations for different sizes of copper tubes as well as various internal enhancements and fin designs. As a result, simulation results are very accurate.
Shabtay notes that the simulations are for the heat exchanger only. HXSim does not calculate the system efficiencies that are calculated in a program such as HPDM from ORNL or VapCyc from OTS. Nor does it easily allow for optimization studies such as the MOGA projects that OTS provides to its clients.
“Nonetheless, this is a professional grade simulation software program that is versatile and accurate. Many laboratory verified-correlations are built into the program,” he says. “Results are available as 3D diagrams, 2D plots or even in tabular formats.”
“HXSim is very useful. One of the best features of HXSim is that it allows for quick comparisons between different tube sizes, tube spacings, fin spacing and refrigerants. It is quite easy to change the coil geometry and the refrigerant and then run a simulation and save the results,” he adds.
As a consultant for the ICA based in the USA, Shabtay is available to assist HXSim users with learning to apply the simulation software to current projects. He will also relay suggestions on how the software could be improved to the software development team at SJTU through the ICA China office.
“HXSim software is specific to copper tubes as used in round tube plate fin (RTPF) heat exchangers,” says Kerry Song of ICA China, “including simulations of smaller diameter tubes. The simulation software allows the user to select from an extensive database of correlations. The user can select from a menu of different-sized tubes and detailed microfin geometries. There is also a large data base of fin designs included in the package. Our focus on copper tubes and aluminum fins allows users to access state-of-the-art copper tube technology from a broad range of tube suppliers and plate-fin manufacturers.
“This combination of depth and breadth of menu choices helps designers to bring products to market faster. We are constantly adding new correlations for combinations of MicroGroove tubes and new refrigerants, including low GWP refrigerants, natural refrigerants and refrigerant blends,” Song says.
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Toward the Next Generation
Simulation programs are as commonplace to heat exchanger engineers today as sliderulers were to engineers in the 1950s and calculators in the 1970s.
The technology has matured to the point where the end user does not have to be an expert at writing code for scientific computations. User friendly interfaces and outputs also help free up the time of end users. Now end users can spend more time on creative work and less time on repetitive tasks.
The programs are now fast and accurate. Packages of software specific to heat exchangers are available to designers as indicated above. Additionally, for those familiar with scientific computing, there are more general software packages that can be used to develop programs dedicated to specific computing problems. Several such research projects were described in a recent issue (Volume 11, Issue 1) of the MicroGroove Update newsletter [23] and in the “In the Spotlight” column below on the TPTPR Virtual Conference recently organized by researchers at the University of Padua.
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References
Main Article References
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[2] "Net Zero Carbon Goals Assisted by Microgroove Copper Tubes," MicroGroove Update, Volume 10, Issue 2.
[3] C. C. Hiller and L. R. Glicksman, Improving Heat Pump Performance
via Compressor Capacity Control -Analysis and Test, VOWS. I and
II, MIT Energy Laboratory Report No. MIT-EL 76-001, 1976.
[4] “The Oak Ridge Heat Pump Models: I. A Steady-State Computer Design Model for Air-to-Air Heat Pumps” S. K. Fischer and C. K. Rice.
https://www.optimizedthermalsystems.com/index.php/products/vapcyc
[10] Jiang, H., Aute, V., Radermacher, R., 2002, “A User-Friendly Simulation and Optimization Tool for Design of Air-Cooled Heat Exchangers”, Proceedings of 16th International Compressor Engineering and 9th International Refrigeration and Air Conditioning Conferences.
[11] Jiang H, Aute V, Radermacher R. 2006, CoilDesigner: a general-purpose simulation and design tool for air-to-refrigerant heat exchangers, Int. J. Refrig. 33 (7): 1356-1369
[12] Singh, V., Aute, V. and Radermacher, R., 2008 "Numerical approach for modeling air to refrigerant fin-and-tube heat exchanger with tube-to-tube heat transfer"International Journal of refrigeration, 31(8),1414-1425
[13] Singh, V., Aute, V. and Radermacher, R., 2009. "A heat exchanger model for a fin-and-tube heat exchanger with an arbitrary fin sheet" International Journal of Refrigeration.
[14] Holland, J.H., 1975. Adaptation in Natural and Artificial Systems: An Introductory Analysis with Applications to Biology, Control and Artificial Intelligence. MIT Press, Cambridge, MA, USA.
