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Controlling Scale Formation in Evaporators
Scale formation in freshwater generators is a problem encountered by crew onboard ships. Scales adversely affect freshwater generation capacity of the plant. Chemical like 'VAPTREAT' (From Unitor) is dosed to the feed of the evaporator in a measured quantity (it is calculated from the capacity of the plant) to ensure that no scale is deposited during freshwater production. Dosage can be doubled to dissolve the existing salts which are already formed before. Foaming, and subsequent carryover of salts along with the vapours, also can be minimized by addition of chemicals. Some of the methods for controlling scale formation in evaporators are described below.
Use of Low Pressure Evaporator Plant
Operation of the evaporator is at a temperature below 80 Degree Celsius so that calcium carbonate scale predominates. This is a soft scale which is easily removed and is not as poor a conductor of heat as other type of scales.
This type of treatment consist of permanent magnets, followed by a filter, is installed in the feed line of the evaporator. The charge of the salts is altered when the seawater passes through the strong magnetic field. This prevents the amalgamation of salts, during precipitation inside the evaporator, the salt then goes out with the brine.
Use of Flexing Elements
This method applies the principle of expansion and contraction of metals with temperature. Here heating elements made up of monel metal is used. According to the pressure variation inside the evaporator, temperature also varies. This variation in temperature cause flexes in the element, which results in cracking off the scales. Care must be taken not to subject these elements to high pressure, which may result in metal failure.
Continuous Chemical Treatment
Here chemical compounds are dosed into the evaporator feed line continuously for water treatment to prevent scale formation and foaming.
Organic Polyelectrolyte Combined with Anti-foam
This can be used in evaporators producing water for drinking purposes, for minimizing scale formation. The chemical to be continuously fed to the feed line of the evaporator, using a metering pump, hence overdosing can be avoided. Rate of chemical dosing is about 1 to 8 ppm, which depends on the output capacity of the evaporator and density of the contents inside. The chemical used is alkaline in nature and instruction in Material Safety Data Sheet (MSDS) to be followed while application.
Polyphosphate Compounds with Anti-foam
This minimizes foaming and calcium carbonate scale formation. The chemical is non acidic, non toxic, easy to handle and comparatively cheap. Dosing is carried out in evaporator feed line at a rate of 2 - 4 ppm and can be used in units producing potable water. One disadvantage of this method is that, it forms a sticky sludge when used at temperatures near 100 Degree Celsius, thereby limiting its use in low pressure evaporators only, where temperature is well below 100 Degree Celsius.
This is a non toxic, stable, hygroscopic chemical which becomes acidic when combined with water. If used in prescribed proportions, this compound prevents formation of magnesium hydroxide and calcium carbonate scales. It must be fed to the evaporator using plastic tubes, since it is corrosive in nature.
Evaporator can be circulated with descaling chemicals available in the market and opened up at regular intervals for manual cleaning. Interval of chemical re-circulation and cleaning can be adjusted after evaluating the condition of the tubes or plates inside.
- Marine Engineering Study Materials
Study Materials on Marine Internal Combustion Engines, Compressors, Heat Exchangers, Air Conditioning and Refrigeration System, Main Propulsion System, Auxiliary Power Generation and Distribution System, Water Treatment, Fresh Water Generator, etc.