Design of a Compressor Load Stand Capable of Supplying Two-Phase Refrigerant at Two Intermediate Pressures

The development of compressors with refrigerant injection ports provides a less complex and less costly alternative to implementing multi-stage compressors with economization. The ports can be used to inject economized refrigerant during the compression process, which provides the desired cooling effect and decreases the work required to compress the gas per unit mass. Experiments have shown that injecting liquid or low quality refrigerant is effective for reducing the compressor exit temperature and improving system reliability, while injecting refrigerant vapor improves the cooling or heating capacity of the system. However, very little information is available for cycles operating with injection states between these limits of liquid and vapor injection. Theoretical work suggests that cycle performance with two-phase refrigerant injection can provide greater improvements in COP than vapor injection. In addition, experimental work has shown that increasing the number of stages in an economized cycle with a multi-stage compressor improves the cycle performance, and theoretical work suggests that increasing the number of injection ports would have a similar effect. Therefore, this paper presents the design of a compressor load stand for testing compressors with multiple injection ports. The load stand is based on a traditional hot gas bypass configuration but is capable of supplying refrigerant to injection ports at two different pressures between the compressor suction and discharge pressures. In addition, the state of the injected refrigerant can be controlled such that it is either superheated vapor or a saturated liquid-vapor mixture. To guide the design of the bench and size system components, a model was developed to predict the system performance with a commercially available R-410A compressor that has a single injection port. The model is used to predict the range of injection conditions that can be achieved with the load stand over a range of operating conditions. Finally, preliminary test results for the load stand operating without injection are presented, and the experimentally measured compressor performance is compared to the performance data published by the compressor manufacturer

Modeling of a Hot Gas Bypass Test Block for Centrifugal Compressors

The increasingly competitive building equipment and control industry pushes manufacturers to devote more resources each year to research and development, continually improving the performance and efficiency of their products to develop and maintain a competitive edge. Compressor development is an expensive endeavor because of prototyping and testing costs, but the cost and time required for testing can be minimized by developing a model of the compressor test block to predict its behavior with a given prototype compressor at specified operating conditions. This paper presents a thermodynamic model of a hot gas bypass test block used to evaluate centrifugal compressor performance at a compressor development facility. The test block uses cooling towers to reject the heat of compression to outdoor air, and experience has shown that the range of achievable compressor test conditions can be limited by outdoor air temperature and humidity, which control the heat rejection rate. Therefore, one goal of the model development was to provide a means for evaluating the feasibility of tests at given ambient conditions. By incorporating models of the cooling towers into the test block model, test operators now are able to predict the range of compressor suction and discharge conditions that can be tested under the current outdoor air conditions. A second goal of the model was to assist in selecting the orifice plate used in the orifice flow meter that measures mass flow through the compressor. Operators previously had to make an educated guess as to the best orifice plate size in advance of running the tests, but the model now identifies the orifice diameters that result in pressure drops within the desired range, minimizing the trial and error involved in testing. The model assumes that the system operates at steady-state conditions and uses a compressor map to model expected prototype compressor performance. Therefore, this paper focuses on the condenser and cooling tower models, which are the most important elements for predicting the impact of outdoor conditions on cycle performance. It is shown that the resulting model achieves reasonable agreement with experimental data and provides a useful orifice selection routine

Performance of an R-410A Room Air Conditioner Modified for Use with R-1234ze

This paper presents the results of a senior design project that challenged a team of undergraduate students to reduce the environmental impact of a room air conditioner (RAC) by reducing its energy consumption and/or use of high-GWP refrigerants. Over the course of an academic year, the team was able to investigate, design, model, evaluate, and build a prototype improved RAC. The team began by reviewing literature on approaches that have been proposed to meet or exceed existing energy efficiency and refrigerant selection regulations. Based on these findings and the specified needs of the project sponsor, the team evaluated the appropriateness of different concepts for improving the existing R-410A RAC design and decided to pursue modifications to adapt the unit for R-1234yf. The first step in the redesign process was to develop a thermodynamic model of the existing system. Because very little information was known about the performance of the individual components in the existing RAC, some rough performance estimates were obtained through measurements. The model of the existing system was then modified to provide the same cooling capacity as the original unit but using R-1234yf, and a replacement compressor was selected based on the model results. After the replacement compressor and resized capillary tubes were installed in the RAC, the team was asked to test the prototype unit using R-1234ze instead of R-1234yf. Therefore, the model was modified to predict the cooling capacity of the unit using R-1234ze as the working fluid. The unit was tested using an environmental chamber to simulate the outdoor air conditions and a large room as the indoor environment. Although this setup could not ensure steady-state operation, air temperature measurements indicated that the room temperature did not vary more than 1.5°F over 12 minutes of RAC operation. The cooling capacity calculated based on experimental measurements agreed within 3% of the model predictions. While the team was able to modify the RAC to operate with R-1234ze and was able to predict the unit’s performance with reasonable accuracy, the modifications required a significantly larger compressor and capillary tubes. Therefore, the project clearly illustrated that fitting within the space and weight constraints of window units presents a significant challenge to implementing R-1234ze in RACs

