Two-stage Cycle Research Articles

Experimental investigation of a direct driven radial compressor for domestic heat pumps

The presence of oil in domestic heat pumps is an obstacle toward higher efficiency, particularly for enhanced surface evaporators and for advanced concepts based on two-stage cycles. Very compact direct driven radial compressors supported on oil-free bearings represent a promising alternative. This paper presents the derivation of the specifications, the choice for an appropriate refrigerant fluid and the design of a proof of concept prototype with the various tradeoffs between the impeller characteristics to follow the seasonal heat demand, the bearing and rotordynamics for a stable operation. Heat pump simulation results, the design of the impeller as well as the layout of the experimental facility and first experimental results are presented. An impeller with a tip diameter of 20 mm has been tested at rotational speeds of up to 210 krpm reaching pressure ratios in excess of 3.3 and efficiencies above 78%. The refrigerant chosen for this first experimental approach is HFC 134a.

Optimization of homogenizing mode for aluminum alloy AA7075 using calorimetric and microstructural studies

Modes of heat treatment are developed for homogenizing cast structure of commercial aluminum alloy AA7075 with the aim of improving its process ductility. The temperature is optimized with the help of differential scanning calorimetry. Computations and studies of microstructure are used to optimize the hold time for the given grain size from the standpoint of minimizing the consequences of dendritic segregation over grain bodies and of dissolution of particles of second phase. A two-stage cycle of homogenizing (470°C, 18 h + 490°C, 7 h) is suggested for cast alloy AA7075 with the initial mean grain size of 200 μm in cast condition.

Automated modelling of complex refrigeration cycles through topological structure analysis

We have developed a computational method for analysis of refrigeration cycles. The method is well suited for automated analysis of complex refrigeration systems. The refrigerator is specified through a description of flows representing thermodynamic sates at system locations; components that modify the thermodynamic state of a flow; and controls that specify flow characteristics at selected points in the diagram. A system of equations is then established for the refrigerator, based on mass, energy and momentum balances for each of the system components. Controls specify the values of certain system variables, thereby reducing the number of unknowns. It is found that the system of equations for the refrigerator may contain a number of redundant or duplicate equations, and therefore further equations are necessary for a full characterization. The number of additional equations is related to the number of loops in the cycle, and this is calculated by a matrix-based topological method. The methodology is demonstrated through an analysis of a two-stage refrigeration cycle.

Experimental Energetic Analysis of the Subcooler System in a Two-Stage Refrigeration Facility Driven by a Compound Compressor

This paper presents a detailed experimental analysis of the energetic effects of using a subcooler system in a two-stage refrigeration facility driven by a compound compressor. The analysis is carried out by comparing the experimental performance of the two-stage cycle working with the subcooler system to the experimental performance of the two-state cycle working without the subcooler system in an evaporating temperature range between −36°C and −21°C and in a condensing temperature range between 31°C and 49°C using R-404A as the working fluid. Experimental results show that the amount of refrigerant used to obtain the subcooling, which is injected between the compression stages, causes an increment in the interstage working pressure, in power consumption, and in the cooling capacity of the cycle. This paper discusses and analyzes these modifications.

Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO 2 refrigeration cycle

In this paper, a new two-stage configuration of ejector-expansion transcritical CO 2 (TRCC) refrigeration cycle is presented, which uses an internal heat exchanger and intercooler to enhance the performance of the new cycle. The theoretical analysis on the performance characteristics was carried out for the new cycle based on the first and second laws of thermodynamics. Based on the simulation results, it is found that, compared with the conventional two-stage transcritical CO 2 cycle, the COP and second law efficiency of the new two-stage cycle are about 12.5–21% higher than that of conventional two-stage cycle. It is also concluded that, the performance of the new two-stage transcritical CO 2 refrigeration can be significantly improved based on the presented new two-stage cycle. Hence the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamically and technical point of views. A regression analysis in terms of evaporator and gas cooler exit temperatures has been used, in order to develop mathematical expressions for maximum COP, optimum discharge and inter-stage pressures and entrainment ratio.

Methodological Aspects of Evaluating Tribological Properties of Ball Bearings

The paper presents selected results of tribological tests on ball bearings. The elements for tests were obtained from their manufacturer. Two groups of ball bearings were evaluated: 134-781TNG-2RS and CBK 441TNG. Tribological tests were conducted with SMT-1 device in two-stage cycle. The first stage involved tests based on the authors’ own method. In the second stage the tests were conducted according to the manufacturer’s recommendations (methodology). The obtained results indicate varied tribological characteristics of the tested ball bearings, both in reference to the parameters of the external function and the applied test method.

