Technical Literature

Improved heat pump designs reduce fossil-fuel consumption and eliminate HFCs, helping to achieve net-zero-carbon goals. The evaporator coil described in this case study is suitable for cold climate heat pumps. It uses R290 refrigerant and 5 mm diameter inner-grooved copper tubes. Heat is transferred from the outdoor evaporator to an insulated outdoor water tank for domestic hot water or space heating. Performance is demonstrated using computer simulations. The L-Type evaporator block consists of 54 tube lengths with four tubes per row across the airflow direction. The heat exchanger has capacity of 14 kW for a mass flow rate of 196 kg/h and airflow rate of 10,400 m3/h. Results are compared for various operating conditions and designs. As an early adopter of the latest tube technologies for use in new heat pump designs, Lordan actively collaborates with partners in commercial, industrial and residential applications.

Many equipment designers have discovered that smaller-diameter copper tubes meet key design ‎criteria for use with natural refrigerants. New software allows for heat exchanger performance to be ‎simulated quickly and accurately for hydrocarbon refrigerants. Speedy computations allow for the ‎exploration of a wide range of designs. Basic design principles are illustrated using HXSim Version 3.3 ‎software tool from the International Copper Association, simulating natural refrigerants in air-‎conditioners with MicroGroove copper tubes. In another case study, CoilDesigner® software is ‎combined with a Multi-Objective Genetic Algorithm in the optimization of a condenser for a ‎residential-freezer, using R600a refrigerant and MicroGroove copper tubes, including diameters of 3 ‎mm, 4 mm and 5 mm. The International Copper Association offers assistance in bringing ecofriendly ‎products optimized for higher efficiency, reduced refrigerant charge and materials savings.‎

MicroGroove Paper at ATMO Europe 2019, Warsaw, ‎Poland: Optimisation of transcritical R744 gas coolers ‎with ‎Microgroove smaller diameter copper tubes

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The basic and main function of Fin-and-Tube Heat Exchanger Simulation Software is to give accurate calculation results for predicting the capability of fin-and-tube heat exchangers in steady state condition, such as heat exchange capacity, pressure drop of refrigerant side or air side and etc. once the input parameters are given. The Fin-and-Tube Heat Exchanger Simulation Software can provide a graphical user interface to help users to enter data and observe simulation results, and both 3D and 2D graphic user interfaces are available.

R744 gas coolers remove heat from the high density, low viscosity, supercritical fluid (SCF) in the transcritical refrigeration cycle. Designs vary widely with respect to operational pressures, tube wall thickness, alloy composition and internal enhancements as well as tube-fin geometry and, most importantly, tube diameter, which can range from 5 mm to 9.52 mm (0.197 in. to 0.375 in.). Since the SCF can flow through smaller diameter tubes with little increase in pressure drop, the use of such tubes is advantageous, allowing the design of compact and efficient gas coolers. Close to the critical point, small changes in pressure or temperature result in large changes in density, so coping with and managing the SCF at the outlet of the gas cooler is vitally important. In this presentation, typical gas coolers are assessed in terms of performance objectives and the end user experience.

https://www.slideshare.net/EdaIsaksson/advantages-of-small-diameter-tubes-in-transcritical-refrigeration-cycles

Hydrocarbons are highly attractive for use in refrigerator and freezer applications but their flammability necessitates strict “use conditions” with respect to refrigerant charge. Sub-Zero, Inc. makes high-end, built-in residential refrigerators, freezers and wine coolers. The company used a proprietary heat exchanger design and simulation software tool from Optimized Thermal Systems to identify feasible 5-mm inner-grooved copper tube coil designs. The baseline design used 6.35-mm copper tubes. For an R600a residential application, simulations identified 5-mm tube coil designs with significantly less internal tube volume than the baseline coil. One 5-mm design reduced the internal tube volume by as much as 41 percent as compared to the baseline, along with a potential 57 percent reduction in coil footprint, while maintaining a near equal airside pressure drop. Sub-Zero is reinventing its residential appliances to allow for more efficient use of natural refrigerants using MicroGroove smaller diameter copper tubes

The Seventeenth International Refrigeration and Air-Conditioning Conference included 41 sessions (R-01 through R-41) with hundreds of papers. PDF versions of all of these papers are now openly accessible and downloadable by searching for the paper number in the Conference Tool. (https://www.conftool.com/Purdue2018/sessions.php) Research relating to MicroGroove includes laboratory experiments, theory and modeling, and design case studies. The most pertinent papers relating to MicroGroove are highlighted in this article.

