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A Review of the Influence of Microfin Enhancements on the Condensation Heat Transfer Coefficient for Small Diameter Tubes

White Paper by Burr Oak Tool Inc. and Optimized Thermal Solutions (2015)

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This white paper reviews the relationship between condenser coil tube diameter, internal tube enhancements, and the resultant influence on heat transfer coefficient (HTC). Helical microfins are the primary enhancement of interest, specifically for condenser applications for tube diameters ranging from 9.5 mm to 3 mm outer diameter (OD). Several other tube enhancements, such as herringbone fins and surface microstructures, are also considered.

New Copper-Based Heat Exchangers for R744 Refrigerant, Part I: New Technologies for Tubes and Coils

Yoram Shabtay et al. (Presented at Ohrid 2015.)

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The replacement of current refrigerants with zero ozone depletion potential (ODP) and virtually zero global warming potential (GWP) refrigerants has important implications for heat exchangers, heat pumps, air conditioners and refrigeration system design, and the materials choices in these designs. The use of CO2 as a refrigerant requires components to withstand higher pressures compared to conventional refrigerants. Part I presents critical information about smaller-diameter copper tubes and other heat-exchanger technologies, and Part II discusses design principles and presents example case studies for CO2. Seamless copper tubes with or without inner-grooves can be fabricated from a high-strength copper-iron alloy, reducing wall thickness and thus cost. Coil processing can usually be performed with existing manufacturing equipment since the high strength alloys are brazeable and weldable. Corresponding fittings made from the high strength alloys are also available. Since the volume of CO2 required to achieve the same cooling effect is much lower than for HFCs, components and tubing can be smaller than in conventional installations. In practice, accommodating the high pressures of CO2 systems is advantageous because the smaller diameter tubes used to withstand higher pressures also reduce system size and materials requirements. CuFe2P alloy tubes at small diameters are further advantageous for use in high-pressure CO2 cascade, transcritical and secondary-loop refrigeration systems due to their high strength without increasing wall thickness in the transmission lines.

New Copper-Based Heat Exchangers For R744 Refrigerant, Part II: System Design and Case Studies.

Yoram Shabtay et al. (Presented at Ohrid 2015.)

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Part I presented critical information on how round inner-grooved small-diameter copper tube and newly developed flat copper microchannel tube can be used with alternative refrigerants and especially CO2. Seamless copper tubes with or without inner-grooves can be fabricated from a high-strength copper-iron alloy, reducing wall thickness and thus cost. Corresponding fittings made from the high strength alloys are also available. Part II examines and presents critical information and case studies relating to system design. Since the volume of CO2 required to achieve the same cooling effect is much lower than for HFCs, components and tubing can be smaller than conventional installations. In practice, accommodating the high pressures of CO2 systems is advantageous because the smaller diameter tubes used to withstand higher pressures also reduce system size and materials requirements. CuFe2P alloy tubes at small diameters are further advantageous for use in high-pressure CO2 cascade, transcritical and secondary-loop refrigeration systems due to their high strength without increasing wall thickness in the transmission lines.

New Copper-based Heat Exchangers for Alternative Refrigerants

Yoram Shabtay et al. (Presented at Purdue 2014, paper 2570).

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The ongoing global effort to replace current refrigerants with zero Ozone Depletion Potential (ODP) and virtually zero Global Warming Potential (GWP) refrigerants has important implications for heat exchangers, air conditioning system design, and the materials choices in these designs. Natural refrigerants with higher flammability, CO2, HFOs, and HFC – HFO blends each place different requirements on the heat exchanger design, whether it be for higher equipment efficiency, to reduce refrigerant charge, to operate to much higher operating pressures or temperatures, to prevent corrosion or to avoid leakage. This paper presents critical information on how heat exchangers based on round inner-grooved small-diameter copper tube and newly-developed flat copper microchannel tube can be applied in air conditioning equipment using new alternative refrigerants. These technologies have synergies with key refrigerant performance characteristics enabling multiple application opportunities, and they address operating energy efficiency degradation from mold growth on total Life Cycle Climate Performance (LCCP).

Investigation of Application of Suction Line Heat Exchanger in R290 Air Conditioner with Small Diameter Copper Tube

Tao Ren et al. (SJTU and ICA, Presented at Purdue 2014, paper 2231).

