Refrigerant Pressure Research Articles

Development of Mathematical Model to Predict the Co-Efficient Of Performance in A Refrigeration System Working with R404a

This paper exhibits an orderly way to deal with build up the numerical model for anticipating the coefficient of execution of R 404a which is an eco benevolent refrigerant and broadly utilized in refrigeration and airconditioning ventures. The procedure parameters are loop width, curl pitch and loop length. Reaction surface procedure (RSM) is utilized to build up the scientific model.Analysis of Variance (ANOVA) system is utilized to check the ampleness of the created model. The created numerical model can be utilized adequately at 95% certainty level. The coefficient of execution of R404a in a fume pressure refrigeration framework has been examined in detail.

Experimental study on refrigerant maldistribution in a fin-and-tube evaporator for a direct expansion air-conditioning system

Abstract In this paper, the design concept of the face split evaporator with a retrofitted suction header and choked flow in the capillary tubes was adopted to improve a direct expansion air-conditioning system with a designed air flow rate of 20,000 m3/h. The novel design of the direct expansion air-conditioning system was experimentally tested and validated. Face split evaporator reduced the refrigerant pressure drop in the suction header and the choked flow design limited the maximum flow rate across each branch circuit, which both contributed to the uniform refrigerant flow distribution of the improved system. Consequently, the cooling capacity and COP of the improved system was 10.5% and 13.4% higher than the original system. With the refrigerant mass flow rate of the improved system varying from 1.33 kg/s to 2.05 kg/s, the measured cooling capacity ranged 240–350 kW and the ratio of the system COP to the ideal COP of the reversed Carnot cycle with the same evaporating and condensing temperatures maintained at 0.72–0.74. The experimental results demonstrated that the improved system performed a relatively uniform refrigerant flow distribution among the branch circuit of the evaporator regardless of the refrigerant mass flow rate. This study is beneficial to the optimization design of the multi-circuit evaporator for a direct expansion air-conditioning system with a large air flow rate in engineering applications.

Refrigerant condensation in vertical pipe minichannels under various heat flux density level

Abstract This work is a study of the condensation process taking place in the conditions so far unproven. It was devoted to selected problems accompanying the condensation phenomenon occurring in a mini scale. The work is a continuation and development of the authors' research to-date. Paper describes the flow condensation of high-pressure refrigerants through channels made of stainless steel AISI 304 and AISI 316L with various inner diameter di. A new heat transfer universal correlation was developed on the basis of own experimental data for high pressure refrigerants condensation in pipe minichannels.

Experimental study on an inverter heat pump air conditioner with a vapor-bypassed evaporator

Abstract This paper presents an experimental study on the performance of an inverter heat pump air conditioner with a vapor-bypassed evaporator. The vapor-bypassed evaporator, in which a phase separator is set in the middle of the refrigerant circuit to bypass vapor refrigerant, can reduce the refrigerant pressure drop and enhance the system overall performance. The tested heat pump system can be operated at two modes, i.e., vapor-bypassed (VB) mode and conventional heat pump (CH) mode. The testing results indicate that as the heating capacity ranges 1900–4900 W, the COP of VB mode is improved by 4.1%–11.6% over CH mode under the H1 condition (indoor: 20.0/15.0 °C; outdoor: 7.0/6.0 °C). When operated under the H2 condition (indoor: 20.0/15.0 °C; outdoor: 2.0/1.0 °C; frosting condition), the system at VB mode yields 11.2% higher average heating capacity and 4.4% higher COP over CH mode at a fixed compressor speed of 95 Hz. Generally, using the vapor-bypassed evaporator has a great potential for improving the performance of a heat pump air conditioner especially under high heating capacity conditions.

