Because of the difference in their construction, methods of cleaning both types of heat exchangers (radiator and tubular) differ in some respects. Radiator-type heat exchangers are cleaned by chemical means because mechanical cleaning is not satisfactory for this type heat exchanger. Chemical cleaning of radiator-type units is discussed in Engineman 3 & 2, NAVEDTRA 10541 (current edition). Tubular heat exchangers, on the other hand, are cleaned by mechanical means.

In both types of heat exchangers, loose foreign matter such as seaweed, sand, and dirt may be removed by blowing steam through the element in a direction opposite to the normal flow of water. When an element is badly clogged, care must be exercised not to admit steam at a pressure exceeding the maximum specified for the element. If a film of oil or grease is evident, the element should be cleaned like an oil cooler element. Leakage from the CASING of a radiator-type heat exchanger may be caused by a damaged gasket. If so, the heat exchanger should be re-moved from the piping in order that flange faces may be tightened evenly after a new gasket is installed. If there is any reason to suspect that there are leaks in a heat exchanger element, the best method for locating them is by an air test.

This test may be accomplished as follows:

1. Remove the element from the casing.

2. Block off the discharge side of the element.

3. Attach a pressure gage to the inlet line of the element.

4. Supply low-pressure air to the inlet side of the element. Remember: Air pressure must NEVER exceed design pressure for the element.

5. Immerse the element in a tank of water.

6. Check for bubbles.

An element of a heat exchanger may also be tested hydrostatically by filling the element with water under pressure and checking for leaks. Emergency repair of leaks in the element of a radiator-type heat exchanger can be made as shown in figure 3-13. When emergency repairs to the radiator-type heat exchanger are necessary, they may be made with the use of soft solder and a small torch or soldering iron. Extreme care must be taken to prevent the surrounding area from being overheated, thus causing the existing solder to melt. Small radiator-type heat exchangers

Figure 3-13.Emergency repair of a tube leak in a radiator type heat exchanger.

should be replaced as soon as a leak develops, if a replacement is available. The presence of one leak, unless caused by dropping or accidental puncture, indicates that other areas in the heat exchanger may be eroded.

In shell-and-tube heat exchangers, a leaking tube must be replaced as soon as possible. In an emergency, a faulty tube may be blocked off by inserting a special plug at each end, until the tube can be replaced. An air lance or water lance should be used to clean the tubes of a shell-and-tube heat exchanger. If the scale has hardened in the tubes, a round bristle brush or soft rubber plugs may be used to clean the tubes. When clean-ing the tubes by mechanical means, avoid damaging the protective coating inside the tubes. These tubes should never be polished, as the tar-nish on the tubes acts as insulation to prevent further corrosion. Removing the tarnish will also reduce the tube wall thickness and over a period of time and a number of cleanings, could suffi-ciently reduce tube strength, resulting in tube failure. For the proper procedures for cleaning shell and tube type heat exchangers and the safety precautions, use the PMS maintenance requirements cards, the manufacturers technical manual and Naval Shipss Technical Manual, chapter 254.