[15] Goldberg, D. E., 1989. Genetic Algorithms in Search, Optimization and Machine Learning. Addison-Wesley Publishing Company, Reading, Mass.
[16] Engelbrecht, A.P., 2007. Computational Intelligence: An Introduction, 2nd Edition, Wiley, Chichester, West Sussex, UK. 628 pages. Chapter 9 Genetic Algorithms, 143-174.
[17] Aute, V., Radermacher, R., Naduvath, M.V., 2004. Constrained multi-objective optimization of a condenser coil using evolutionary algorithms. In: International Refrigeration and Air Conditioning Conference at Purdue, West Lafayette, Indiana.
[18] Cotton, N., Stillman, H., Nasuta, D., Shabtay, Y. 2019. Optimization of copper-tube coils for energy-efficiency and charge reduction in heat pumps, air conditioners and refrigerators, The 25th IIR International Conference of Refrigeration, Montreal.
[19] Li, Z., Aute V., (2021) “Enhanced Integer Permutation based Genetic Algorithm for Optimization of Tube-Fin Heat Exchanger Circuitry with Splits and Merges”
[20] HXSim The software has been made available at no charge to qualified users under the auspices of the International Copper Association. It can be downloaded from the microgroove.net website. There are detailed instructions for downloading and running the software available at www.microgroove.net/HXSim.
[21] Liu J, Wei W.J., Ding G.L., Zhang C.L., Fukaya M, Wang K.J., Inagaki T. 2004, A general steady state mathematical model for fin-and-tube heat exchanger based on graph theory, Int. J. Refrig. 24 (8): 823-833.
[22] Ding, G.L., Ren, T., Zheng, W., Gao, F. 2011. Simulation-Based Design Method for Room Air Conditioner with Smaller Diameter Copper Tubes. 23rd IIR International Congress of Refrigeration, Prague, Czech Republic. Paper 186.
[24] MicroGroove Update, Volume 11, Number 1 (2021).
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Refrigerants Take Center Stage at TPTPR2021 |
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New Research on Tubes and Heat Exchangers, Too. |
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The International Institute of Refrigeration (IIR) Commission B1 on Thermodynamics and Transfer Processes has been extremely active in the areas of working groups (WG), IIR conferences, IIR-co-sponsored conferences and commission business meetings. Since the first year of the new millennium, past and future Commission B1 conferences on the Thermophysical Properties and Transport Properties of Refrigerants conferences, also known as “TPTPR,” took place as follows: |
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TPTPR Conference
6th
5th
4th
3rd
2nd
1st
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Place
Vicenza, Italy (virtual)
Seoul, South Korea
Delft, the Netherlands
Boulder, United States
Vicenza, Italy
Paderborn, Germany
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Year
2021
2017
2013
2009
2005
2001
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The theme of TPTPR2021 was “Low GWP Refrigerants: 10 Years After.” The year 2021 is 10 years after the first European F-GAS Regulation fixed the phaseout of refrigerants having GWP > 150 in automotive air conditioning and started an era of intense research and technological activity on low GWP refrigerants.
Coincidentlaly, ICA’s MicroGroove campaign debuted in 2011 at the AHR Expo in Las Vegas. Prior to its Las Vegas debut, technical papers on designing heat exchangers with 5 mm diameter copper tubes and heat exchanger simulations were presented in 2010 [1, 2] as well as 2013 at the Fourth TPTPR Conference [3]. Refrigerants indeed have been “in the spotlight” for the past ten years. The 2021 TPTPR Conference was an opportunity to take inventory of this rapid progress and set goals for the future.
Papers were presented at TPTPR2021 on natural refrigerants (e.g., propane and carbon dioxide) as well as HFOs and various refrigerant blends. Laboratory experiments on smaller-diameter copper tubes and tube-fin heat exchangers were presented as well as simulations of heat exchangers and heat pump systems.
Indeed, heat pumps were examined from many angles as an important technology for decarbonization of buildings. A IIR conference on phase change materials (PCM) was held concurrently. Many papers described synergies between PCM and heat pumps. Indeed, there was much overlap between TPTPR and PCM, especially at the system level.
There is just no way to do justice to the entire conference in this short review. In all there were fifty papers organized into thirteen sessions (and that does not include the PCM papers!). The final program is available at no charge and the complete proceedings can be purchased online.
