Seawater evaporators are designed to produce distilled water from sea water for use as boiler feed water, engine cooling water and domestic purposes.
The two basic types of evaporator, namely the flash and submerged element or coil type, both suffer from the problem of scale formation on their heat exchange surfaces. This scale impairs heat transfer, thereby reducing the efficiency of the evaporator and eventually necessitating evaporator shut-down for either chemical or mechanical cleaning. Therefore there is a requirement for an effective method of controlling or preventing scale formation.
The two types of scale that occur in sea water evaporators are the alkaline hardness scales, and calcium sulphate scales. The former is composed of calcium carbonate, magnesium hydroxide and occasionally magnesium carbonate. Other compounds are found in scales however, resulting from absorption or co-precipitation. The nature and composition of the scale formed is dependent on the operating temperature of the evaporator and to a lesser extent on the brine concentration in the evaporator.
The ZI-CHEM® range of specialist scale deposit inhibitors are comprised of specially prepared formulation of low molecular weight synthetic polymers designed to give maximum protection against all common types of scale, throughout the entire temperature range of existing evaporators. It is equally effective for flash or submerged element types.
These products prevent scale formation by a “threshold” mechanism, i.e. a small amount of treatment when added to the seawater prevents precipitation of a large amount of hardness salts. This causes a delay in precipitation of all alkaline hardness at the time when precipitation would normally occur.
Also, the normal crystalline structure is so distorted that a fine sludge occurs, which does not compact but remains in suspension. This can be easily removed using the brine blowdown or deconcentrator.
With non-alkaline hardness such as calcium sulphate these programs give a stabilizing effect to the supersaturated solution, resulting in up to two times the normal solubility of calcium sulphate being achieved.