Experimental study on an innovative multifunction heat pipe type heat recovery two-stage sorption refrigeration system

An innovative multifunction heat pipe type sorption refrigeration system is designed, in which a two-stage sorption thermodynamic cycle based on two heat recovery processes was employed to reduce the driving heat source temperature, and the composite sorbent of CaCl 2 and activated carbon was used to improve the mass and heat transfer performances. For this test unit, the heating, cooling and heat recovery processes between two reactive beds are performed by multifunction heat pipes. The aim of this paper is to investigate the cycled characteristics of two-stage sorption refrigeration system with heat recovery processes. The two sub-cycles of a two-stage cycle have different sorption platforms though the adsorption and desorption temperatures are equivalent. The experimental results showed that the pressure evolutions of two beds are nearly equivalent during the first stage, and desorption pressure during the second stage is large higher than that in the first stage while the desorption temperatures are same during the two operation stages. In comparison with conventional two-stage cycle, the two-stage cycle with heat recovery processes can reduce the heating load for desorber and cooling load for adsorber, the coefficient of performance (COP) has been improved more than 23% when both cycles have the same regeneration temperature of 103 °C and the cooling water temperature of 30 °C. The advanced two-stage cycle provides an effective method for application of sorption refrigeration technology under the condition of low-grade temperature heat source or utilization of renewable energy.

The performance analysis of a two-stage transcritical CO2cooling cycle with various gas cooler pressures

A theoretical analysis of a two-stage transcritical CO2 cooling cycle is presented. The effect of a two-stage cycle with intercooling process on the system coefficient of cooling performance is presented for various gas cooler pressures. However, the performance comparison between one-stage and two-stage cycles is presented for same operating conditions. Gas cooler pressure, compressor isentropic efficiency, gas cooler efficiency, intercooling quantity and refrigerant outlet temperature from the gas cooler are used as variable parameters in the analysis. It is concluded that the performance of the two-stage transcritical CO2 cycle is approximately 30% higher than that of the one-stage transcritical CO2 cycle. Hence, the two-stage compression and intercooling processes can be assumed as valuable applications to improve the transcritical CO2 cycle performance. Copyright © 2008 John Wiley & Sons, Ltd.

Microcalorimeter Instruments for the Spectrum-R(X)G and NeXT Missions

X-ray spectrometers utilizing a microcalorimeter array are presently under study for the Russian Spectrum R-G (or Spectrum-X-Gamma) mission, which is to be launched in 2011, and for the Japanese NeXT (New X-ray Telescope or Non-thermal energy eXploration Telescope) mission, whose launch is expected to be in 2012 to 2015. The primary instrument of Spectrum R-G is eROSITA, which will make an all sky survey in the 0.1–10 keV range using an array of seven telescopes and X-ray CCD cameras. The mission also carries smaller instruments, a wide-field monitor (Lobster) and a hard X-ray telescope (ART). We are proposing SXC—the Spectrum-X Calorimeter—to obtain spatially-resolved precision spectra of a number of nearby massive clusters of galaxies during an initial 6-month pointed phase, and to obtain a detailed spectral map of the soft X-ray diffuse background during the 4-year survey phase. The NeXT mission is a combination of wide band X-ray spectroscopy provided by multi-layer coating, focusing X-ray mirrors and pixel detectors, and high resolution soft X-ray spectroscopy by microcalorimeter instrument, SXS—the Soft X-ray Spectrometer. The effective area of the SXS is about 20 times larger than that of SXC at the iron K line energy (6.7 keV) while the solid angle of the field of view is by a factor of 15 smaller. One of the major scientific objectives of SXS is to determine turbulent and/or macroscopic velocities in the hot gas of distant clusters of galaxies. Both of the instruments will use 6×6 microcalorimeter array similar to the one launched on Suzaku, while both will adopt a 3He Joule Thomson cooler and two-stage Stirling cycle in the cryogenic systems. The 3He Joule Thomson cooler provides a thermal guard to liquid He but it can also work as a 1.8 K heat bath for the adiabatic demagnetization refrigerator.

