Figure 1-10.—Two-pass SW/DW beat exchanger with single-tube sheets

Figure 1-9.-Single-pass SW/DW heat exchanger with double-tube sheets. electronic components is transferred to the primary cooling system, which contains either seawater or chilled water from an air-conditioning plant. In both cases (figs. 1-9 and 1-10), the heat exchangers are of the shell- and tube-type in which the secondary coolant (distilled water) flows through the shell, while the primary coolant (seawater or chilled water) flows through the tubes. A single-pass counterflow heat exchanger (fig. 1-9) is more efficient than the double-pass heat exchanger, because there is a more uniform gradient of temperature difference between the two fluids. In figure 1-9, 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 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 1-9, the preferred method of double-tube sheet construction is shown. Single-tube sheet construction is shown in figure 1-10. Double-tube sheets are used at both ends of a tube bundle. A void space between the sheets prevents contamination of the distilled water and permits the monitoring of water loss because of tube leakage. You should be on the lookout to detect leakage at the “telltale drains,” which indicates a failure of a tube joint. The type of water leaking out indicates whether the failure is in the primary or the secondary system. The telltale drains should never be plugged or capped off. A leak in one of the tubes shows up as a loss of water in the secondary side of the liquid coolant system, because it operates at a higher pressure than the primary side. his is intentional. This ensures that the distilled water is not contaminated with seawater when a leak develops in a heat exchanger. Figure 1-10.—Two-pass SW/DW beat exchanger with single-tube sheets. 1-8