Performances of thermostatic and electronic valves controlling the compressor capacity

The performance of the energy consumption of an electronic valve and a classical thermostatic valve has been compared when these expansion valves are adopted in a vapour compression plant subjected to a cold store. The main aim is to verify experimentally which type of expansion valve would be preferable from energy point of view when a classical thermostat or a fuzzy logic algorithm are used as the control system for the refrigeration capacity. The fuzzy logic-based control is able to modulate continuously the compressor speed through an inverter. The results show that with a fuzzy algorithm, the thermostatic expansion valve allows an energy saving of about 8% in comparison with the electronic valve. When on–off control is used, the electric energy consumption obtained both with the electronic valve and with the thermostatic valve is comparable

Determination of the compressor optimal working conditions

The replacement of environmentally unfriendly refrigerants and the energy saving demand have recently caused changes of the components and operation of the vapour compression plants; in particular, the compressors have been experiencing upgrades and modifications. The compression systems are usually designed for working under maximum load conditions, but most of the time these plants work under partial load conditions with compressor on–off cycles regulated by a thermostat. As for the variable speed compressor, the speed is continuously controlled to match the compressor capacity to the load required; this allows to save energy when compared to the thermostatic control. The aim of this paper is to identify the compressor current frequency that optimizes the energy, exergy and economy aspects. The determination of the optimum frequency for each working condition is key to build a control algorithm that allows the compressor speed to be continuously regulated by an inverter. This analysis has been applied to the reciprocating and scroll compressors and high energy savings have been achieved

Flow boiling of carbon dioxide: Heat transfer for smooth and enhanced geometries and effect of oil. state of the art review

This paper presents a state-of-the-art review on flow boiling of carbon dioxide, including experimen- tal studies and correlations for smooth and enhanced tubes, with pure CO 2 and CO 2 /lubricant mixtures. Specifically, 5223 CO 2 heat transfer coefficient data in smooth tubes are collected, and the effect of the operating conditions is discussed. Additional 883 data points in microfin tubes and 1184 experimental heat transfer coefficients in smooth tubes with CO 2 /oil mixture are also collected, and the influence of the microfin structure and of the oil presence on the heat transfer mechanism is analyzed. The statistical analysis has highlighted that the CO 2 -based correlation of Fang et al. is very accurate ( MAE = 5.1%) for the smooth tube database, whereas the heat transfer coefficients in microfin tubes are satisfactorily predicted ( MAE = 30.5%) with the model of Mehendale. Among the available cor- relations for CO 2 /oil mixture in smooth tubes, the method of Gao et al. provides the highest accuracy ( MAE = 63.2%)

Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation

This paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved

An evaluation of R22 substitutes performances regulating continuously the compressor refrigeration capacity

This paper presents the results of an experimental analysis which compares in terms of energetic performances the refrigeration capacity control obtained by means of a variable-speed compressor with the on/off control deriving from a classical thermostatic device. The compressor considered is semi-hermetic reciprocating and is a component of a vapour compression refrigeration plant subjected to a commercially available cold store. The compressor working with the fluids R22, R507 and R407C and designed for a revolution speed corresponding to the compressor supply current nominal frequency of 50 Hz, has been tested varying the frequency in the range 30–50 Hz. In this range, the most suitable working fluids proposed as substitutes of the R22 as the R407C (R32/R125/R134a 23/25/52% in mass), the R507 (R125/R143A 50/ 50% in mass) and the R417A (R125/R134a/R600 46.6/50/3.4% in mass) have been tested. The results show that, using the R407C, it is possible an average an electric energy consumption about 12% smaller when an inverter is employed to control the compressor refrigeration capacity instead of the thermostatic control which imposes on/off cycles on the compressor, working at the nominal frequency of 50 Hz. So the R407C confirms its superiority in comparison with the R417A and R507; only the R22 shows a better performance

State of the art of evapotranspiration models for plant cultivation in open fields, greenhouse systems and plant factories

The scarcity of water, the need to reduce of pesticides, the demand for on-site production of vegetables are moving the interest from greenhouse cultivation to indoor farming. Compared to greenhouses, indoor farms allow to reduce considerably the water consumption, requiring more energy, which could be provided by renewable sources. In order to assess the convenience of such a system, accurate preliminary calculations are needed for productivity, energy requirements and costs as a function of the type of cultivation and the operating conditions. While some knowledge (e.g. production rate or cooling system performance) are available from open literature, some specific predictive methods are required. Based on the few works available in literature about indoor farming, evapotranspiration rate resulted as a critical term. An assessment of different methods based on literature data with a critical analysis of their effectiveness based on several aspects (level of fidelity of the model, complexity in the calibration and use, potential strengths and weaknesses) is proposed in this work