Experimental investigation of an innovative dual-mode chemisorption refrigeration system based on multifunction heat pipes

In this paper, an innovative dual-mode multifunction heat pipe type chemisorption ice maker was designed, in which the compound adsorbent of activated carbon–CaCl 2 was used to improve the mass and heat transfer performance of adsorbent. For this test unit, the heating, cooling and heat recovery processes between two adsorbent beds were performed by multifunction heat pipes without additional power consumption. Two operation modes were possible for the advanced chemisorption refrigeration system. The first operation mode was a highly efficient mass and heat recovery sorption cycle where driving heat source temperature was about 145 °C. The second operation mode was a two-stage heat recovery sorption cycle in which available driving heat source temperature was about 103 °C. The experimental results showed that the first operation mode cycle can increase the coefficient of performance (COP) by 69% when compared with basic cycle. The second operation mode cycle can operate effectively with relatively low-grade generation temperature, and the performance of the two-stage heat recovery cycle was improved by more than 23% when compared with conventional two-stage cycle under the same generation temperature of 103 °C and cooling water temperature of 30 °C.

Experimental Energetic Analysis of the Liquid Injection Effect in a Two-Stage Refrigeration Facility Using a Compound Compressor

The objective of this paper is to detail an experimental analysis of the energetic implications of using a liquid injection system to produce desuperheating between the compression stages in a two-stage refrigeration facility. The analysis is carried out by comparing the experimental performance of the two-stage cycle working with and without the liquid injection system. The experimental analysis of the two-stage refrigeration cycle is performed in an evaporating temperature range between −36°C and −20°C and in a condensing temperature range between 30°C and 47°C using R-404A as working fluid. The discussion compares the cycle working with liquid injection to the two-stage configuration without this intermediate system by using experimental data from condensing and evaporating pressure variation tests in a two-stage refrigerating plant driven by a compound compressor and working with one of the fluids most widely-used in Europe for low-temperature applications, R-404A. Both experimental and theoretical results show that the use of the liquid injection to produce desuperheating between compression stages produces an increment of the interstage pressure in the cycle, the consequences of which are variations of the cooling capacity and the absorbed power consumption of the two-stage cycle.

Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator, respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO 2 cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8–11 MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated. A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle.

Thermodynamic Investigation of Two-Stage Absorption Refrigeration System Connected by a Compressor

The present work is to analyze a two-stage cycle based on the ammonia-water absorption system, with intermediate compression. The two generators of the system are heated by geothermal energy at low temperature. The study shows that this system makes it possible at lower generator temperature, under the limits permitted by the systems suggested up to now. For Tg = 335 K, Tc = Ta = 308 K and Te = 263 K, based on the electric consumption, the system efficiency is 8.2. The comparative study of the hybrid system and vapor compression systems shows the superiority of the proposed system. Supplied by the geothermal sources of the Tunisian south, the system makes it possible to obtain for a pilot geothermal station, a production of 75 tons of ice per day. The greenhouse gas emissions should thus be reduced by about 2.38 tons of CO2 per day. Therefore, based on the typical geothermal energy sources in Tunisia which present a global refrigeration potential of 4.4 MW, the daily quantity of ice that could be produced is about 865 tons. The greenhouse gas emissions should thus be reduced by about 10,000 tons of CO2 per year.

Surface Waviness Controlled Grinding of Thin Mold Inserts Using Chilled Air as Coolant

The ongoing trend of miniaturization in electronic related parts, which is an average of two times every 5 to 7 years, introduces machining challenges. Surface grinding of thin miniature parts is an ardent task due to its warpage, induced by high specific grinding energy (10–100 J/mm3). Therefore, coolant is used to avoid thermal damage, improve surface integrity, and prolong wheel life. However, coolant, the incompressibility media, introduces high forces at the grinding zone, that create dimensional and shape instability. This article presents the results of a new grinding method when using chilled air as coolant media for thin mold insert, two-stage vapor compression refrigeration cycle was adopted for production of chilled air of temperature, −35°C; pressure, 0.2–0.3 MPa; and flow rate, 0.4 m3/min. Along with chilled air, traces of vegetable oil mist was also impinged to the grinding zone. Both chilled air and vegetable oil mist were applied through two independent paths of a specially designed twin compartment nozzle for maximizing the penetration.

The influence of wet/dry treatment on pore structure-the correlation of pore parameters, water retention and moisture regain values

Three types of regenerated cellulosic fibres, normal lyocell (CLY1), crosslinked lyocell (CLY2 and CLY3), and micromodal (μCMD) were treated with five two-stage cycles comprised of wetting in water at 40 °C for 6 h, and drying at 105 °C for 6 h. The resulting changes in fibre pore characteristics were then estimated with Inverse Size Exclusion Chromatography, water retention values (WRV), and moisture regain values (MRV). Reductions in fibre pore volumes after wet/dry treatments were observed to the extents of 20, 13, 18, and 0% in CLY1, CLY2, CLY3 and μCMD, respectively. Corresponding reductions were also observed in water retention and moisture regain after the treatments. WRVs decreased by 21, 28, 24 and 14%, and MRVs decreased by 48, 43, 34 and 40% in CLY1, CLY2, CLY3 and μCMD, respectively. The changes observed in pore characteristics, WRVs and MRVs are attributed to changes that occur in the microstructures of fibres. It is proposed that structural rearrangements occur in fibres during repetitive wetting and drying that leads to decreased water sorption.

