Experimental and numerical study on microchannel and round-tube condensers in a R410A residential air-conditioning system

The effect of different type of condensers on the performance of R410A residential air-conditioning systems was investigated in this study. Two R410A residential air-conditioning systems, one with a microchannel condenser and the other with a round-tube condenser, were examined experimentally, while the other components of the two systems were identical except the condensers. Two condensers had almost same package volumes. The two systems were operated in separate environmental chambers and their performance was measured in ARI A, B, and C conditions. Both the COP and cooling capacity of the system with the microchannel condenser were higher than those for the round-tube condenser in all test conditions. The refrigerant charge amount and the refrigerant pressure drop were measured; the results showed a reduction of charge and pressure drop in the microchannel condenser. A numerical model for the microchannel condenser was developed and its results were compared with the experiments. The model simulated the condenser with consideration given to the non-uniform air distribution at the face of the condenser and refrigerant distribution in the headers. The results showed that the effect of the air and refrigerant distribution was not a significant parameter in predicting the capacity of the microchannel condenser experimentally examined in this study. Temperature contours, generated from the measured air exit temperatures, showed the refrigerant distribution in the microchannel condenser indirectly. The temperature contours developed from the model results showed a relatively good agreement with the contours for measured air exit temperatures of the microchannel condenser.     

Modeling and experimental investigation of accumulators for automotive air conditioning systems

A stream analysis model was developed to simulate the behavior of accumulators and their influence on the automotive air conditioning (A/C) systems. It allows a comprehensive steady state simulation with a set of input conditions such as refrigerant vapor mass flow rate and pressure at the inlet of an accumulator. In this study, the refrigerant/oil mixture is R134a/PAG oil which are totally miscible, but could be any air conditioning refrigerant/oil, including carbon dioxide (CO₂)/oil. The model accounts for all major effects inside the accumulator, such as friction, bends, sudden expansion, sudden contraction and heat exchange. The outputs are vapor quality, pressure and temperature at various positions of accumulator. In order to verify the mathematical model, experiments are performed in an experimental setup made up of real size automotive air conditioning components. The simulated results agree well with the experimental data. The simulation and experimental results show an important function of accumulators that is to determine the vapor quality into compressor, and thus has influence on the performance of whole automotive A/C systems.     

Identification and control of multi-evaporator air-conditioning systems

Modern multi-evaporator air-conditioners (MEACs) incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and conventional expansion valves. In this paper, feedback controller design for the MEAC system is first addressed through experimental identification. The identification produces a low-order, linear model suitable for controller design. The feedback controller employed is multi-input–multi-output-based and possesses a cascade structure for dealing with the fast and slow dynamics in the system. To determine appropriate control parameters, conditions that establish the stability for the cascade design are given. Due to the deficiency in control inputs, the proposed control structure exhibits steady-state errors in the superheat responses which in turn can produce unacceptable steady-state superheats. To resolve this issue, the reference superheat settings are determined via an optimization procedure so that the resultant steady-state superheats become acceptable. The control experiments indicate that the proposed controller can successfully regulate the indoor temperatures and maintain the steady-state superheat temperatures at acceptable levels.     

Development and validation of “grey-box” models for refrigeration applications: A review of key concepts

“Grey-box” modelling combines the use of first-principle based “white-box” models and empirical “black-box” models, offering particular benefits when: (a) there is a lack of fundamental theory to describe the system or process modelled; (b) there is a scarcity of suitable experimental data for validation or (c) there is a need to decrease the complexity of the model. The grey-box approach has been used, for example, to create mathematical models to predict the shelf life of chilled products or the thermal behaviour of imperfectly mixed fluids, or to create models that combine artificial neural networks and dynamic differential equations for control-related applications. This paper discusses the main characteristics of white-box, black-box and their integration into grey-box models, the requirements and sourcing of accurate data for model development and important validation concepts and measures.     

