Refrigerant Circuitry Research Articles

Heat exchanger circuitry optimization using an enhanced integer permutation-based genetic algorithm in low-GWP reversible heat pump applications

Tube-fin heat exchangers (HXs) are key components in air-conditioning and heat pump applications. Advanced heat exchanger design optimization tools are desired to adapt to the industrial transition from conventional refrigerants to low Global Warming Potential (low-GWP) refrigerants. The performance of heat exchangers is strongly influenced by the refrigerant circuitry. This research presents an enhanced integer permutation-based Genetic Algorithm (IPGA) to optimize the circuitry of tube-fin heat exchangers. To remedy the limitation of the previous IPGA, which cannot generate designs with splitting and merging of circuits, a new chromosome, as well as new generic operators, are developed. The enhanced IPGA uses effective chromosome representations and GA operators to guarantee the chromosome (genotype) can be mapped to valid heat exchanger designs (phenotype) and incorporates real-world manufacturability constraints to ensure the optimal designs are manufacturable with the available tooling. Previous circuitry optimizations focus on improving component-level performance under a specific operating condition, i.e., the heat exchanger either works as a condenser or an evaporator. The optimal circuitry obtained under air conditioning (AC) mode cannot guarantee optimal performance when used in heat pump (HP) mode. To improve the dual-mode performance of reversible heat pumps, a bi-objective optimization problem formulation is implemented to achieve optimal system performance in both AC and HP modes. A case study using six refrigerants, i.e., R410A, R452B, R454B, R32, D2Y60, and L41a, shows that optimal heat exchangers yield up to 6.1 % energy efficiency ratio (EER) improvement under AC mode and up to 8.5 % coefficient of performance (COP) improvement under HP mode. The use of this design approach can ensure a preferable performance of reversible heat pumps during both cooling and heating modes.

Circuitry optimization using genetic programming for the advancement of next generation refrigerants

In this study, a new evolutionary method, which can handle the implementation of genetic operators with unrestrained number and locations of splitting and merging nodes for the optimization of heat exchanger circuitries, is developed. Accordingly, this technique expands the search space of previous optimization studies. To this end, a finned-tube heat exchanger simulator is structured around a bijective mathematical representation of a refrigerant circuitry (the tube–tube adjacency matrix), which is used in combination with traversing algorithms from graph theory to recognize infeasible circuitries and constrain the evolutionary search to coherent and feasible offspring. The performance of three refrigerants, namely R32, R410A, and R454C, commonly used in air-conditioning applications was assessed for the optimized circuitries of a 36-tube evaporator while converging to a given cooling capacity, degree of superheating, and heat source boundary conditions. At a given output capacity and air outlet temperature, larger coefficient-of-performance improvements (up to 9.99% with reference to a common serpentine configuration) were realized for zeotropic refrigerant mixtures, such as R454C, where appropriate matching of the temperature glide with the temperature variation of the air yielded the possibility of further reducing the required compression ratio under the corresponding operating conditions. Hence, it was demonstrated that low-GWP zeotropic mixtures with temperature glide can realize a performance comparable to that of R32 and higher than that of R410A by approaching the Lorenz cycle operation.

Refrigerant circuitry optimization of finned tube heat exchangers using a dual-mode intelligent search algorithm

• Presented a novel dual-mode search algorithm for refrigerant circuitry optimization. • Optimization scheme is implemented in two separate modules. • The knowledge-based computational module provides an optimal number of circuits. • The permutation-based computational module gives the best possible tube connection. This paper presents a novel dual-mode search algorithm for refrigerant circuitry optimization, which is implemented in two modules, namely, the knowledge-based computational module (KBCM) and the permutation-based computational module (PBCM). The KBCM determines the appropriate number of tubes for each heat exchanger circuit based on the related non-uniform airflow velocity distribution, whereas the PBCM establishes the optimal or near-optimal sequence in which tubes are linked. The method is designed to maximize the heat exchanger capacity while minimizing the computational costs by excluding infeasible circuits. By combining the KBCM and PBCM techniques, significant gains of 9.64%, 9.87%, 6.96%, and 7.16% in the heat exchanger capacity are observed for the part-load cooling conditions according to the European standard EN14825, particularly for outdoor unit heat exchangers with a non-uniform airflow distribution.

