A double-pass heat exchanger is generally used ensure that tools, such as screwdrivers and wire brushes, when there is limitation on the installation of the heat are not used in such a way that they may scratch or mar exchanger. This type of heat exchanger is less efficient the tube surfaces. than a single-pass exchanger and is subject to internal undetectable leakage across the flow divider in the Over a period of time, electrolysis, which results inlet-outlet  water  box. because of dissimilar metals in the cooling system, will slowly dissolve the insides of various components in the It is to your advantage to maintain a log on the performance of the heat exchangers installed in the cooling  systems  with  which  you  are  involved.  By recording  the  operating  characteristics,  you  will  have the data you will need to later analyze the performance of  the  heat  exchanger.  The  performance  is  monitored by observing the temperature gradient (AT) between the inlets  and  outlets  of  the  two  fluids.  The  overall effectiveness  of  a  heat  exchanger  is  determined  by comparing its primary inlet temperature to its secondary outlet temperature. For example, with no change in the primary or secondary flow and if the heat transfer capacity  of  the  heat  exchanger  drops,  the  AT  will increase. By  comparing  the  AT  readings  taken periodically with the clean heat exchanger AT readings, you can detect the deterioration of a heat exchanger because of fouling. By maintaining a record of both input and output pressure gradient you will be able to determine which side of the heat exchanger is fouled. Heat exchangers must periodically be cleaned. The secondary  section  (distilled  water)  is  cleaned  by circulating chemicals through the secondary cooling system to remove any buildup of scale deposits, which accumulate on the surface of the tubes. The procedure for routine cleaning of the primary section of the heat exchanger  is  to  first  secure  the  sea  connections  to prevent  flooding.  In  some  cases,  an  inspection  port  in the water box can be opened to remove any foreign matter lodged inside and against the tubes. If you are unable to get at the ends of the heat exchanger to remove the  water  boxes,  then  you  must  remove  the  heat exchanger from its location and place it on the deck or a suitable work surface. Mark each unit removed so that it   can   be   positioned   in   its   proper   place   during reassembly. With the water boxes removed, an air lance should be passed through each tube and the passages washed out. Where severe fouling exists, a water lance should be pushed through each tube to remove foreign matter  attached  to  the  tube  walls.  Where  extreme fouling exists, special cleaning equipment operated by personnel skilled in their use is required. The ship’s engineering  officer  is  the  best  person  qualified  to determine which procedure to use and whether the job can be performed aboard ship or if it must be transferred to a repair facility. You should take precautions to primary seawater cooling system. (Electrolysis is not a problem in chilled water systems to the extent that it is in seawater systems.) The type of metal used in the fabrication of the heat exchanger tubes is the deciding factor as to the use of zincs anodes or zincs. Zincs are disks, rods, bars, or plates made of zinc metal that are installed inside the heat exchanger’s water boxes. When zincs are installed, the electrolytic action is concentrated on the zinc and not on the metal of the heat exchanger tubes. As electrolysis dissolves the zincs instead of the heat exchanger tubes, they should be replaced. (The purity of distilled water inhibits electrolysis in the secondary system.) In an older cooling system, you should be on the lookout for thin pipes in the seawater side of the cooling system. You can check forbad pipes by gently tapping the empty pipes with the ball of a ball-peen hammer. A bad piece of pipe will make a dull sound and dimple as it is struck lightly. This work should be scheduled for overhaul while the ship is inport. The heat exchangers in the distilled water cooling systems  that  cool  electronic  equipment  are  either liquid-to-air or coolant-jacket type of heat exchangers. The  liquid-to-air  heat  exchangers  are  mounted  inside cabinets  containing  the  heat-producing  electronic components. A cabinet fan circulates the air across the heat exchanger and to the heat source in an airtight circuit.  In  the  coolant-jacket  type  of  heat  exchangers, the  distilled  water  is  circulated  through  an  integral  water jacket  in  a  large  heat-producing  component  such  as  a power-amplifier  tube,  a  plate  transformer,  or  load isolators.  Vent  and  drain  connections  are  provided  to permit  the  venting  of  trapped  air  and  the  draining  of water. Temperature gauges may be provided in the inlet and outlet piping to check the performance of the heat exchanger.  Label  plates  indicate  the  water  flow direction  through  each  cabinet.  Flow  regulators  (orifice plate   or   constant-flow   device)   usually   provide   a constant  flow  of  coolant  to  the  individual  component, cabinet, or bay of electronic equipment to be cooled. On critical  electronic  components  that  would  be  damaged without coolant to remove the heat, coolant flow and temperature  switches  monitor  the  coolant. 1-9