Evaporative cooling towers

All values per tonne of good casting or in mg/1 effluent For foundry C, waste water quality after treatment consisting in a distilling plant and cooling-evaporation towers ... 

Water-cooling in towers operates on the evaporative principles, which are a combination of several heat/mass transfer processes. The most important of these is the transfer of liquid into a vapor/air mixture, as, for example, the surface area of a droplet of water. Convective transfer occurs as a result of the difference in temperature between the water and the surrounding air. Both these processes take place at the interface of the water surface and the air. Thus it is considered to behave as a film of saturated air at the same temperature as the bulk of the water droplet. 

The raffinate from the solvent extraction plant is pumped to a basin with two evaporative towers, from which is sent to a heat exchanger. Most of the hot raffinate is recycled to the top of the towers and about 66 % of the volume of water is evaporated, by cooling with an air flow in a countercurrent mode. The concentrated raffinate is sent to the heavy metals removal stage. The recycled raffinate flow in the tower is determined by the concentration of the hot raffinate. 

Heat rejected to the environment by most industries is of Httle consequence. Cooling flows of air or water are deployed over equipment or through heat exchangers and the relatively small quantities of heat are dissipated to the surrounding air. Small cooling towers, often of the evaporative type, have become ubiquitous in an industrial faciUty. 

Constmction of new power plants in the coal region of the western United States presents serious problems in states whose laws dictate zero effluent. In these plants, cooling-tower water withdrawn from rivers cannot be returned to them. In these situations, cooling-tower effluent is purified by distillation (vapor-compression plants have predominated) and by a combination of distillation and membrane technology. The converted water then is used as boiler feedwater the plant blowdown (effluent) is evaporated from open-air lined pools, and pool sediment is periodically buried back in the coal mine with the flue ashes. 

Although 600 m of water is used to make a metric ton of fertilizer, 150—240 m to make a tonne of steel, 480 m to make a tonne of gasoline, and 1000 m to make a tonne of acetate fiber, Htde if any is required chemically in any of these processes. Recycling can reduce industrial requirements by a factor of 10—50. Much of this water, particularly that for cooling, and often that for washing, can be saline. Some petroleum refiners have used salt water to remove heat (water s principal role in gasoline production), and some have actually produced table salt by evaporation in cooling towers. 

Cooling by means of evaporative cooling towers is required to maintain a constant temperature of 30—40°C. At higher temperatures, the deposit is rougher, impurity effects are more pronounced, lead codeposition is favored, and the manganese dioxide formed at the anode iacreases and tends to adhere rather than fall to the bottom of the cell. 

Some use is being made of lower grade heat sources, such as moist air from driers, and the constmction of auxiHaries, such as condensers, integral with the evaporator body. A further step is elimination of the conventional condenser—cooling tower—vacuum pump circuit by recirculating last-effect Hquor over the equivalent of a cooling tower built as an integral part of the evaporator body. 

Evaporative condensers (Fig. 11-88) are widely used due to lower condensing temperatures than in the air-cooled condensers and also lower than the water-cooled condenser combined with the cooling tower. Water demands are far lower than for water-cooled condensers. The chemical industry uses shell-and-tube condensers widely, although the use of air-cooled condensing equipment and evaporative condensers is on the increase. 

A slight variation of the above method accounts for increases in the solvent content of the gas stream between the inlet and the outlet of the tower and assumes that the evaporation of solvent tends to cool the hquid. This procedure offsets a part of the temperature rise that... 

SI 1992/2225 Notification of Cooling Towers and Evaporative Condensers Regulations... 

Calculating the heat transfer and water evaporation rates are illustrated by the following example. A cooling tower eools 900 gpm of water from 95 to 85 F. The problem is to determine what the heat rejeetion is, and also what is the evaporation rate. The heat rejeetion is ealeulated as follows ... 

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. 

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