Performance assessment of R1234ze(E) as a low GWP substitute to R410A in fin-and-tube heat exchangers

Several energy efficiency regulations worldwide are forcing air conditioning and heat pump manufacturers to replace the halogenated refrigerants currently used in these systems, such as R410A, with more climate friendly alternatives that have a lower Global Warming Potential (GWP). This study explores how the performance of R1234ze(E) compares for an R410A based fin-and-tube evaporator coil. A total of 36 experiments were completed for both refrigerants, by means of a high-fidelity experimental facility, to validate a segment-by-segment heat exchanger model. For R410A, the mean absolute percent error between the experimental and simulated capacity was found to be 0.9%, and it was 1.4% for R1234ze(E). Simulations were then carried out, using the model, with R1234ze(E), where the fin density and refrigerant circuitry of the original heat exchanger were altered independently. By increasing the fin density from 15 to 20 fins per inch (FPI), the capacity increased by 4.9%, and refrigerant side pressure drop decreased by 4.5%. For the refrigerant circuitry optimizations, a maximum increase in capacity of 5.6% was observed when compared to the baseline, but at the expense of 7.5 times higher pressure drop. Alternatively, a separate circuitry resulted in a 38% pressure drop reduction and 2.5% reduction in capacity. The results demonstrated the relative sensitivity of capacity and refrigerant side pressure drop for R1234ze(E) on only two heat exchanger geometrical parameters. This sensitivity suggests potential for additional optimizations to heat exchanger designs for low-GWP refrigerants by performing further parametric analysis on heat exchanger geometry.

Design of a Numerical Simulator for Finned-Tube Heat Exchangers with Arbitrary Circuitry

A numerical simulator that can handle a finned-tube heat exchanger with a complex refrigerant circuitry is presented. The model is based on a tube-by-tube approach in which each tube is considered as the control volume in the analysis of the mass, energy, and momentum conservation. A mathematical representation of the refrigerant circuitry is also developed using the concepts in graph theory. The tube–tube adjacency matrix provides a unique representation of the tube connectivity and can also be utilized for constraints handling. With the implementation of the constraints, nonphysical and infeasible circuitries are removed from the solution space, which is advantageous for an evolutionary search for an optimum solution. The numerical simulator is verified against two sets of experimental data, and the calculated heat duty and pressure drop are within ±6% and ±20%, respectively. Sample simulations of the performance of different refrigerants in three circuitry configurations, and circuitry optimization are conducted to investigate the reliability and effectiveness of the tube-tube adjacency matrix and simulation model in the search for an optimal solution.

Validation of a multi-circuit heat exchanger model for evaluating the effect of refrigerant circuitry on cross-fin conduction in evaporator mode

Fin-and-tube heat exchangers can have active circuits directly adjacent to inactive circuits at part load conditions, leading to cross-fin conduction. Cross-fin conduction needs to be accounted for in heat exchanger modelling to accurately predict heat exchanger performance. This study explores how refrigerant circuitry can influence cross-fin conduction in multi-circuit evaporator coils. Our advanced segment-by-segment heat exchanger model is validated against two-phase refrigerant data obtained from our experimental facility. Three, custom designed, multi-circuit fin-and-tube heat exchanger coils with distinct circuities were used for validation purposes, including an interleaved, vertical and block circuited coils, respectively. A total of 162 experiments with several refrigerant and air inlet conditions, at full load (all circuits active) and part load (some circuits active) were carried out. At full load, for all coils, differences in coil capacity between simulated and experimental capacities were no greater than 10%, for part load the differences are larger. For the test coil with interleaved circuitry, operating with 3 of 8 circuits active, the mean average percentage error (MAPE) between experimental and simulated capacity was 2.7% when cross-fin conduction was considered, increasing to 7.7% when it was ignored. Similarly, for test coil with block circuitry, operating with 2 out of 4 circuits active, the MAPE between experimental and simulated capacity was 1.7% when cross-fin conduction was considered, and 3.4% when ignored. Results suggest that the effect of cross-fin conduction on coil capacity is directly proportional to number of inactive circuits, and it is more pronounced on interleaved circuitry compared to block circuitry.

