Mc f6a

where p (kg/m3) is the bulk density of food and x (m) the thickness of the bed of food.

The drying time in the constant-rate period is found using phdMi - Mc)

where t (s) is the drying time, Mt (kg/kg of dry solids) the initial moisture content, and Mc (kg/kg of dry solids) the critical moisture content.

For water evaporating from a spherical droplet in a spray dryer, the drying time is found using t =


where px (kg/m3) is the density of the liquid, r (m) the radius of the droplet, Mf (kg/kg of dry solids) the final moisture content.

In the falling-rate period, moisture gradients change throughout the food and the temperature slowly increases from the wet-bulb temperature to the dry-bulb temperature as the food dries. The following equation is used to calculate the drying time from the start of the falling-rate period to the equilibrium moisture content using a number of assumptions concerning, eg, the nature of moisture movement and the absence of shrinkage of the food:

where Me (kg/kg of dry solids) is the equilibrium moisture content, M (kg/kg of dry solids) the moisture content at time t from the start of the falling-rate period, Ps (mmHg) the saturated water vapor pressure at the wet-bulb temperature, and Pa (mmHg) the partial water vapor pressure.

The velocity of the air needed to achieve fluidization of spherical particles is calculated using vf

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