measured heat transfer coefficients for no strip, a PVC strip, and a porous nickel strip
were 6200 W/m2 K, 6900 W/m2 K, and 10,200 W/m2 K, respectively. The effectiveness of
including Yau et al. (1986) and Marto et al. (1988). Although drainage strips have proved
effective under laboratory conditions, their use in an actual condenser tube bundle may
not be feasible.
Example 5
Horizontal integral-fin tubes are to be designed to condense steam, ethylene glycol and
R-113 at atmospheric conditions.
(i) If the maximum circumference flooded is to be 50 %, calculate the number of fins
per meter that should be provided if the outer tube diameter is 19 mm and rectan-
gular fins of 0.25 mm thickness are used.
(ii) If the outer diameter and fin thickness are changed to 21.05 mm and 0.5 mm,
respectively, find the new fin density for each of the three fluids.
(iii) Calculate the maximum fin density for steam, ethylene glycol and R-113, condens-
ing on 19-mm outer-diameter tubes, fitted with 0.25-mm-thick rectangular fins.
Solution
(i) For 50 % flooding, β = 90 in eq 89. The interfin spacing s is then given by
4σ
s=
.
do ρl g
The values of the ratio σ/ρl at atmospheric pressure for the three fluids have been quoted
by Marto (1988) as follows:
61 106 m3/s2
Steam
34 106 m3/s2
Ethylene glycol
11 106 m3/s2.
R-113
Thus for steam, the spacing is s = 1.3 mm.
The fin density is 645 fins/m.
The above calculations repeated for ethylene glycol and R-113 give fin densities of 1021
and 2057, respectively.
(ii) Repeating the calculations with 21.05-mm-outer diameter and 0.5-mm-thick fins
gives densities of 595 fins/m for steam, 863 fins/m for ethylene glycol, and 1402
fins/m for R-113.
(iii) For total flooding, the angle b in eq 89 would be 180; then the fin spacing s is given
by
2σ
s=
,
do ρl g
which shows the fin spacing is one-half the values obtained in part (a). Thus for steam
condensing on 19-mm outer-diameter tube, s = 0.65 mm. For a fin thickness of 0.25 mm,
the fin density is 1111 fins/m. The corresponding figures for ethylene glycol and R-113
work out to be 1626 and 2717. The value of 1111 fins/m for steam confirms Webb's (1994)
statement that condensation of steam on a 19-mm-diam. tube having a 0.25-mm-fin thick-
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