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 MIL-HDBK-1003/13A
If there is no danger of freezing and the collector loop consists of all
copper flow passages, then ordinary water would be the choice for collector
fluid. If freezing conditions are encountered, there are a number of designs
that should be considered before it is decided to use a heat transfer oil or
antifreeze mixture. These freeze protection schemes are summarized here
using Figure 2-4 as the basic open loop type collector circuit.
a.
Drain Down or Drain Back Method - The water in the collector is
drained out of the system, or into a tank near the collector, or
into the main storage tank when temperatures in the collector
approach freezing. This scheme requires automatic valves to dump
the water and purge air from the system. Often a larger pump will
be required to overcome the system head and re-prime the
collectors. A way to avoid automatic (solenoid) valves is to drain
the collectors whenever the pump shuts off. This still requires a
larger pump. Three-way valves exist that can use city water
pressure to reprime the system; otherwise pumps must be used. Some
drainback systems only drain the water to a small tank near the
collectors thus requiring only a small additional pump. Heat
exchangers may be required to separate potable water from
nonpotable water.
b.
Heat Tapes - Electric resistance heat tapes are thermostatically
activated to heat the water. This scheme requires extra energy and
is not completely reliable. Insertion of heat tapes into
preconstructed collectors may be difficult.
c.
Recirculation Method - In this method the control system of Figure
2-4 merely turns on the pump if freezing approaches. In this way,
warm water from storage circulates through the collectors until the
freezing condition is over. The only extra component needed is a
freeze sensor on the collector which is a minimum cost item ($5-
$10). However, by circulating heated water, the capacity of
storage decreases and less is available the following day. This
method is probably the most reliable of the three since it does not
depend on additional electrical valves or heating tape, provided
that back up power is available to operate pumps in the event of
power failure.
If the preceding methods are not acceptable or if the choice of water is not
acceptable due to concern about corrosion, then a heat transfer fluid must be
used. The heat transfer fluid must be used with a heat exchanger in a
"closed--loop" configuration as shown in Figure 2-4.
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