electronic equipment being cooled, a demineralize, a temperature control valve, monitoring equipment with its associated alarms, and the heat exchanger, which is shared  with  the  primary  system.  The  secondary  system is  a  closed-loop  water  system,  as  compared  to  the seawater  system,  which  is  a  one-pass  or  open-loop system. TYPE I COOLING SYSTEM We are now ready for a more detailed look at the types of cooling systems. Let’s begin by looking at the Type I system. Starting with the distilled water pumps (fig. 1-6), distilled water under pressure flows to the temperature  regulating  valve.  The  temperature regulating valve is installed to partially bypass distilled water  around  the  seawater-  to-distilled-water  heat exchanger  so  that  a  constant  water  temperature  can  be supplied  to  the  electronic  equipment.  As  the temperature in the distilled water increases, more water is directed to the heat exchanger and less to the bypass line.  ‘his  maintains  the  output  water  temperature constant. The standby heat exchanger is usually of the same design and is used when the online heat exchanger is  inoperable  or  experiencing  maintenance. The size of the heat exchanger is designed to handle the full cooling load of the electronic equipment plus a 20 percent margin. From the heat exchanger, the water then goes through various monitoring devices, which check the water temperature and flow. These two things depend   upon   the   requirements   of   the   electronic equipment being coded. After the water moves through the equipment, it is drawn back to the pump on the suction side. In this way, a continuous flow of coolant is maintained in a closed-loop system. An expansion tank is provided in the distilled water system  to  compensate  for  changes  in  the  coolant volume, and to provide a source of makeup water in the event of a secondary system leak. When the expansion tank is located above the highest point in the secondary system and vented to the atmosphere, it is called a gravity tank. If it is below the highest point in the secondary  cooling  system,  then  it  is  called  a compression tank, because it requires an air charge on the tank for proper operation. ‘The  demineralize  is  designed  to  remove  dissolved metals,  carbon  dioxide,  and  oxygen.  In  addition,  a submicron falter (submicron meaning less than one millionth of a meter) is installed at the output of the demineralizer to prevent the carry-over of chemicals into the system and to remove existing solids. TYPE II COOLING SYSTEM ‘he secondary system of the Type II cooling system (fig. 1-7) is similar to the Type I secondary coolant system and uses many of the same components. The major difference is in the operation of the CW/DW heat exchanger. The secondary coolant is in series with the SW/DW  heat  exchanger  and  automatically  supplements the  cooling  operation  when  the  SW/DW  heat  exchanger is unable to lower the temperature of the distilled water to  the  normal  operating  temperature.  The  CW/DW temperature  regulating  valve  allows  more  chilled  water to flow in the primary cooling system to the CW/DW heat exchanger. This causes the temperature in the secondary system to go down. Normally, this action only occurs in the event of high seawater temperatures encountered   in   tropic   waters.   The   CW/DW   heat exchanger is also used in the event of an SW/DW heat exchanger malfunction. TYPE III COOLING SYSTEM The Type III secondary cooling system (fig. 1-8) also operates in a similar manner to the Type I system. The major difference is in the way that the temperature of  the  secondary  coolant  is  regulated.  A  three-way temperature regulating valve is not used. A two-way temperature regulating valve is used in the primary cooling   loop   to   regulate   the   temperature   of   the secondary  loop. The duplicate CW/DW heat exchanger is installed parallel to the first heat exchanger and is used as a standby heat exchanger. In the event that a malfunction occurs requiring the first heat exchanger to be removed from service, the standby exchanger can be put into service by manipulating the isolation valves associated with the two heat exchangers. COOLING SYSTEM COMPONENTS You should be able to identify and describe the operation of the individual components of a typical cooling system. This will help you to perform the required system maintenance and trouble isolation. You should never neglect the cooling system, because it will quickly deteriorate to a point where only extreme and costly maintenance will restore the system to its proper performance. HEAT  EXCHANGERS In the liquid coolant heat exchangers, heat that has been  absorbed  by  distilled  water  flowing  through  the 1-7