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
