Page 93 - Modelling in Transport Phenomena A Conceptual Approach

4.2. FLOW PAST A FLAT PLATE 73

The Schmidt number is
v
sc = -
DAB
- 15.54 x = 0.56
-
2.79 x 10-5
For water at 25 "C (298 K) : PSat = 0.03165 bar
Assumptions
1. Steady-state conditions prevail.
2. Ideal gas behavior.

Analysis
To determine which correlation to we, we mwt first calculate the Reynolds number:
ReL = -
L VOO
U

Since both laminar and turbulent conditions exist, the use of Eq. (F) in Table 4.2
gives
(Sh) = (0.037Re4,/5 -871) SC~'~

= b.037 (1.54 x 106)4/5 - 8711 (0.56)1/3 = 2000
Therefore, the average mass transfer coeficient is

The number of moles of HzO (A) evaporated in unit time is
h~ = A (kc) [CTt - cA(air)]
= A (IC,) (cyt - 0.6 cyt) = 0.4A (IC,) cyt

Saturation concentration of water, cyt, is
psat
Cgat = -
A
A RT
- 0.03165 = 1.28 x kmol/ m3
-
(8.314 x 10-2)(25 + 273)
Hence, the rate of water loss is
rit~ = ~AMA =0.4A(kC)cytM~
= (0.4)(12 x 6)(4.65 x 10-3)(1.28 x 10-3)(18)(3600) = ll.lkg/h