b = bc
Porous
Drainage
Strip
Condensate Flooding
(no drainage strip)
Condensate
Flooding
(porous drainage strip)
Figure 21. Effect of porous drainage strip on condensate flooding. Adapted
from Marto (1988).
reached by Honda et al. (1983) when they found their data for methanol and R-113 under
static and dynamic conditions to be essentially the same.
More recent experiments on flooding by Masuda and Rose (1987) have shown that the
liquid is not only retained on the lower side of the tube, but also on the upper portion in
the form of a small liquid wedge between the flanks of the fins and the tube surface
between adjacent fins.
Rudy and Webb (1983a, 1985) and Honda et al. (1983) developed a theoretical expres-
sion for the condensate flooding angle b (Fig. 20b) by equating the surface tension and
gravity forces on the liquid retained between the fins. The expression is
4σ
β = cos-11 -
(89)
doρl gs
where σ = surface tension
do = tube outside diameter
ρl = condensate density
s = gap between fins.
Equation 89 shows that the flooding angle β increases with increasing surface tension σ
and with decreasing fin spacing s.
The amount of condensate retained between the fins can be reduced by attaching a
rectangular porous plate to the bottom of the tube. This attachment, known as a drainage
strip, increases the downward capillary force, drawing the condensate into its pores.
Figure 21 illustrates schematically the reduction in condensate flooding achieved with a
porous drainage strip. A solid plate can also serve as a drainage strip, but it is not as
effective as the porous plate. For example, Honda and Nozu (1987a) measured the effect
fin density of 2000 fins/m and a fin height of 1.13 mm. With no drainage strip, they found
that 62 % of the tube circumference was flooded. When a polyvinyl chloride strip (solid)
of height 12.6 mm was used, the percentage of circumference flooded was reduced to 57, a
reduction of about 8 %. However, when a porous nickel strip of the same height (12.6 mm)
was attached, only 32 % of the tube circumference was flooded, giving a reduction of
about 48 %. Clearly the porous strip is much more effective than a solid strip. The
31
Previous Page