Experimental evaluation of the thermal conductivity for additive manufactured materials. New test facility concept and preliminary tests

Items for heat transfer made by metal foams or additive manufactured structures allow to create special components for several applications (i.e. fast response PCM, complex and light HEXs working at high temperatures for the aerospace sector). For their design thermo-physical properties data are required, such as thermal conductivity. To accurately measure this parameter with conventional procedures for metallic items (e.g. flash methods), the specific heat and density should be measured each time depending on the actual composition of the porous media (estimation is not yet reliable and porosity not known). The scope of this paper is to validate a test facility and the relative method for the data reduction to proof the possibility to measure directly the thermal conductivity with samples of small size. The experiments, carried-out with square samples (30 30 mm2), allow to measure a range of thermal conductivity between 5 and 50 W/m K. The main aspect of the experimental method is the calibration of the heat losses towards the environment by means of a reverse technique. The assessment of the results against samples of known materials was good

Thermodynamic Analysis of a Multi-Ejector, CO2, Air-To-Water Heat Pump System

Abstract Nowadays, air conditioning systems for residential and office buildings, contribute largely to the energy consumptions and to the direct and indirect emissions of greenhouse gases. Carbon dioxide (CO2) could be an interesting option to replace traditional HFCs in space heating applications, due to its environmentally friendly characteristics: zero ODP and extremely low GWP, but, in order to spread its use, improvements in performances are needed. In fact, CO2 requires transcritical cycles with high expansion losses. The use of an ejector can reduce these losses and improve the performances up to 30% (depending on the performances of the ejector itself and on the operating conditions). In the a/c applications, characterized by variable operating conditions, multi-ejector systems could be used, where some ejectors work in parallel, in different combination, varying the operating conditions. Currently, a project of DTE-PCU-SPCT Department of ENEA and Industrial Engineering Department of Federico II University of Naples, is in progress, in order to evaluate experimentally the effect of several ejectors geometries on the global performance of a CO2 heat pump working with a transcritical cycle. As a part of this project, a complete heat pump system for production of hot water for sanitary use and for space heating is tested to investigate the effect of the ejector size on the balancing of the global performance of the whole system

Seasonal performance assessment of sanitary hot water production systems using propane and CO2 heat pumps

[EN] This paper presents an experimental analysis and performance evaluation of a ground source heat pumpsystem providing heating/cooling to an office building, located at the Universitat Politècnica de Valènciain Spain. The experimental data and a detailed description of the analysis tool used were presented ina previous paper for the first six operational years. This paper provides the adaptation of such analysistool to the new configuration (heat pump with two compressors working in tandem), and provides anupdated complete reference data sets over more than eleven years which can be used by researchers formodel validation purposes.The work of M. Tammaro on electric heat pumps is supported by the Next Heat Pump Generation project (funded by the European Commission in the 7th Framework Programme, grant number 307169 - European Heat Pump Association, 2013), which is gratefully acknowledged.Tammaro, M.; Montagud, C.; Corberán, J.; Mauro, A.; Mastrullo, R. (2017). Seasonal performance assessment of sanitary hot water production systems using propane and CO2 heat pumps. International Journal of Refrigeration. 74:222-237. https://doi.org/10.1016/j.ijrefrig.2016.09.0262222377

Adiabatic frictional pressure gradient during flow boiling of pure refrigerant R1233zd and non-azeotropic mixtures R448A, R452A and R455A

The research on two-phase flow characteristics of refrigerants is of primary importance in several fields, such as air conditioning and refrigeration systems. Therefore, the determination of the pressure drop during flow boiling is important for the correct design of evaporators and heat spreaders systems. This paper presents a collection of experiments on flow boiling pressure drop using pure refrigerant R1233zd and new low-GWP refrigerant mixtures R448A, R452A and R455A. All tests were performed in adiabatic conditions, in a smooth horizontal stainless-steel tube having an internal diameter of 6.0 mm and a thickness of 1.0 mm. The effect of operating parameters, such as (bubble) saturation temperature (from 25 to 65 °C) and mass flux (from 150 to 600 kg/m2s) is investigated and discussed, and the performance of the chosen fluids is also compared. Finally, an assessment of existing prediction methods is carried-out to find the most suitable correlations for two-phase pressure drop evaluation