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In last two decades, the upheaval in refrigeration sector has led to replacement of conventional refrigerants with non-ozone depleting and low global warming potential refrigerants. Most of alternate refrigerants which have low global warming potential, have usually low coefficient of performance (COP). To overcome the problem of low COP, a novel Vortex Tube integrated Vapour Compression cycle (NVTCR) has been proposed. The paper emphasizes on improvement in thermodynamic performance of a simple Vapour Compression cycle (VCR) by utilizing a vortex tube in the scheme of VCR. The proposed cycle operates at three pressure levels viz. lowest pressure (in evaporator), intermediate pressure (in vortex tube) and highest pressure (in condenser). A computer program has been formulated in EES software for the analysis. The results have been computed for NVTCR and compared with VCR and Vortex Tube cycle based on Keller model (VCRK) for R134a. The results show that the COP of NVTCR (with liquid vapour heat exchanger (LVHE)) is higher than the COP for both VCR and VCRK. The COP (corresponding to Te = -25℃ and 10℃ respectively) of VCR, VCRK, NVTCR and NVTCR (with LVHE) varies between 1.59 to 4.16, 1.7 to 4.24, 1.65 to 4.20 and 1.74 to 4.28 respectively. The effect of various parameters like evaporator temperature, condenser temperature, vortex tube nozzle efficiency (ηv), cold mass fraction (μ), bifurcation of cold mass fraction stream (z) for subcooling in heat exchangers (installed after condenser and separator) and effectiveness of liquid vapour heat exchanger (εlvhe) on COP of NVTCR have been examined. The effect of εlvhe on COP of NVTCR is maximum among the parameters εlvhe, ηv, μ and z considered for analysis. It is observed that percentage increase in COP of the NVTCR (with LVHE) with respect to COP of VCR varies by 4.46% and 10.05% corresponding to εlvhe equal to 0.1 and 1 respectively. [ABSTRACT FROM AUTHOR]

Condenser water supply temperature (CWST) resets are control strategies that reduce chiller plant energy consumption. Optimizing the CWST based on parameters such as outside wet bulb temperature, chiller load, and chilled water supply temperature (CHWST) setpoints can minimize total power across chiller plant components. An algorithm was developed for finding the optimal CWST for a given chiller plant. The algorithm generates lookup tables that show the optimal CWST and minimal power for a set of operating conditions. The results of the algorithm suggest three beneficial sequences of operation: resetting the CWST setpoint based on weather and cooling demand, running multiple cooling towers, and distributing chiller loads evenly. [ABSTRACT FROM AUTHOR]

Simple analysis of the thermodynamics of the refrigeration cycle suggests that lowering the condenser pressure should result in decreased compressor energy and that the lowest possible pressure is constrained by the temperature of the available heat sink (usually the ambient condition). Thus optimization of the condenser pressure seems straight-forward. However, in practice there is a more complex trade-off, as lowering condenser pressure increases the condenser fan energy. The trade-off is particularly acute when variable speed drives are used to control the condenser fans. In addition, there can be other constraints such as expansion valve performance. This paper demonstrates how the theoretical optimal condensing temperature can be defined using a mathematical model that takes into account the characteristics of the condenser, the condenser controls, the refrigeration load, and the ambient temperature. The model is used to explore practical condenser pressure control strategies for a year of operation of a case study refrigerated warehouse facility. The strategies were the optimum defined by the model and alternative control strategies including fixed head pressure set-point, minimum approach to the ambient temperature, and fixed condenser fan speed. The alternative controls are much simpler to implement and often the penalty relative to the optimum strategy is small. [ABSTRACT FROM AUTHOR]

This paper explains the development of a chiller power estimation method, based on understanding the processes nature, and building each term in the equation with intention to reflect some distinct phenomena and to realize the sense of its value. The analysis is started with a study of refrigerant isentropic compression, continued with estimating the power losses in centrifugal variable speed single-stage compressor, and completed by evaluating heat dissipation in frequency drive and motor. The results have been successfully verified with chillers of different sizes, achieving good accuracy over wide ranges of condenser water temperatures and loads. [ABSTRACT FROM AUTHOR]

