Calcium sulphate deposition on heated metal surfaces
The accumulation of undesired matter at heat transfer surfaces
(fouling) is a severe problem to industry. The growth of calcium
sulphate dihydrate on heated metal surfaces has been examined and a
mechanism for the effect of surface roughness on the amount of
deposition has been proposed.
A novel piece of equipment was designed and constructed in which
the growth of crystals on heat transfer surfaces with different surface
roughnesses could be observed under controlled solution conditions.
The test section was a transparent rectangular channel into Rhich three
25 mm diameter test pieces could be inserted such that the polished
faces were planar with the rear face of the channel. The back faces of
these test pieces were heated by contact with hot Mater.
Using Reynolds numbers of 300 to 13,500 and calcium sulphate
solutions with bulk concentrations from 20 to 50 mH Ca , no gross
effect due to surface roughness was seen. However a limited effect,
which distinguished grit-blasted surfaces from polished surfaces, was
found in experiments with a bulk concentration from 28 to 33 mH Ca .
In all of the experiments it was observed that the presence of bubbles
enhanced crystal growth. It was also found that the amount of
deposition formed on any surface decreased with decreasing dissolved
oxygen content of the bulk solution.
It is suggested that a bulk concentration of approximately 33 mH
Ca is a critical level of supersaturation, which corresponds with the
so-called metastable limit of supersaturation. The surface roughness
effect may be associated with two factors. Firstly, as the critical
supersaturation is approached crystal growth is enhanced at certain sites. in particular the edges of bubbles. Secondly, very rough
surfaces. such as grit-blasted surfaces. more readily support and
initiate bubble formation and consequently the grit-blasted surface
shows greater growth.
Additional work with a different test rig. using a stagnant
solution, indicated that suppression of bubble formation during an
experiment, either by initially degassing the surface or by
pressurising the system, decreased the amount of crystal growth.
This work suggests that a surface roughness effect exists which is related to the presence of bubbles at the surface. Consequently the
amount of deposition can be decreased either by controlling the
formation of the bubbles (e.g. by completely degassing both the
solution and the surface. and pressurising the system). or by polishing
the surface. However. the surface roughness effect was most apparent
between the very coarse, grit-blasted surfaces (R ~ 2.90 um) and the a
medium finish 180 or 240 Grit surfaces (R ~ 0.23 um). a Little or no
further improvement was obtained in polishing to a mirror finish. i. e.
0.25 or 0.1 um (R ~ 0.03 um). a
further polishing beyond removing gross roughness would not appear to
Therefore. for industrial purposes,
yield sufficient benefit to justify the large cost involved.
It is recommended that further work be undertaken to determine
whether the deposition which occurs in the absence of bubbles is
influenced by the surface roughness, and whether other chemical system
and surfaces behave similar to the calcium sulphate - stainless steel
system studied in this work. Moreover, additional work should be
undertaken to examine the effect of surface physical properties on
crystal and bubble nucleation and growth.