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 innovative air-conditioning load forecasting model based on RBF neural network and combined residual error correction

Accurate air-conditioning load forecasting is the precondition for the optimal control and energy saving operation of HVAC systems. They have developed many forecasting methods, such as multiple linear regression (MLR), autoregressive integrated moving average (ARIMA), grey model (GM) and artificial neural network (ANN), in the field of air-conditioning load prediction. However, none of them has enough accuracy to satisfy the practical demand. On the basis of these models existed, a novel forecasting method, called ‘RBF neural network (RBFNN) with combined residual error correction’, is developed in this paper. The new model adopts the advanced algorithm of neural network based on radial basis functions for the air-conditioning load forecasting, and uses the combined forecasting model, which is the combination of MLR, ARIMA and GM, to estimate the residual errors and correct the ultimate foresting results. A study case indicates that RBFNN with combined residual error correction has a much better forecasting accuracy than RBFNN itself and RBFNN with single-model correction.     

Modeling absorption of pure refrigerants and refrigerant mixtures in lubricant oil

This paper addresses the problem of absorption of refrigerant vapor in a stagnant layer of lubricant oil. The bulk motion of the solute is described in terms of apparent diffusion coefficients that encompass both molecular diffusion and possible macroscopic motion induced by liquid density instability and surface tension. In absorption of refrigerant mixtures, diffusion in the vapor and liquid phases are coupled with a thermodynamic model for interfacial equilibrium. Results are compared with experimental data available in the literature for absorption of several refrigerants in polyol ester oil (POE68). The adequacy of the formulation is assessed in the light of its basic assumptions and performance of the model.     

An overall performance index for characterizing the economic impact of faults in direct expansion cooling equipment

Air conditioning is a non-critical application for fault detection and diagnosis (FDD) where decisions about servicing faults should involve the use of economics. Existing methods for evaluating impacts of faults on equipment performance only consider some individual factors such as the equipment coefficient of performance (COP) or cooling capacity. This paper develops an overall economic performance degradation index (EPDI) for air conditioning equipment that includes the combined effects of degradations in COP, cooling capacity, and sensible heat ratio (SHR). EPDI quantifies the performance degradation caused by faults based on economics so it can be used as part of the decision making process in an overall FDD system. Furthermore, EPDI can be used along with estimates of typical field performance degradations to assess the economic benefits associated with the application of automated FDD. A case study is presented where EPDI was applied to measurements for an existing unit where faults were artificially introduced.     

The reliable simple one-dimensional 64-CPWTR model applied to the two-dimensional heat transfer problem of an insulated rectangular duct in an air conditioning or refrigeration system

The heat-transfer characteristics of an insulated long rectangular or square duct are analyzed by using the one-dimensional plane wedge thermal resistance (PWTR) model and plate thermal resistance (PTR) model in this study. It is found that the errors generated by the PWTR model are all positive and the errors generated by the PTR model are all negative. Thus, the combined plate wedge thermal resistance (CPWTR) model generated by paralleling PWTR and PTR models with the proportion factors of α=0.6 vs. β=0.4 (64-CPWTR model) can neutralize the positive and negative errors and obtain very accurate results in comparison with the two-dimensional numerical solutions analyzed by the CFD software. The errors generated by the one-dimensional 64-CPWTR model are within 1% for practical sizes and practical insulated thickness in air conditioning and refrigeration systems. Thus, the engineer can obtain very reliable heat transfer results when applying the one-dimensional 64-CPWTR model to an insulated rectangular duct.     

Control optimisation of CO2 cycles for medium temperature retail food refrigeration systems

This paper describes a detailed procedure into the investigation of optimised control strategies for CO₂ cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO₂ system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.     

