Heat exchangers scale formation

The precipitation of calcium carbonate in the boiler involves a number of discrete processes, including nucleation, followed by the formation of microcrystals which compete to grow into larger crystals (accretion) that eventually form layers of dense, crystalline scale on the heat exchange surface. 

New boilers, heat exchangers, and other equipment must be provided with a PCC program to remove oil, grease, mill scale, pipe-threading compounds, drawing compounds, and other detritus that otherwise prevent the formation of clean, passivated metal surfaces and encourage... 

Any of a number of items of equipment used to determine the rate of deposition of biofoulants, scale formation or corrosion debris, in laboratory studies of new products, or in actual field use. Typically using a heated tube to simulate actual heat-exchanger conditions. 

Inhibition of the precipitation of calcium carbonate is one potential commercial application for 0-(carboxymethyl flnulin. The formation of calcium carbonate crystals is a major problem in boilers, heat exchangers, saltwater desalination, gas and oil production, laundry, and other fields. As the water temperature increases, the solubility of calcium carbonates decreases, resulting in crystalli-zation/scale formation/incrustation, decreasing the efficiency of the operation and increasing costs. 

Morse, J.W., de Kanel, J., and Craig, H.L., Jr. A literature review of the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on OTEC heat exchangers. Ocean Engineering (in press). 

The temperature and flow conditions within the heat exchanger will determine the location at which these various stages occur. For instance the supersaturation and crystallite formation may occur in the bulk fluid with the growing crystals moving towards the wall to form the deposit. The movement of foulant will under these circumstances, follow the processes described in Chapter 7 for particulate deposition. It is possible that due to the level of turbulence within the system, that some (or possibly many) of the crystallites formed are swept into re ons where the solution is not supersaturated. Under these conditions the particles will redissolve. On the other hand crystallisation may occur near or at the heat transfer surface. The presence of nucleation sites on a solid surface may encourage the formation of scale on the surface. Under these circumstances the process is largely governed by the mechanics of the crystallisation process. 

Crystallisation or scale formation on heat exchanger surfaces is influenced by a number of system variables (see Chapter 8) including temperature, pH supersaturation conditions and the flow velocity. In general these are fixed parameters and may not be changed without affecting the requirements of the process. 

The most common situation requires cleaning the intricate internal surface of steam-generating and heat exchange equipment where exposure to water has led to formation of corrosion products and scale on the heat exchanger surfaces. Generally the equipment can be filled with a liquid solvent and at the proper time emptied by opening a drain valve. 

Heat Exchangers, The heat exchangers will be of the shell-and-tube type, with the pile-water inside the tubes and the cooling tower water outside. Steel tubes are contemplated, with corrosion inhibitors in both the water streams. Scale formation should not be troublesome since the make-up cooling water will be softened before introduction into the system. 

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