What follows is a sampling of a few papers along with short synopses. The papers are identified by number here. For full titles, authors and affiliations, please see the final 2021 TPTPR program.
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MicroGroove Simulations with HXSim Software
Authors from ICA and SJTU presented MicroGroove research at the Fourth TPTPR Conference in 2013 in a paper was titled “Fin Design for Fin-And-Tube Heat Exchanger with Microgroove Small Diameter Tubes for Air Conditioner” (Paper TP-071).
In the years since, ICA has continued to support MicroGroove research, including the development of tube and fin correlations for various refrigerants, allowing for extremely accurate simulations. (See the main article in this issue.) ICA has returned to TPTPR in 2021 with a paper titled “Simulation of the effects of copper tube diameter on refrigerant charge reduction in split AC systems and refrigerated cabinets” by Shabtay, Gao and Song (Paper 1969 at the 2021 TPTPR Conference). The slideshow and a video of the presentation are available on the HXSim landing page of the MicroGroove.net website.
The 2021 paper presents two cases of MicroGroove heat exchanger design. The first case study compares outdoor condenser units with copper-tube outer diameters of 7 mm vs. 5 mm in a nominal 3500 W split AC system with R32 refrigerant. The second compares cooling cabinet condensers with tube diameters of 9.52 mm vs. 5 mm using R290 refrigerant and similar capacity.
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Simulations from SINTEF Energy
Another impressive paper was titled “Comparison of Refrigerant Charge Requirements in Optimized Fin and Tube Evaporator versus a Plate Heat Exchangers” (Paper 1923). Authors were from the Norwegian University of Science and Technology and SINTEF Energy, Trondheim, Norway; and Fraunhofer-Institute for Solar Energy Systems, Freiburg, Germany. R290 heat pump systems were optimized for minimal charge using a tube-fin evaporator and a brazed plate condenser. Three different 5 mm tubes were tested for the 5 kW evaporator, including a smooth tube and two different microfinned tubes. MF1 had an internal diameter of 4.32 mm, 35 fins, and spiral angle of 15°; MF2 had an internal diameter of 4.26 mm, 56 fins and spiral angle of 37° degrees. Consequently, MF2 had a significantly higher internal area for heat exchange (2.63 versus 1.50). The use of microfinned tubes reduced the total charge by 33.6 percent and 50.3 percent for MF1 and MF2, respectively.
This research demonstrates the difference between smooth tubes and microfinned tubes for small diameter MicroGroove tubes; and indeed, the type of microfins can also have a significant effect on the charge reduction. The researchers used in-house code referred to as “HXSim” for simulations. The code was developed by SINTEF Energy and has no connection with the ICA’s HXSim simulation software. According to the references in this article, the SINTEF Energy software is described in a paper titled “Simulation of extended surface heat exchangers using CO2 as refrigerant” from the Proceedings of the 4th IIR-Gustav Lorentzen Conference on Natural Working Fluids held at Purdue in 2000.
The upshot is that when using 5 mm MicroGroove Tubes in an evaporator with R290 as a refrigerant, the type of internal enhancement has a significant effect on the possible charge reduction that can be achieved. This is especially important when using a flammable refrigerant such as propane (R290).
It should be noted that ICA’s HXSim software allows for various types of tube enhancements to be selected. The different correlations are included in the software and the tube enhancement can be selected from a menu.
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Laboratory Measurements on Microfin Copper Tubes
TPTPR is of course noted for its papers on fundamental measurements of thermophysical and transport properties. These laboratory measurements are crucial for accurate simulations of heat exchangers. The correlations are dependent not only on the tube diameters and internal enhancements but also on the refrigerant. When a new refrigerant is adopted, refrigerant properies for various tube geometries must be experimentally measured. The University of Padova is especially noted for such research. A paper titled “Critical Vapour Quality Measurement during R1233zd(E) Flow Boiling in a Microfin Tube” (Paper 2205) was presented at 2021 TPTPR.
The microfin tube under investigation had a 4.2 mm fin tip diameter, 40 fins, 0.15 mm high with an apex angle equal to 42° and helix angle equal to 18°. These careful experiments were carried out, varying the mass flow rate and measuring the vapor quality and heat flux by means of thermocouples placed along the length of the tube. According to the authors, “Further experimental campaigns with different refrigerants and tubes can help in building a large database of values of critical vapour quality to assess the predicting abilities of the proposed models.”
