The effect of refrigerant blends 410A and 507 on the operation of an ejector cooling system -ECS-is theoretically studied with the aid of a validated multi-geometry ejector mathematical model. For a system cooling capacity of 1 kW, a set of possible design conditions were obtained by means of a parametric study varying the generator, condenser and evaporator temperatures from 50ºC to 70ºC; 30ºC to 40ºC and 5ºC to 15ºC. The ejector's lower U and values are obtained when R410A is employed, meaning higher primary fluid mass flow rates and smaller ejectors, respectively. ECS's upper COP s values are achieved at higher T GE and T EV with lower T CO . An ECS operating with these blends has a higher COP s using R410A, around 0.53 for T GE , T CO and T EV of 70ºC, 30ºC and 10ºC, respectively. The system using R507 has a similar but slightly lower performance. Therefore, the systems employing R410A and R507 are good options when a maximum generation temperature of 70ºC is available from a thermal source, either solar or industrial. As well, an ECS working with R410A and R507 experiences higher pressures and has a robust construction. NomenclatureCOP coefficient of performance (dimensionless) d diameter (m) ECS ejector cooling system F f friction factor (dimensionless) h specific enthalpy (kJ/kg) l distance, length (m) m mass flow rate (kg/s) p pressure (MPa) Q heat flow rate (kW) T temperature (ºC) U entrainment ratio, 2 m / 1 m , (dimensionless) W mechanical power (kW) ejector area ratio, (d m /d t ) 2 , (dimensionless) efficiency (dimensionless) Subscripts CO condenser d diffuser e main nozzle exit EV evaporator GE generator m mixing chamber n main nozzle, between main nozzle exit and mixing chamber inlet p r reversible pump s system t main nozzle throat 1…6 thermodynamic cycle states Jorge I. Hernandez et al. / Energy Procedia 57 ( 2014 ) 3021 -3030 3023
Ligado a la crisis económica de Estados Unidos que estalló a finales de 2007, el Valle del Mezquital ha experimentado un alto retorno migratorio. En esta región, la migración internacional había definido parte fundamental de la dinámica económica desde finales del siglo pasado. El autor plantea la interrogante: ¿De qué manera se relacionan con la actividad campesina los migrantes internacionales retornados desde Estados Unidos al Valle del Mezquital? En su respuesta, concluye la emergencia de un proceso de recampesinización forzada por las condiciones de la agricultura y el mercado de trabajo local.
In the socio-anthropological literature on the social change linked to the migration of Mexican indigenous populations, first country-city and then international, a kind of gray area is perceived, in which the linearity of the process seems to be decoupled. In particular, I refer to the contrasting narratives that at one pole have the indigenous atavistic and apathetic to change, presented by the indigenous ethnographies of the last century; and, on the other, the indigenous person who in contemporary anthropologies appears as appealing to novelty, consumption, overcoming poverty, educational improvement and everything involved in the search for well-being. In this paper, I propose that an alternative to investigate this gray area is to pay attention to the temporal substrates involved in said changes, to the ways in which indigenous communities manage to articulate the old and the new unevenly and discontinuously in the construction of their contemporaneities and , above all, to the multiple ways in which the search for ways to earn a living linked to constant mobility, were awakening humble aspirations and dreams among migrant workers and their communities that, committed to these images of the future, foreshadowed and foreshadowed the to come in your present practice.
Bibliografía sobre: Futuros vividos: orientaciones y prácticas de futuro en tiempos inciertos
Presentación del dossier número 51 de la revista Andamios dedicado al tema Futuros vividos: orientaciones y prácticas de futuro en tiempos inciertos
The objective of this work is to evaluate the structural integrity of the feed-effluent heat exchangers of the Hydrocracking of the Luján de Cuyo Refinery, belonging to Repsol-YPF S.A. This equipment operates under a hydrogen load at a pressure of 2133 psi and at a temperature that varies between 300°C (572°F) and 400°C (752°F). An inspection performed during the plant maintenance shutdowns revealed several cracks on the channel of one of the equipment. These cracks had a depth of 2.4 in and a length covering almost all the circumference of the channel. It was concluded after several studies that during the 27 years service period and being exposed to temperatures of above 300°C (572°F), the material suffered an embrittlement process due to a phenomenon known as Temper Embrittlement, and that at room temperature, the material was within the range of brittle fracture. The commencement of the crack can be associated to the presence of hydrogen, to the effects of local stress concentration and to the plant maintenance shutdowns. It was estimated that probably the growth of the cracks was produced by the thermal stresses originated during the plant starting and the plant maintenance shutdowns, and that it is facilitated by the embrittlement of the material and by the presence of hydrogen. A risk evaluation was performed to analyze the feasibility of the continuous operation of the equipment under the described metallurgic conditions. By accepting a criterion of risk decision, it was concluded that the operation under such conditions was "not desirable" and that situation must be accepted by a high rank executive officer. A heat exchanger was shut down from service and used to perform several tests to determine its condition and therefore to provide the necessary technical background to the management decision making. This study was highly significant because the remaining four heat exchangers were in similar metallurgical conditions than the equipment removed from operation and they should continue with their service operation. Several studies were performed such as Charpy V tests, fracture tests and stress analysis calculations to determine the value of the stress intensity factor (J). These studies were then used as technical background and support material for decision making in regards to the continuity of the service operation of the equipment for a certain period of time and under certain operational conditions. Introduction The objective of this study is to evaluate the structural integrity of the feed-effluent heat exchangers of the Hydrocracking of the Luján de Cuyo Refinery belonging to Repsol-YPF S.A. and the associated risk of keeping this equipment in operation. This study was conducted with the purpose of determine if the mentioned equipment were suitable to continue providing service until the next scheduled shutdown of the unit. The Hydrocracking has two exchange ensembles with three heat exchangers each, named LX E4 A/A, A/B, A/C and B/A, B/B, B/C (See figure 1). These exchangers are subjected to a pressure value of 2133 psi, the circulating fluid inside the equipment is gas Oil with hydrogen. The temperature of operation of each exchanger is shown in table 1. The metallurgic of each one of the components is shown in table 2. The LX E4 A/A and B/A (see figure 1) were replaced in 1997 with new equipment, built with a similar engineering design than the original. After the non-destructive inspection tests of the other four pieces, several cracks were detected. In particular, the worst crack was found at the channel of the LX E4 B/C, contained on a plane perpendicular to the axis of the equipment and having a depth of 2.4 in and a length that covered almost the complete head circumference. (See figure 2). It is probable that the crack originated under the cladding on an area where stress concentrations might occur associated to the changes in the plate thickness of the head.
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