A four-bed mass recovery adsorption refrigeration cycle driven by low temperature waste/renewable heat source

The study deals with an advanced four-bed mass recovery adsorption refrigeration cycle driven by low temperature heat source. The proposed cycle consists of two basic adsorption refrigeration cycle. The heat source rejected by one cycle is used to power the second cycle. Due to the cascading use of heat and cooling source, all major components of the system maintain different pressure levels. The proposed cycle utilize those pressure levels to enhance the refrigeration mass circulation that leads the system to perform better performances. The performance of the proposed cycle evaluated by the mathematical model at equilibrium condition and compared with the performance of the basic two-bed adsorption refrigeration cycle. It is seen that the cooling effect as well as COP of the proposed cycle is superior to those of the basic cycle. The performances of the cycle are also compared with those of the two-stage cycle. Results also show that though the cooling effect of the proposed cycle is lower than that of two-stage cycle for heat source temperature less than 70 °C, the COP of the cycle, however, is superior to that of two-stage cycle for heat source temperature greater than 60 °C.

Catecholamines inEntamoebae: Recent (re)discoveries

Free-living and enteric amoebae have similar two-stage life cycles, and both organisms depend on being able to monitor environmental conditions to determine whether to continue multiplying as trophozoites, or to differentiate into dormant or transmissible cysts. Conditions that support high trophozoite densities might also be expected to select for mechanisms of information exchange between these cells. We recently determined that trophozoites of at least one species ofEntamoeba release and respond to catecholamine compounds during differentiation from the trophozoite stage into the cyst stage. It turns out that this is not an entirely novel finding, as a number of previous studies have demonstrated parts of this story in free-living or enteric amoebae. We briefly review here major points of the previous studies and describe some of our recent results that have extended them.

Thermoeconomic optimization of a two stage combined refrigeration system: a finite-time approach

A finite-time thermoeconomic performance analysis based on a new kind of optimization criterion has been carried out for a two-stage endoreversible combined refrigeration cycle model. The optimal performances and design parameters that maximize the objective function (cooling load per total cost) are investigated. In this context, the optimal temperatures of the working fluids, the optimum performance coefficient, the optimum specific cooling load and the optimal distribution of the heat exchanger areas are determined in terms of technical and economical parameters. The effects of the economical parameter that characterizes the investment and energy consumption costs on the general and the optimal performances have been discussed.

Improving the length-fractionation of DNA during capillary electrophoresis.

The present study develops a path-lengthening strategy for capillary electrophoresis of short double-stranded DNA molecules, in an aqueous solution of neutral polymer (hydroxypropylmethylcellulose). Tests of the dependence of fractionations on pulse times reveal the operation of at least one mechanism in addition to increase in effective path length. Electrophoresis is performed in the following two-stage cycles (cyclic electrophoresis): The first analysis-stage of each cycle is a constant field (forward) capillary electrophoresis. This analysis-stage reveals the length distribution of the shortest DNA molecules not previously analyzed. The second, enhancement-stage of each cycle is zero-integrated field electrophoresis (ZIFE). The enhancement-stage improves the DNA length-fractionation for the next DNA molecules to be analyzed. A slight reverse migration occurs in the enhancement-stage. Increase in both peak separation and peak sharpness contribute to improvement in the length-fractionation of DNA molecules.

Cycles de frigopompes à absorption en cascades matérielles — détermination du nombre d'étages optimal pour le mélange ammonoiac–eau

This study is devoted to the modelling of absorption heat pumps using an ammonia-water mixture, for air conditioning. The mixture is separated in a distillation column and the components are remixed in a number of evaporation-absorption cells, linked in a material cascade. We compare the performance of three structures, with one, two or three stages, and we determine the influence of various parameters: final desorption temperature, cold source temperature, the number of theoretical separation plates, and the thermal pinch of the main exchanger. This last factor exerts only a small influence. the two-stage cycle appears to be the most efficient: it is even more efficient than the three-stage cycle. The higher rectification losses and recirculation rates for the three-stage cycle lower its performance. The three-stage cycle becomes the most efficient for the production of cold at a lower environmental temperature and with a greater number of theoretical separation plates.