Scale formation in evaporators

Sample calculations in reports, 462 Saran, 437, 440-442 Sawing for equipment fabrication, 447 Scale formation in evaporators, 355-360 Scaling for equipment cost estimation, 169-171 Scaling factors for heat transfer, 586-587 Scale-up for equipment specifications, 36-39 Schedule number for pipe, 493 Screen, cost of 567 Self insurance, 264-265 Sensitivity of results for pipe sizing, 367-368 Separators, cost of 559-561 Sequential analysis, 771-772 Series compound-amount factor, 227... 

During the operation of the tower, water is lost by evaporation, water droplets entrained in the outgoing air, and in a water purge, called blowdown. To reduce carry-over of water droplets the air flows across drift eliminators. The water droplets impinge on the drift eliminators and then flows down to the bottom of the tower. The droplet water loss is about 0.2% of the incoming water [11]. After leaving the drift eliminators, air flows up and out of the tower. Evaporation of water into air transfers heat from the water to the air. Cooling the water requires about 1.0 % evaporation for every 5.56 C (10.0 "F) drop in the water tempera-ture[l 1]. To reduce scale formation in the tower because of dissolved calcium or... 

The discussion has concerned the transfer of sensible heat and the associated scale formation. In addition, an important area for processing is when fouling accompanies boiling heat transfer. The problem is encountered in such processing as the crystallization of final products, such as sugar and table salt or in the desalination of seawater by flash evaporation. There is a potential problem of scale formation in boiler plant, but the technology is such that in modem boiler plant, the problem is well controlled by the use of chemical additives. 

Minimizing Scale Formation in Saline Water Evaporators... 

The presence of dissolved calcium and magnesimn compounds (termed hardness) leads to scale formation in boilers and fouling of evaporative cooling systems, and is the biggest canse of plant water problems. Other dissolved soUds may be considered as impurities, depending on their concentrations and the intended water use. 

A number of options for controlling scale formation are used in plant operations around the world. One approach is use of mechanical means, including thermal shock. Although rare today (ca 1997), this practice can be found ia use with the few obsolete submerged tube evaporators. 

Blowdown discards a portion of the concentrated circulating water due to the evaporation process in order to lower the system solids concentration. The amount of blowdown can be calculated according to the number of cycles of concentration required to limit scale formation. Cycles of concentration are the ratio of dissolved sohds in the recirculating water to dissolved solids in the makeup water. Since chlorides remain soluble on concentration, cycles of concentration are best expressed as the ratio of the chloride content of the circulating and makeup waters. Thus, the blowdown quantities required are determined from... 

Larger scale preparations (up to 0.100 mole) have been carried out on the bench top (in the hood ) under inert atmosphere conditions (a slow nitrogen purge through a oil bubbler), using the appropriate scale-up in reactant and solvent quantities. The reaction is complete in about 72 hours. The bulk of the solvent can be removed by evaporation at or below room temperature under reduced pressure on the bench top [heating should be avoided because it leads to significant product decomposition and the formation of HMn(CO)5 and Mn2(CO)io impurities]. The final purification of the product should be accomplished on the vacuum line. 

Scale deposition was investigated with and without boiling in an apparatus designed to approximate to some extent conditions in commercial evaporator tubes, yet in which observation of scale formation is possible. 

That the highest performance evaporator could be used for sea water. By performance was meant heat transfer coefficient not in B.t.u./hr./° F./sq. ft. but in B.t.u./ hr./° F./dollar of installed cost. For a long time, the long-tube vertical (LTV) evaporator has best fitted this description, at least under favorable operating conditions, such as at relatively high temperature differences (usually), and with little scale formation. 

In the. first run, acid feed rate was gradually reduced without immediate evidence of scale formation and then was cut off entirely. Heat transfer coefficients started to drop immediately and at the end of the week the tubes were coated with calcium carbonate scale. In this first run, the pH entering the evaporator was 8.3 and leaving it was 8.4. Langelier, Caldwell, and Lawrence have measured the equilibrium pH above which a sea water concentrate is supersaturated with respect to calcium carbonate and magnesium hydroxide (4). Under the conditions of this test, the following conditions were encountered ... 

The next series of tests was made to prove out the seeding method of scale prevention. The first test was made under the same conditions as for the acid trials. Once the mechanical problems of recycling the seeds in the pilot plant were solved, it was found possible to prevent scale formation completely, if the evaporating liquid contained 0.5% calcium carbonate solids. The solids were made initially by slowly adding soda ash to sea water. 

The hollow nano-objects may be synthesized by the joint evaporation of carbon and a metal component in liquid. These nano-objects are formed when products get into the zone of high plasma temperatures repeatedly. This process may be presented as that proceeding in two stages. At the first stage a metal crystal with the more thermally stable surface film forms. At the second stage the product gets repeatedly into the zone of high temperatures (up to 12000 K) due to the turbulent movement of liquid with this product within the reactor (Fig. 3). This results in evaporation of metal from the thermally stable shell and formation of the nano-scaled hollow structure (Fig. 4). 

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