Zigzags and Wire Mesh
The 2021 TPTPR included several papers exploring original ideas that could best be classified as paradigm busters. One such paper is titled “A Computational Approach to Design and Investigation of Finless Zigzag Shaped Tubes Heat exchanger” (Paper 1965) from researchers at Saga University and Kyushu University in Japan. Although the idea of a micro bare-tube heat exchanger has a lengthy history, Paper 1965 tests the proposition that a Zigzag Shaped Tubes Heat eXchanger (ZSTHX) could outperform a Parallel Tubes Heat eXchanger (PTHX). The short answer is “yes but it depends” on the zigzag patterns and pressure drops. The cases included various air velocities, gaps between tubes and zigzag tube angles.
Not all the papers at TPTPR were on the vapor compression refrigeration cycle. There were two full sessions dedicated to Absorption / Adsorption processes. Paper 1959 titled “Unsteady Evaporation of Water from Wire Mesh Structures at Sub-Atmospheric Pressures” was presented in the session “TPTPR12: Enhanced heat transfer” (Paper 1965 on zigzag tubes was also in the TPTPR12 session.) This study of water evaporation by Volmer and Schnabel from the Fraunhofer Institute for Solar Energy Systems, Freiberg Germany is a good example of original out-of-the-box thinking on the timeless phenomenon of water evaporation. The topic is of interest in the development of adsorption heat pumps and chillers with water as refrigerant. It studies the use of copper wire mesh structures as exemplary capillary structures. The researchers seek to determine the dynamic patterns formed by the refrigerant within the porous structure, how dewetting dynamics interact with heat transfer dynamics during the evaporation process, and how structure geometry (especially pore size) affects this interaction.
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R290 for Domestic Hot Water
Heat pump water heaters are poised to be a key technology for the electrification of residential heating aound the world. It would be even better if domestic hot water could be produced using a natural refrigerant such as propane (R290) which has an ultralow GWP.
The problem is that propane is flammable, so the refrigerant charge of the heat pump must be greatly reduced. In Paper 1944, researchers from Advantix and Politecnico di Milano tested various options for replacing R134a with R290 in domestic hot water (DHW) designs. After reviewing the history of refrigerants in this application and comparing the thermophysical characteristics of R290 with R134a, various components are compared, including tube materials for the evaporator unit, which sits on top of the DHW unit; and for the condensing tubes that wrap around the storage tank. The main challenge of the project is to “respect the stringent limit of 150 g of refrigerant loaded inside the heat pump. A microchannel evaporator was compared with a MicroGroove evaporator. These two exchangers were sized according to the same operating conditions to guarantee comparable performances. Prototype systems were then tested in a climate chamber connected with a wide range of measurement tools. The main design criteria was to reduce the heating time, according to European Standard EN 16147. For complete test results, see Paper 1944. One remarkable conclusion is stated as follows:
“The baseline heat pump works with a charge of 920 g of R134a, corresponding to 1.32 ton of equivalent CO2 potentially released in atmosphere. The machine operating with R290, with a GWP of 3, potentially could affect the atmosphere releasing 150 g of refrigerant, which in terms of CO2 a value less than 0.5 kg, corresponding to the 0.03% with respect to R134a.”
OEMs are also evaluating the low-GWP refrigerant R-454B as a replacement for R-410A in heat pumps. Researchers from the University of Bologna modeled each component of a complete heat pump system. They reported on the results in Paper 1920 titled "Numerical Modelling and Seasonal Performance Analysis of Air-to-Water Heat Pumps Using Low-GWP Refrigerant R-454B as an Alternative to R-410A.”
Different configurations were simulated allowing for a determination of the heat pump seasonal efficiency ratio (SEER) and it was determined that R-454B is indeed a valid alternative. This model can be run in Excel Spreadsheets using VBA (Visual Basic for Applications), which is an easy to learn programming language that runs inside of Excel and other Office programs. An example case study is based on a fin-and-tube V-shape heat exchanger as condenser for a commercial-sized (160 kW) air-to-water heat pump in the cooling mode. The finned coil has copper tubes (8 mm internal diameter, 0.3 mm thickness) arranged on 4 rows, coupled to louvered aluminium fins (1.8 mm pitch, 0.1 mm thickness). A brazed plate heat exchanger was used as an evaporator. Using R-454B rather than R-410A in the same unit, the energy efficiency improved up to 5 percent and the seasonal average EER could be enhanced by up to 2.5 percent. For more details, see Paper 1920.
