UFC 3-440-01
14 June 2002
Figure 3-1. Closed-Loop Antifreeze System
3.2.1.2.1
Collector Loop Check Valve. The check valve shown in the collector
loop is required to prevent "reverse thermosiphoning". This phenomenon can occur on
cold nights when the collector loop is not active. Warm solution from the lower part of
the loop (usually located indoors) becomes buoyant and rises toward the top of the loop
where it becomes colder. This cold, denser solution then drops to the bottom of the
loop, often passing through the heat exchanger and removing energy from the storage
loop. Extreme cases have resulted in frozen heat exchangers. Care should be taken to
locate the check valves so that the fluid in the collector loop can be drained if
necessary.
3.2.1.2.2
Piping and Component Protection. Fluid problems and associated
corrosion and maintenance issues are a common cause of closed-loop system failure.
However, results from the testing of degraded, uninhibited propylene glycol indicate that
with proper design, a closed-loop system may run without fluid maintenance for up to
20 years. Designers should ensure that non-ferrous piping and components are used
whenever possible, that no air is allowed to be drawn into or contained within the
system, and that the expansion tank and pressure relief valves are correctly sized to
prevent loss of solution and opening of the collector loop in the event of high pressure
stagnation.
3.2.1.2.3
Collector Loop Air Vent. The manual air vent shown at the top of the
collector loop allows air that has been released from solution to be purged. Propylene
glycol has a strong affinity for air, and dissolved oxygen in solution can greatly impair
system performance by contributing to corrosion.
3.2.1.2.4
Mixing Valves. Mixing valves are typically used to provide a high
3-2
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