The ultralow Global Warming Potential (GWP) of propane (R290) and isobutane (R600a) refrigerants make them highly attractive for refrigerator and freezer applications, although their flammability necessitates strict use conditions with respect to refrigerant charge. Copper tubes with smaller diameters are widely used to reduce refrigerant charge. The process of downsizing copper-tube diameters involves detailed simulations and prototype construction as well as testing and validation. A proprietary heat exchanger design and simulation software tool (Jiang et al., 2006) was used to evaluate the performance of and optimize the design of domestic refrigerator condenser coils made with 5-mm outer-diameter copper tubes. Optimization was accomplished through the use of reduced order models, meta-models and a multi-objective genetic algorithm (MOGA). Reducing refrigerant charge was the primary objective. Secondary objectives included the reduction of the total footprint and the total tube-andfin material mass. The baseline design used 6.35-mm O.D. copper tubes with a minimum wall thickness of 0.41 mm, i.e., quarter-inch tubes with 0.016-inch wall thickness. The new designs use wavy-herringbone fins with reduced fin thicknesses as compared to the baseline design. Other variables included the horizontal and vertical spacing of the tubes; number of tubes per bank; fin density; wavy fin pattern depth; tube length; and tube circuitry. For an R600a residential application, reduced internal volume was considered to be more important than the airside pressure drop. A Pareto chart is presented of optimized values from the design space. Compared to the baseline design, the best 5- mm design reduced the internal tube volume by 41 percent, along with a 57 percent reduction in coil footprint. Additionally, test data to validate the performance of prototype coils is presented with emphasis on the design, construction and manufacture of the heat exchanger coils.

This study experimentally investigated the flow boiling heat transfer and pressure drop of R1234ze(E) in a horizontal small-diameter microfin tube with an outer diameter of 2.5 mm and equivalent diameter of 2.1 mm. The boiling heat transfer and pressure drop were measured in the mass velocity range of 100–400 kg/(m2 s) and in the heat flux range of 5–20 kW/m2 at a saturation temperature of 15 °C. The heat transfer coefficient increased as the quality increased because of increasing the forced convection in the pre-dryout region. The heat transfer coefficient increased as the heat flux increased in the low-quality region, but decreased when the heat flux increased past a certain point because the thin liquid film at the fin tips had dried. The heat transfer coefficient increased as the mass velocity increased and exhibited the highest value at a mass velocity of 200 kg/(m2 s). The measured heat transfer coefficient agreed well with previous correlations only in the dominant region of the forced convection evaporation. The frictional pressure drop increased as the mass velocity and vapor quality increased, and the measured values agreed well with previously reported correlations for conventional-diameter microfin tubes.

This study experimentally investigated the condensation heat transfer and pressure drop characteristics of R1234ze(E) inside horizontal small-diameter 4.0 mm OD microfin tubes having three different types of fin geometries. The specifications of the three fin geometries were 40 fins with a fin height of 0.18 mm and a helix angle of 18°, 50 fins with a fin height of 0.15 mm and a helix angle of 12°, and 50 fins with a fin height of 0.12 mm and a helix angle of 25°. The experiments were carried out for a range of mass velocities from 50 to 400 kgm-2 s -1 , and at a saturation temperature of 35 °C. The effects of fin geometries such as the number of fins, fin height, and helix angle on the heat transfer and pressure drop were investigated. The heat transfer coefficient increased as the number of fins increased for the lowest mass velocity. Fin height was most effective on heat transfer enhancement at higher mass velocities. The heat transfer coefficient and pressure drop of the microfin tubes were compared with those of smooth tube and were evaluated in terms of the enhancement factor of heat transfer. The measured heat transfer coefficient and pressure drop were compared with previous correlations, and the results were verified practical effectiveness of the previous correlations for small-diameter microfin tubes.

This paper presents some new experimental data on R1234yf saturated flow boiling inside a 4 mm horizontal smooth tube: the effects of heat flux, refrigerant mass flux, and mean vapor quality are investigated separately to point out the different heat transfer mechanism contributions (i.e., nucleate boiling or/and forced convection boiling). The experimental tests were carried out at a saturation temperature equal to 10 °C, refrigerant mass flux from 200 to 600 kg m-2 s -1 , heat flux from 15 to 30 kW m-2 , and at increasing vapor quality up to incipient dryout. The measurements are here reported in terms of boiling heat transfer coefficient and frictional pressure drop. Furthermore, the R1234yf performance is compared against R1234ze(E) and R134a, since the substitution of R134a with low GWP refrigerants is one of the most important actual challenge for refrigeration and air conditioning, and R1234ze(E) and R1234yf seem to be very promising substitutes of it. Finally, the experimental heat transfer and frictional pressure drop data are used to assess some classical literature correlations.

This paper presents the comparative analysis of HFC134a and its low GWP substitutes HFO1234yf, and HFO1234ze(E) in saturated vapour condensation inside a 4 mm ID horizontal smooth tube. The experimental tests were carried out at 30, 35, and 40°C of saturation temperatures, with refrigerant mass flux in the range 100 - 600 kg m-2s -1 at decreasing vapour quality. A transition point from gravity-dominated and forced convection condensation was found in the range of the equivalent Reynolds number 10,000  20,000. The experimental heat transfer coefficients in the forced convection condensation regime were very well predicted by the Akers et al. (1959) model, whereas the Friedel (1979) correlation was able to reproduce the frictional pressure drop data in the whole experimental range. HFO1234yf and HFO1234ze(E) exhibit heat transfer coefficients and frictional pressure drops similar to those of HFC134a and both the HFO refrigerants seem to be very promising as long-term low GWP substitutes for HFC134a.

