摘要:

In industrial drying applications, efficient transfer of heat and mass between a drying medium and the material being dried is very critical for the overall economics of the operation. Impinging jets are therefore widely used for their enhanced tmnsport characteristics, especially for drying of continuous sheets of materials such as paper and textiles. In such applications, a thin sheet of material, as wide as 6m in cross machine direction, speeds at velocities as high as 90 km/hr under high velocity jets emerging from a confining surface parallel to the material surface. Many variables and effects need to k considered for proper design of such impinging jet systems: nozzle geometry and size, nozzle configuration, location of exhaust pons, nozzle-to-surface separation, jet-to-jet separation, cross flow, jet exit velocity and surface motion. For permeable materials, additional enhancement of heat and mass transfer that occur when some of the impinging gas is removed through the material makes this option an atmctive one.

ISSN:0737-3937    

DOI:10.1080/07373939308916894

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The literature on simulation models for multi-cylinder paper dryers is surveyed, 20 such models being reviewed. Models for intraweb heat and mass transfer are treated extensively, a separate chapter being devoted to them. In some of the models the specific physical transfer phenomena are modelled in detail, whereas in other models a more macroscopic approach is adopted. Normally the latter approach requires the incorporation of a large number of empirical constants into the model to describe the complicated heat and mass transfer processes involved and to obtain a reasonable agreement between the model and the measured data. In order to develop the simulation models further without use of an inordinately large number of empirical constants, the different physical phenomena which occur in the dryer must be modelled in greater detail, bnsed both on fundnmental theory and onempirical correlations.

ISSN:0737-3937    

DOI:10.1080/07373939308916895

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The diffusion of water vapour through pulp and paper was investigated experimentally in a diffusion cell. The diffusion cell consisted of two chambers differing in relative humidity. Any given sample was placed as a banier between the two chambers. Two pulp qualities and eleven paper grades were included in the investigation. The diffusivities measured ranged from 2.1.10-8m2/s to 5.4.10-6m2/. Effective vapour diffusivity was found to correlate with the density of the sheet. For five of the paper or pulp grades measurements were performed a t different temperatures and relative humidities. In the range of relative humidities investigated, effective vapour diffusivity was found to be independent of relative humidity.

ISSN:0737-3937    

DOI:10.1080/07373939308916896

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Experimentally determined mass transfer coefficients for wood surfaces are often much lower than theoretically and experimentally determined coefficients for other surfaces. An investigation of this is made by comparing mass transfer coefficients for water and saturated non-wood surfaces, with coefficients measured for wood surfaces. It is found that measurements on wood surfaces are usually evaluated by assuming that Fick's law describes diffusion in wood. As this is not always true, it is proposed that the mass transfer coefficients measured on wood surfaces are low because the internal mass transfer in wood is not well understood. New measurements are presented which support this conclusiou.

ISSN:0737-3937    

DOI:10.1080/07373939308916897

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The objective of much experimentation in drying is to build or discover a suitable model for a drying system. In this study, an attempt is made to formulate an approach to the problem of iteratively improving models, guided by the inefficiences of an initial tested model. This model involved both moisture diffusion coefficient and external mass transfer coefficient A statistical analysis was applied to the residual populations created by fitting a model to the experimental data, so as to exhibit possible model inadequacies. Inspection of the tendencies produced. suggested what modifications were necessary for improvement. The cycle was repeated until an adequate model was found.

ISSN:0737-3937    

DOI:10.1080/07373939308916898

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The design problem of a multiproduct dehydration plant involving continuous conveyor-belt dryers utilised for the dehvdration of agricultural products over a fmite tim horizon, is presented. The peculiarities of the problem under demand-varying market patterns and time-variable market prices of raw materials are underlined. Limitations imposed by storage capacity, custormer orders, and constraints dictated by the process itself, are also taken into consideration. The overall problem is cast into a MINLP formulation involving the total annual cost of the plant as its objective function. This was infered by deducing the mathematical model of conveyor-belt dryers. Repeated evaluation of the total annual cost for various structures and operating points, allowed the determination of overall performance characteristic operational and structural curves. approximated by suitable mathematical expressions. A test case is presented in order to demonstrate the effectiveness of the proposed approach.

ISSN:0737-3937    

DOI:10.1080/07373939308916899

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Experiments have been carried out to determine a relation between mechanical properties, microstructure and drying conditions of faba bean seeds and their damages during single-stage drying. Results showed that physical damages of beans strongly depend on drying parameters, i.e.drying time and temperature, air flow rate and variety of seeds. It was revealed that some seed parts: the hilum and embryonal axis play a special role in the cracking mechanism of seeds.

ISSN:0737-3937    

DOI:10.1080/07373939308916900

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The design equations for batch drying of shrinking hygroscopic materials in a fixed-bed were solved numerically for the specialcase of time-varying inlet air flow rate. The model presentedconsiderable flexibility to manage the time-varying drying conditions. Almost the same drying residence times as those obtained with constant air flow rates can be achieved using a square-wave type variation of the air flow rates, depending mainly on the degree of flow rate reduction. Two important features are demonstrated theoretically as well as via numerical experiments: when the drying air flow rate is reduced, the cumulative air consumption for drying is also reduced, regardless of the controlling mass transfer process and the degree of air flow rate reduction. In addition, a minimum in the total consumption of air exists at least under external or combined internal and external control of the mass transfer process.

ISSN:0737-3937    

DOI:10.1080/07373939308916901

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The solution of classical diffusion equation based on the assumption of average moisture diffusion coefficient did not adequately represent natural convection drying of rough rice in thin vertical columns exposed on both sides to hot air. Instantaneous moisture diffusivity coefficients determined from experimental drying curves decreased continuously with an increase in exposure duration and were linearly related to moisture ratio. The proponionality constant which was called apparent moisture diffusion coefficient was distinctly related to air temperature, relative humidity, and initial moisture content of rough rice. The modified moisture diffusion model using the instantaneous moisture diffusion coefficient was found to best represent the moisture removal from bulk rough rice.

ISSN:0737-3937    

DOI:10.1080/07373939308916902

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Kinetics of the convection drying process of flat tile has been investigated experimentally in an industrial tunnel dryer. Several velocities of wet tile movement through the dryer were tested to obtain maximum allowable drying rate curve. As there are various models to describe the kinetics of convection drying, finding a model that would fairly well approximate the kinetics of the whole drying process was part of the research. Especially the polynomial and exponential models were tested.

ISSN:0737-3937    

DOI:10.1080/07373939308916903

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor