Dryers enthalpy balances

Enthalpy balances for the dry layers and the wet layer can be formulated along with a pertinent drying rate equation. Formulation by Beckwith and Beard results in three ordinary differential equations that describe the dry fabric temperature, the wet layer temperature and most importantly, the moisture content of the total fabric as a function of timeQJ. By predetermining the fabric speed through the dryer, residence time can be converted to dryer length. 

The heat required to remove 1 kg of water is a measure for the energy costs of the dryer. Under the assumption of steady state, this heat can be determined by mass and enthalpy balances as follows The amount of moisture L to be extracted from the drying good is found with the help of the water balance of the good in the dryer ... 

When the dryer is seen as a heat exchanger, the obvious perspective is to cut down on the enthalpy of the air purged with the evaporated water. Minimum enthalpy is achieved by using the minimum amount of air and cooling as low as possible. A simple heat balance shows that for a given heat input, minimum air means a high inlet temperature. However, this often presents problems with heat-sensitive material and sometimes with materials of constmction, heat source, or other process needs. AH can be countered somewhat by exhaust-air recirculation. 

Solution Answering the questions above involves an overall mass and energy balance. Only the mass and enthalpy of the streams need to be considered to answer the two questions above. Only the streams entering the overall process need to be considered. In this example, wet-basis moisture content (and therefore total mass flow rate including moisture) will be used. Since the same mass of air flows in and out of the dryer, there are no equations to solve for the dry air. 

If the sorption isotherm is temperature-independent the heat of sorption is zero therefore a number of sorption isotherm equations used in agricultural sciences are useless from the point of view of dryer calculations unless drying is isothermal. It is noteworthy that in the model equations derived in this section the heat of sorption is neglected, but it can easily be added by introducing Equation 3.59 for the solid enthalpy in energy balances of the solid phase. 

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