Internal hysteresis experienced on a high pressure syn gas compressor

A vibration instability phenomenon experienced in operating high pressure syn gas centrifugal compressors in two ammonia plants is described. The compressors were monitored by orbit and spectrum analysis for changes from baseline readings. It is found that internal hysteresis was the major destabilizing force; however, the problem was further complicated by seal lockup at the suction end of the compressor. A coupling lockup problem and a coupling fit problem, which frettage of the shaft, are also considered as contributors to the self excited vibrations

Accounting and Economic Rates of Return: a Dynamic Econometric Investigation

Many studies have questioned empirical utilization of accounting data as internal rates of return would be more consistent with the relevant economic concept. The paper investigates the dynamic relationships between different measures of accounting rates of return (ARRs) and different approximations for the internal rates of returns (IRRs). In contrast with the prevailing case-study investigations, one considers a panel for quoted Brazilian firms in the manufacturing industry along the 1988-3/2003-2 period. Granger causality tests are considered and even though the results are not completely clear cut, some discernible uni-directional patterns emerge. In particular, there seems to be informational content between economic and accounting rates of return, between ROA (Net Profits/Total Assets) and PM (Gross Profits/ Operational Income), and internal rates of return. This seems to indicate that there is some validity in using accounting rates of return in certain economic studies.

Tacit Collusion under Imperfect Monitoring in the Canadian Manufacturing Industry: An Empirical Study

The paper undertakes a cross-sectoral analysis of a salient empirical implication of the model of tacit collusion advanced by Abreu et al (1986). Specifically, the prevalence of a first order Markovian process for alternating between price wars and collusive periods is assessed by means of non-parametric tests. The analysis focuses on 30 different industries in Canada. The evidence provides weak support for optimal collusion in one industry, which is consistent with the idea that such collusive arrangements are unusual.tacit collusion, game theory, Canada, price war

Measuring Market Conduct in the Brazilian Cement Industry: a Dynamic Econometric Investigation

Indirect assessments of market conduct have become widespread in the New Empirical Industrial Organization-NEIO literature. Recently, Steen and Salvanes (1999) provided a flexible dynamic econometric formulation of the approach advanced by Bresnahan (1982) and Lau (1982). The present paper considers a similar approach as applied to regional cement markets in Brazil under more favorable data availability and it also attempts to address part of the critiques that usually emerge with respect to the NEIO literature. In particular, issues pertaining to structural stability and yet the control for the number of competing firms are addressed. The evidence clearly indicates non-negligible and distinct market power in the different regions and yet distinct conduct patterns in the short and long-run.

Impact of season of harvest on in vitro gas production and dry matter degradability of Acacia saligna leaves with inoculum from three ruminant species

In vitro gas production (IVGP) and dry matter degradability (IVDMD) of Acacia saligna leaves (ASL) from four seasons were studied under arid Egyptian conditions as a 4×3 factorial experiment (4 seasons×3 ruminant species). Incubations were completed using rumen liquid collected immediately after slaughter from sheep, cattle and buffalo, in order to investigate differences among ruminants in their ASL fermentation capacity. Samples of ASL were collected during the last 2 months of each season, being autumn, winter, spring and summer (between the 5th and 12th week of each season). Dried samples of ASL were incubated for 24 h in each of the three buffered rumen liquors, using a syringe technique, to determine IVGP and IVDMD. The crude protein content of ASL was lower (P<0.01) in summer (143 g/kg DM) than autumn (171 g/kg DM), winter (177 g/kg DM) and spring (182 g/kg DM). In winter, ASL had lower neutral detergent fibre, acid detergent fibre, acid detergent lignin and cellulose than in other seasons (P<0.05), but there were no differences among seasons in ash and hemicellulose contents. Condensed tannin (CT, as quebracho tannin equivalent) contents of ASL were higher (P<0.001) in summer (113 g/kg DM) versus the other seasons, with the lowest value during winter (63 g/kg DM). Gas production after 24 h was higher (P<0.05) with buffalo rumen fluid, versus cattle or sheep, in all seasons except winter. IVGP with buffalo rumen fluid was not affected by season but, with cattle and sheep, IVGP was higher (P<0.01) in winter. IVDMD was higher in winter and spring, and lower in summer and autumn, within all species, and higher (P<0.001) values were general with buffalo versus other species. IVGP was positively (P<0.05) correlated with IVDMD, but there was no consistent relationship between IVGP or IVDMD and chemical composition of ASL. Rumen fluid from buffalo, cattle and sheep have different capacities to ferment A. saligna leaves, and differences among species were smallest in winter, when the fibre and CT contents of ASL were lower. © 2005 Elsevier B.V. All rights reserved

Cavitation effects on the pressure distribution of a squeeze film damper bearing

High speed motion pictures have revealed several operating regimes in a squeeze film damper. Pressure measurements corresponding to these distinct regimes were made to examine their effect on the performance of such dampers. Visual observation also revealed the means by which the pressure in the feed groove showed higher amplitudes than the theory predicts. Comparison between vapor and gaseous cavitation are made based on their characteristic pressure wave, and the effect this has on the total force and its phase

Cooperative catalysis by silica-supported organic functional groups

Hybrid inorganic–organic materials comprising organic functional groups tethered from silica surfaces are versatile, heterogeneous catalysts. Recent advances have led to the preparation of silica materials containing multiple, different functional groups that can show cooperative catalysis; that is, these functional groups can act together to provide catalytic activity and selectivity superior to what can be obtained from either monofunctional materials or homogeneous catalysts. This tutorial review discusses cooperative catalysis of silica-based catalytic materials, focusing on the cooperative action of acid–base, acid–thiol, amine–urea, and imidazole–alcohol–carboxylate groups. Particular attention is given to the effect of the spatial arrangement of these organic groups and recent developments in the spatial organization of multiple groups on the silica surface

The neural mechanisms of mindfulness-based pain relief: a functional magnetic resonance imaging-based review and primer.

The advent of neuroimaging methodologies, such as functional magnetic resonance imaging (fMRI), has significantly advanced our understanding of the neurophysiological processes supporting a wide spectrum of mind-body approaches to treat pain. A promising self-regulatory practice, mindfulness meditation, reliably alleviates experimentally induced and clinical pain. Yet, the neural mechanisms supporting mindfulness-based pain relief remain poorly characterized. The present review delineates evidence from a spectrum of fMRI studies showing that the neural mechanisms supporting mindfulness-induced pain attenuation differ across varying levels of meditative experience. After brief mindfulness-based mental training (ie, less than 10 hours of practice), mindfulness-based pain relief is associated with higher order (orbitofrontal cortex and rostral anterior cingulate cortex) regulation of low-level nociceptive neural targets (thalamus and primary somatosensory cortex), suggesting an engagement of unique, reappraisal mechanisms. By contrast, mindfulness-based pain relief after extensive training (greater than 1000 hours of practice) is associated with deactivation of prefrontal and greater activation of somatosensory cortical regions, demonstrating an ability to reduce appraisals of arising sensory events. We also describe recent findings showing that higher levels of dispositional mindfulness, in meditation-naïve individuals, are associated with lower pain and greater deactivation of the posterior cingulate cortex, a neural mechanism implicated in self-referential processes. A brief fMRI primer is presented describing appropriate steps and considerations to conduct studies combining mindfulness, pain, and fMRI. We postulate that the identification of the active analgesic neural substrates involved in mindfulness can be used to inform the development and optimization of behavioral therapies to specifically target pain, an important consideration for the ongoing opioid and chronic pain epidemic