Influence of refrigerant circuitry arrangement of three-row finned tube evaporator on heat transfer performance under uniform wind speed distribution

The EVAP-COND software was used to simulate 8 different refrigerant circuitry of the three-row tube heat exchanger under different superheat and outlet saturation temperature conditions. The influence of different refrigerant circuitry on the heat transfer performance of the evaporator was explored. And the influence of superheat and outlet saturation temperature on the evaporator and its changing law were analyzed and discussed. The results show that: Cross arrangement facilitated heat transfer. The dual-flow solution was better than the single-flow solution. And countercurrent arrangement was conducive to heat exchange. The best solution was g, and the worst solution was a. Under the conditions of this study, the heat transfer of curve g was 44.78% and 36.20% higher than curve a, respectively.

Multi-objective optimization of low-GWP mixture composition and heat exchanger circuitry configuration for improved system performance and reduced refrigerant flammability

The recently introduced hydrofluoroolefin (HFO) refrigerants, including R1234yf and R1234ze(E), have significantly lower global warming potentials (GWPs) than traditional hydrofluorocarbon (HFC) refrigerants like R410A. However, prior tests show that direct drop-in of pure R1234yf or R1234ze(E) into equipment designed for R410A results in a decrease in heat exchanger capacity and the system coefficient of performance. The primary reason is the lower in-tube heat transfer performance of R1234yf and R1234ze(E) compared with that of R410A. To address this issue, previous studies have mixed the mildly flammable HFC R32 with HFOs to improve system performance, with HFC R125 also added to suppress flammability. Previous studies selected compositions based on simple cycle analyses and did not consider modifications of the heat exchanger circuitry configuration to adapt to the new refrigerants. This study presents a novel multi-objective optimization approach to design a refrigerant composition that maximizes energy efficiency within flammability and GWP limits. The approach in this work simultaneously optimizes mixture composition and heat exchanger circuitry configuration. A case study on a rooftop unit indicates that, compared with mixture-only optimization, simultaneous optimization of mixture and heat exchanger circuitry yields a 5.9% improvement in cycle efficiency and a 48.6% reduction in refrigerant flammability with a GWP of 268. Circuitry optimization using refrigerants with different temperature glides shows that the larger the temperature glide is, the larger EER improvement is obtained. The results show that zeotropic blends with a large temperature glide are more sensitive to the refrigerant circuitry than pure refrigerants and may suffer significant performance degradation with subpar heat exchanger circuitry design. The proposed optimization approach is generally applicable to mixtures with any number of components. Using this approach to design a HVAC system can yield higher system efficiency within flammability and GWP constraints.

Numerical modeling of an outdoor unit heat exchanger for residential heat pump systems with nonuniform airflow and refrigerant distribution

Multirow finned tube heat exchangers have been extensively used as an outdoor unit in heat pump applications. Several factors, including nonuniform airflow distribution, refrigerant circuitry arrangement, fin and tube configurations, and operating conditions, have a significant influence on its thermohydraulic performance. For accurate outdoor unit heat exchanger modeling and analysis, developing a robust and experimentally validated numerical model reflecting the influence of realistic nonuniform airflow and refrigerant distribution is necessary. This paper presents a general-purpose numerical model, implemented in MATLAB, for a multirow finned tube heat exchanger using a tube-by-tube modeling approach based on the log mean enthalpy difference method. State-of-the-art heat transfer and pressure drop correlations are used for each refrigerant-side's flow regime and airside fin configuration. An alternative iteration loop is also incorporated to solve the complex circuitry of the heat exchanger under wet conditions. Simulation results of the current model yield consistent and more real heat transfer, pressure drop, and temperature contours using nonuniform airflow velocity profiles than those of the simplified models that assumed uniform airflow. Therefore, higher accuracy of the performance evaluation for complex refrigerant circuitry was achieved. Furthermore, current model is able to identify the exit vapor quality that allows proper selection of tubes and mass flow rates to avoid unnecessary superheat or sub cooling. The model is also validated with available experimental data, and the maximum error is within ±10.0%.

