Evaporator

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Evaporator scale
There are numerous types of evaporators all working to produce pure water with concentrated sea-water as waste. This concentration effect can lead to the formation of damaging scales within the evaporator. Over concentration is usually prevented by having a continuous stream of sea-water passing through the unit thus maintaining a satisfactory dilution of the sea-water side of the evaporator. However, because of the high salt content, when sea-water is elevated to temperatures above 30 C scales can begin to form on heat transfer surfaces. Additionally as the majority of evaporators operate under vacuum there is a tendency for the make-up water side to foam, which can give rise to carry-over and contamination of the pure water stream.

Four scales which are principally found in evaporators are;
Calcium Sulphate (CaSO4)-1200ppm, scale formation is principally on density, remains in solution below 140oC and/or 96000ppm.The worst scale forming salt forming a thin hard grey scale

Magnessium Hydroxide Mg(OH)2
remains in solution below 90oC

Magnessium Bi-Carbonate 150ppm
soluble below 90oC, forms a soft scale, prevention by keeping operating temperature of evaporator below 90oC
Above 90oC
breaks down to form MgCO3 and CO2 and then Mg(OH)2 and CO2

Calcium bicarbonate Ca(HCO3)2 180ppm
Slightly solube, above 65oC breaks down to form insuluble calcium carbonate forming a soft white scale. scale formation prevented by chemical treatment Ca(HCO3)2 = Ca + 2HCO3
2HCO3 = CO3 + H20 + CO2

If heated up to approximately 80oC

CO3 + Ca = CaCO3

If heated above 800C

CO3 + H20 = HCO3 + OH
Mg + 2OH = Mg(OH)2

Hence if sea water in the evaporator is heated to a temperature below 80oC calcium carbonate predominates. If it is heated above 80oC then magnesium hydroxide scale is deposited.

Sodium Chloride 32230 to 25600ppm -generally ignored
Soluble below 225000ppm forms a soft encrustation, free ions promote galvanic action. It is unlikely to precipitate and is easily removed

Supersaturation
This is where the concentration of dissolved salts exceed their solubility at the particular temperature encountered and precipitation begins to occur. When deposition occurs under these conditions heavy scale deposits can rapidly build up and lead to a loss of heat transfer efficiency. Scale deposition due to supersaturation is often localised in areas of elevated temperature such as heat transfer surfaces in heat-exchangers. This is because of localised over concentration of salts with respect to the temperature of the thin water layer at the surface of the metal. Scale deposition can therefore occur on heat-exchange surfaces even when the conditions in the bulk of the water are not scale forming.
Methods of controlling and minimising scale (evaporators)

    Use low pressure evaporation plant-Operating at temperature below 80oC so that calcium carbonate scale predominates. That is a soft scale easily removed and not such a poor conductor of heat.
    Use magnetic treatment-A unit consisting of permanent magnets, preceded by a filter., is installed in the evaporator feed line. The water passes through a strong magnetic field which alters the charge on the salts so that amalgamation of the salt crystals, formed during precipitation in the evaporator , is prevented and the salt then goes out with the brine.
    Use flexing elements-a heating element made of thin gauge monel metal built like a concertina may be used. The advantage of such an element is that when pressure, and hence temperature , vary slightly the element flexes considerably thereby cracking off scale effectively and permitting longer running periods of the evaporator between shut downs. Care should be made not to submit the element to over pressurisation.
    . Use continuous chemical treatment

Treatments

    Ferrous chloride-FeCl2- Keeps pH too low for Mg(OH)2 and CaCO3 to decompose to form scale.
    sodium sulphate-Na(SO4)2 or HCl and corrosion inhibitor alternative;Rubber lining necessary to protect shell from acidic water.
    Sodium polyphosphate
        -Prevents scale formation below 80oC
        - Mixture of different phosphates
        -sludge (coagulent and antifoam) conditioners
        -can be used with potable water.
    Polyelctrolytes
        -Prevent scale above 80oC
        -safe with potable water.