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The use of small diameter copper tube is an effective way to reduce refrigerant charge for R290 air conditioner, but may reduce system performance. To improve the performance of R290air conditioner with small diameter copper tube, this paper presents an investigation of application of a suction line heat exchanger. A theoretical analysis is proposed to investigate the effect of the suction line heat exchanger on capacity and coefficient of performance, and a simulation based analysis is processed to investigate the effect of suction line heat exchanger on refrigerant charge of condenser. To verify the results of analysis, experiments on a prototypeR290 air conditioner are carried out, and the results show that the suction line heat exchanger improves the cooling capacity by 5.3%and system efficiency by4.5%, and reduces the refrigerant charge by 6%, which agree well with that of the theoretical and simulation based on analysis.

Influence of Oil on Heat Transfer Characteristics of R410A Flow Boiling in Conventional and Small Size Microfin Tubes

Haitao Hu et al. (SJTU and ICA, Presented at Purdue 2014, paper 2347).

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In order to extend the existing heat transfer coefficient correlation of R410A-oil mixture flow boiling in conventional size (7.0 mm) microfin tube to be suitable for widely used small diameter tubes (4.0~5.0 mm), the experiments of R410A-oil mixture flow boiling inside three small diameter microfin tubes with different outside diameters of 4.0~5.0 mm and different microfin structures were performed. For the tested tubes with different diameter, the decrease of tube diameter may weaken the deterioration effect of oil on heat transfer at intermediate and high vapor qualities. For the fixed outside diameter microfin tubes with different microfin structures, larger fin height and contact area of liquid with tube wall may enhance the heat transfer for oil-free R410A, but result in smaller enhancement effect of oil at low vapor qualities and smaller deterioration effect of oil at intermediate and high vapor qualities for R410A-oil mixture. A general correlation to predict the heat transfer coefficients of R410A-oil mixture flow boiling inside conventional size and small diameter microfin tubes was developed, and it agrees with 94% of the experimental data of R410A-oil mixture in 4.0 mm ~ 7.0 mm microfin tubes within a deviation of ±30%.

Diffluence Characteristics Investigation of Distributors Used in Heat Pump Type Air Conditioner with Microgroove Tubes

Guoliang Ding et al. (SJTU and ICA, Eleventh IEA Heat Pump Conference 2014, Montréal (Québec) Canada, Paper AO-119093.

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Well design and application of a distributor is important to obtain a reasonable distribution of refrigeration flow among the multi paths of a heat exchanger in heat pump type air conditioner. In this paper, theoretical and experimental studies on the diffluence characteristics of distributors used in heat pump type air conditioners with microgroove tubes were done. The main factors influencing diffluence performance of distributors were summarized by theoretical analysis. The effects of the main factors on diffluence performance of typical distributors were investigated by experiments with air and water as the working fluid instead of actual refrigerants. Based on the theoretical analysis and experimental results, improvement methods for the existing distributors were proposed, and the improvement effects were verified by experiments

Stefano Filippini – List of Papers

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Fin Design for Fin-And-Tube Heat Exchanger with Microgroove Small Diameter Tubes For Air Conditioner

Yifeng Gao et al. (SJTU and ICA, Paper No. TP-071 presented at IIR-TPTPR 2013)

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Optimal louver fins suitable for 5 mm diameter tubes are designed by Computational Fluid Dynamic-based method in this study. Based on the design result, a set of fin-and-tube heat exchangers with 5 mm diameter tubes are tested to develop correlations to predict the performance of new fin-and-tube heat exchanger. According to the experimental results, it is found that water bridge occurs at the bottom of fin with hydrophilic coating, which did not occur in fin-and-tube heat exchangers with 7 mm or 10.33 mm diameter tubes in previous studies. Based on the data, correlation of j is developed to predict the heat transfer rate of fin-and-tube heat exchanger with 5 mm diameter tubes. The mean deviations of the proposed j correlation are 6.5%.

