Modification And Testing Of Biomass Dryer

If the rate of drying is plotted against time, three distinct drying period emerge as shown in figure 2.

Heating up Period (A - B)
Part of the heat input initially goes to heating u the material and progressively increasing the sensible heat of the contained moisture.  This then lead to a greater proportion of evaporating water from the material surface which in turn increases the drying rate.  It is often a negligible proportion of the overall drying cycle.
     Constant Rate Period (B - C)
This is the constant rate – drying period.  During his period, the surface of the slid remains saturated with liquid water, since the rate of removal of moisture from the surface is counter balanced by the rate of capillary transfer of water to the surface of the solid.  The driving force causing vapor movement through the stagnant air film is the water vapor gradient between the drying surface and the main air stream.  The rate of mass transfer can be expressed as given by Hall (2007) as,
"D" _"w" /"D" _"t"   "  =  KgA (ps - pa)                                                               (2.1)"     (Hall, 2007)
Where
dw⁄dt= drying rate kg/hr
    Kg  =  mass transfer coefficient (Kg/hr-m2atm)
    A  =  drying surface area m2
    Ps  =  water vapor pressure at surface temperature (ATM)
    Equation (2.1) can also be expressed in terms of humidity’s as
"dw" /"dt"  "= "  "- kg" /"t"  ("Hs-Ha" )"                                                               Equation " ("2.2" )   
where
Kg1  =  mass transfer coefficient (Kg/hr-m2)
Hs  =  absolute humidity at surface temperature (Kg/Kg)
Ha  =  Humidity of air kg/kg or gm/kg
The rate of heat transfer to the drying surface is given by (Hall, 2007) as,
"d" _"q" /"d" _"t"   "  = hcAC (Ta-Ts)"                         Equation 2.3
where
d_q/d_t    =  Rate of heat transfer (j/hr)
    hc  =  convective heat transfer coefficient (j/hr-m2 oC)
    A  =  area of heat transfer (m2)
Ta  =  Dry build temperature of air (oC)
Ts  =  Temperature of drying surface (wet bulb temperature of air)
     Since the rate of mass transfer can be said to be rate of change of moisture content drying product, the equation (3) could be written as
    "d" _"q" /"d" _"t"   "  "=    -  hcA (Ta     - Ts)                        Equation 2.4
where   
"d" _"m" /"d" _"t"  = drying rate kg/kg dry solid
    A  =  effective drying surface area m2/Kg dry solid
The convective heat transfer coefficient for air blowing parallel to a surface is given by (Charm 2008) as
    Hc  =  0.0128G08
Where
hc  =  Convective heat transfer C
G  =  Mass velocity of air Kg/hr-m2.