Cleaning action of spherical projectiles in tubular heat exchangers

Projectiles of different properties, i.e. shape, materials, hardness, can be propelled through the heat exchanger tubes to mitigate fouling when used at the early stage of deposition process. In the present study, fouling experiments have been performed in which CaSO4 was used as foulant and plain tubes as heat transfer surfaces. The rate of projectile injection was also varied to discern its impact on the cleaning. The experimental results showed that at fouling conditions, the inner layer of the deposit, i.e. near the heated surface, is sintered while the outer layer of the deposit, i.e. near the solution side, is porous and crystalline. As the fouling process reaches an asymptote, the whole fouling layer is sintered. Sintering changes the whole fouling layer to a robust and non-porous structure which does not allow the projectile to remove crystals from the deposit layer anymore. Two mechanisms are simultaneously dominant once projectiles are injected (i) sintering of the fouling layer near the tube surface and (ii) removal of precursors from the deposit layer near the solution side due to projectiles. If the removal rate is faster than the sintering rate then it is expected to end up with a very thin sintered layer and vice versa. Furthermore, using high injection rates will minimize the deposit thickness and once an asymptote is reached then it is not recommended to use projectiles anymore.