An experimental study of performances of bottle cooler using carbon dioxide as refrigerant

Transcritical carbon dioxide microchannel heat pump water heaters: Part II – System simulation and optimization

The objective of the present study was to determine the performance of CO{sub 2} microchannel evaporators and gas coolers in operational conditions representing those of residential heat pumps. A set of breadboard prototype microchannel evaporators and gas coolers was developed and tested. The refrigerant in the heat exchangers followed a counter cross-flow path with respect to the airflow direction. The test conditions corresponded to the typical operating conditions of residential heat pumps. In addition, a second set of commercial microchannel evaporators and gas coolers was tested for a less comprehensive range of operating conditions. The test results were reduced and a comprehensive data analysis, including comparison with the previous studies in this field, was performed. Capacity and pressure drop of the evaporator and gas cooler for the range of parameters studied were analyzed and are documented in this report. A gas cooler performance prediction model based on non-dimensional parameters was also developed and results are discussed as well. In addition, in the present study, experiments were conducted to evaluate capacities and pressure drops for sub-critical CO{sub 2} flow boiling and transcritical CO{sub 2} gas cooling in microchannel heat exchangers. An extensive review of the literature failed to indicate any previous systematic study in this area, suggesting a lack of fundamental understanding of the phenomena and a lack of comprehensive data that would quantify the performance potential of CO{sub 2} microchannel heat exchangers for the application at hand. All experimental tests were successfully conducted with an energy balance within {+-}3%. The only exceptions to this were experiments at very low saturation temperatures (-23 C), where energy balances were as high as 10%. In the case of evaporators, it was found that a lower saturation temperature (especially when moisture condensation occurs) improves the overall heat transfer coefficient significantly. However, under such conditions, air side pressure drop also increases when moisture condensation occurs. An increase in airflow rate also increases the overall heat transfer coefficient. Air side pressure drop mainly depends on airflow rate. For the gas cooler, a significant portion of the heat transfer occurred in the first heat exchanger module on the refrigerant inlet side. The temperature and pressure of CO{sub 2} significantly affect the heat transfer and fluid flow characteristics due to some important properties (such as specific heat, density, and viscosity). In the transcritical region, performance of CO{sub 2} strongly depends on the operating temperature and pressure. Semi-empirical models were developed for predictions of CO{sub 2} evaporator and gas cooler system capacities. The evaporator model introduced two new factors to account for the effects of air-side moisture condensate and refrigerant outlet superheat. The model agreed with the experimental results within {+-}13%. The gas cooler model, based on non-dimensional parameters, successfully predicted the experimental results within {+-}20%. Recommendations for future work on this project include redesigning headers and/or introducing flow mixers to avoid flow mal-distribution problems, devising new defrosting techniques, and improving numerical models. These recommendations are described in more detail at the end of this report.

Experimental study on the impact of indoor unit airflow velocity on the performance of an automotive heat pump system with a suction line heat exchanger

An experimental study was investigated to show the effect of heat exchangers of capillary tube with suction line on compression refrigeration system performance which work on R-134a as refrigerant. Three types of heat exchangers (Coil, Concentric, Lateral) with capillary tube where made to use in refrigeration system. The experimental setup is measured for various volumetric flow rates for refrigerant, different air speeds for evaporator. So iterative method to find the actual process in capillary tube - suction line heat exchanger. The experimental results showed that at low refrigerant flow rate 15kg/hr, the maximum coefficient of performance for lateral heat exchanger which was 2.992 and minimum coefficient of performance for coil heat exchanger which was 2.876. While in case of high refrigerant flow rate 22.5kg/hr, the maximum COP for coil heat exchanger was 3.6, which was the highest value for all heat exchangers using in the system.

Transcritical carbon dioxide microchannel heat pump water heaters: Part I – validated component simulation modules

New heat pump water heater system using CO_2 has been developed to spread the electrical heat pump water heater from the point of view of the global warming prevention. The developed system consists of a water tank, the refrigeration circuit with a suction line heat exchanger (SLHX), a bypass line and heat recovery heat exchanger (HRHX) for the defrost. This system has three operating modes; hot water storage mode, heating up mode and defrost mode using HRHX. The SLHX can make the inlet quality of the evaporator lower than the conventional cycle without the SLHX, therefore amount of excessive refrigerant can be stored in the air side heat exchanger (evaporator) without accumulator under the various condition. In the defrost mode, the HRHX makes defrosting capacity increase and the air heat exchanger are used as the condenser.

