IMPINGING JET DRYER

ABSTRACT

In coating and gravure printing, an impinging jet nozzle with high thermal efficiency for drying of coated film was developed.

Trial production 0f 40 kinds of nozzle enables to develop a high-performance impinging jet nozzle with heat transfer coefficient 1.5 times larger than that of current slit nozzle, through measurement of heat transfer coefficient, visualizations of air flow and heat transfer, and measuremenu of jet velocity and turbulence distribution. The purpose of the trial production was to expand a range of high heat transfer and promote turbulence compared with the current nozzle.

Paying attention to mass transfer within gravure ink coated film, drying characteristic of the film was analyzed by numerical solution of a set of equations governing the drying process in which concentration dependencies 0f the diffusion coefficient and the equilibrium vapor pressure were considered.

Applying these analyses. an industrial scale dryer with excellent drying efficiency has finally been developed.     

HEAT AND MASS TRANSFER IN IMPINGEMENT DRYING

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.

Here, we review the above effects and offer predictive correlations from literature which may be used in the design of high velocity impinging jet systems.     

EFFECT OF NEAR-WALL MODELLING ON PREDICTION OF IMPINGEMENT HEAT TRANSFER

Due to enhanced transport characteristics, impinging jets are widely used in industry to dry large surface area products such as paper and textiles. The present numerical study concerns the modelling of convective heat transfer for impingement drying. Flow and heat transfer under a confined two dimensional turbulent air jet impinging on a flat surface were modelled by solution of two-dimensional Navier-Stokes and energy equations. The turbulence model used was the high-Re number version of the well known two-equation (κ-ε) model and numerical solution was by the upwind finite difference scheme. The specific objective was to evaluate the accuracy of schemes for modelling the near-wall turbulent flow.

The mean flow properties such as centerline velocity decay and the pressure distribution at the impingement surface show no dependence on the near-wall model used. Heat transfer predictions were found to be quite sensitive to the choice of near-wall model. Best agreement between predictions and experiments was obtained for a Chieng-Launder type model with a new modification, use of k_p instead of k_v in the calculation of τ_w.     

MULTI-STAGE SPRAY DRYING METHOD IN MANUFACTURING HEAVY DUTY DETERGENTS

Collision of droplets; counter-current spray dryer; drying rate; heat transfer; nonphosphated detergent; spray drying

The spray drying method of non-phosphated granular detergents is studied to decrease the amount of agglomerate particles. The formation of agglomerates is mainly influenced by the concentration of droplets in spray cloud and the water content of droplets at the time of collision. The overlaps of different spray clouds should be de- creased.

The drying rate near the nozzle zone is considerably faster than that calculated by Ranz-Marshal's equation. According to these phenomena, “Multi-stage spray drying” is developed, which is characterized by in stalling plural spraying stages in a spray dryer.

Consequently, non-phosphated detergents are manufactured with the same powder properties and productivity as phosphated detergents.     

A STUDY OF CLOSED CYCLE DRYING FOR THE COATED FILM WITH ORGANIC SOLVENT ON THE CONTINUOUS SHEET MATERIALS

The conventional drying of the coated film with organic solvent on continuous sheet materials is usually performed in open or one pass drying system using air as the drying medium. The concentration of evaporated organic solvent in the drying system must be low enough to prevent explosion and large volume of off gas from dryer to solvent recovery system is required, resulting in poor heat economy for all the plant

To improve the heat economy of the plant it had been proposed to use a closed cycle drying system, where an inert gas, e.g. nitrogen, is used as the drying medium. High concentration of the organic solvent in the recycle gas mixture may then be used, which results in smaller volume of recycle gas and possibility of applying an inexpensive solvent recovery system of dehumidification. This investigation includes two important problems to realize the closed cycle drying

(1)Measurement of solvent (toluene) evaporation rate from coated film in the gas mixture of toluene andnitrogen over a wide range of toluene/nitrogen ratios (0-1.0) and drying temperature using bench scaleapparatus

(2)Development of a contactless sealing method, using an inert gas, for closed cycle drying of continuoussheet materials. The results of preliminary tests demonstrate its feasibility.     

