HEAT EXCHANGERS

The type III secondary liquid-cooling system also operates in a similar manner to the type I secondary liquid-cooling 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, but a two-way temperature-regu- lating 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. If a malfunction occurs that requires 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. LIQUID-COOLING SYSTEM COMPONENTS The main components of liquid-cooling systems are heat exchangers, expansion tanks, seawater strain- ers, temperature-regulating valves, flow regulators, flow-monitoring devices, circulating pumps, demin- eralizes, oxygen analyzers, and coolant-alarm switch- boards. In some systems, there are specialized components to monitor cooling water to the electronic equipment. You should be able to identify and describe the operation of the individual components of a typical liquid-cooling system to help you perform the re- quired 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 it to its proper performance. HEAT EXCHANGERS In liquid-cooling heat exchangers, the heat that has been absorbed by distilled water flowing through the electronic components is transferred to the pri- mary cooling system, which contains either seawater or chilled water from an air-conditioning plant. In both cases, the heat exchangers are of the shell and tube type in which the secondary coolant (DW) flows through the shell, while the primary coolant (SW or CW) flows through the tubes. A single-pass counterflow heat exchanger is more efficient than the double-pass heat exchanger, because there is a more-uniform gradient of temperature dif- ference between the two fluids. The primary coolant (SW/CW) flows through the tubes in the opposite direction to the flow of the secondary coolant (DW). Heat transfer occurs when the seawater flows through the tubes, extracting heat from the distilled water flowing through the shell side of the heat exchanger. The distilled water is then directed by baffles to flow back and forth across the tubes as it progresses along the inside of the shell from inlet to outlet. In figure 2-4, the preferred method of double-tube sheet con- struction is shown. Single-tube sheet construction is shown in figure 2-5. 2-6