CHAPTER
7 Writing these symbolically, analogous to q = UA DT, we have
where w is the mass being transferred kg s^{1}, A is the area through which the transfer is taking place, k'_{g} is the masstransfer coefficient in this case in units kg m^{2} s^{1} , and Y is the humidity difference in kg kg^{1}. 
Unfortunately the application of masstransfer equation is
not as straightforward as heat transfer, one reason being because the
movement pattern of moisture changes as drying proceeds. Initially,
the mass (moisture) is transferred from the surface of the material
and later, to an increasing extent, from deeper within the food to
the surface and thence to the air. So the first stage is to determine
the relationships between the moist surface and the ambient air and
then to consider the diffusion through the food. In studying the surface/air
relationships, it is necessary to consider mass and heat transfer simultaneously.
Air for drying is usually heated and it is also a major heattransfer
medium. Therefore it is necessary to look carefully into the relationships
between air and the moisture it contains.
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