Page 182 - Modelling in Transport Phenomena A Conceptual Approach

162 CHAPTER 6. STEADY-STATE MACROSCOPIC BALANCES

Analysis

System: Wet cloth and the coke can
The inventory mte equation for energy becomes
Rate of energy in = Rate of energy out (1)

Let the steady-state temperature of the cloth and that of coke be T,. The mte of
energy entering the system .is given by

Rate of energy in = AH(~)(T, - T,) (2)
in which AH and T, represent the heat transfer area and air temperature, respec-
tively. On the other hand, the rate of energy leaving the system is expressed in the
form
Rate of energy out = ?i~ FA + (cp)~(Tm - T,)] (3)
where ?i~ represents the rate of moles of water, i.e., species A, evaporated and is
given by
=
fi~ AM (kc) (CA, - CA, (4)
in which AM represents the mas transfer area. Substitution of Eqs. (i?), (3) and
(4) into Eq. (1) and using
AH = AM

CA, 0
L >> (&)A(T, - T,)
gives

The ratio (kc)/(h) can be estimated by the use of the Chilton-Colbum analogy, i.e.,
3, =3M, 03

The use of Eq.

where the properties p, Cp, Pr and Sc belong to air, species B. The concentration
of species A at the interface, CA,, is given by
Dsat