A chilled water plant optimization program has been implemented on central plants at Texas A&M University, College Station, Texas. Due to the conflict conclusions drawn by other researchers, there was a debate on the advantage of applying variable condenser water flow control sequence. A calibrated component-based model is used to evaluate the energy savings potential with variable condenser water flow control sequence. The simulation result indicates that maximum achievable energy savings at the load profile of case study plant is only 1.03%. If counting the requirement of cooling tower minimum water flow, the energy saving will be negative which means the annual power consumption may increase by 3.4%. Considering the increased complex of control sequence, it is not recommended to implement variable condenser water flow control sequence for case study project. [ABSTRACT FROM AUTHOR]

A desiccant wheel integrated with condenser water cooling and geothermal energy from depleted hydrocarbon wells is investigated using an empirical desiccant wheel model to simulate the dehumidification performance. This system reduced the simulated yearly energy input required to cool and dehumidify 1 cfm (1L/s) of outside air by 43.1% compared to the use of cooling coils in College Station, TX. The yearly CHW energy and cost savings in a 202,467 ft² (18,809 m²) case study building are 2,436 MMBtu (2,572 GJ), and $37,183, with a 21.2% reduction in total CHW energy consumption. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of liquid overfeed and direct-expansion (DX) refrigeration systems and includes component performance and system balance. This project includes the design of two refrigeration circuits, liquid overfeed flooded and DX with a common condenser, and analyzes the performance of the heat exchangers and their impact on the overall circuit's performance. Several design techniques and methods are considered, including geometry and construction of heat exchangers. The performance of the two circuits is compared for four operating temperatures; -20°C (-4°F), -10°C (14°F), 0°C (32°F), and 5°C (4°F); and analyzed to ascertain circuit operation. DXevaporators are often utilized operating at-10°C(14°F) and below for relatively small duty systems, below 25kW(7.1 TR). However, liquid overfeed flooded evaporators are rarely seen practically for systems in this range. The performance of the DX andliquid over feed flood edevaporatorsis also analyzed for small duties at the four operating conditions. [ABSTRACT FROM AUTHOR]

Ammonia refrigeration systems are typically large energy users and hence good targets for energy efficiency improvements. Common opportunities to reduce energy use and improve energy efficiency include reducing refrigeration load, increasing suction pressure, employing dual suction, decreasing minimum head pressure setpoint, increasing evaporative condenser effectiveness, and reclaiming heat. Estimating savings from these measures is often difficult because of the complexity of the equipment and controls. However, the fundamental energy response of ammonia refrigeration systems to these measures can be estimated using a simplified model. An understanding of these responses is often an important first step to designing or optimizing the performance of actual systems. Thus, this paper develops a simplified model for simulating a single-stage ammonia refrigeration system and uses the model to estimate how energy use changes in response to the opportunities mentioned above. The results show how system energy use and potential energy savings vary with ambient wet-bulb temperature. The results give designers and operators insight into possible savings opportunities. [ABSTRACT FROM AUTHOR]

This study investigates the optimization of the cooling tower condenser-water leaving temperature using a component-based model. This model consists of a chiller, a condenser-water pump, and two cooling towers. The chiller is modeled with a Gordon-Ng model for vapor-compression chillers with variable-condenser flow. The cooling tower is simulated with an effectiveness-NTU model. The pump power is calculated from the pump flow rate, pump head, and efficiency. This optimization problem is formulated as that of minimizing the total power of the chiller, pump, and fans by selecting an optimal cooling tower condenser-water leaving temperature at given weather conditions, chiller load, chilled-water leaving temperature, and condense-water flow rate. The model is applied in an example chiller CW system, and a generalized reduced gradient solver is used to search the optimal cooling tower approach setpoint. Simulation results show that the optimal cooling tower approach reset schedule can be approximated with two straight lines. Significant energy savings could be achieved if compared with the scenario with a constant cooling tower condenser-water leaving temperature. Further simulations show that the chilled-water leaving temperature, chiller part-load ratio, and the climate zones the plant locates in have a minor effect on the optimal approach reset schedule. A higher condenser-water flow rate per cooling ton leads to a higher optimal cooling tower approach, but this effect can be neglected for a system with a constant CW flow rate. The approach setpoint reset schedule that yields optimal control depends on the performance characteristics of the chiller and the cooling tower. [ABSTRACT FROM AUTHOR]

