The objective of this study was to compare the degree of CaCO3 scale s
uppression provided by various anti-sealants belonging to different ch
emical categories. The scaling system examined consisted of a freely f
alling film of a hot aqueous solution of Ca(HCO3)(2) flowing down the
outer surface of a vertical pipe. Evaporative air-cooling of the falli
ng film released gaseous CO2 from the water, thereby creating a substa
ntial CaCO3 scaling potential. Scale suppression effectiveness was det
ermined by comparing the amount of scale deposited on a test pipe fed
with water dosed with an anti-sealant with the amount deposited on an
identical reference pipe, fed with the same water but without anti-sea
lant dosage. In the absence of an inhibitor, the scale layer was found
to grow linearly with time, past an induction period. In the presence
of an anti-sealant, the length of the induction period was extended a
nd scale growth was arrested at an asymptotic limiting thickness. Scal
e suppression was more effective in the case of feed waters having a r
elatively higher temperature and a somewhat lower scaling potential, a
s compared to feed waters at a lower temperature and a somewhat higher
scaling potential. No major difference could be discerned among the t
ested anti-sealants in the falling film system; all showed a substanti
ally similar scale suppression effectiveness, within a narrow range of
concentrations (0.2 to 0.5 ppm). Preliminary results of tests, aiming
to check the applicability of the above conclusions to membrane syste
ms, are also presented.