Emissions and environmental impacts from air-conditioning and refrigeration systems

The impacts of air conditioning and refrigeration systems on stratospheric ozone are primarily linked to release of ozone-depleting refrigerants. Their contributions to global warming stem both from release of refrigerants and from emission of greenhouse gases (GHGs) for associated energy use. Because the energy-related component has a significantly higher warming impact, phaseout of hydrofluorocarbon (HFC) refrigerants with less efficient options will increase net GHG emissions. The same conclusion applies for perfluorocarbon (PFCs), though they are less commonly used as refrigerants. Integrated assessment of ozone depletion, global warming, and atmospheric lifetime provides essential indications in the absence of ideal refrigerants, namely those free of these problems as well as safety, stability, compatibility, cost, and similar burdens. This study examines the trend in refrigerant losses from chiller use. It documents both substantial progress in release reductions and the technical innovations to achieve them. It contrasts the impacts of current refrigerants with alternatives and with the chlorofluorocarbons (CFCs) they replaced. The study examines the sensitivity of efficiency to charge loss. It also summarizes thermodynamic and environmental comparisons of options to show that phaseout decisions based on chemical composition alone, without regard to attributes of individual substances, can result in greater environmental harm than benefit.     

Experimental investigation of oil retention in air conditioning systems

In air conditioning and refrigeration systems a small amount of oil is carried with the refrigerant and is retained in the system components. Oil retention characteristics in the condenser, evaporator, liquid and suction lines were measured and are presented and discussed here. Refrigerants R22, R410A, and R134a with miscible and non-miscible lubricants were considered to investigate oil retention physics in the widest possible range of transport properties. A parametric analysis in the suction line showed that oil retention depends on the oil mass fraction, vapor refrigerant mass flux, mixture viscosity ratio and orientation of the pipe. In the suction line, an increase in mixture viscosity of about 55% caused a rise in oil retention in the range of 50%, depending on the oil mass fraction. Oil retention in the upward vertical suction line is about 50% higher than in the horizontal line at similar conditions.     

Experimental performance of a direct evaporative cooler operating during summer in a Brazilian city

This paper presents the basic principles of the evaporative cooling process for human thermal comfort, the principles of operation for the direct evaporative cooling system and the mathematical development of the equations of thermal exchanges, allowing the determination of the effectiveness of saturation. It also presents the results of experimental tests in a direct evaporative cooler that take place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, and the experimental results are used to determinate the convective heat transfer co-efficient and to compare with the mathematical model.     

Multivariable control-oriented modeling of a direct expansion (DX) air conditioning (A/C) system

A dynamic mathematical model for a DX A/C system has been developed. The dynamic model, written in state-space representation which was suitable for designing multivariable control, was linearized at steady state operating points. The linearized model has been validated by comparing the model simulation results with the experimental data obtained from an experimental DX A/C system. The simulated results agreed well with the experimental data, suggesting that the model developed was able to capture the transient characteristics of the DX A/C system modeled. It is expected that the model developed can be useful in designing a multi-input multi-output (MIMO) controller to simultaneously control indoor air temperature and humidity in a space served by a DX A/C system.     

The impact of evaporator fouling and filtration on the performance of packaged air conditioners

The goal of the study presented in this paper was to evaluate the impact of different filter types on the performance of three typical packaged air conditioners under both clean and fouled conditions. In a companion paper, combinations of six different levels of filtration and four different coils were tested under clean and fouled conditions. From the tests, it was found that fouling has a relatively small impact on air-side effective heat transfer coefficient, but can have a large impact on coil pressure drop. Data from the experimental study were used in developing simulation models for the three packaged air conditioners. Simulations show that the equipment cooling capacity is reduced with fouling primarily because of a decrease in air flow due to the increased pressure drop. In most cases, EER (energy efficiency ratio) was reduced with fouling primarily due to increased fan power. However, the changes in EER were relatively small, in the range of 1–10%. Equipment having low efficiency filters had higher EER after fouling than equipment with high efficiency filters, because high efficiency filters result in significantly higher pressure drops than low efficiency filters. The impact of the evaporator side fan efficiency was found to be significant. The energy penalty associated with high efficiency filters was reduced greatly when fan efficiency increased. Although high efficiency filters cause higher energy penalties they provide considerably better air quality. The quantity of dust passing through the coil with an MERV14 filter was approximately 30 times less than the dust passing the coil with an MERV4 filter. This difference was doubled when the MERV14 filter was compared to a case with no filter in place.     