This work presents a semi-empirical simulation of an automotive climate control system equipped with a transcritical vapor compression cycle running on carbon dioxide. The cycle components (a compressor, a throttling valve, an evaporator, a gas cooler, a suction accumulator and a suction line heat exchanger) were modeled to study the operation of the system, in the steady-state regime, under high ambient temperatures. The model took into account the severe conditions of tropical climates since the temperature at the inlet of the gas cooler is one of the predominant factors in the transcritical cycle performance. To assess the performance of the cycle, the thermodynamic model, reduced to a set of non-linear algebraic equations, was solved by a modified Newton-Raphson method. Reasonable agreement was found when results predicted by the model were compared with experimental data available in the literature.

In CO2 transcritical refrigeration cycles, fin-and-tube coils are still considered possible gas cooling devices due to their lower cost when compared with recent aluminium minichannel heat exchangers. In spite of the very high working pressures, an off-the-shelf coil with four ranks of 3/8 in. (9.52 mm) copper tube and louvered fins was found suitable to work with high R-744 pressures and has been studied as a gas cooler in a test rig built for testing carbon dioxide (CO2) equipment operating with air as a secondary fluid. The test rig consists of two closed-loop air circuits acting as heat sink and heat source for the gas cooler and evaporator, respectively. The tested refrigerating circuit consists of two tube-and-fin heat exchangers as the gas cooler and the evaporator, a back-pressure valve as the throttling device, a double-stage compound compressor equipped with an oil separator, and an intercooler. A full set of thermocouples, pressure transducers, and flowmeters allows measurement and recording of all the main parameters of the CO2 cycle, enabling heat balance to be performed for both air side and refrigerant side. Tests focused on two different gas coolers, with continuous and cut fins, and on two different circuit arrangements. Tests on each heat exchanger were run at three different inlet conditions, for both CO2 and air. A simulation model was developed for this type of heat exchanger and three models (Dang and Hihara 2004; Gnielinski 1976; Pitla et al. 2002) proposed for the CO2 supercritical cooling heat transfer coefficients were implemented and compared in the code. The model results are compared with the experimental data for the finned coil; emphasis is given to the effect of heat conduction through fins between adjacent tube ranks on system efficiency. In the paper, the experimental results for transcritical CO2 entering the gas cooler at 87.0°C (7.911 MPa), 97.6°C (8.599 MPa), and 107.8°C (9.102 MPa) with air inlet temperatures of 20.3°C, 21.5°C, and 23.0°C, respectively, are presented. By using a coil with fins modified to reduce the heat conduction, a 3.7% to 5.6% heat flux improvement was gained. This improvement can be clearly translated in terms of coefficient of performance (COP), since a low value of the CO2 temperature at its outlet increases the cooling capacity. Considering a reference cycle with the same operating conditions, a 5.7% to 6.6% increase of COP can be obtained.

The suction line heat exchanger is introduced in a mobile air conditioning system in order to achieve higher energy efficiency. Recent studies reveal that improvement in the system with suction line heat exchanger greatly depends on refrigerant properties and operating conditions. In this work, suction line heat exchanger was incorporated in a mobile air conditioning system and the spring force in the block-type thermostatic expansion valve of the modified system was adjusted (1) to match with the cooling capacity and (2) to match with the suction degree of superheat of the original system as two different cases. The refrigerant R134a was used in this study. The optimum charge of the mobile air conditioning system using R134a with suction line heat exchanger was experimentally found to be 740 g which was 19.4% higher than that of the original system. At optimum charge level before tuning the thermostatic expansion valve, the coefficient of performance (COP) of the system was increased by 2.8%, while the cooling capacity was reduced by 6%. After retuning the thermostatic expansion valve to match the original cooling capacity, the COP of the system improved by 11.8% with 10.5% lesser power consumption than that of the original system. With retuned thermostatic expansion valve for the original degree of superheating, the cooling capacity and COP of the system were increased by 6.7% and 16.2%, respectively. From this experiment, it is concluded that the mobile air conditioning system using R134a as a refrigerant with suction line heat exchanger performs better with suitable retuning of thermostatic expansion valve for the same degree of superheating than that is tuned for same cooling capacity.

