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1950-51 Theatre Catalog, 9th Edition, Page 374 (352)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 374
Page 374

1950-51 Theatre Catalog, 9th Edition, Page 374

FIGURE SeThe chilled-water arrangement for removing heat from spray water is often a closed circuit where air is not absorbed and. hence. corrosion is no problem.

Corrective Measures

To prevent inside surface corrosion of air washers and water-circulation systems, a combination of chemicalsealkali and sodium chromate-should be fed into the spray water. Actually, sodium dichromate acts as supply for the chromate; it combines with the alkali to form sodium chromate.

In some plants these chemicals are fed intermittently in daily doses. Best practice employs a chemical feeder that operates whenever the spray waterfs circulation pump does. This maintains the water in the most favorable condition at all times.

Fig. 2 shows a further development of the simplified air washer in Fig. 1. Incoming air can be either cooled or heated. Cooling is effected by spray water, which goes through the cooler in the refrigerating system.

This system consists of a compressor for the refrigerating gas (ammonia, carbon dioxide or some other refrigerant) and a condenser for liquefying the gas. From here the liquefied gas passes through expansion valve to the cooler, where it expands to absorb heat from the spray water.

This process introduces another circulating-water systemethe one that carries water for condensing the refrigerant. In very small plants this water may be drawn from the city supply and pass through the condenser and out to the sewer. Some cities have ordinances forbidding this use of water because it is so wasteful.

One way to conserve the refrigerant condenser water is to recirculate it through a cooling tower, Fig. 2. In the tower the water, flowing down over baffles and fill, comes in contact with an air stream and is thereby cooled. This contact with air lowers pH, and produces a corrosive water. Without countermeasures, corrosion damage in cooling tower, piping and refrigerant condenSers may well become a problem.

A somewhat different air-washer arrangement, Fig. 3, employs a chilledwater circuit. Spray water is not directly cooled by the refrigerating system but, instead, sprays over coils carrying the chilled water in a closed circuit. Here the spray water needs chemical treatment because of its contact with air. But the closed circuit probably does not need this protection. I

What about an evaporative condenser? Fig. 4 shows a system where the refrigerant is condensed in an evaporative condenSer. A circulating pump keeps water moving through spray nozzles that play the water over the refrigerant coils. An air blast, forced through the chamber housing coils and sprays, cools the circulating water. In this arrangement chemical treatment is added to the two waters that come in contact with air: (1) the air washeris spray water (2) the circulating water in the evaporative condenser.

Brine Treatment

In refrigerating systems employing brine, sodium chromate provides an effective and economical defense against corrosion. It requires a chromate con FIGURE 4-Circulating water in an evaporative condenser becomes corrosive by absorption of C02 and other acidic compounds from the air. Protection must be provided.

centration of about 2000 ppm for calcium brine and. 3200 ppm for sodium brine. Since chromate has an acidifying action, caustic soda may also be used to raise and hold pH of the treated brine at 7.5 to 8.5. At the same time, the caustic combats lowering of the pH caused by any absorption of 002. Periodic analyses and regularly adjusted chemical feeding are the only sure ways to prevent corrosion.

Ammonia escaping through a leak into the brine makes it too alkaline. If alkalinity tests indicate that this is happening the leak should be located and repaired as quickly as possible.

In handling chromates and caustic soda considerable care must be exercised. These chemicals can irritate or burn the skin or injure the face and eyes.

Scale Prevention

Suppose in addition to being corrosive, the water is hard, that is, contains large amounts of calcium and magnesium salts. Because of the evaporation that takes place in cooling towers, air washers and evaporative condensers, these mineral salts tend to concentrate in the same way as in a boiler. Eventually they come out of solution and form scale.

Over and above this scale is the coating that may form on heat-transfer surfaces from the dust particles, bacteria and pollen absorbed by water. Such organic growths give water an offensive odor, which in turn is imparted to the air. Proper chemical treatment can reduce such difficulties.

Just what does chemical treatment for scale prevention involve? There are several acceptable approaches. A zeolite softener on the make-up water supply' can prevent or minimize scale troubles by softening the water. Surfaceactive materials, such as certain polyphosphates, may achieve the same result. Bleeding a small stream of water from the circulating system tends to keep the concentration of dissolved mineral salts below the point where they precipitate as scale.

Bleeding or blowdown, with other water losses, affect the amount of chromate and alkali needed for the anticorrosion treatment. As an example, moisture condensed from air passing through the air washer, Fig. 1, drops into the well at the bottom of the chamber and causes a continuous small overHow. This overflow carries away some of the chemicals, and pH of the water drops unless a continuous automatic chemical feeder is provided.

In cooling towers some circulating water is carried oh by drift or windage. In mechanical draft towers rate of such loss runs to about 0.2% of circulation rate. For natural draft this loss ranges to about 2% depending on wind velocity. Spray ponds have an even higher loss from this cause.

But no matter what the particular airconditioning equipment used, in handling the water or brine an active, closely followed chemical-treatment program can extend service life by reducing or preventing troubles from corrosion or scale formation.

1950-51 Theatre Catalog, 9th Edition, Page 374