Theoretical and experimental analysis of a vertical direct expansion geothermal evaporator using CO2 as refrigerant

In direct expansion ground coupled heat pumps (DX-GCHPs), the borehole heat exchanger (BHE) plays a major role, as it is the component where heat transfer between the ground coupled heat pump (GCHP) and the soil occurs. To address the current challenges of using CO2 as refrigerant in DX-GCHP systems, theoretical and experimental investigations were performed on a single U-tube vertical direct expansion BHE using CO2 as the refrigerant. First, an experimental setup of a transcritical CO2 DX-GCHP built at CanmetENERGY laboratories was used to monitor, collect, and analyze data for refrigerant flow, pressure drop, and ground temperature along the boreholes. The CO2 evaporation process in the BHE for different mass flow rates per borehole was then experimentally analyzed. Later, a previously developed theoretical model of the BHE was validated using a set of experiments collected with the current test apparatus. Finally, the model was used for the analysis of the effect of the following parameters on the performance of the BHE: (i) the refrigerant flow rate, (ii) the borehole wall temperature and average fluid temperature over the test period during the evaporation process, (iii) the pressure drop, (iv) the required power for CO2 circulation, and (v) the evaporating temperature change in the BHE.

Wetting behavior and interfacial tension of a refrigerant oil in air and refrigerant atmospheres

Abstract In the present study, the wettability of ground AISI 321 steel and PTFE surfaces with a polyalkylene glycol-based lubricant in air and 1,1,1,2-tetrafluoroethane (R-134a) atmospheres is characterized. For this, spreading velocities are determined from videos recorded during the continuous dosing of lubricant on the solid surfaces. The results are compared with interfacial tension data for the lubricant measured by the pendant-drop method in air and R-134a atmospheres. Significantly higher wettability was found in the presence of refrigerant atmosphere, on the steel surface and along its grinding grooves, whereas no consistent temperature-dependent trends could be identified. The interfacial tension of the lubricant decreases with increasing temperature at ambient air atmosphere as well as with increasing R-134a pressure at a fixed temperature. The temperature-dependent behavior of the interfacial tension for a given refrigerant pressure is affected by the temperature itself and by the resulting solubility of the refrigerant in the lubricant.

Effect of compound sloshing conditions on pressure drop and heat transfer characteristics for FLNG spiral wound heat exchanger

Floating liquefied natural gas (FLNG) is a promising floating production equipment which integrates the gas extraction, liquefaction, storage, loading and unloading of offshore liquefied natural gas. The spiral wound heat exchanger (SWHE) is one of the most important installations of FLNG and the effect of compound sloshing on the performance of the spiral wound heat exchanger attracts a lot of attention. In this work, an experimental device of dual mixed refrigerant (DMR) and a multi-phase four-stream calculation code for SWHE are constructed to study the performance of FLNG SWHE. Quantitative analysis of temperature, pressure drop and enthalpy affected by sloshing periods, displacements and types are completed. Compared with temperatures, pressure drop characteristics of SWHE are much more significantly influenced under sloshing conditions, especially for compound heaving and pitching conditions. The maximum fluctuation range of gas-phase mixed refrigerant pressure drop is from −11.08% to 24.96% for steady state. The temperature of gas-phase mixed refrigerant increases by 6.45 °C and the heat exchange decreases by 37.90 W under compound heaving and pitching conditions.

Performance of heat pumps using pure and mixed refrigerants with maldistribution effects in plate heat exchanger evaporators

Abstract This paper presents a combined plate heat exchanger (PHE) - heat pump simulation framework for the evaluation of flow maldistribution in PHE evaporators and its effect on the cycle thermodynamic and economic performance. A case study of heat pump integration for waste heat recovery purposes in data centres was chosen to demonstrate the utilization of the simulation tool. The analyses were made for the pure fluids butane and propane, and for the zeotropic mixtures propylene/butane at (0.5,0.5) mass composition and CO2/dimethyl ether (DME) (0.2,0.8) as refrigerants. Both liquid/vapour maldistribution and the effect of end plates were considered in the heat exchanger models. Results show that butane is most sensitive to maldistribution, with a maximum Coefficient of Performance (COP) reduction of 5.9%, while propane experiences the lowest reduction of 2.5%. The different sensitivity of the working fluids to maldistribution was found to be related to the evaporator design, refrigerant pressure drop, and fluid properties. Last, the results of the economic analysis show that a higher specific cost of heat is obtained when considering maldistribution effects.

