Effect of circumferential pin thickness on condensate retention as a function of vapor velocity on horizontal pin-fin tubes

Abstract

This paper reports the effect of circumferential pin thickness on retention angle as a function of vapor velocity on four pin-fin tubes (varying in circumferential pin thickness from 0.5 mm to 2.0 mm). Three fluids (namely water, ethylene glycol and R-141b) with high, intermediate and low surface tension to density ratios were tested. Experimentation was performed by providing downward air (to simulate vapor) through a vertical wind tunnel with velocities from 0 to 18 m/s. By providing small holes on the upper side of tube a continuous flow of condensate was ensured along the circumference of the test tubes. At low approaching zero vapor velocity; an increase in circumferential pin thickness caused a decrease in retention angle (an angle measured from top of test tube to the point of flooded flank in circumferential direction) in case of pin-fin tubes for all fluids tested. At high vapor velocity; the role of vapor shear was less effective on the upper half of pin-fin tubes when compared to the equivalent integral-fin tube (i.e. with same longitudinal fin spacing, tooth thickness, tooth height, inner and outer diameter as that of pin-fin tubes). This less effective role of vapor shear on the upper half of pin-fin tubes, when compared with integral-fin tube is thought to be due to the presence of trapped condensate in between the horizontal cuts (circumferential pin spacing) available on pin-fin tubes, which tends to resist the downward motion of the condensate. On the lower half of test tubes (as in case of test fluid R-141b) vapor velocity showed no effect on retention angle for all pin-fin tubes while for the case of equivalent integral-fin tube, retention angle was decreased with the increase of vapor velocity.