This paper presents an absorption refrigeration cycle which is driven by capillary pumping. Conventional absorption refrigeration cycles require solution pumps to transfer the solution from a low-pressure absorber to a high-pressure generator. In contrast, the cycle proposed in the present study involves a low-pressure generator and a low-pressure absorber. The low-pressure generator consists of a heat transfer tube with rectangular grooves, which provide a path for the refrigerant vapor, and a cylindrical capillary wick inserted into the heat transfer tube. The inside part of the wick and the cooling tubes constitute the absorber. The working solution in the absorber penetrates the wick, after which the refrigerant evaporates due to the heat load transmitted to the heat transfer tube. This evaporation creates a pressure difference between the evaporated refrigerant vapor and the solution at the liquid-vapor interface, and the high-pressure refrigerant vapor condenses into liquid phase as it cools inside the condenser. In this situation, the high-pressure liquid refrigerant expands via the expansion valve and evaporates into the evaporator, which results in a decrease in temperature. The low-pressure refrigerant vapor which evaporates into the evaporator is absorbed into the solution inside the absorber. In this cycle, only the refrigerant vapor generated in the generator and the liquid refrigerant in the condenser are kept at high pressure. Therefore, there is no need to use a solution pump to transport the solution. For the purposes of this study, experimental equipment was manufactured in order to verify the effectiveness of the proposed cycle. The experimental equipment consisted of a condenser, an evaporator, a low pressure generator and an absorber, where the generator and the absorber were separated by a porous wick. Furthermore, in order to simplify the experiment, the refrigeration test was performed by using a single-component refrigerant as the working fluid. The resulting decrease in pressure due to capillary pumping was between 3.3 and 4.7 kPa, and a decrease in temperature of 8.0 to 13.2 K was achieved. [ABSTRACT FROM AUTHOR]

This paper provides new selection charts for the sizing of adiabatic capillary tubes operating with alternative refrigerants. The mathematical model is based on conservation of mass, energy, and momentum of fluids in the capillary tube. After the developed model is validated by comparison with the experimental data reported in literature, selection charts that contain the relevant parameters are proposed for sizing adiabatic capillary tubes. The selection charts are presented for some alternative refrigerants and a wide range of operations. These newly developed selection charts can be practically used to select capillary tube size from the flow rate and flow condition or to determine mass flow rate directly from a given capillary tube size and flow condition. [ABSTRACT FROM AUTHOR]

Despite the higher operating pressure, carbon dioxide has attracted many researchers' attention because of its higher density, favorable heat transfer characteristics, and environmental friendliness. In order to design components and systems for carbon dioxide, a performance database should be established. As part of the efforts for developing such a performance database, a set of tests were conducted for a fin-and-tube type heat exchanger serving as a gas cooler in the transcritical cycle by using a specially designed test facility. Furthermore, to provide a better understanding of the performance ofa carbon dioxide gas cooler a set of parametric measurements at various inlet air temperatures and velocities, refrigerant mass flow rates, and operating pressures were conducted. The test results of one CO2 gas cooler are reported in the current paper. In the second phase of the study, fin-and-tube type heat exchangers (as a condenser and an evaporator) and micro channel heat exchangers (as a gas cooler a condenser, and an evaporator) will be constructed and tested. Then experimental results of those studies will serve as our design database for the CO² heat exchangers. [ABSTRACT FROM AUTHOR]