Improved genetic algorithm-based prediction of a CO2 micro-channel gas-cooler against experimental data in automobile air conditioning system

Abstract For the prevailing CO2 micro-channel gas-cooler (MCGC) mathematical modeling research, a big challenge for an efficient numerical model is the trade-off between fast calculation and high-precision considering the specific properties of CO2. The main purpose of this work is to propose a regression method with multiple variables for CO2 MCGC model based on the combination of Distributed Parameter Model (DPM) and Genetic algorithm (GA). The GA based prediction model of MCGC is established, which takes correlation coefficients and mean squared error into consideration. A test rig of CO2 MCGC is developed, and the experimental data used to validate the MCGC model are collected. The fixed-point iteration algorithm is utilized to reduce the complexity of simulation to accelerate the iteration speed. Moreover, an improved Taylor series expansion-based method for fast calculation of working fluid properties was proposed. The model precision was analyzed by comparing the prediction results with conducted experimental data. The improved CO2-to-air MCGC model enables to predict refrigerant outlet temperature and pressure drop within a maximum deviation of 1.2 °C and 2 kPa against the experimental data.

Dual mixed refrigerant LNG process: Uncertainty quantification and dimensional reduction sensitivity analysis

Abstract The dual mixed refrigerant (DMR) liquefaction process is complicated and sensitive compared to the competitive propane pre-cooled mixed refrigerant liquefied natural gas (LNG) process. When any uncertainty is introduced to the process operation conditions, it is necessary for the DMR process to be re-optimized to maintain efficient operation at a minimal cost. However, in actual operation, re-optimization is a challenging task when the process operational input variables are varied, typically owing to the lack of information regarding the nature, impact, and levels of uncertainty. Within this context, this study investigates the uncertainty levels in the overall energy consumption and minimum internal temperature approach (MITA) inside LNG heat exchangers with variations in the operational variables of the DMR processes. Moreover, a global sensitivity analysis is conducted to identify the influence of random inputs on the process performance parameters. The required energy is significantly influenced by the variations in the variables in the cold mixed refrigerant (approximately 63%), while changes in the warm mixed refrigerant (WMR) section only slightly affect the uncertainty of the required specific energy. Furthermore, the probability distribution of the approach temperature (MITA1) inside the WMR exchanger is mainly affected by changes in the compositions of methane, ethane, and propane, as well as the high pressure of the cold mixed refrigerant (approximately 97%). Conversely, the flow rate of ethane and low pressure of the WMR significantly affect the uncertainty of the approach temperature (MITA2) inside the cold mixed refrigerant exchanger.

Design and assessment of an experimental facility for the characterization of flow boiling of azeotropic refrigerants in horizontal tubes

An experimental facility was designed to measure pressure drop and heat transfer coefficient during flow boiling of azeotropic refrigerants in horizontal tubes. The apparatus is made of the refrigerant circuit, including the test section; the water circuit, to provide the power for evaporation; the glycol-water circuit, to fix the refrigerant pressure. The test section is a counterflow tube-in-tube heat exchanger (refrigerant inside, water outside). Plant assessment involved the measurement of pressure drop and heat transfer during R134a flow boiling in a smooth tube (outer diameter 9.56mm, inner diameter 8.92mm). Such a case study is a benchmark for the availability of a wide experimental database in the literature, which has been also summarized in many correlations. Then further tests involved a microfin tube. The experimental conditions were: evaporation temperature, 5°C; mass flux, 111÷333kg/m2s; average quality, 0.15÷0.93; heat flux, 8.8kW/m2 and 17.6kW/m2. The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The comparison with the literature shows satisfactory agreement with the major findings and enables using the data for the assessment of the existing models to predict both the pressure drop and the heat transfer coefficient.

Analysis of refrigerant pipe pressure drop of a CO2 air conditioning unit for vehicles

Abstract This study investigates the pressure drop in refrigerant pipes of a CO2 automotive air conditioning unit. A test rig to measure the pressure drop of arbitrarily shaped pipes is presented. As boundary conditions at the inlet of the test section, mass flow, pressure and enthalpy of the refrigerant are independently adjustable to values that cover the whole operating range of a CO2 automotive air conditioning system for cars in the premium segment. Numerical analysis using CFD and a one-dimensional modelling tool is carried out. The pressure drop for the suction pipe reaches values of up to 500 mbar. CFD simulation predicts the pressure drop with an error of 25.7% for single-phase flow whereas the deviation using one-dimensional models is 46.5%. During two-phase flow the error increases to 55.8% with one-dimensional models and 42.4% using CFD.

