The expansion tank serves a threefold purpose in a
liquid cooling system.
First, it maintains a positive
pressure required on the circulating pump inlet for
proper operation of the circulating pump. Second, the
expansion tank compensates for changes in the coolant
volume because of temperature changes. Third, it vents
air from the system and provides a source of makeup
coolant to compensate for minor losses because of
leakage or losses that occur during the replacement of
radar equipment served by the system. The tank maybe
either a gravity tank or a pressurized tank.
When an expansion tank is used as a gravity tank
(fig. 1-11), it is located above the highest point in the
distilled water system.
This provides sufficient pressure to the suction side
of the circulating pump. This also ensures a flow of
water from the tank into the system when makeup water
is required. The tank is provided with a sight glass to
check the level of water in the tank. The sight glass
should normally show the tank to be two-thirds to
Figure 1-11.-Gravity expansion tank.
four-fifths full. The glass should be redlined at
four-fifths of the tank capacity. A vent pipe is located
on the top of the tank to prevent air pressure from
building up in the system. A valve and funnel
connection with cap is located on the top of the tank to
provide a means for filling the system with distilled
water. A low-level alarm switch is usually set at 20
percent of tank capacity. When the fluid level in the tank
lowers to 20 percent of the full level, visual and audible
alarms actuate at the alarm switchboard to warn
personnel when the system is low on distilled water. If
the tank should run out of water, then air is drawn into
the system, which results in increased corrective
maintenance on the system to remove the trapped air or
possible pump damage and/or failure of high power
The pressurized expansion tank (fig. 1-12) is
normally located near the circulating pump suction in
the return main of the secondary liquid cooling system.
The pressurized tank is airtight and is charged with
compressed air to an appropriate pressure from the
ships low-pressure air system. In some systems, a hose
Figure 1-12.Pre-ssure expansion tank.