Surface moisture transfer

Countercurrent flow of gas and sohds gives greater heat-transfer efficiency with a given inlet-gas temperature. But cocurrent flow can be used more frequently to diy heat-sensitive materials at higher inlet-gas temperatures because of the rapid coohng of the gas during initial evaporation of surface moisture. 

Vibrating-conveyor dryers are suitable for free-flowing solids containing mainly surface moisture. Retention is limited by conveying speeds which range from 0.02 to 0.12 m/s. Bed depth rarely exceeds 7 cm, although units are fabricated to carry 30- to 46-cm-deep beds these also employ plate and pipe coils suspended in the bed to provide additional heat-transfer area. Vibrating dryers are not suit le for fibrous materials which mat or for sticky sohds which may ball or adhere to the deck. 

De Rosnay et al. (2000) assessed the reliability of schemes that parameterize land surface processes to find the correspondence between calculated mean annual fluxes of energy and moisture depending on detailed consideration of the vertical structure of soil. These schemes are used in general circulation models of the atmosphere (GCMAs). The calculations testify to the strong dependence of fluxes on vertical resolution. The 11-layer scheme parameterizing heat and moisture transfer in the top 1 mm thick layer of soil was found to be adequate. 

The Lu number is the ratio between the mass diffusion coefficient and the heat diffusion coefficient. It can be interpreted as the ratio between the propagation velocity of the iso-concentration surface and the isothermal surface. In other words, it characterizes the inertia of the temperature field inertia, with respect to the moisture content field (the heat and moisture transfers inertia number). The LUp diffusive filtration number is the ratio between the diffusive filtration field potential (internal pressure field potential) and the temperature field propagation. 

The first, termed the Constant Rate Period, is the initial drying phase in which surface moisture exceeds a critical level and rate is controlled by surface area. When the level of moisture falls below the critical level, it begins to be controlled by mass transfer from inside the solid mass this is called the First Falling Rate Period. As drying proceeds, mass transfer is not able to supply moisture to the surface of the solid mass at a rate equal to the drying rate, and the free water content at the surface goes to zero. At this time, the surface temperature rises rapidly, and a receding evaporation... 

Temperature influences the rate of drying in a number of ways. The principal reason for kiln drying at high temperatures is to increase the rate of moisture transfer to the wood surface. Raising the temperature dramatically enhances the rate of diffusion of water molecules across cell walls. The rate of diffusion increases with temperature at approximately the same rate, as does the saturated vapour pressure (Table 8.1 Figure 8.9b). 

With a slow-drying impermeable species a fan speed as low as 1.5-2.0 m s may be sufficient as the moisture eontent at the surface drops quickly below the fibre saturation point - no mass flow - and there is no point in installing overly powerful fans just to strip off surface moisture for the first few minutes of a long kiln schedule (>14 days) thereafter the slow rate of transfer of moisture from the centres of the boards to their surfaces becomes more important than the rate of evaporation. 

In general, the process line consists of a preparation section where coal is crushed and screened to the desired particle size of --3 mm for briquetting. The fine coal is then transferred to a drying facility where the water content is reduced so that no surface moisture remains. Depending on the coal source, crushing... 

Simpson, W. T. Liu, J. Y. An Optimization Technique to Determine Red Oak Surface and Internal Moisture Transfer Coefficients during Drying, Wood Fiber Sci., 1997, 29(4), p. 312-318. 

The CsCl (>99.9% purity) is dried at 200° under high vacuum and then melted in a silica container. The latter step reduces the possibility of recontamination by surface moisture and makes weighing and complete sample transfer easier in the dry box. 

Transfer of energy (mostly as heat) from the surrounding environment to evaporate the surface moisture... 

Surface mass transfer coefficient is high enough so that the material moisture content at the surface is in equilibrium with the air drying conditions. 

For design purposes the overall heat transfer coefficients of this type of dryer, which depend largely on the film coefficient between the inner jacket wall and the solids, which in turn depend largely on the solid characteristics, can be assumed to be around 50 W/(m K) (Moyers and Baldwin, 1997). However, higher values are expected for products that have surface moisture (van t Land, 1991). 

Miller et al. [30], Friedman and Marshall [9], and Seaman and Mitchell [47] have done considerable amount of research for the evaluation of U a. The volumetric coefficient U a is the product of the heat transfer coefficient based on the effective area of contact between the gas and the solids, and the ratio a of this area to the volume of the dryer. When a considerable amount of surface moisture is removed from the solids and their temperature is unknown, a good approximation of (At)m is the logarithmic mean between the wet-bulb depressions of the drying air at the inlet and outlet of the dryer [35]. 

Gas velocity has a dominant effect on removing surface moisture. Increasing the gas velocity increases the drying rate. However, gas velocity has no effect at all for particles with high internal resistance to moisture transfer. High internal moisture resistance dominates at the end of the falling rate. 

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