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Notes 1775
Note: Data adjusted to 25%
Fig. 8. Temperature-corrected data for clod cards run at different velocities in tapwater (O?%O), brackish water (20%0),
and seawater (40%0). The data points are superimposed on a response surface based on Eq. 4 with k proportional to
W.s and with AC calculated from Eq. 7 for Puerto Peiiasco seawater. Note that experimental data points fall on the
response surface except at lowest velocity where free convection affects dissolution rates of the cards.
convenient for use with marine systems since
all the information required can usually be obtained
from Figs. 4 and 5. Our forced convection
data can also be described by the expression
NSh = 0 . 059NgtN SC N - (9)
Ns,, is the Sherwood number (kLID,), NRe the
Reynolds number (L VP/~), and Nsc the Schmidt
number (p/pDJ In the dimensionless groups,
k is the mass transfer coefficient evaluated with
Eq. 3 (with AC evaluated at solution temperature
and salinity), D, the volumetric molecular
difhtsion coefficient, p the viscosity, p the
liquid density, and L a geometry term determined
by the shape and orientation of the object.
The geometry term introduced by Pasternak
and Gauvin (1960) was used, where L
= (surface area of body) + (perimeter of projected
area perpendicular to flow). The mounting
card was included in the calculation of the
geometry term. The average value of L in these
studies was 5.4 cm.
Fluid properties were evaluated based on the
bulk solution, with density and viscosity data
from Riley and Skirrow ( 197 5). Diffusion co-
efficients were calculated from equations given
by Cussler (1984), Perry and Chilton (1973),
and Dickson and Whitfield (198 1). Despite the
large amount of information in the literature
on scale formation in seawater, no experimental
values for CaSO, diffusion coefficients in
seawater could be located to compare with the
computed values. At 25”C, the spread in the
values of the difhtsion coefficients for the different
salinities was on the order of 5%.
Figure 9 compares our clod card data and
those of Glenn and Doty (1992) with equations
for spheres and flat plates. Glenn and Doty did
not include a number of details relating to their
clod cards and calibration conditions (e.g. block
dimensions, water temperature, and salinity),
so we did not use their data in obtaining Eq.
9. With respect to other geometries, the clod
card data are roughly the same magnitude as
predicted by the sphere equation (Pasternak
and Gauvin 1960), but the slope is different,
possibly due to the presence of the mounting
card. The flat-plate equation (Skelland 1974)
parallels the clod card data but is substantially
lower.
Both Doty (197 1) and Muus (1968) expe-