Through Air Drying Characteristics of Machine-Formed Semi-Permeable Paper

Hashemi, S. J.; Gomes, Vincent G.; Crotagino, R.H.; Douglas, W. J. M.

doi:10.1080/07373939708917237

Cited by 9 publications

(14 citation statements)

References 7 publications

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“…Extensive earlier work [9][10][11] has established that for the through drying conditions used here the through flow air is nearly saturated when exiting a 30 g/m 2 sheet containing water in the pores, i.e., when the moisture content is above that of the fiber saturation point, 0.87 kg/kg for the pulp used here. 4-8 show consistent behavior, with two notable features.…”

Section: Integration Of Moisture Profile Resultsmentioning

confidence: 99%

“…[1,2] As investigating a considerable variety of alternatives for such combinations could be carried out most effectively using a dryer simulator that modeled both drying techniques in a hybrid dryer, it would be useful in future to have a model for through air drying that treated thickness direction gradients of the relevant variables. [9][10][11] It follows that the relative role of thickness direction diffusion on reducing the moisture gradient would become progressively less important as the drying air temperature is increased to industrial levels. The first step in development of such a model would be validation, for low intensity drying conditions, using the numerous profiles obtained here.…”

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confidence: 99%

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Mechanism of Through Air Drying of Paper: Application in Hybrid Drying

2003

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During through air drying the internal sheet drying history develops as a succession of local moisture content profiles in the sheet thickness dimension. These profiles were determined by interrupting the through drying of 180 g/m 2 6-ply sheets, then quickly delaminating and determining the moisture content of each 30 g/m 2 ply. On the through flow inlet side of the sheet, large moisture differences, about 0.4-0.6 kg/kg dry per 30 g/m 2 ply, develop and move into the interior of the sheet provided that local moisture content is in the range 0.45-1.75 kg/kg. On the through flow exit side, moisture gradients only become significant for moisture content below about 1.25 kg/kg. From the through flow exit side of the sheet much of the water is removed by diffusion to the flow entry side where most evaporation into the through flow occurs. The kinetics of through drying reflect the interaction between these two mechanisms, local evaporation into the through flow and thickness direction diffusion. The moisture diffusivity-moisture content relation controls this balance. In any combination with cylinder drying to give some kind of hybrid dryer, through drying should be at the dry end where air permeability is highest and cylinder drying rate is lowest.

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Section: Integration Of Moisture Profile Resultsmentioning

confidence: 99%

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confidence: 99%

Mechanism of Through Air Drying of Paper: Application in Hybrid Drying

2003

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“…al. [19]. This facility enables operation at constant air through flow rate, G, as the sheet goes from wet to dry, a substantial advantage over the much simpler operation at constant pressure drop across the sheet, which produces results of limited relevance to process fundamentals.…”

Section: Methodsmentioning

confidence: 99%

“…Figure 2 shows a typical evolution in paper moisture content in cylinder drying where typically the sheet passes over 40 to 80 steam heated cylinders depending on the efficiency of dryers, paper basis weight and paper machine speed. Previous studies with semi-permeable papers [19,20] indicated that using through air drying at the moisture content at which the sheet enters paper machine dryer sections not only requires a very high fan power for air flow through the paper, but may degrade product due to local nonuniformity in drying.…”

Section: Drying Of Kraft Papermentioning

confidence: 99%

A Hybrid Drying Process: Cylinder Drying with Through Air After-drying

Hashemi,

Douglas

2005

Trans. Of the XIIIth Fund. Res. Symp. Cambridge, 2005

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BACKGROUND Cylinder drying and air drying of paperHigh paper machine capital costs provide a permanent incentive for increased productivity. Pikulik et al. [1] discussed the evolution of machine speed for printing grades to about 2000m/min. Karlsson [2] considered potential developments for the all too common bottleneck of cylinder drying. Extensive studies at McGill examined the possible integration of impingement air drying with cylinder drying, providing one source concerning multiple technique drying, Bond et al. [3], in hybrid dryer sections, Hashemi and Douglas [4,5]. Hybrid drying for printing paper recently became an industrial reality with a dryer section consisting of alternating cylinder and high intensity impingement air drying at the Nordland Papier mill in Germany, Sundqvist and Anderson [6]. Thus, although impingement air drying was long considered applicable only to tissue and toweling in Yankee dryers, machine builders now propose the integration of these two techniques to produce hybrid dryer sections for heavier grades.The other basic type of air drying, i.e. through air drying, is distinguished by the manner of air -sheet contacting. In through drying the moisture removal is obtained by drawing hot unsaturated air or combustion gas through the permeable web. For the heat and mass transfer of drying, this intimate contact between the drying medium and wet fibres decreases the distance for transport from the thickness of the sheet for cylinder or

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“…The second major study of the basic aspects was by Chen [1994] and Chen and Douglas [1997]. The most recent investigation of the characteristics of through air drying of paper was by Hashemi et al [1994Hashemi et al [ , 1997aHashemi et al [ ,b,c, 1999 with machine produced paper, of grades from tissue to heavier papers such as newsprint, fine paper and sack paper. The most recent investigation of the characteristics of through air drying of paper was by Hashemi et al [1994Hashemi et al [ , 1997aHashemi et al [ ,b,c, 1999 with machine produced paper, of grades from tissue to heavier papers such as newsprint, fine paper and sack paper.…”

Section: Through Air Drying Characteristicsmentioning

confidence: 99%

Paper Drying: A Strategy for Higher Machine Speed. I. Through Air Drying for Hybrid Dryer Sections

Hashemi¹,

Douglas²

2001

Drying Technol

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For all grades of uncoated paper heavier than tissue, the search for new drying technology has recently become directed towards finding combinations of air drying with cylinder drying which have advantageous techno-economic characteristics. Air drying exists in two forms, through air drying and impingement air drying. An advantage intrinsic to through air drying is exceptionally high drying rates because the distance for heat and mass transport processes is reduced from the thickness of the sheet to the dimensions of pores and fibres. Use of through air drying combined with traditional cylinder drying into multiple technique dryer sections is constrained by two basic characteristics, the cost of the pressure drop for air flow through the sheet, and the tendency for through air drying to produce undesirable local moisture nonuniformity during drying. These two characteristics of through drying are documented and strategies for limiting these disadvantages are identified.

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Through Air Drying Characteristics of Machine-Formed Semi-Permeable Paper

Cited by 9 publications

References 7 publications

Mechanism of Through Air Drying of Paper: Application in Hybrid Drying

Mechanism of Through Air Drying of Paper: Application in Hybrid Drying

A Hybrid Drying Process: Cylinder Drying with Through Air After-drying

Paper Drying: A Strategy for Higher Machine Speed. I. Through Air Drying for Hybrid Dryer Sections

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