Step-by-Step Coil Design: Principle of Designing Fin-and-Tube Heat Exchanger with Smaller Diameter Tubes for Air Conditioner

Wei Wu et al. (SJTU and ICA, presented at Purdue 2012)

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Several useful design principles as applied to smaller diameter copper tubes are described in this paper. In particular, the knowledge-based evolution method (KBEM) has been developed into a step-by-step procedure that simulates and optimizes every aspect of the heat exchanger design, from tube spacing to fin type to tube circuitry. A methodical approach to the design of heat exchangers using smaller diameter copper tubes is summarized by the following steps: 1) Determine the best ratio of transverse tube pitch to longitudinal tube pitch by fin efficiency analysis; 2) Optimize transverse tube pitch and longitudinal tube pitch by analysis of performance and material cost; 3) Optimize fin pattern by comparing performances of fins with different patterns through CFD-based simulations; 4) Test the performance of heat exchanger with smaller diameter tubes; 5) Develop empiric equations for predicting performance of heat exchanger with smaller diameter tubes.

Copper v Aluminum: Simulation-Based Comparison of Optimized AC Coils Using Small Diameter Copper and Aluminum Micro-Channel Tubes

John C. Hipchen et al. (Copper Alliance and OTS, presented at Purdue)

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New research compares an RTPF heat exchanger with a brazed-aluminum multichannel (BAM) heat exchanger. Until now, there have been few studies comparing MicroGroove technology directly with aluminum MicroChannel technology. For that reason, the ICA sponsored a research project that allows for meaningful comparisons of the performance of these disparate systems. The method of comparison is simple. A search was made for a state-of-the-art, best-in-class BAM heat exchanger. The performance specifications were then identified and set as a target for the RTPF heat-exchanger. The design space was searched for candidate RTPF designs that met the performance specification.

Propane as a Refrigerant: Developing Low Charge R290 Room Air Conditioner by Using Smaller Diameter Copper Tubes

Guoliang Ding et al. (Copper Alliance and SJTU, presented at Delft 2012)

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The research describes a new case study on an R290 room air conditioner with 5 mm tubes. R290 is considered an eco-friendly natural refrigerant R290 because it has zero ozone depletion potential (ODP) and virtually zero global-warming potential (GWP). Moreover, it does not give out any toxic decomposing agents on combustion and is compatible with the materials and lubricants used in air conditioning. In the case study, the refrigerant charge for systems using 5-mm tubes was decreased in comparison to systems using 7-mm or 9.52-mm diameter tubes; and the cooling capacity was enhanced by optimization of heat exchanger as well as the matching of the entire system. Experimental results confirm the simulation results.

Optimization of Heat Exchanger: Simulation-Based Design Method for Room Air Conditioner with Smaller Diameter Copper Tubes

Guoliang Ding et al.  (SJTU and ICA, presented at IIR-ICR-2011)

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Smaller diameter copper tubes are beneficial in reducing the weight of tube and fin materials and the volume of refrigerant charge in room air conditioners while maintaining performance. This paper presents a simulation-based design method for air conditioners with smaller diameter tube. The new method combines heat exchanger simulator and knowledge-based evolution method optimizer to design and optimize air conditioner heat exchangers with smaller diameter tubes. The design method is illustrated in detail through an example of an air conditioner in which 5 mm tubes replace 7 mm tubes in the evaporator and replace 9.52 mm tubes in the condenser. The cost reduction is estimated at 17 percent while the performance deviation is less than one percent.

Copper Coils For R744: Air Cooled Heat Exchangers For CO2 Refrigeration Cycles

Stefano Filippini, Umberto Merlo,  (LU-VE, presented at IIR-ICR-2011, paper 295)

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The article illustrates first the key points of air cooler unit design, the differences compared to HFC products, and underlines the necessity of having low internal volume. DX air coolers and pump air coolers are analyzed.

Reduced Refrigerant Volume: New Finned Heat Exchanger Development With Low Refrigerant Charge

Stefano Filippini, Umberto Merlo, (LU-VE, presented at IIR-ICR-2011, paper 296)

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Many types of air cooled heat exchangers are used in the HVACR sector. The paper presents the results of development in new compact finned tube geometry 20x17.32mm. The technology employs 5.0 mm diameter copper tubes and advanced louvered fins for condenser applications.

