Although heat exchangers vary in design, they are all subject to similar casualties. The principal difficulties which may prevent heat exchangers from functioning properly are excessive scale deposits on the cooler element, clogged cooler elements, or cooler leakage.

Figure 3-11.A cooling water system.

A gradual increase in the freshwater temperature is usually an indication of EX-CESSIVE SCALE on a cooler element. As scale formation increases, there is a gradual increase in the pressure difference between the inlet and outlet of the heat exchanger. Scale deposits generally form faster on the saltwater side than on the freshwater side, because of the greater amount of dissolved salt present in the water. Complete prevention of scale formation is not possible, but steps can be taken to reduce its formation by using proper cleaning methods and procedures.

Seawater discharge temperature should be maintained below a specified limit (130F), because the rate of scale formation is increased as the temperature increases. The water used in closed cooling systems must be as pure as possible. Distilled water is recommended for a freshwater cooling system, but since distilled water is not absolutely pure, additional steps must be taken to control acidity and alkalinity. The treat-ment used to control these factors will not remove scales already formed, but it will prevent further precipitation of scale-forming slats. You will find details for water treatment in closed water systems in chapter 233, NAVSHIPS Technical Manual, and in most engine instruction manuals.

Not only the hard deposits chemically precipitated from the circulating water, but also such items as marine life, grease, and debris of various types may CLOG OR RESTRICT COOLER ELEMENTS. The principal causes of

Figure 3-12.Types of heat exchanges.

cooler clogging by loose foreign matter are faulty seawater strainers, dirty freshwater, excessive lubrication of the pumps, and leaking oil coolers. To prevent the entry of sea debris, a punctured screen in a seawater strainer must be replaced as soon as possible. Obviously, the use of dirty freshwater will hasten the clogging of a cooler element. Grease and oil may enter the cooling system and the film deposited on the cooler element will reduce the capacity of the cooler. Grease may come from grease cups which are used on some water pumps to lubricate bearings. If the cups are turned down too much or too often, grease is forced into the circulating water. A hole in the element of an oil cooler permits oil to flow into the cooling system. Any source of oil or grease should be located and repairs made as soon as possible. Corrosion or erosion of the element in a heat exchanger, as well as operation at excessive pressure, may cause LEAKS. These leaks can develop either in the element or in the casing. Leakage from the cooler casing can usually be detected by inspection. Element leaks, however, are more difficult to detect. Any noticeable decline or rise in the freshwater tank level, with the temperature remaining normal, usually indicates leakage.

A hole made by corrosion in a cooler element indicates that corrosion probably exists throughout the element, and a thorough inspec-tion should be made. Corrosion can be prevented to a large extent by using the prescribed freshwater treatment, inspecting as necessary and venting the cooler to remove entrapped air.

Holes due to erosion are usually caused by particles of grit (sand, dirt, etc., resulting usually from operation in shallow water) striking an element at high velocity. Grit is for the most part so fine that it passes easily through the strainer. If the strainer is defective, even the larger particles of grit may enter the cooler.

Erosion by water at high velocity may also result in holes in a cooler element. This occurs when water flow has to be increased above the rated capacity in order to maintain a desired freshwater temperature. Whenever it is found necessary to greatly increase the water flow, the cooler should be cleaned.

If the designed maximum operating pressure (indicated on the exchanger name plate) is ex-ceeded, leaks are apt to result. Excessive pressure is likely to occur in conjunction with clogging, because additional pressure is necessary to force a given quantity of water through a clogged element.