This paper presents the design of model-based nonlinear dynamic observers of an evaporator and a condenser for advanced control of air-conditioning and refrigeration systems. Control-oriented low-order models are derived for the evaporator and the condenser that are spatially distributed. Based on sensor measurements of the evaporating temperature, the nonlinear observer for the evaporator can be used to estimate the heat transfer rate and the length of the two-phase section that cannot be measured directly in transient processes. Based on sensor measurements of the condensing temperature, the nonlinear observer for the condenser can be used to estimate the length of the two-phase section and the length of the subcooling section that cannot be measured directly. The simulation results and experimental testing are also presented. In air-conditioning and refrigeration applications, the nonlinear observers can be utilized to synthesize feedback linearization nonlinear control and provide accurate refrigerant charge inventory estimation and optimization for transient processes. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the impacts of air-side fouling and cleaning on the performance of various condenser coils used in unitary air-conditioning systems. A total of six condenser coils with different fin geometries and depths were tested. Performance tests were conducted at three different conditions: clean-as-received, after fouling, and after cleaning. In all cases, it was observed that fouling was mostly confined to the frontal face of the heat exchanger as reported in previous investigations. The amount of deposited dust was more dependent on fin geometry for single-row heat exchangers than for double-row heat exchangers. The predominant effect of fouling was to cause a more significant increase in air-side pressure drop than degradation in heat transfer performance. For the single-row heat exchangers, the pressure drop increased by 28% to 31%, while the heat transfer performance decreased by 7% to 12% at the standard air face velocity of 1.53 m/s depending on fin shape. For the double-row heat exchangers, the pressure drop increased by 22% to 3 7%, and heat transfer performance decreased by only 4% to 5% at the same air face velocity. Once the contaminated coils were cleaned according to a manufacturer-specified cleaning procedure, the original performance of the heat exchangers was recovered almost completely. The pressure drop was restored to within 1% to 7% and the heat transfer performance could be recovered to within 1% to 5% of the originally clean heat exchangers. Therefore, it is concluded that a periodic application of .specified cleaning procedures will be effective in maintaining the thermal performance of condenser coils. [ABSTRACT FROM AUTHOR]

Air-cooled condensers are routinely designed for a variety of applications, including residential air-conditioning systems. Recent attempts at improving the performance of these heat exchangers have included the consideration of microchannel tube, multilouver fin heat exchangers instead of the more conventional round tube-plate fin designs. Such microchannel tube, multilouver fin heat exchangers have a large number of geometric parameters that affect the performance of the condenser. The present work provides a systematic procedure to optimize the performance of such air-cooled microchannel condensers with refrigerant blend R-4I0A for minimum material and energy consumption. Flow regime-based models from the literature and the authors' prior work are adapted to model condensation of refrigerant blend R-410A in the microchannel tubes under consideration. A variety of refrigerant-side pass arrangements are also considered to help tailor the pass-wise performance to maximize heat transfer within the allowable pressure drop constraints. Similarly, correlations for airflow over multilouver fins are used to predict the air-side behavior Features of the air-side geometry such as fin pitch, height, and louver details are varied to obtain the optimal configuration. Through the analysis of several cases, it is shown that material savings can result through Judicious choice of .surface area and refrigerant-side flow area allocations. The results from this study can be used to guide the design of air-cooled condensers for a wide range of design conditions. [ABSTRACT FROM AUTHOR]

In this paper the salient features of an experimental investigation on heat transfer enhancement by twisted-tape inserts inside a horizontal R-22 condenser are reported. The test condenser has four test sections connected in series, and each test section has an inside diameter of 12.7 mm and length of 950 mm. Three twisted-tape inserts with twist ratios, y, of 14.95, 9.05, and 5.90 were put, one-by-one, in the entire length of the test condenser. The refrigerant mass velocity G was varied from 210 to 372 kg/s-m² in four steps. Enhancement in the average heat transfer coefficient h was attained up to 25% for the twisted-tape insert with the twist ratio y of 5.90, as compared to the plain tube flow. It was also found that the twisted-tape inserts are more effective at low vapor mass velocities inside the tubes. An empirical correlation equation was also developed to calculate the heat transfer coefficients. This equation predicts the experimental heat transfer coefficient within an error band of ±30% for a tube with the twisted-tape inserts. [ABSTRACT FROM AUTHOR]