Tube-fin heat exchangers (HXs) are widely used in air-conditioning and heat pump applications. The performance of these heat exchangers is strongly influenced by the refrigerant circuitry, i.e. the refrigerant flow path along the different tubes in the HX core. Since for a given number of tubes, the number of possible circuitries is exponentially large, neither the exhaustive search nor traditional optimization algorithms can be used to optimize the circuitry for a given application. Researchers have previously used Genetic Algorithms (GAs) coupled with a learning module or other heuristic algorithm to solve this problem, but there is no guarantee that the resulting circuitry can be manufactured in a cost-effective manner. In this paper, we present an integer permutation-based GA approach for solving the circuitry optimization problem. A finite volume heat exchanger simulation tool is used to simulate the performance of different circuitries generated by the optimizer. The novel genetic operators are designed such that all chromosomes generated by GA can be mapped to a valid circuitry design. As a result, the proposed approach can explore the solution space more efficiently than a conventional GA. The manufacturability aspect is handled using a constraint-dominated sorting technique in the fitness assignment stage. The analyses of several case studies show that the constrained integer permutation-based GA can generate circuitry designs with capacities superior to those obtained manually and meanwhile guarantee good manufacturability. Overall, a 2.4-14.6% increase in heat exchange capacity is observed by applying the new optimization method to an evaporator from a A-type indoor unit. Comparison with other optimization methods in literature shows that the proposed approach exhibits higher quality optimal solutions than other methods.

Heat pumps are an energy efficient way to provide adequate heating to an indoor space. In contrast to electric or gas heating, a heat pump works to draw the “free” heat from the cold outdoor ambient and transfer thermal energy to the heated space. When the outdoor ambient is cold and humidity conditions are right, however, frost will start to develop on the heat pump’s outdoor coil. Frost on the coil surface blocks air flow through the coil thus reducing the heat pump’s efficiency and overall performance. Control of frosting and defrosting is particularly important to the successful use of heat pumps in cold climates.

To restore the unit’s heating capacity and efficiency, a defrost cycle is needed to remove the accumulated frost developed on the coil. While defrosting restores heat pump efficiency, this period of operation itself requires additional power and ultimately results in an energy penalty. Improving defrost performance and/or reducing the number and duration of required defrost periods would significantly improve heat pump operation.

Heat pump researchers and manufacturers have spent a great amount of effort on the topic of defrosting. A comprehensive review of efforts within the last 15 years was conducted to summarize existing defrosting technologies and identify those solutions that may be more feasible or readily commercialized to reduce the defrost penalty. This summary highlights technologies that may reduce or eliminate the negative impacts of the defrosting period including approaches using hot gas bypass, coil coatings and advanced control strategies. The potential impact of the identified technologies is energy and demand savings, improved performance for comfort, and removal of a significant barrier to widespread adoption of air-source heat pumps in all climates

Reduced internal volume of coils made with smaller-diameter copper tubes allows for a reduction in refrigerant charge compared to conventional heat exchangers. Especially for propane and isobutane systems, the smaller refrigerant volume allows for the desired cooling capacities to be reached without exceeding refrigerant charge limitations.

Recently obtained system test data results provided by a North American manufacturer of light-commercial refrigeration equipment are presented. MicroGroove copper-tube heat exchangers are used along with natural refrigerants. Performance comparisons are made for several models of cooling cabinets currently sold to end-users in North America. For ambient temperatures of 75 ºF and 55% RH, conventional designs using R134a and R404A refrigerants are compared with new designs using R290 (propane) and R600a (isobutane) refrigerants, respectively. The effects of variables such as heat exchangers size, refrigerant charge and energy efficiency are also examined.

The use of natural refrigerants in refrigeration equipment necessitates the redesign of heat exchangers. In particular, R744 heat exchangers must withstand high pressure because of the extraordinary high operating pressures of R744 systems; and R290 heat exchangers must minimize refrigerant volume because of the R290’s Level-3 flammability rating.

In this paper, several case studies are presented from various coil makers and refrigeration equipment makers to illustrate the design principles and manufacturing methods. Successful designs have been realized using high strength copper tube alloys and the development of new manufacturing equipment for smaller diameter copper tubes. Recent development in design simulations as well as real world applications will be presented.

Comparing various diameters of copper tubes with the same wall thicknesses, burst pressures are much higher for the smaller diameter tubes.

Copper tubes up to ten feet in length with wall thicknesses of 0.040 inches (about one millimeter) have been hydraulically expanded to obtain a mechanical fit against aluminum fins with 10 fins per inch.

This presentation will describe the performance R744 gas coolers and evaporators currently produced in North America for real-world commercial and industrial applications and will discuss the practicality of manufacturing such systems in volume. It will also present new high-strength tube alloys and research on new copper tube technologies.

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.

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.

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.

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).

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.

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%.

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

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.