Based on a detailed steady-state system and component modeling, a rooftop unit system design was developed that is can achieve an integrated energy efficiency rating higher than 20. Fin-and-tube and microchannel heat exchangers were modeled using a segment-to-segment approach, and an AHRI 10-coefficient compressor map used to simulate compressor performance. The system modeling is based on a component-based modeling approach, which facilitates flexible simulation of complicated system configurations. Starting with a baseline system having integrated energy efficiency rating of 16.6, numerous technical options were extensively investigated, i.e., varying compressor sizes, heat exchanger fin densities, fin-and-tube or microchannel heat exchanger, suction line heat exchanger, desiccant wheel, tandem compressor (TD), variable-speed compressor (VS), and condenser evaporative pre-cooling; an innovative system configuration was developed by combining a tandem compression system with a variable-speed compression system. The combined system can achieve a high integrated energy efficiency ratio as well as process the outdoor ventilation air over an extensive range. The design concept for a 20-ton (70.4-kW) unit, as well as a 10-ton (35.2-kW) unit was successfully evaluated. All selected components are readily accessible on the market, and performance predictions were validated against existing rooftop unit products at the rating condition. This article illustrates a potentially cost-effective high integrated energy efficiency ratio rooftop unit design. In addtion, extensive building energy simulations were conducted using EnergyPlus to predict seasonal energy saving potentials and peak power reductions using the high integrated energy efficiency ratio rooftop unit in 16 U.S. cities in comparison to a rooftop unit with a minimum efficiency.

This paper considers the influence of internal heat exchangers to the efficiency of a refrigerating system. These internal heat exchangers(liquid-suction or suction-line heat exchangers) can, in some cases, yield improved system performance while in other cases they degrade system performance. A steady state mathematical model is used to analysis the performance characteristics of refrigeration system with internal heat exchanger. The influence of operating conditions, such as gas cooler pressure and evaporation temperatures, superheat in the evaporator and temperature of gas cooler outlet, to optimal dimensions of the heat exchanger is also analyzed in the paper. The main results were summarized as follows : the mass flowrate of R744, inner diameter tube and length of internal heat exchanger, and effectiveness have an effect on the cooling capacity, compressor work and RCI(Relative capacity index) of this system. With a thorough grasp of these effect, it is necessary to design the R744 compression refrigeration cycle using internal heat exchanger.

The aim of this study was to test a tri-partite gas cooler for CO2 heat pumps (5-13 kW). The proposed heat exchanger allowed the simultaneous and separate production of space heating and domestic hot water with an integrated design, simplifying the heat pump layout and piping requirements. The experimental campaign was dedicated to the performance of the gas cooler at various operation conditions. These operation modes are: i) only space heating, ii) only domestic hot water iii) simultaneous production of domestic hot water and space heating. The results of the test campaigns were analysed in order to determine potential improvements or required redesign of the heat exchanger. The heat exchanger solution was integrated to novel prototype of tri-generation CO2 heat pump system.

In carbon dioxide transcritical air-conditioning and heat pump systems, the high-pressure-side heat exchanger operating at supercritical pressures is usually called as gas cooler. The carbon dioxide gas cooler displays much difference from the traditional heat exchangers employing constant property fluids. The commonly used logarithmic mean temperature difference (LMTD) and effectiveness—heat transfer unit (ε-NTU) fail for the gas cooler design calculation as the carbon dioxide properties change sharply near the critical or pseudo-critical point in the heat transfer processes. The new effective heat transfer temperature difference expression for variable fluid property derived by the authors is verified by numeric simulation of the carbon dioxide gas cooler. Moreover, the available correlated models for the cooled carbon dioxide supercritical heat transfer are used to simulate the gas cooler. Detail analysis is made for the deviations among the different models, and for the distributions of local convective coefficient, heat flux, and local temperature of carbon dioxide along the flow path in the gas cooler. Copyright © 2002 John Wiley & Sons, Ltd.

A study on the performance of multi-stage heat pumps using mixtures

This paper investigates the performance of the refrigerants R-407C and R-22 with and without a suction line heat exchanger. The cyclic and steady state performance of both refrigerants is investigated in an air conditioner/heat pump. The combined impact of cyclic and steady state performances is evaluated through the cooling and heating seasonal performances. The results show that R-407C has a 4.3% lower cooling seasonal performance and a 1.5–7.0% lower heating seasonal performance, depending on the climate in question. The steady state tests indicate that there is no benefit from using a suction line heat exchanger for either fluid. However, R-22 and R-407C benefit from the suction line heat exchanger during cyclic tests. Utilizing the suction line heat exchanger reduces the performance degradation associated with cyclic operation by 5.5% and 8.1% for R-22 and R-407C, respectively.