Theoretical investigation of a novel hybrid refrigeration cycle based on the partial thermal isochoric compression

Abstract The objective of this work is the investigation of a novel refrigeration cycle which utilizes both electricity and heat input. This hybrid cycle compresses the refrigerant by using a conventional compressor in the low-pressure stage and a novel isochoric compression in the high-pressure stage by adding heat in the system. Between the compression devices, there is an intercooler which is vital for reducing the heat input demand. The isochoric process is performed in a closed vessel where refrigerant pressure increases by adding heat, according to the gas laws. Different working fluids are studies like CO2, R134a, R404A, R152a, R290 and R32. According to the final results, the electricity consumption is reduced by the incorporation of the partial thermal compression compared to the conventional system with only mechanical compression. The CO2 and the R32 seem to be the most promising refrigerants for utilization in the novel suggested cycles. For the case with 75% mechanical compression and 25% thermal compression, the electricity savings with CO2 are found 15.61% for the case with the evaporating temperature at −25 °C and the heat rejection temperature at 35 °C. More specifically, in this scenario for 100 kW refrigeration production there is a need for 47.29 kW electricity and 19.72 kW heat input, while the system COP is 1.492 and the exergy efficiency 41.01%.

High pressure refrigerants condensation in vertical pipe minichannels

Abstract The paper consists a description of thermal and flow experimental research on high-pressure refrigerants condensation process in vertical minichannels. The influence of mass flux density, heat flux density, the vapour quality and minichannel's internal diameter on the condensation process was investigated. Under the same conditions, R404A, R407C, R410A refrigerants were condensed. The study also consists a summary of data obtained during condensation process of all three investigated refrigerants. The results of experimental studies were compared with the results of theoretical modeling according to the dependencies developed by other authors. The compliance of the modeling data with the results of the experiment was considered unsatisfactory. Based on the results of experimental data, heat transfer coefficient and pressure drop prediction models for low heat fluxes (q up to 5000 W/m2) were obtained.

Numerical study on heat transfer performance of micro-channel gas coolers for automobile CO2 heat pump systems

Abstract This study aims to investigate the supercritical heat transfer characteristics of compact micro-channel gas coolers applied in automobile CO2 heat pump systems. A simulation model of automobile gas cooler was developed by using segment-by-segment method, and validated by experimental results. Using this model, under the typical heat pump operation conditions, the temperature distributions along the CO2 refrigerant flow path were predicted, and the variation of heat transfer coefficient was analyzed. Furthermore, the impacts of operation conditions such as refrigerant inlet pressure and temperature, as well as configuration parameters such as depth and pass number on the heating performance were discussed. Results show that the established model has a good prediction accuracy for heating capacity of gas cooler. It is found that there is unfavorable air temperature inhomogeneity on the airside outlet surface of gas cooler.

Study on the heat transfer performance of boiling in vertical buried tube of direct expansion ground source heat pump

Abstract Two kinds of physical phenomena of phase change heat transfer and gas-liquid two-phase flow co-exist in the buried tube of the direct expansion ground source heat pump system. The performance of boiling heat transfer process in vertical U-tube was discussed in this study. Firstly, the models of the refrigerant flow equation and the phase change heat transfer process in the vertical U-tube, the heat transfer process internal and outside well heat transfer were established. Based on the models, the changes of the refrigerant parameters along the length of tube were simulated and analyzed during boiling heat transfer process. The simulation results showed that the pressure and temperature of refrigerant rise in the downward tube, while the pressure and temperature of refrigerant declined rapidly in the upward tube. So the evaporation kept slow rate in the downward tube but fast in the upward tube. The effect of heat transfer in upward tube is better. At the same time, the influence of the soil initial temperature is simulated. as the soil initial temperature declining, the changes of the refrigerant temperature and pressure in the buried tube are smaller. And the changes of the soil temperature field with the running time and the different depth of the borehole are also simulating studied. The simulation results show that the temperature around the buried tube decreases along with the operating time, and the temperature diffusion radius around the buried tube increases in a descent gradient.

