Falling rate drying period

The following diffusion equation can be used to simulate a single droplet drying (falling rate period) in a hot moving airflow ... 

Falling-rate period is a drying period during which the instantaneous diying rate continually decreases. 

Estimates of both the constant-rate and the falling-rate periods are needed to estimate the total drying time for a given diying operation. If estimates for these periods are available, the total diying time is estimated by summing as... 

The total drying time is the sum of the durations of the eonstant rate and falling rate periods and is often simplified into two linear portions (Figure 4.27). The period taken for X to reaeh Xc is known as the eonstant rate time, Oqr, whilst that to go from Xc to X is the falling rate time, 0pR. The sum of these two quantities gives the total drying time 6i. 

Falling needle viscometer, 27 738 Falling rate drying, 9 109-110, 113-115 Falling rate period, 9 97 Falling rod viscometer, 27 738 False Irish moss, common and scientific names, 3 188t... 

At the conclusion of the first falling rate period it may be assumed that the surface is dry and that the plane of separation has moved into the solid. In this case, evaporation takes place from within the solid and the vapour reaches the surface by molecular diffusion through the material. The forces controlling the vapour diffusion determine the final rate of drying, and these are largely independent of the conditions outside the material. 

The rate of drying Rc over the constant rate period is equal to the initial rate of drying in the falling rate period, so that Rc = mfc. 

In the falling-rate period, the surface is no longer completely wetted and the rate of drying steadily falls. In the previous analysis, it has been assumed that the rate of drying per unit effective wetted area is a linear function of the water content, so that the rate of drying is given by ... 

For the falling rate period the batch drying time is given by... 

When drying is entirely in the falling rate period with rate equation... 

Drying Time over Constant and Falling Rate Periods with Constant Gas Conditions 237... 

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 ... 

Figures 2 show a comparison of convective drying with or without microwaves [23], Whilst for convective drying there are definite eonstant rate and falling rate periods, when microwaves are added the form of the eurves ehanges [24-25],...

J3 Diffusion and Heat Conduction in the Porous Network, Giving the Drying Rate for the Falling Rate Period... 

Whitaker [20,21] has developed an analysis of heat and mass transfer for the drying of rigid materials that gives a description of the falling rate period. This model presented here has been verified by Wei et al. 

As drying proceeds, the point is reached where the evaporating surface is unsaturated. The point of evaporation moves into the solid, and the dry process enters the second falling-rate period. The drying rate is now governed by the rate of internal moisture movement the influence of external variables diminishes. This period usually predominates in determining the overall drying time to lower moisture content. 

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