Refrigerant Circuitry Research Articles

Numerical model for microchannel condensers and gas coolers: Part II – Simulation studies and model comparison

For a microchannel heat exchanger (MCHX), given the working conditions, main geometric data of the fin and tubes, heat transfer and face areas, there are multiple choices for the refrigerant circuitry and aspect ratio. Numerical studies using the Fin1Dx3 model, presented in Part I, are undertaken in order to assess the impact on the heat transfer of these design parameters for a microchannel gas cooler. The effect of fin cuts in the gas cooler performance has also been studied numerically as function of the refrigerant circuitry, where it has been found that an optimum circuitry for the use of fin cuts exists. Finally, with the aim of presenting the Fin1Dx3 model as a suitable design tool for MCHX, the model has been compared against the authors’ previous model (Fin2D) and other representative models from the literature in terms of accuracy and computational cost. The Fin1Dx3 model has reduced the simulation time by one order of magnitude with regard to Fin2D, and in terms of accuracy deviates less than 0.3%.

Numerical model for microchannel condensers and gas coolers: Part I – Model description and validation

The present work presents a model (Fin1Dx3) for air-to-refrigerant microchannel condensers and gas coolers, with any refrigerant circuitry. The model applies a segment-by-segment discretization to the heat exchanger, adding in each segment a novel bi-dimensional discretization to the fluids flow, fin and tube wall. Fin1Dx3 introduces a new approach to model the air-side heat transfer by using a composed function for the fin wall temperature, which allows to take into account more fundamentally the heat conduction between tubes. The proposed model accounts for: 2D longitudinal heat conduction in the tube wall, the heat conduction between tubes along the fin, and the unmixed air influence on performance. The paper presents the heat exchanger discretization, the governing equations, the numerical scheme employed to discretize equations and the solving methodology. The model has been validated against experimental data for both a condenser and a gas cooler, resulting in predicted capacity errors within ±5%.

Refrigerant circuitry design of fin-and-tube condenser based on entropy generation minimization

This study presents a numerical model that is applicable to fin-and-tube condensers with complex refrigerant circuitry, and can be used to describe the exergy loss on the refrigerant side. The model was validated by comparing numerical results with experimental data for an R410A multi-pass condenser. The modeled and measured heat transfer rates were observed to agree with an error of less than 10%. Refrigerant-side performance was analyzed according to the design parameters using the numerical model, and appropriate performance evaluation criteria (PEC) were established for the refrigerant side of the condenser. A methodology was developed for refrigerant circuitry design based on entropy generation minimization (EGM). The resulting refrigerant circuit design enhanced heat transfer performance and lowered entropy generation in comparison to simple refrigerant circuitries.

In-tube condensation performance of refrigerants considering penalization terms (exergy losses) for heat transfer and pressure drop

This paper presents Performance Evaluation Criteria ( PEC) dubbed Penalty Factor ( PF) and Total Temperature Penalization ( TTP) for condensation inside tubes. This general methodology allows for the ranking of refrigerants for condenser applications based on their heat transfer potentials. Among halocarbons, higher pressure fluids have lower PF and TTP values than medium and lower pressure refrigerants, with ammonia outperforming all refrigerants. These PEC are also used to develop simple theoretical models for the optimization of condenser refrigerant circuitry. As examples, two condenser optimization cases are considered, with the theoretical predictions comparing favorably with simulation results from a detailed heat exchanger model.

Experimental validation of model predictions on evaporator coils with an emphasis on fin efficiency

Simultaneous heat and mass transfers occur when an evaporator coil works in a wet condition due to the condensation of moist air. In evaporator modeling, overall fin efficiency is commonly used in heat balance and mass conservation equations to calculate sensible and latent heat transfers. In this paper, to analyze the simultaneous heat and mass transfer on the evaporator coils, a new concept, i.e. sensible and latent fin efficiencies, is introduced. The result shows that the latent fin efficiency is generally not equal to the sensible or the overall fin efficiencies. The proposed 1-D wet fin efficiency model has been validated by the extensive comparison of the model predictions against the experimental data on evaporator coils with various configurations that include simple and complex refrigerant circuitry.