The effect of refrigerant circuitry on the performance of a freezer with a tube-on-sheet evaporator

In this work, the effect of flow circuitry on the performance of a household freezer with a tube-on-sheet evaporator is investigated. The evaporator used in the freezer is a tube-on-sheet type where the tubes are bent and attached underneath the shelves, as in the serpentine heat exchanger. The evaporator is composed of six shelves that are located inside the cabin with a particular distance between each. Four different refrigerant circuitries are modeled to identify the one that provides a homogeneous temperature distribution while consuming less energy, and experiments are conducted by switching the refrigerants flow route between the shelves. The uniform temperature distribution at around the target temperature and the energy consumption of the entire system while achieving this target are monitored as the major performance measures of the investigation. The tests are conducted according to IEC 62552:2015, standard clause 8. The experiments found that the change of the flow circuitry has a crucial impact on the temperature distribution’s uniformity and energy consumption. The type in which the refrigerant circuitry between the shelves is 2-1-6-5-4-3 has the best performance in terms of temperature distribution and energy consumption.

The hybrid method for the plate-finned tube evaporator design process

The plate-finned tube evaporator performance, in terms of heat transfer rate and refrigerant pressure drops, is influenced by several choices done during the design process to be carried out complying with different constraints. In this paper different refrigerant circuitry layout options together with other parameters variations were investigated with the purpose of supporting designers. Performance predictions were calculated using the hybrid method as well as appropriate Performance Evaluation Criteria (PEC) were adopted to select the best layout. The hybrid method has been here revised and improved aiming at modelling heat exchangers with an approach closer to real configurations that include complex circuit layouts. The method was chosen as its main advantage (high accuracy in the results with low computational costs) allowed to easily perform operating conditions modifications to be compared.

Analysis of the heat transfer characteristics of the liquid-vapor separation condenser based on a condensate growth model in horizontal tube

A theoretical model for horizontal tubes was proposed to analyze heat transfer performance of the condenser with liquid-vapor separation device in this paper. The model was on basis of convection boundary condition and effects of gravity and shear force to express the feature of the condensing film growth inside a horizontal smooth circular tube. In order to validate the model, a set of relevant experiments were carried out to gauge the heat transfer coefficients of condensers with liquid-vapor separation device. The predicted heat transfer coefficients showed a nice agreement with these experimental results. The refrigerant circuitry of a condenser with liquid-vapor separation device was designed according to the principle of equal mass flux with predicted results. Besides, the heat transfer performance of a condenser unit with liquid-vapor separation device in actual operating condition was calculated and analyzed. The influences of refrigerant vapor mass flux and saturation temperature on the performance were investigated and the enhancement of the liquid-vapor separation device on heat transfer was verified.

