Evaporation of spray water is the dominant process of heat rejection in evapoartive condensers. The amount of heat transfer attributible to the difference in dry-bulb temperatures between the condenser tubes and the air is minor. Because the makeup water contains some minerals and other chemicals, and because the water vapor leaves the condenser with none of these impurities, the concentration of foreign materials in the spray water tends to increase. To keep the concentration of impurities under control, blowdown of some of the spray water should be provided whenever the condenser operates. Manufacturers of evaporative condensers typically recommend that the blowdown rate be approximately equal to the evaporation rate. The rate of makeup water is thus twice the blowdown rate.
Almost all sources of makeup water must be treated in some way to avoid one or more of the problems that beset the surfaces of the condenser that are in contact with water. These difficulties include scaling, fouling, and corrosion. Scaling is the deposit of a hard layer of minerals, usually calcium carbonate (CaCO3), on the tube surfaces. A layer of scale 0.8 mm (1/32 in) thick on the tubes can be expectecd to reduce the capacity of the condenser by 30%. This layer of scale is usually due to high mineral concentration in the makeup water and if calcium carbonate precipitates it can form scale on the tubes. Generally a CaCO3 concentration of less than about 170 ppm will be satisfactory. It should not be implied, however, that softened water with a mineral concentration of perhaps 30 ppm is desirable. Softened water may result in excessive corrosion.
Fouling usually refers to the accumulation of nonscale solids, such as dirt, silt, sand, algae, fungi, and bacteria.
Corrosion is a distressing problem in galvanized steel because it often takes the form of deteriorating the zinc coating, which exposes the steel to oxidation. Corrosion is an electrochemical process where an electrical potential develops between two different metals. When current flows as a result of this difference in voltage in the presence of an electrolyte, such as water with dissolved solids, one of the metals dissolves. A particularly sensitive situation is where the zinc coating on steel has deteriorated, because it is between these points that current will flow. Controlling the pH value of the spray water is particularly helpful in retarding corrosion. The pH value is an indicator of the alkalinity or the acidity of a solution, with a pH value of 7 being defined as neutral. Maintenance of a pH value of between 6 and 8 is usually recommended. Corrosion inhibitors approved by the condenser manufacturer are also treatments to prevent corrosion.
A problem called white rust has appeared in the past decade and may be associated with the prohibition on the use of chromates for corrosion protection. White rust is the accumulation of a white, waxy, nonprotective zinc corrosion product on galvanized surfaces. Typically it will appear suddenly and progress rapidly over the wetted, galvanized steel components of the condenser. If not corrected, white rust may lead to premature failure of the galvanized coating. One of the most effective methods of preventing white rust is to assure the passivation of the condenser during initial operation. Passivation is the natural formation of a protective, light crystalline film on the zinc surface. A recent trend in water treatment has been toward greater alkalinity (high pH), which some experts suspect promotes white rust. Indications are that soft water (less than 30 ppm total hardness) combined with high pH valves exacerbate the problem. Passivation is facilitated by initial use of untreated water and also by the use of phosphates. A water treatment expert should be consulted for the unique approach applicable to local conditions.