圆形自由水射流冲击换热及喷嘴布置

引言 水射流冲击冷却由于具有较高的换热能力,广泛应用于机械和化工行业,以实现工件的快速冷却和控制工件的温度变化.在大型轴类工件(工件直径D=1000～3000 mm)喷水冷却装置中,多喷嘴圆孔自由水射流以特定阵列布置冲击至工件表面,相对短暂的沸腾换热结束后,阵列自由水射流即以强制对流方式实现工件冷却.因此,自由水射流冲击换热特性及多喷嘴布置形式对喷水冷却装置的结构设计至关重要.     

DRYING CHARACTERISTICS OF SLOT JET REATTACHMENT NOZZLE AND COMPARISON WITH A SLOT JET NOZZLE

Slot Jet Reattachment (SJR) nozzle is an extension of the Radial Jet Reattachment (RJR) concept used to provide high heat and mass transfer while minimizing flow exerted forces on the reattachment surface. The SJR is a slot jet nozzle with a bottom plate attached to it, which is machined to direct impinging flow at different angles to the surface. The drying characteristics of the SJR nozzle with four exit angles on a paper sample were studied for three Reynolds numbers, three temperatures and two initial moisture contents. Dry air was used as the jet fluid. Correlations to predict drying rates and moisture content for the SJR nozzle as a function of exit angle, temperature, Reynolds number and drying time, for a given initial moisture content, were developed. A comparison of the drying characteristics and net forces of the slot jet and SJR nozzles was also performed under the same flow power and surface peak pressure.     

Effect of Turbulence on Heat and Mass Transfer in the Atomization Zone

One of possibilities to extend operating efficiency of spray dryers is to increase turbulence of the drying agent flow. In the literature no quantitative data describing this phenomenon are available.

In the paper results of experimental investigations on the effect of turbulence on heat and mass transfer during atomization are discussed. The scope of experiments covered the analysis of changing evaporation capacity. temperature of gas and atomized material and particle size distribution as a function of distance to the atomizer. It was proven that an increase of air flow turbulence could cause 20-25% increase of evaporation capacity.     

DRYING CHARACTERISTICS OF SLOT JET REATTACHMENT NOZZLE AND COMPARISON WITH A SLOT JET NOZZLE

ABSTRACT

Slot Jet Reattachment (SJR) nozzle is an extension of the Radial Jet Reattachment (RJR) concept used to provide high heat and mass transfer while minimizing flow exerted forces on the reattachment surface. The SJR is a slot jet nozzle with a bottom plate attached to it, which is machined to direct impinging flow at different angles to the surface. The drying characteristics of the SJR nozzle with four exit angles on a paper sample were studied for three Reynolds numbers, three temperatures and two initial moisture contents. Dry air was used as the jet fluid. Correlations to predict drying rates and moisture content for the SJR nozzle as a function of exit angle, temperature, Reynolds number and drying time, for a given initial moisture content, were developed. A comparison of the drying characteristics and net forces of the slot jet and SJR nozzles was also performed under the same flow power and surface peak pressure.     

HEAT AND MASS TRANSFER IN IMPINGEMENT DRYING

ABSTRACT

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.

Here, we review the above effects and offer predictive correlations from literature which may be used in the design of high velocity impinging jet systems.     

Enhancement of Drying Rate in Paper Machines with Multi-Functional Radial Jet Reattachment Blow Boxes — Theoretical Analysis

Multi-functional radial jet reattachment blow boxes deliver a dry air into the pockets of the dryer section of a paper machine by impinging directly onto the unsuppned sheet in the open diaw. The air flow also maintains or improves the sheet swbility.

The iadial jet reattachment blow box technology pmvides a significant enhancement in drying rates. The theoretical drying results are presented in terms of average drying rates, moisture and temperature pmfiles along the drying section with and without the presence of the multi-functional radial jet reaaachment blow boxes. The thcorclical results indicate that enhancements of drying rates within the range of 8% to 25% are feasible depending on ihe operating conditions and machine configuration.     

