Scale Removal and Descaling
For HydroFLOW to be able to descale a system, certain conditions
MUST
exist.
System descaling is dependant upon the following conditions:
In a scaling system there are three processes that are at work:
Heterogeneous crystallization,
homogeneous crystallization, and scale returning to solution
once the solute has become unsaturated.
Heterogeneous crystallization occurs
primarily on surfaces that are subject to increasing temperatures. As
not all the solute is in contact with the heating surface,
supersaturated liquid will be carried away by convection and circulating
currents to other surfaces. Scaling on other surfaces will continue
until the saturation point is achieved.
Homogeneous crystallization occurs in
large vessels containing high volumes of solute, with a relatively small
surface area. As the solute is being heated, the solution becomes
supersaturated. The surface area is not sufficient to provide all the
nucleation necessary. The solute reaches a critical condition. At this
point any source of energy, such as turbulence in the solute, will cause
homogeneous nucleation. All the material that can precipitate does so at
once. Large numbers of small crystals are formed. These crystals have a
high surface charge that causes them to adhere to all the surfaces,
including cold surfaces. The fine crystals which have adhered to the
surfaces will then become the nuclei for heterogeneous crystallization
in subsequent heating cycles.
The third process is the return to solution
of the scale deposits. After the solute has become unsaturated due to
cooling or pressure change, a quantity of the deposits will be returned
to solution. The surface scale that had been formed is not as stable as
the crystals that have been formed in suspension, due to the uneven way
that nucleation has occurred on the surfaces.
Descaling can occur only if the water in
contact with the scaled surface is unsaturated and is able to dissolve
the carbonates to form bicarbonates.
The presence of CO2 is
necessary for the formation of bicarbonates. The CO2, which is present
in solution in the water, comes from two sources, one from the air in
contact with the water and the other from the decomposition of
bicarbonates due to the heating process.
Descaling of a heat exchanger using HydroFLOW relies on
turbulence. This is
because the temperature of the water is increasing and would normally
only deposit scale. If turbulence is present, the water experiences
pressure changes that cause the water to change rapidly from
supersaturated to an unsaturated condition. While unsaturated, the water
will dissolve the scale on the surfaces, and in the supersaturated
condition, the deposits will grow in suspension due to the presence of
the clusters generated by the HydroFLOW applied electric field.
In every system containing solute, there is a balance of
scale-formation and scale-solution. In a system where the balance favors
scale-formation, the system will experience scaling. In a system where
the balance favors scale-solution, the system will remain free of scale.
HydroFLOW simply tips the balance in favor of
the scale-solution, by providing a large quantity of unsaturated
solution that dissolves the existing surface scale. This process is
repeated, dissolving surface scale and forming suspended stable
crystals. The heterogeneous crystallization is replaced by homogeneous
crystallization. However, in this case homogeneous crystallization
occurs as soon as the solute becomes supersaturated, due to the presence
of a large quantity of clusters generated by HydroFLOW. As a result,
the old scale will ultimately completely return to solution and is
converted to stable individual crystals. These stable amorphous crystals
can be removed by filtration in circulating systems. In open systems
they will pass harmlessly out with the flow. |
