EFFECT OF BLOW BOXES ON EVAPORATION DURING CYLINDER CONTACT IN A PAPER MACHINE DRYER SECTION

In this paper an inexpensive configuration for a cylinder blow box (CBB) and its effect on the evaporation rate are described. The objective of this work was to determine the effect of the blow boxes on drying intensity on a paper machine. The pilot machine trials were simulated with a computer program. Based on these results two cases were simulated for a commercial paper machine with and without blow boxes on single cylinders. The results revealed that when using the blow boxes on single cylinders, the drying rate can be increased by up to 15%. The potential for using this kind of configuration is biggest at the beginning of a dryer section. A simple construction of blow boxes makes this technique useful especially in boosting the drying performance of older paper and board machines. The exhaust air from the boxes can be utilized in ventilation of dryer pockets.     

盘式连续干燥器的传质传热模型

建立了盘式连续干燥器内加热盘面物料的传质模型,给出了干燥器内料环高度、盘面积料量及干燥时间的计算方法.应用颗粒传热理论和扩散理论,导出了每道料环的传热计算公式,由该迭代公式可逐步计算出每道料环的出料温度、出料湿含量及干燥速率.扩散理论中的惟一经验参数——搅拌参数N_mix,在本模型中可由不同进料工况下的多组实验数据加以确定.最后给出了应用该模型进行计算的实例,结果表明,理论计算值与实测值吻合较好,误差一般在20%以内.     

THE DEPENDENCE OF PAPER MACHINE DRYING DRYING CHARACTERISTIC ON THE MACHINE GEOMETRY

ABSTRACT

The effect of the dryer cylinder diameter on the dryer specific evaporation rate is examined theoretically. It is observed that, in the range of cylinder diameters applicable in practice, the dryer specific evaporation rate is nearly independent of the selected cylinder diameter when the dryer geometry remains unchanged. In practice, however, the dryer geometries have changed slightly according to the cylinder diameter.

TO clarify the effects of the various component factors pertaining to the dryer geometry, this paper presents a drying characteristic, i.e.. a curve showing the dependence of the drying rate on the condensing steam temperature. The drying characteristic is given in the form of a nomogram to facilitate the evaluation of the effect of the ratio between the free draw length and the cylinder diameter, and the wrapping angle, to the dryer specific evaporation rate. Further, by means of an example, the specific evaporation rates obtainable for machine geometries currently used by various machine manufacturers are compared.     

SIZE SELECTION OF VACUUM CONTACT DRYER WITH MECHANICALLY MIXED PARTICULATE MATERIAL

The paper presents methods of modelling and simulation of vacuum contact drying. The selection of dryer size is discussed on the basis of experimental results and theory. Use of the thermal conductivity of the bed as a parameter of the model is proposed.     

DETERMINATION OF THE EVAPORATION OF WATER DURING WOOD DRYING BY MEANS OF HEAT FLUX MEASUREMENT

ABSTRACT

Since the only measured value that is derived from the wood for controlling the kiln drying process is the mean moisture content, it is essential to develop new techniques for the measurement of additional process parameters. When the drying rate, which could be such an additional parameter, is seen in conjunction with other process variables, conclusions on the instantaneous drying behaviour of the wood are possible. A simple and practical way for determining the drying rate is based on the relationship between the drying rate and the heat-flux for evaporation. A measurement of the heat-flux by means of a heat-flux sensor allows the calculation of the drying rate.     

PAPER DRYING: A STRATEGY FOR HIGHER MACHINE SPEED. II. IMPINGEMENT AIR DRYING FOR HYBRID DRYER SECTIONS

For printing and heavier grades, combining cylinder and impingement air drying into a multiple technique dryer section can enable higher productivity through higher machine speed. The large differences in local moisture content and temperature across the sheet which develop quickly under high intensity impingement drying provide the potential for reducing drying time by sheet reversal between impingement drying cylinders. This advantage was determined experimentally under low intensity impingement drying conditions. Use of the micro-scale based McGill dryer simulator for determining the advantage from sheet reversal between impingement drying cylinders was demonstrated for both laboratory and industrial impingement drying intensities. For completing the drying of 205 g/m² linerboard from 0.3 to 0.5 kg/kg dry under 400°C air jets of Re 20000, drying time is about 30% less with sheet reversal between two impingement drying cylinders than for a single, larger diameter cylinder. This extensively validated dryer simulator enables determining advantageous design specifications and operating conditions for hybrid dryer sections involving combinations of cylinder and impingement air drying, a concept with potential to become common industrial practice.     

REDUCTION OF SECONDARY HEAT LOSSES IN YANKEE DRYER

In this paper the possibilities of secondary heat recovery in the ventilating system of a paper dryer were die-ed. To reduce of secondary heat losses the replacement of air as drying medium by superheated steam was proposed. This proposal let us to replace heat recovery ventilating system with heat recycling system.For the Yankee dryer the energy analyses of these ventilating system were done. Obtained results are showing that this approach provides a reduction of amount of the drying medium as well as secondary heat losses.     

PAPER DRYING: A STRATEGY FOR HIGHER MACHINE SPEED. I. THROUGH AIR DRYING FOR HYBRID DRYER SECTIONS

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.     

PAPER DRYING: A STRATEGY FOR HIGHER MACHINE SPEED. I. THROUGH AIR DRYING FOR HYBRID DRYER SECTIONS

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.     