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Refrigerant Properties
The main focus of TPTPR is of course the measurement of the thermophysical properties of refrigerants. Several key papers on this topic were presented in Session 11 & 13. Noteworthy is a joint paper from Danfoss, IPU, Frascold, Emerson, Tecumseh Embraco and GEA titled “New Package for Generating Refrigerant Equations” (Paper 1867); and a joint paper titled “An Update on the Thermophysical Properties Data Available for Pure Low GWP Refrigerants” (Paper 1985) from the Istituto per le Tecnologie della Costruzione, Consiglio Nazionale delle Ricerche, Padova, Italy; and the Università di Padova.
Opitimizing the System
Significant energy savings can be realized through smart system design at the building and community level. The seasonal and daily demands for heating and cooling can be compared with the available grid electricity, thermal storage and alternative energy generation. Hence the focus on water tanks, phase change materials and ice energy storage. An example is Paper 2038 titled “Ice Thermal Energy Storage for Electricity Peak Shaving in a Commercial Refrigeration/HVAC Unit.” Ice thermal energy storage (ITES) is used in a supermarket, to shave peaks in electricity use. Ice is formed at night-time by when the commercial refrigeration system is only partly loaded; during the day, the “thermal storage” is discharged and operated in parallel to a water chiller, to produce chilled water for air conditioning purposes.
Gustav Lorentzen Conference
There were relatively few papers on R744 as a refrigerant compared to papers on propane and HFO blends. Such research may be best suited for IIR’s biennial Gustav Lorentzen Conference on Natural Refrigerants. The 15th GLC is scheduled for June 2022 and will be brought back to Trondheim, the hometown of Professor Gustav Lorentzen and the cradle for the revival of CO2 as refrigerant.
Artificial Intelligence on the Horizon
Researchers from the Università degli studi di Napoli, presented an intriguing paper titled “Use of Artificial Intelligence in the Refrigeration Field” (Paper 2061). They identified several broad areas of application of artificial intelligence, including the following topics.
- machine learning tools for fault detection and diagnosis (FDD),
- black box digital twin to predict energy consumption and performances,
- AI application for demand defrost,
- optimization algorithms for complex system.
Looking Ahead
Perhaps AI capabilities will be realized by someone who has been inspired by one of the above research topics. The above synopses give just the briefest description of a fraction of the many research papers presented at IIR’s 6th TPTPR Conference. Be sure to download the final program free of charge or download the entire proceedings.
And start now on whatever research will be presented at upcoming conferences or developed into new technologies essential to decarbonization, electrification and HFC reduction. The IIR’s 7th TPTPR Conference will be held in College Park, Maryland, USA in 2025. See the IIR event page for updates.
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"In the Spotlight" References
1. Ding, W., Fan, J. Tao, W., Zheng, W., Gao, F., Song, K., 2010. Development of small-diameter tube heat exchanger: fin circuit design and performance simulation. Proceedings of Conference on Thermal and Environment Issues in Energy Systems, Sorrento, Italy, ASME-ATI-UIT
2. Ding, W., Fan, J. Tao, W., Zheng, W., Gao, F., Song, K., 2010. Development of small-diameter tube heat exchanger: fin design and performance research. Proceedings of Conference on Thermal and Environment Issues in Energy Systems, Sorrento, Italy, ASME-ATI-UIT.
3. Gao, Y., Song, J., Gao, J., Ding, G. 2013. Fin Design for Fin-And-Tube Heat Exchanger with Microgroove Small Diameter Tubes For Air Conditioner, 4th IIR Conference on Thermophysical Properties and Transfer Processes of Refrigerants, Delft, The Netherlands, 2013. Paper TP-071.
4. Skaugen, G. 2000. Simulation of extended surface heat exchangers using CO2 as refrigerant, Proceedings of the 4th IIR-Gustav Lorentzen Conference on Natural Working Fluids, Purdue.
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New Webinar
Webinar Presented by Optimized Thermal Systems on December 7, 2021
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