Drying of Transformer Board

The internal insulation in shell type power transforms is usually ensured by the stacking of plates of board impregnated with oil. A board is in equilibrium with the atmosphere at a moisture content varying from seven to eight percent by weight, but, when the insulating function is required, the water content must be lower than 0.5 %. The aim of the present work is the understanding and the modelling of transformer boards drying.

The first part of this work presents experimental studies showing that the drying of transform board follows two regimes :

- a fast regime in the fim period

- a slow regime, up to 99 percent of dryness.

The effect of heating and the influence of the board thickness on the drying rate also studied. In the second part of this paper, we presnt a model capable to represent the observed kinetics of transformer board drying. This model is based on the diffusion of water vapour in the gaseous phases combined with the transfer of water vapor from the fibres to the -us phase. The due of the average global transfer coefficient can be deduced from the slope of the curve 1 f(drying time), where x     

DEVELOPMENT AND VALIDATION OF A PROCESS SIMULATOR FOR DRYING SUBBITUMINOUS COAL

Drying subbituminous coal has never been practiced commercially. The commercial dryers built to date have been designed for drying surface moisture in conjunction with upstream coal preparation facilities. This type of drying is mainly controlled by input energy and the basis of the design is an energy balance. In drying inherent moisture from subbituminous coal, the thermal conductivity of the coal and the diffusion of molecular water within coal particles impose limitations on the process conditions. Energy input and solids residence time in the dryer have to be controlled properly for simultaneously balancing the heat and mass transfer within the coal particles. Improper control of either parameter can cause fires and explosions during the key steps of the drying process—drying and cooling

In parallel to the Anaconda coal drying pilot plant program, a cross-flow, fluid-bed coal drying/cooling process simulator was developed for: (1) understanding the drying phenomena on an individual particle basis; (2) analyzing potential risks and safety limits, and (3) designing the Anaconda pilot plant program

The development of the process simulator was based on both first principles and laboratory data and can be divided into two phases:

1 Development of a semi-mechanistic drying model for Powder River Basin subbituminous coal employing an analytical solution of the diffusion equation

2.Formulation of a fluid-bed cross-bed cross-flow dryer/cooler simulator employing simultaneous heat and mass transfer

This model was validated against process variables data taken on a 4 tph pilot plant. An operable range, or process envelope, has been developed through the pilot plant experience and the process simulation study. Based on the model predictions, an uncertainly range was defined in the design recommendations of a pioneer coal drying plant in scale-up.     

SCALE-UP OF CONTACT DRYERS

The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer.

This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions.

The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-hygroscopic in the moisture range investigated.     

HIGH INTENSITY DRYING

A mathematical model simulating the heat and mass transfer process during high intensity drying of paper and board has been developed. The model is successful in predicting the vapor pressure developments, pressure driven bulk flow of liquid and vapor, and increased drying rates during high-intensity drying, closely matching the experimental determination.

The model predicts substantial amounts of water removal in the liquid form during high-intensity drying being pushed out of the web by pressurized vapor zone. Water removal by pressure flow of liquid could account for as much as one-third of the total water removed.

Similar to drying under conventional conditions, the existence of a dry zone, wet zone and an intermediate zone with accompanying advancing heat pipe has also been shown for drying under high intensity conditions.     

Steam Drying - Modelling and Applications

The concept of steam drying originates from the mid of the last century. However, a broad industrial acceptance of the technique has so far not taken place. The paper deals with modelling the steam drying process and applications of steam drying with in certain industrial sectors where the technique has been deemed to hove special opponunities.

In the modelling scction the mass and heat transfer proceases are described along with equilibrium, capillarity and sorption phenomena occurring in porous materials during the steam drying process. In addition existing models in the literslure are presented.

The applications discussed involve drying of fuels with high moisture contcna, cattle feed exemplified by sugar beet pulp. lumber. paper pulp. paper and sludges.