Knowledge-Based Evolution Method for Optimizing Refrigerant Circuitry of Fin-and-Tube Heat Exchangers

Fin-and-tube heat exchangers are widely used in HVAC applications, and the optimization of the refrigerant circuitry (RC) is important to improve their performance. The genetic algorithm (GA) is a commonly used optimization method but cannot be directly used for RC optimization. This paper presents a new optimization method based on the GA, which is simpler and more effective for RC optimization than existing methods. The optimization method for RC optimization presented in this paper is a knowledge-based evolution method (KBEM) that includes an improved genetic algorithm (IGA) and domain knowledge-based RC search methods. The IGA, as the basis of the KBEM, uses a tailor-made RC coding method, RC generation method, and improved genetic operators. The knowledge-based RC search methods increase the optimization efficiency of the IGA by reasonably reducing the search space according to the domain knowledge. Case studies show that the KBEM can generate RC designs better than those generated by the existing RC optimization methods, and the KBEM is suitable for various types of fin-and-tube heat exchangers.

Optimization of finned-tube condensers using an intelligent system

The refrigerant circuitry influences a heat exchanger's attainable capacity. Typically, a design engineer specifies a circuitry and validates it using a simulation model or laboratory test. The circuitry optimization process can be improved by using intelligent search techniques. This paper presents experiments with a novel intelligent optimization module, ISHED (Intelligent System for Heat Exchanger Design), applied to maximize capacity through circuitry design of finned-tube condensers. The module operates in a semi-Darwinian mode and seeks refrigerant circuitry designs that maximize the condenser capacity for specified operating conditions and condenser slab design constraints. Examples of optimization runs for six different refrigerants are included. ISHED demonstrated the ability to generate circuitry architectures with capacities equal to or superior to those prepared manually, particularly for cases involving non-uniform air distribution.

An Optimized Design of Finned-Tube Evaporators Using the Learnable Evolution Model

Optimizing the refrigerant circuitry for a finned-tube evaporator is a daunting task for traditional exhaustive search techniques due to the extremely large number of circuitry possibilities. For this reason, more intelligent search techniques are needed. This paper presents and evaluates a novel optimization system called ISHED1 (intelligent system for heat exchanger design). This system uses a recently developed non-Darwinian evolutionary computation method to seek evaporator circuit designs that maximize the capacity of the evaporator under given technical and environmental constraints. Circuitries were developed for an evaporator with three depth rows of 12 tubes each, based on optimizing the performance with uniform and nonuniform airflow profiles. ISHED1 demonstrated the capability to generate designs with capacity equal or superior to that of best human designs, particularly in cases with non-uniform airflow.

Numerical and experimental studies of refrigerant circuitry of evaporator coils

This study attempts to analyze the performances of evaporator coils with complex refrigerant circuitry using a distributed simulation model, which has three elements: branch, tube and control volume. The governing equations for a control volume are presented in the paper together with the computer simulation procedure for branches, tubes and control volumes of a coil. The model predictions on four test coils are validated with experimental data collected under different airflow conditions using R134a as a refrigerant. Using this model, the heat transfer and fluid flow characteristics of the coils are studied. The study shows that while the thermal resistance of refrigerant side is comparable to that of airside, the coil comprehensive performance can be improved by changing the refrigerant mass velocity along the flow path. Compared with a common coil, using a complex refrigerant circuitry arrangement where the refrigerant circuits are properly branched or joined may reduce the heat transfer area by around 5% in coil design. A guideline is proposed for the refrigerant circuitry arrangements to improve the coil performance.

Study on refrigerant circuitry of condenser coils with exergy destruction analysis

A numerical model of air-cooled condenser coils is proposed to investigate the coil performance. In this model, the properties of refrigerant and air, as well as six exergy destruction components associated with different sources of irreversibility are computed locally along the coil based on numerous control volumes. The governing equations for a control volume are presented together with a computer simulation procedure to link all the control volumes together for a coil. Using this model, the coil characteristics in heat transfer, fluid flow and exergy destruction are analyzed with an emphasis on the refrigerant circuitry investigation. The study shows that the thermal resistance of refrigerant side is comparable to that of air side and the coil performance can be improved by varying the refrigerant mass velocity along the flow path. Compared with a common coil, using a complex refrigerant circuitry that the refrigerant circuits are properly branched or joined may reduce the heat transfer area by approximately 5% in coil design. This study also shows that the quantitative analysis on the exergy destruction components gives a clear picture in the coil performance evaluation. The results show that the criterion of exergy destruction ratio conforms well to that of coil heat flux in coil performance evaluation under the specified conditions.