MicroGroove Overview: Benefits of Reduced Diameter Copper Tubes in Evaporators and Condensers

Robert Weed et al. (Copper Development Association, presented at ASHRAE-2011)

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The heat transfer characteristics of smooth and inner-grooved tubes are compared and the benefits of smaller diameter copper tubes reviewed. Various sizes of conventional tubes are compared with smaller diameter tubes, using simulated and actual performance data as well as energy-efficient design options. Copper components also offer antimicrobial properties. Data from bacteria studies supports the use of copper components where antimicrobial properties are required. Recent registration of copper alloys with the U.S. Environmental Protection Agency is discussed as well.

Tube Circuitry Simulations: Development of Small-diameter Tube Heat Exchanger: Circuit Design and Performance Simulation.

Wei-kun Ding et al. (XJTU and ICA, presented at ASME-ATI-UIT-10)

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The performance of condensers made from small diameter tubes was modeled and simulated at the School of Energy & Power Engineering, Xi'an Jiao Tong University, Xi'an, China. Separate papers were published on the tube circuitry and on fin design. The tube-circuitry research compared conventional-diameter (F7mm) tubes with small-diameter (F5mm) tubes. Simulations predict higher refrigerant pressure drops for small-diameter tubes. Nonetheless, by increasing the number of circuit branches, decreasing the single branch length and adding more tubes in the circuit design, refrigerant pressure drop could be kept within a factor of two for almost the same heat exchange rate and a significant saving in tube materials. Reducing the refrigerant pressure drop is a key issue for practical applications.

Fin Design Simulations: Development of Small-diameter Tube Heat Exchanger: Fin Design and Performance Research.

Ju-fang Fan et al. (XJTU and ICA, presented at ASME-ATI-UIT-10).

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Researchers studied fin designs for heat exchangers made with small-diameter (F4mm) tubes. For reference, numerical simulations were conducted on reference fins with conventional-diameter (F7mm) tubes, including various louvered and slotted-fin heat transfer surfaces. Then, based on nearly optimized values, new louvered-fin and slotted-fin structures were proposed and simulated for small-diameter (F4mm) tubes. The fin designs with the small-diameter tubes matched the heat transfer requirements for the reference louvered fin while greatly reducing the copper tube material in the new heat exchanger.

HTCs for Smooth Tubes: Condensation Heat Transfer Characteristic of R410A-oil Mixture inside Small Diameter Smooth Copper Tubes.

Guoliang Ding et al. (SJTU and ICA, presented at ASME-ATI-UIT-10).

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The condensation heat-transfer characteristics of an R410A-oil mixture inside small-diameter smooth copper tubes was investigated experimentally by researchers from the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University, Shanghai, China. The heat-transfer coefficient (HTC) was found to decrease with the increasing oil concentration. For small diameter smooth tubes, it decreased by a maximum of 28.5 percent at an oil concentration of five percent. A correlation was proposed between the heat transfer coefficient and oil concentration for R410A-oil mixture flow condensation inside smooth copper tubes. The proposed correlation agreed with all experimental data within a deviation of –30 percent to +20 percent.

HTCs for Inner-Grooved Tubes: Two-phase Heat Transfer Characteristics of R410A-oil Mixture Flow Condensation inside Small Diameter Microfin Copper Tubes.

Guoliang Ding et al. (SJTU and ICA at ASME-ATI-UIT-10)

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The two-phase heat-transfer characteristics for flow condensation of an R410A-oil mixture inside small-diameter, inner-grooved copper tubes were investigated experimentally by researchers from the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University, Shanghai, China. Effects on heat transfer were negligible at one percent nominal oil concentration but the heat transfer coefficient deteriorated by 25.1 percent at five percent nominal oil concentration. The correlation of Yu and Koyama is recommended to predict the local condensation heat transfer coefficient of R410A-oil mixture inside small diameter microfin tubes.

Less Tube Material in Evaporator Application for Residential AC: Application of Small Diameter Inner-Grooved Copper Tubes in Air-Conditioning Systems.

You Shunyi et al. (Chigo, presented at IIR-RCR-2010)

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Researchers at the Guangdong Chigo Air Conditioning in Foshan, Guangdong, compared the performance of conventional diameter (F7mm) and small-diameter

(F5mm) inner-grooved copper tubes in an evaporator application for a residential air conditioner. Heat-transfer coefficients, fluid pressure drops and system performance were compared. For the F5mm tube, copper usage was lowered by more than 43 percent and cost reduced by around 40 percent while increasing the cooling capacity, energy efficiency ratio (EER) and air volume.