Modification of acceleration signal to improve classification performance of valve defects in a linear compressor

In general, it may be advantageous to measure the pressure pulsation near a valve to detect a valve defect in a linear compressor. However, the acceleration signals are more advantageous for rapid classification in a mass-production line. This paper deals with the performance improvement of fault classification using only the compressor-shell acceleration signal based on the relation between the refrigerant pressure pulsation and the shell acceleration of the compressor. A transfer function was estimated experimentally to take into account the signal noise ratio between the pressure pulsation of the refrigerant in the suction pipe and the shell acceleration. The shell acceleration signal of the compressor was modified using this transfer function to improve the defect classification performance. The defect classification of the modified signal was evaluated in the acceleration signal in the frequency domain using Fisher\'s discriminant ratio (FDR). The defect classification method was validated by experimental data. By using the method presented, the classification of valve defects can be performed rapidly and efficiently during mass production.

Study on a chiller of air conditioning system by sensing refrigerant pressure feedback control with stepless variable speed driving technology

Abstract This study proposes a stepless variable speed driving technology for the refrigeration equipment of the air conditioning system, to realize the energy saving strategies by using the Variable Refrigerant Volume (VRV). The developed technology was applied to the Air Cooled Packaged Water Chiller Unit (ACH). In this study, a Stepless Variable Refrigerant Output (SVRO) technology was developed for the ACH. The proposed technology, SVRO, is a plug-in device attached to the ACH controller. The SVRO is divided into two modes, using the temperature and the pressure sensors as the feedback signals. The signals are transmitted to the control module for calculation by the feedback of different sensors and then regulate the output frequency of the inverter. Therefore, the ACH was tested in a single zone, and the experiment is divided into two types: closedown experiments and non-stop experiments. Moreover, three different case tests were performed and compared by applying the conventional control method and the proposed stepless variable speed driving technology. The results show that the cumulative energy consumption of the developed control technology is lower than that of the conventional method. The experimental results also show that the energy accumulated by the pressure sensor is lower than that of the temperature sensor, which is more energy-efficient. Furthermore, the experimental results of using the pressure sensor combined with the SVRO technique indicate that at least 20% of the energy saving rate was achieved. This verified the energy saving effect of the ACH with the proposed stepless variable speed driving technology.

Reduction in power consumption of refrigerator by using parallel expansion

This study aims to determine the reduction in power consumption of refrigerators by using parallel expansion. A refrigerator with a capacity of 120 litters has been used as a test instrument and R134a acts as a working fluid. Recording of refrigerator energy consumption is based on changes in cooling load temperature from range 3C to -3 C. The parameters observed during the test include suction pressure, discharge pressure, electric current, and weight of refrigerant circulated. The results indicate that installation of parallel expansion device on the refrigerator generates a decreased 1.5% - 4% of the electrical power consumption (ECP) so that it implicates for the save on power consumption required a refrigerator.

Numerical and experimental study on the reduction of refrigerant pressure pulsation within compressor pipes

Abstract The pulsation of compressor pipe refrigerant pressure that occurs during the suction and discharge processes of a compressor is a major cause of vibration and noise. This makes it important to be able to predict the pressure pulsation within the pipe using numerical analysis, leading to a design that can reduce it. In this study, a method is presented to obtain the complex propagation speed and impedance boundary conditions of the refrigerant, which are necessary for the numerical analysis, by using the least square estimation and experimental data. Experimental verification was carried out by installing a total of 14 pressure sensors at 100 mm intervals on the compressor suction pipe connected to the refrigerant supply equipment. To verify the numerical analysis model the pressure pulsation was measured by varying the compressor operating frequency. Also, a pressure-structure interaction analysis was used to investigate whether an elastic structure installed within the pipe would affect any reduction in the pressure pulsation. The results showed that installation of the elastic structure within the pipe resulted in a significant pulsation reduction effect for the first operating frequency, while a slight reduction effect was observed for the second operating frequency. It was found that the pressure pulsation reduction was most significant when the longitudinal natural frequency of the installed elastic structure matches the operating frequency of the compressor.