Drying constant rate period

The drying rate during the constant drying period is then given by ... 

During the period of drying from the initial moisture content w i to the critical moisture content wc, the rate of drying is constant, and the time of drying /,. is given by ... 

In tests on the drying of sand, Wenzel and White 53 1 found that the use of steam rather than air did not alter the general characteristics of the drying process, and that the drying rate during the constant rate period was determined by the heat transfer rate. In these tests, the heat transferred by radiation from the steam and surrounding surfaces was 7.5-31... 

Drying Rates. Drying a frozen material proceeds initially at a constant rate with rapid evolution of water vapor. As Ihe sublimation interface recedes within the product, water-vapor evolution decreases. This is the start of the falling-rate period. When only bound water remains within the cellular structure of the product, the desorption period begins. During the constant-rate period, the sublimation rate can be expressed in terms of the heat of sublimation of ice and the heat-rale equation ... 

For the sake of simplicity and clarity of the presentation, we confine further considerations to the so-called constant drying rate period. In this period, the temperature of the dried body is constant and equal to the temperature of the wet bulb thermometer if the drying condition are stable, i.e.,... 

Mails and Muir (19) treated an experimental pond with fenitrothion and observed that fenitrothion levels in duckweed changed relatively little from 1 to 10 days after treatment. The averages of radioactivity (as fenitrothion) in the plants over that interval were 17480 and 18229 pg/kg in shaded and sunlit ponds respectively, on a dry weight basis. During the period after treatment water content of radioactivity declined continuously, but "average" values taken as the means of initial and 10-day samples were 43 and 37 pg/L for the same ponds. Calculated bioconcentration factors were therefore 406 and 492 fold for the shaded and sunlit ponds. The rate constant ratio (Table III) indicates a steady state prediction of 24 on a wet... 

The drying of solids. VI. Diffusion equations for the period of constant drying rate. Ind. Eng. Cheni., 25 1134-1136. 

Y. Sano [107] described the influence of the film thickness, 8, on the drying course of water-moist polyimide films. In thick films (6 = 1 mm), the liquid-side diffusion plays an important role from the very beginning. The surface concentration quickly drops off to an equilibrium value and the temperature at the film surface increases to the drying air temperature, without reaching a constant steady-state goods temperature. A period of constant drying rate does not appear. 

In contrast, thin films (5 = 50 pm) display, at the beginning of the drying process, a period of constant drying rate at which the evaporation takes place from the free liquid surface. Only when the film is impoverished on solvent, drying is governed by diffusion through the polymer matrix. 

Giving the Drying Rate for the Constant Rate Period... 

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... 

The temperature and computed drying rate histories for a water based drying case in air at a constant flow rate of 2.36 x 10 cm /s (5000ft /min) is shown in Fig. 10.The heat loss term parameter hA is computed to be 17.9 cal/s/°C for this case. An early constant rate period is evident that extends out to the first 30 min of drying in which a drying rate of approximately 40g/s is achieved. 

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