Less Refrigerant in Split AC system: Experimental Research for Lowering Refrigerant Charge with 4mm Tube Heat Exchanger

Jia Qingxian et al. (Midea, presented at IIR-RCR-2010)

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Researchers at the Refrigeration Research Institute of Guangdong Midea

Refrigeration Appliances Group in Shunde, China conducted R410A and R290 performance experiments on split air conditioners with small diameter (F4mm) heat exchanger tubes. Without sacrificing any performance, the 4mm tubes reduced the volume and cost of the heat exchanger and significantly lowered the refrigerant charge.

Performance and Cost Analysis: Performance and Cost Analysis and Research of Air-Cooled Heat ExchangerUsing Small Diameter Copper Tubes

Wu Yang et al. (Tube Supplier, presented at IIR-RCR-2010)

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Researchers reported performance results and a cost analysis for conventional diameter (F9.52mm) and small diameter (F5mm) inner-grooved copper tubes as well as for heat exchangers made with these two tube types. For the same experimental conditions and volume flow, the heat exchange coefficient is about 15% higher for the F5mm inner-grooved copper tube than the F9.52mm inner-grooved copper tube. For the same operating conditions the same windward dimensions, comparing a F5mm copper tube heat exchanger with a F9.52mm copper tube heat exchanger, the former uses 41.8 percent less copper material for the tubes and 50 percent less aluminum foil material in the fins.

Microgroove vs Microchannel:A New Heat Exchanger Geometry for Next Condenser Generation with Ultra Low Refrigerant Charge

Stefano Filippini (LU-VE, presented at IIR-RCR-2010)

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A new coil geometry based on smaller-diameter copper tubes is compared with aluminum microchannel coils. The new design appears to be the best way to lower the internal volume and consequently the refrigerant charge, an issue that is more and more requested in order to ensure environmental sustainability of ACR products.

Abbreviations

ASHRAE 2011: ASHRAE Annual Conference, Montreal, Quebec Canada, June 25-29, 2011.

ASME-ATI-UIT-10: Conference on Thermal and Environmental Issues in Energy Systems, Sorrento, Italy, May 2010. The American Society of Mechanical Engineers (ASME), the Associazione Termotecnica Italiana (ATI), the Unione Italiana di Termofluidodinamica (UIT) and International Centre for Heat and Mass Transfer (ICHMT).  http://www.ichmt.org/asme-ati-uit-10/.

CDA = The Copper Development Association, US regional member of the Copper Alliance. www.coppper.org

Delft 2012 = The Tenth IIR Gustav Lorentzen Conference on Natural Refrigerants at the Delft University of Technology in the Netherlands. www.gl2012.nl

ICA: International Copper Association Ltd., Shanghai Office, Shanghai, China

IIR-ICR-2011: 23rd IIR International Congress of Refrigeration, Prague, Czech Republic, August 2011.

IIR-RCR-2010: Second IIR Workshop on Refrigerant Charge Reduction, Stockholm, Sweden, June 2010.

IIR TPTPR 2013: Fourth IIR Conference on Thermophysical Properties and Transfer Processes of Refrigerants, Delft, The Netherlands, 2013

Ohrid = 6th IIR Conference: Ammonia and CO2 Refrigeration Technologies, Ohrid, 2015.

OTS = Optimized Thermal Systems, College Park, Maryland. www.optimizedthermalsystems.com

Purdue 2012 = Fourteenth International Refrigeration and Air Conditioning Conference at the Purdue Conferences in West Lafayette, Indiana on July 16-19, 2012. https://engineering.purdue.edu/Herrick/Events/2012Conf

SJTU: Shanghai Jiao Tong University, Institute of Refrigeration and Cryogenics, Shanghai, China.

The Copper Alliance = The Copper Alliance® brand represents a network of regional copper centers and their industry-leading members, led by ICA. www.copperalliance.org

XJTU: Xi'an Jiao Tong University, School of Energy & Power Engineering, Xi'an, China