Tube-fin heat exchanger circuitry optimization using integer permutation based Genetic Algorithm

The performance of tube-fin heat exchangers is strongly influenced by the refrigerant circuitry, i.e., the refrigerant flow path along different tubes. Since for a given number of tubes, the number of possible circuitries is exponentially large, neither exhaustive search nor traditional optimization algorithms can be used to optimize the circuitry. Researchers previously used Evolutionary Algorithms (EAs) 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 a novel integer permutation-based Genetic Algorithm (IPGA) for optimizing circuitry with manufacturability and operating constraints. The novel genetic operators are designed such that all chromosomes generated by IPGA can be mapped to a valid circuitry. As a result, the proposed approach can explore the solution space more efficiently than conventional GA. A constraint-dominated sorting technique is used in the fitness assignment stage to handle manufacturability constraints. An exhaustive search on a small heat exchanger proves IPGA can find optimal or near-optimal circuitries using relatively small population size and low number of iterations. The analyses of four case studies show that IPGA can find circuitry designs with capacities superior to those designed based on engineers’ experience and meanwhile guarantee good manufacturability. Overall, a 2.4–14.6% increase in heat exchange capacity is observed by applying IPGA to an A-shaped indoor unit. Comparison with other optimization methods in literature shows that IPGA finds better designs exhibiting higher capacity, lower pressure drop and better manufacturability.

Comparative performance of air-conditioning systems with different refrigerant circuitries in liquid-separation condenser

Liquid-separation condensation is a novel technology to enhance the condenser performance. Four liquid-separation condensers (LSCs) that had the only difference in refrigerant circuitry were designed, and they were respectively installed to the air-conditioning system and experimentally studied. The capillary tube length and the refrigerant charge were adjusted to achieve the optimum energy efficiency ratio (EER) for each system. The cooling capacity, the system power input and EER were compared at the optimum point. It is found that the four systems are most distinguished in LSCs in terms of condensing pressure and subcooling because of different refrigerant circuitries. The exergy analysis was also carried out. The results show that the refrigerant circuity in LSC has great impacts on the exergetic performance of all system components, and the order of highest exergy efficiency of the four systems is different from that of EER. The detailed characteristics of the four LSCs were further discussed at the designing condition and the practical condition in the tests. The pressure drop of LSC is dominated by the last pass that has serpentine tubes. It is concluded that a low penalty factor (PF) in LSC does not guarantee the good system performance. The optimization of refrigerant circuitry in the condenser to improve the system efficiency should use the system performance as the objective at the system level instead of PF.

Optimization of circuitry arrangements for heat exchangers using derivative-free optimization

Optimization of the refrigerant circuitry can improve a heat exchanger's performance. Design engineers currently choose the refrigerant circuitry according to their experience and heat exchanger simulations. However, the design of an optimized refrigerant circuitry is difficult. The number of refrigerant circuitry candidates is enormous. Therefore, exhaustive search algorithms cannot be used and intelligent techniques must be developed to explore the solution space efficiently. In this paper, we formulate refrigerant circuitry design as a binary constrained optimization problem. We use CoilDesigner, a simulation and design tool of air to refrigerant heat exchangers, in order to simulate the performance of different refrigerant circuitry designs. We treat CoilDesigner as a black-box system since the exact relationship of the objective function with the decision variables is not explicit. Derivative-free optimization (DFO) algorithms are suitable for solving this black-box model since they do not require explicit functional representations of the objective function and the constraints. The aim of this paper is twofold. First, we compare four mixed-integer constrained DFO solvers and one box-bounded DFO solver and evaluate their ability to solve a difficult industrially relevant problem. Second, we demonstrate that the proposed formulation is suitable for optimizing the circuitry configuration of heat exchangers. We apply the DFO solvers to 17 heat exchanger design problems. Results show that TOMLAB/glcDirect and TOMLAB/glcSolve can find optimal or near-optimal refrigerant circuitry designs after a relatively small number of circuit simulations.

Method for determining the optimum number of circuits for a fin-tube condenser in a heat pump

Fin-tube heat exchangers are widely used in air-source heat pumps and air-conditioners. Because these heat exchangers are comprised of a number of tubes, there are innumerable ways of organizing the refrigerant circuitry. It is important to determine the number of refrigerant circuits because the circuitry significantly influences the performance of the heat exchangers. A method for determining the optimum number of circuits is proposed in this work. This method is applied to determine the optimum number of circuits for a fin-tube condenser. The performance analyses of fin-tube condensers with various circuit configurations demonstrates that the new method is useful in determining the number of circuits required in a heat exchanger.

