Experimental and Numerical Analysis of a PCM-Supported ...

uncertainties associated with the flood-point concept and prediction and to keep the
design point away from the region at which efficiency rapidly diminishes (just below
the flood point) [32]. Strigle [36] recommended designing packed towers with a 10 to
20 percent margin from the maximum operational capacity. Since MOC is usually
about 5 percent below the flood point, the Strigle’s criterion is equivalent to designing
at 76 to 86 percent of flood point velocity. This criterion is therefore less conservative
than the flood point criterion [32].
Packed towers are also designed that the pressure drop at any point in the tower
does not exceed a recommended maximum value. Maximum pressure drop criteria
for packed towers are listed by Kister [32]. The generalized pressure drop correlation
(GPDC) is the classic sizing method for packed towers and is used in many
industries. It is, however, based mostly on the small pilot tower data. As long as the
correlation is applied to small packing, it appears to give reasonable results, but
when the performance of large packing in large towers is assessed, then the results
appear overly optimistic [89]. The majority of designers prefer the flood point criterion
over the MOC [32]. For more conservative designs, the maximum pressure drop
criterion is recommended [32] to be used with the flood point criterion or MOC
criterion.
Strigle [36] stated that the use of random dumped tower packing is not customary in
water cooling towers. Because of the pressure drop available, the maximum packed
depth for large size plastic packings is under 1.8 m. Some figures are provided for
the height of gas phase transfer unit for random packing cooling towers. The
average values for packing sizes 2.5, 3.8, 5, and 7.75 or 8.9 cm, are 0.4, 0.46, 0.56,
and 0.76m respectively. In addition, the average packed depths specified must make
allowance of around 20% to 40% greater than these values for large size tower
packings to account for lower quality distribution of both liquid and gas [36]. He
added that, cooling towers which operated with natural draft are hyperbolic and may
require an overall height of 67m. The fill that used in such a column must be quite
open to avoid any significant pressure drop. Such fill can have a T- or V- shape
cross section molded from perforated plastic sheets. Corrugated sheets made of
asbestos and cement are popular in large natural draft towers, noting that asbestos
is banned for its carcinogenic effects. A ceramic cellular block that is stacked into the
tower also is used. However, these fills provide a small interfacial area per unit
volume so mass transfer is rather low per unit of packed depth.
4.4 Modeling solid-liquid phase change
Typically, three methods have been developed for fixed domain class solid-liquid
phase change problems: temperature based method, the enthalpy-based method,
and the apparent heat capacity [18].
89