On the motion of a classical charged particle

We show that the Lorentz-Dirac equation is not an unavoidable consequence of energy-momentum conservation for a point charge. What follows solely from conservation laws is a less restrictive equation already obtained by Honig and Szamosi. The latter is not properly an equation of motion because, as it contains an extra scalar variable, it does not determine the future evolution of the charge. We show that a supplementary constitutive relation can be added so that the motion is determined and free from the troubles that are customary in Lorentz-Dirac equation, i. e. preacceleration and runaways

Economic Stress of International Students: What Counselors Should Know

College students encounter high levels of stress due to intensive demands from developmental and academic tasks. In addition to the stress induced by developmental and academic tasks, economic stress adds substantial distress to college students. Economic contraction is known to bring up mental health concerns in society. Financial stress and diminished optimism are affected by distressful economic conditions. This study examined the different perceptions of economic stress between American students and international students of one English for Speakers of Other Language (ESOL) program. International students in this study sustained equally high economic stress but were more sensitive to the economic downturn. The results provide an opportunity to inform counselors how to better work with international students regarding their economic stress

30 Years of Progress toward Increased Biomass Yield of Switchgrass and Big Bluestem

Breeding to improve biomass production of switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii Vitman) for conversion to bioenergy began in 1992. The purpose of this study was (i) to develop a platform for uniform regional testing of cultivars and experimental populations for these species, and (ii) to estimate the gains made by breeding during 1992 to 2012. A total of 25 switchgrass populations and 16 big bluestem populations were planted in uniform regional trials at 13 locations in 2012 and 2014. The reference region was USDA Hardiness Zones 3 through 6 in the humid temperate United States. Significant progress toward increased biomass yield was made in big bluestem and within upland-ecotype populations, lowland-ecotype populations, and hybrid-derived populations of switchgrass. Four mechanisms of increasing biomass yield were documented: (i) increased biomass yield per se, (ii) later flowering to extend the growing season, (iii) combined later flowering from the lowland ecotype with survivorship of the upland ecotype in hybrid-derived populations, and (iv) increased survivorship of late-flowering lowland populations in hardiness zones that represent an expansion of their natural adaption zone. Switchgrass exhibited all four mechanisms in one or more improved populations, whereas improved populations of big bluestem were likely influenced by two of the four mechanisms. The uniform testing program was successful at documenting increases in biomass yield, identifying the mechanisms for increased yield, and determining adaptation characteristics and limitations of improved populations

30 Years of Progress toward Increased Biomass Yield of Switchgrass and Big Bluestem

Breeding to improve biomass production of switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii Vitman) for conversion to bioenergy began in 1992. The purpose of this study was (i) to develop a platform for uniform regional testing of cultivars and experimental populations for these species, and (ii) to estimate the gains made by breeding during 1992 to 2012. A total of 25 switchgrass populations and 16 big bluestem populations were planted in uniform regional trials at 13 locations in 2012 and 2014. The reference region was USDA Hardiness Zones 3 through 6 in the humid temperate United States. Significant progress toward increased biomass yield was made in big bluestem and within upland-ecotype populations, lowland-ecotype populations, and hybrid-derived populations of switchgrass. Four mechanisms of increasing biomass yield were documented: (i) increased biomass yield per se, (ii) later flowering to extend the growing season, (iii) combined later flowering from the lowland ecotype with survivorship of the upland ecotype in hybrid-derived populations, and (iv) increased survivorship of late-flowering lowland populations in hardiness zones that represent an expansion of their natural adaption zone. Switchgrass exhibited all four mechanisms in one or more improved populations, whereas improved populations of big bluestem were likely influenced by two of the four mechanisms. The uniform testing program was successful at documenting increases in biomass yield, identifying the mechanisms for increased yield, and determining adaptation characteristics and limitations of improved populations

How can the First ISLSCP Field Experiment contribute to present-day efforts to evaluate water stress in JULESv5.0?

The First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), Kansas, US, 1987–1989, made important contributions to the understanding of energy and CO2 exchanges between the land surface and the atmosphere, which heavily influenced the development of numerical land-surface modelling. Now, 30 years on, we demonstrate how the wealth of data collected during FIFE and its subsequent in-depth analysis in the literature continue to be a valuable resource for the current generation of land-surface models. To illustrate, we use the FIFE dataset to evaluate the representation of water stress on tallgrass prairie vegetation in the Joint UK Land Environment Simulator (JULES) and highlight areas for future development. We show that, while JULES is able to simulate a decrease in net carbon assimilation and evapotranspiration during a dry spell, the shape of the diurnal cycle is not well captured. Evaluating the model parameters and results against this dataset provides a case study on the assumptions in calibrating “unstressed” vegetation parameters and thresholds for water stress. In particular, the responses to low water availability and high temperatures are calibrated separately. We also illustrate the effect of inherent uncertainties in key observables, such as leaf area index, soil moisture and soil properties. Given these valuable lessons, simulations for this site will be a key addition to a compilation of simulations covering a wide range of vegetation types and climate regimes, which will be used to improve the way that water stress is represented within JULES