A DDC-based capacity controller of a direct expansion (DX) air conditioning (A/C) unit for simultaneous indoor air temperature and humidity control – Part II: Further development of the controller to improve control sensitivity

The development of the novel direct digital control (DDC)-based capacity controller for a direct expansion (DX) air conditioning (A/C) unit having variable-speed compressor and supply fan to simultaneously control indoor air temperature and relative humidity (RH) in a conditioned space served by the DX A/C unit has been reported in Part I of the two-part series. The results of preliminary controllability tests for the novel capacity controller presented in Part I, however, suggested that the controller developed was operational, with acceptable control accuracy but rooms for improvement with respect to control sensitivity. This paper, the second part of the two-part series, reports on the further development of the controller to improve its control sensitivity and the associated controllability test results. Both control accuracy and reasonable control sensitivity were achieved by incorporating a traditional Proportional–integral (PI) controller into the DDC-based capacity controller.     

The coefficient of performance of an ideal air conditioner

In this paper we present the expression for the COP of an ideal air conditioner in terms of the temperature of air at the inlet and outlet of the air conditioner.     

A comprehensive design and rating study of evaporative coolers and condensers. Part II. Sensitivity analysis

Sensitivity analysis can be used to identify important model parameters, in particular, normalized sensitivity coefficients; by allowing a one-on-one comparison. Regarding design of evaporative coolers, the sensitivity analysis shows that all sensitivities are unaffected by varying the mass flow ratio and that outlet process fluid temperature is the most important factor. In rating evaporative coolers, effectiveness is found to be most sensitive to the process fluid flow rate. Also, the process fluid outlet temperature is most sensitive to the process fluid inlet temperature. For evaporative condensers, the normalized sensitivity coefficient values indicate that the condensing temperature is the most sensitive parameter and that these are not affected by the value of the mass flow ratio. For evaporative condenser design, it was seen that, for a 53% increase in the inlet relative humidity, the normalized sensitivity of the surface area increased 1.8 times in value and, for a 15 °C increase in the condenser temperature, the sensitivity increased by 3.5 times. The performance study of evaporative condensers show that, for a 72% increase in the inlet relative humidity, the normalized sensitivity coefficient for effectiveness increased 2.4 times and, for a 15 °C increase in the condenser temperature, it doubled in value.     

Performance analysis of liquid desiccant dehumidification systems

Desiccant systems find applications in a very large variety of industrial and daily usage products including the new HVAC installations. An overview of liquid desiccant technology has been presented in this paper along with a compilation of experimental performance data of liquid desiccant dehumidifiers, empirical dehumidification effectiveness and mass transfer correlations in a useful and easy to read tabular format. The latest trends in this area suggest that hybrid systems are of current interest to HVAC industry, not only for high latent load applications but also for improving indoor air quality. The paper presents a comprehensive comparative parametric analysis of packed bed dehumidifiers for three commonly used desiccant materials viz. triethylene glycol, lithium chloride and calcium chloride, using empirical correlations for dehumidification effectiveness from the literature. The analysis reveals significant variations and anomalies in trends between the predictions by various correlations for the same operating conditions, and highlights the need for benchmarking the performance of desiccant dehumidifiers.     

CFD analysis of ejector in a combined ejector cooling system

One-dimensional ejector analyses often use coefficients derived from experimental data for a set of operating conditions with limited functionality. In this study, several ejector designs were modelled using finite volume CFD techniques to resolve the flow dynamics in the ejectors. The CFD results were validated with available experimental data. Flow field analyses and predictions of ejector performance outside the experimental range were also carried out. During validation, data from CFD predicted the entrainment ratios with greater accuracy on definite area ratios, although no shock was recorded in the ejector. Predictions outside the experimental range—at operating conditions in a combined ejector–vapour compression system—and flow conditions resulting from ejector geometry variations are discussed. It is found that the maximum entrainment ratio happens in the ejector just before a shock occurs and that the position of the nozzle is an important ejector design parameter.