Steam drying is compared to flue gas drying of biofuels prior to combustion in a boiler. With reference to a current insrallation in Sweden. the exergy losses. as manifested by loss of co-generation cupacity. are discussed. The energy saving potentid when using steam drying of sugar beet pulp as compared to other possible plant configurations is demonstrated.

Mechanical vapour recompression applied to steam drying is analysed with reference to reponed dau from industriul plsnts. Finally. environmcntul advantages when using steam drying are presented.     

ESTIMATION OF FUNCTIONS IN DRYING EQUATIONS

In drying problem, particularly for drying foodstuff, modelling is very difficult. Many physical effects have to be taken into account for mass transfer ; then mass transfer coefficient varies

In different models unknown functions must be estimated. It is particularly the case in simple models of drying using average values of water content, where the mass transfer varies versus mean water content in falling rate period. On the other hand in the “diffusion model” we have the same problem concerning the diffusion coefficient which must be also estimated

The method we propose in this paper for these two models : simple and “diffusion model” of drying consists from measurements of temperature and water content of the product to search a numerical approach of the unknown function. This method uses optimization techniques on computer and least squares criterion between model values and experimental data

Results are given for the “diffusion model” applied to shelled corn drying to find the diffusion coefficient and for a simple 11107 del applied to plum drying to find the mass transfer coefficient.     

A DISTRIBUTED PARAMETER APPROACH TO THE DYNAMICS OF ROTARY DRYING PROCESSES

A nonequilibrium distributed parameter model for rotary drying and cooling processes described by a set of partial differitial equations with nonlinear algebraic constraints is developed in this work. These equations arise from the multi-phase heat and mass balances on a typical rotary dryer. A computational algorithm is devekped by employing a polynonial approximation ( orthogonal collocation) with a glotal splinc technique leading to a differential-algebraic equation ( DAE) system. The numerical solution is carried out by using a standard DAE solver.

The two- phase-flow heat transfer coelficient is computed by introducing a correction factor to the commonly accepted correlations. Since interaction between the falling particles are considered in the correction factor,the results are more reliable than those computed by assuming that heat transfer between a single falling particle and the drying air is unaffected by other particles. The heat transfer computations can be further justified via a study on the analogies between heat and mass transfer.

The general model devloped in this work is mathematically more ritorous yet more flexible that the lumped parameter models established by one of the authors (Douglas et al., (1993)). The three major assumptions of an equilibrium operation, perfect mixing and constant drying raic, are removed in the distributed parameter model.

The simulation results are compared with the operational data from an industrial sugar dryer and predictions from earlier models. The model and algorithm successfully predict the steady state behaviour of rotary dryers and collers. The generalized model can be applied to fertilizer drying processes in which the assumption of constant drying rate is no longer valid and the existing dynamic models are not applicable.     

圆形自由表面水射流冲击换热特性

对圆形自由表面水射流冲击换热特性进行了系统的实验研究,总结了喷距、射流出口速度以及相变等因素对换热的影响,并对其形成机理进行了分析,得到了驻点传热系数以及局部传热系数径向分布的关联式.     

INDUSTRIAL THROUGH-AIR DRYING OF NONWOVENS AND PAPER BASIC PRINCIPLES AND APPLICATIONS

The requirements for drying and thermobonding equipment working on the basis of the hot air flow-through system vary considerably due to the large variety of nonwovens qualities, production methods and required production speeds. It is therefore necessary to construct diverse drying systems in order to be able to offer appropriate machines for the desired purposes. Optimum heat and mass transfer from the penetrating air to the permeable material to be dried is ensured by through-air drying

Following Hans Fleissner's invention of the through-air system for drying of textiles and its practical application in the perforated drum dryer for more than 50 years a second engineering breakthrough for the paper and nonwovens industry has been achieved by Gerold Fleissner with the high-tech through-air drum

This paper presents various possibilities of air flow through the material and provide criteria to facilitate selection of the proper drying system