Wetting-drying

Acid Deposition. Acid deposition, the deposition of acids from the atmosphere to the surface of the earth, can be dry or wet. Dry deposition involves acid gases or their precursors or acid particles coming in contact with the earth s surface and thence being retained. The principal species associated with dry acid deposition are S02(g), acid sulfate particles, ie, H2SO4 and NH HSO, and HN02(g). Measurements of dry deposition are quite sparse, however, and usually only speciated as total and total NO3. In general, dry acid deposition is estimated to be a small fraction of the total... 

Wet/dry process. Lime slurry absorbs SO2 in vertical spray dryer forming CaSO —CaS, H2O evaporated before droplets reach... 

One report (13) describes the procedure for spinning dry asymmetric ceUulose acetate fiber with a bore skin. Such fibers are spun in a modified dry-spinning process in which a volatile Uquid (methyl formate) is used as the ceUulose acetate solvent. The bore coagulating Uquid is isopropyl alcohol, which is subsequentiy removed. The advantages of these dry fibers over most ceUulose acetate membranes are that they can be stored dry, they are wet-dry reversible, they can be sterilized and packed dry, and they are ready for use without removal of preservatives. 

Allowing DRI to become wet does not necessatily cause it to overheat. When large pdes of DRI are wetted with rain, the corrosion reactions are limited to the outer surface area of the pde and the resultant heat from the corrosion reactions is dissipated into the atmosphere. However, if water penetrates into the pde from the bottom, or if wet DRI is covered with dry DRI, the heat from corrosion reactions can budd up inside the pde to the point where rapid reoxidation begins. Corrosion occurs significantly faster with salt water than with fresh water. DRI saturated with water can cause steam explosions if it is batch charged into an electric arc furnace. 

For waterproofing, sodium silicate concentrations below 30% are adequate concentrations between 35 and 70% are used for strength improvement. Grouts having 35 vol % or higher silicate resist deterioration on freeze—thaw or wet—dry cycles. Water permeability of sands can be reduced from 10 to 10 cm/s. Unconfined compressive strengths of stabilized sand can vary from 103 to 4130 kPa (15—600 psi) the normal range is between 690 and 1380 kPa. 

Wet-dry coohng towers incoi porating these designs are being used for large-tower industrial applications. At present they are not available for commercial applications. 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

There is no limit at which the concentration of these ions in solution becomes safe. However, as concentration decreases, severity of attack almost always decreases. Concentration may occur by evaporation in alternate wet-dry conditions. 

Fig. 14-9. Wet/dry precipitation collector and flow chart for analysis of samples. (DI HjO distilled water). Source "NADP Quality Assurance Report," Central Analytical Laboratory, Illinois Institute of Nafural Resources, Champaign, 111., March 1980.

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Another type of crossflow cooling tower is the wet-dry tower, which consists of a normal crossflow tower over which a few air coils are placed. The hot water is first cooled by an air cooled heat exchanger and then drops to the wet cooling tower where more cooling is obtained by the evaporative mechanism. Figures 5 and 6 provide examples. In contrast, deck-filled towers contain tiers of splash bars or decks to aid in the breakup of water drops to increase the total water surface and, subsequently, the evaporation rate. 

Figure 5. Single structure type wet-dry cooling tower.

Figure 5-5N. Perforated propeller. Occasionally recommended for wetting dry powders, especially those that tend to form into lumps. Courtesy of Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.

Total Dissolved Solids (TDS)—Over 5,000 ppm can adversely affect thermal performance and may be detrimental to wood in the alternately wet/dry areas of the tower. 

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