Low GWP R134a replacements for small refrigeration (plug-in) applications

This paper presents a performance evaluation of two low global warming potential (GWP) refrigerants, R1234yf and R1234ze(E), as replacements for R134a. A theoretical comparison is first presented based on the thermodynamic properties of the refrigerants. Tests are carried out in a representative vending machine system (cassette) at two representative outdoor ambient temperatures of 32.2°C (efficiency test), and 40.5°C (capacity test) and return air temperatures of 2°C, 20°C and 38°C. Under drop-in conditions, R1234yf shows performance similar to R134a. For R1234ze(E), a refrigerant with lower volumetric cooling capacity, testing was done using a larger compressor to obtain capacity recovery. A simulation model is used to analyze the system design changes, which can lead to further improvement in the performance of low GWP refrigerants. Overall, the results indicate that with minor design changes, such as use of a suction-line/liquid-line heat exchanger and modifications to the refrigerant circuitry, these refrigerants can match the performance of R134a.

Rooftop air-conditioning unit performance improvement using refrigerant circuitry optimization

This study demonstrates the performance improvement of an air-to-air 7.5 Ton (26.4 kW) rooftop unit (RTU) achieved by optimizing an evaporator's refrigerant circuitry using evolutionary algorithms. We measured its cooling capacity and Coefficient of Performance (COP) in an environmental chamber where we also measured the in-situ air velocity profile using Particle Image Velocimetry (PIV).We used a detailed heat exchanger model to simulate the performance of the original evaporator operating with the measured air distribution, and then we used an evolutionary algorithm-based optimization module embedded in the simulation tool to optimize the refrigerant circuitry for the measured inlet air distribution.The optimized refrigerant circuitry design was implemented in a new prototype evaporator, which replaced the original evaporator in the tested RTU system. Laboratory tests of the new system showed an improvement of 2.2 ± 1.5% in capacity and 2.9 ± 1.5% in COP compared to the performance of the original system, which was optimized by the manufacturer using conventional methods prior to the onset of this study.

Mitigation of air flow maldistribution in evaporators

Flow maldistribution in evaporators can lead to significant performance and capacity degradation. A significant amount of work has previously been published to overcome this issue. For a fixed air side maldistribution, various methods have been proposed to significantly reduce the effect on performance. Refrigerant compensation has been proposed to reduce the effects of maldistribution in variable air flow systems. However, little work has been found in open literature relative to modifying the refrigerant circuitry to make it less vulnerable to variable air-side maldistribution. The purpose of this paper is to fill this gap in the open literature with a simple and easy to understand case study. The performance of a proposed interleaved circuitry approach, and active refrigerant flow control are compared to standard coil circuitry for different cases of maldistribution. The results show that the interleaved circuitry recovers less of the performance losses than equalization of the exit superheats by active control of refrigerant distribution. However, the implementation cost in an actual system is expected to be much lower and the long term reliability is expected to be better than for an active control approach

A dual-mode evolutionary algorithm for designing optimized refrigerant circuitries for finned-tube heat exchangers

Heat exchanger performance is strongly influenced by the refrigerant circuitry, i.e., the connection sequence of the tubes. This article describes an evolutionary computation-based approach for designing an optimized refrigerant circuitry used in an intelligent system for heat exchanger design. The technique used in this design employs two methods to generate designs implemented separately in two modules: the knowledge-based evolutionary computation module and the symbolic-learning-based evolutionary computation module. The optimization example presented in this article employed each module independently and used the combined approach to demonstrate the performance of each module and the power of the combined module approach. The best circuitry designs determined through these optimization runs yielded substantial capacity improvements over the original design; the symbolic-learning- and knowledge-based modules returned circuitry designs that improved the heat exchanger capacity by 2.6% and 4.8%, respectively, while the combined module approach resulted in a circuitry design that improved the capacity by 6.5%.