EXPERIMENTAL RESEARCH ON MASS TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Abstract

The influences of bed thickness, particle diameter, rotating speed of the basket and operating gas velocity on the mass transfer between gas and particles in the centrifugal fluidized bed dryer (CFBD) were examined. The experimental correlation used to calculate the mass transfer coefficient in the centrifugal fluidized bed dryer was obtained.     

THERMAL CONTACT CONDUCTANCE OF A COATED PAPER/METAL INTERFACE

A better understanding of the parameters associated with heating of the coated paper during drying process would permit a more accurate design and control of the process to achieve an improved coated paper quality and printability. Following the application of the coating, the coated paper passes through non-contact dryers (i.e. infrared dryers and impinging hot air dryers). When the coating has coalesced, further drying is achieved with steam-heated, contact dryers. One parameter affecting the heat transfer from a metallic dryer drum to the coated paper is the thermal contact conductance at the interface between the coated paper and the drum. In this paper, the thermal contact conductance of a coated paper/metal interface is determined and compared to that of the uncoated paper. Two types of base stock and one type of coating are considered. The thermal contact conductance values are given as a function of the applied interface pressure.     

THERMAL CONTACT CONDUCTANCE OF A COATED PAPER/METAL INTERFACE

A better understanding of the parameters associated with heating of the coated paper during drying process would permit a more accurate design and control of the process to achieve an improved coated paper quality and printability. Following the application of the coating, the coated paper passes through non-contact dryers (i.e. infrared dryers and impinging hot air dryers). When the coating has coalesced, further drying is achieved with steam-heated, contact dryers. One parameter affecting the heat transfer from a metallic dryer drum to the coated paper is the thermal contact conductance at the interface between the coated paper and the drum. In this paper, the thermal contact conductance of a coated paper/metal interface is determined and compared to that of the uncoated paper. Two types of base stock and one type of coating are considered. The thermal contact conductance values are given as a function of the applied interface pressure.     

MODELING AND OPTIMIZATION OF A DRYER

This study deals with the modeling and the search for the optimum production capacity of a tunnel-dryer of the Californian type. The model is obtained from the heat and mass transfer balances at two levels, the product (fruits) level and inside the tunnel itself. The optimum condition is given by the maximum production capacity of a plum dryer satisfying the final product quality. This choice is determined by the fact that a large amount of fruit must be dried in a short time to have a minimum of loss. Recirculation of a proportion of the exhaust air improves the dryer efficiently in terms of energy.     

初始饱和度对微波冷冻干燥过程传热传质的影响

以非饱和含湿牛肉为例,进行了微波冷冻干燥实验研究。获得了干燥时间与物料初始饱和度近似成正比的结论。与升华面模型的计算结果比较表明,升华冷凝模型更符合实验规律,证实了非饱和含湿多孔介质微波冷冻干燥时升华冷凝区的存在。     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

ABSTRACT

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

MEASUREMENTS OF THERMAL CONTACT CONDUCTANCE FOR A PAPER/METAL INTERFACE AND EFFECTIVE CONDUCTIVITY OF MACHINE SAMPLES

ABSTRACT

The thermal contact conductance for a paper/metal interface and the effective thermal conductivity of paper samples were determined using an experimental contact conductance apparatus. The effects of pressure and moisture content on the thermal contact conductance and the effective thermal conductivity were investigated. The samples considered in the experiments consisted of paper machine samples from different sections of the drying section. The results are compared to those of the handsheets prepared in our laboratory.     

ON THE STUDY OF TIME-VARYING TEMPERATURE DRYING—EFFECT ON DRYING KINETICS AND PRODUCT QUALITY

The ability of heat pump dryer to produce controlled transient drying conditions, in terms of temperature, humidity and air velocity, has given it an edge over other drying systems. Exploiting this characteristic, we studied and compared the effect of different temperature-time profiles on the quality of agricultural products in a tunnel heat pump dryer capable of providing up to 14.6 kW of cooling capacity. The product quality refers to the color change of the products. Samples of banana and guava were dried in batches in a two-stage heat pump dryer. The effects of the starting temperature of a selected profile and the cycle time on both drying kinetics and product quality were studied. It was observed that by employing a step change in drying air temperature with the appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color.     

ANALOGY BETWEEN HEAT AND MASS TRANSFER IN THREE SPOUTED BED ZONES DURING THE DRYING OF LIQUID MATERIALS

This work, presents an experimental study of simultaneous heat and mass transfer in spouted bed zones during the drying of liquid materials. Distilled water was used as the liquid material and glass beads as inert material. The effect of the inlet gas temperature (100 to 140 °C), the feed air flow rate relative to air flow at minimum spouting (1.1 to 1.3), and liquid feed flow rate to the bed were investigated. The similarity between heat and mass transfer was analyzed using the Lewis and Chitton-Colburn analogies. Results showed that the ratio between the heat and mass transfer Chilton-Colbum factors lay between 1,31 and 1.74.     

ANALOGY BETWEEN HEAT AND MASS TRANSFER IN THREE SPOUTED BED ZONES DURING THE DRYING OF LIQUID MATERIALS

ABSTRACT

This work, presents an experimental study of simultaneous heat and mass transfer in spouted bed zones during the drying of liquid materials. Distilled water was used as the liquid material and glass beads as inert material. The effect of the inlet gas temperature (100 to 140 °C), the feed air flow rate relative to air flow at minimum spouting (1.1 to 1.3), and liquid feed flow rate to the bed were investigated. The similarity between heat and mass transfer was analyzed using the Lewis and Chitton-Colburn analogies. Results showed that the ratio between the heat and mass transfer Chilton-Colbum factors lay between 1,31 and 1.74.