title: Axial Variations and Entry Effects in a Pressure Screen

authors: Atkins, Martin John

subjects: Pressure Screening

abstract: Pressure screens are used for contaminant removal and fibre length fractionation in the production of pulp and paper products.  Axial variations and entry effects in the screen are known to occur and these variations have not been adequately quantified.  This thesis describes a fundamental study of the axial variations of several factors that occur within an industrial pressure screen; namely, pulp consistency, fibre length distribution, rotor pressure pulse, and feed annulus tangential velocity.

Axial variations of pulp consistency in the screen annulus and the accept chamber of the screen were studied using an internal radial sampling method.  Localised pulp samples were taken and evaluated and common measures of screen performance such as fibre passage ratio and fractionation efficiency were calculated along the screen.  Consistency generally increased along the length of the screen although under certain conditions the consistency toward the front of the screen was lower than the feed consistency.  A two passage ratio model that incorporated forward and reverse passage ratio was derived to elucidate the flow of both fibre and fluid through the screen and their effects on overall screen performance.  The passage of fibre through the screen decreased with screen length which generally had a positive effect on the fractionation efficiency toward the back of the screen.  The passage of individual fibre length fractions was also studied and it was found that long fibre had a much lower passage than short fibre which caused the average fibre length in the annulus to increase.  

Rotor induced pressure pulse variations along the screen length were also investigated.  The magnitude of the pressure pulse was significantly lower (up to 40 %) at the rear of the screen.  The variation in pressure caused by the rotor is due to a Venturi effect and the shape of the rotor.  The relative velocity of the fluid and the rotor, called the slip factor, also directly affects the size of the pressure pulse in the annulus.  The slip factor decreases along the length of the screen due to the increase in tangential velocity of the fluid.  Pressure pulse data was also used to estimate the instantaneous aperture velocity and back-flush ratio.  The instantaneous aperture velocity was calculated to vary considerably from the superficial aperture velocity by up to 5 m/s in the forward direction and 10 m/s in the reverse direction.   
Computational Fluid Dynamics (CFD) was used to model tangential velocity changes in simplified screen annuli with axial through flow.  For a smooth screen rotor the mean tangential velocity increased over the entire length of the annulus without reaching a maximum value.  A step and bump rotor were modelled and the shape of the pressure pulses showed good agreement with experimentally measured pulses.  The mean tangential velocity and the entrance length were found to be heavily dependant on the screen rotor used.

text: Axial Variations and Entry Effects in a Pressure Screen Pressure screens are used for contaminant removal and fibre length fractionation in the production of pulp and paper products.  Axial variations and entry effects in the screen are known to occur and these variations have not been adequately quantified.  This thesis describes a fundamental study of the axial variations of several factors that occur within an industrial pressure screen; namely, pulp consistency, fibre length distribution, rotor pressure pulse, and feed annulus tangential velocity.

Axial variations of pulp consistency in the screen annulus and the accept chamber of the screen were studied using an internal radial sampling method.  Localised pulp samples were taken and evaluated and common measures of screen performance such as fibre passage ratio and fractionation efficiency were calculated along the screen.  Consistency generally increased along the length of the screen although under certain conditions the consistency toward the front of the screen was lower than the feed consistency.  A two passage ratio model that incorporated forward and reverse passage ratio was derived to elucidate the flow of both fibre and fluid through the screen and their effects on overall screen performance.  The passage of fibre through the screen decreased with screen length which generally had a positive effect on the fractionation efficiency toward the back of the screen.  The passage of individual fibre length fractions was also studied and it was found that long fibre had a much lower passage than short fibre which caused the average fibre length in the annulus to increase.  

Rotor induced pressure pulse variations along the screen length were also investigated.  The magnitude of the pressure pulse was significantly lower (up to 40 %) at the rear of the screen.  The variation in pressure caused by the rotor is due to a Venturi effect and the shape of the rotor.  The relative velocity of the fluid and the rotor, called the slip factor, also directly affects the size of the pressure pulse in the annulus.  The slip factor decreases along the length of the screen due to the increase in tangential velocity of the fluid.  Pressure pulse data was also used to estimate the instantaneous aperture velocity and back-flush ratio.  The instantaneous aperture velocity was calculated to vary considerably from the superficial aperture velocity by up to 5 m/s in the forward direction and 10 m/s in the reverse direction.   
Computational Fluid Dynamics (CFD) was used to model tangential velocity changes in simplified screen annuli with axial through flow.  For a smooth screen rotor the mean tangential velocity increased over the entire length of the annulus without reaching a maximum value.  A step and bump rotor were modelled and the shape of the pressure pulses showed good agreement with experimentally measured pulses.  The mean tangential velocity and the entrance length were found to be heavily dependant on the screen rotor used.

year: 2007

title: Book review: Linguistic imperialism continued.

authors: Barnard, Roger

subjects: book review

abstract: This article reviews the book “Linguistic imperialism continued”, edited by Robert Phillipson.

text: Book review: Linguistic imperialism continued. This article reviews the book “Linguistic imperialism continued”, edited by Robert Phillipson.

year: 2010

title: Māori and English textbooks designed for language learners at intermediate level: A comparison in terms of the occurrence and use of insights from research on discourse analysis

authors: Fester, Anthea

subjects: New Zealand

abstract: Over the past two decades, research in the area of discourse analysis has expanded rapidly and has yielded insights that could usefully inform the teaching and learning of languages. In this paper, we report on the results of a comparative study of a number of commercially available textbooks designed for learners of English and learners of te reo Māori at intermediate level in terms of the extent to which they can be shown to have been influenced by specific aspects of discourse analysis research. In the textbooks designed for learners of English, there is some evidence that the writers are aware of, and have been influenced by some aspects of discourse analysis research, particularly in the teaching of writing skills. In the textbooks designed for learners of te reo Māori, however, there is almost no sign of influence of research of this type. In view of the importance of the teaching and learning of te reo Māori in relation to the maintenance of the language, and in view of the fact that many young people in New Zealand now learn a range of academic subjects through the medium of te reo Māori, this raises issues that we believe need to be addressed.

text: Māori and English textbooks designed for language learners at intermediate level: A comparison in terms of the occurrence and use of insights from research on discourse analysis Over the past two decades, research in the area of discourse analysis has expanded rapidly and has yielded insights that could usefully inform the teaching and learning of languages. In this paper, we report on the results of a comparative study of a number of commercially available textbooks designed for learners of English and learners of te reo Māori at intermediate level in terms of the extent to which they can be shown to have been influenced by specific aspects of discourse analysis research. In the textbooks designed for learners of English, there is some evidence that the writers are aware of, and have been influenced by some aspects of discourse analysis research, particularly in the teaching of writing skills. In the textbooks designed for learners of te reo Māori, however, there is almost no sign of influence of research of this type. In view of the importance of the teaching and learning of te reo Māori in relation to the maintenance of the language, and in view of the fact that many young people in New Zealand now learn a range of academic subjects through the medium of te reo Māori, this raises issues that we believe need to be addressed.

year: 2007-11-21