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MIL-HDBK-1003/13A
There are several preventive measures which will eliminate or at least
minimize galvanic and pitting corrosion in collector systems which use an
aqueous collector fluid.  The best method to prevent galvanic corrosion is to
avoid using dissimilar metals.  Where this is not possible or practical, the
corrosion can be greatly reduced by using nonmetallic connections between the
dissimilar metals, thus isolating them.  Galvanic protection in the form of a
sacrificial anode is another method of protecting the parent metals.  Also,
use of similar metals reduces the problems of fatigue failure caused by
thermal expansion.  Pitting corrosion is essentially eliminated if copper
absorber plates are used.  Corrosion inhibitors can minimize pitting
corrosion in aluminum absorbers.
The types of heat transfer fluids available may be divided into two
categories, nonaqueous and aqueous.  Silicones and hydrocarbon oils make up
the nonaqueous group, while the aqueous heat transfer fluids include
untreated potable (tap) water, inhibited-distilled water, and inhibited
glycol/water mixtures.  The potable tap water and inhibited distilled water
do not, of course, offer freeze protection.  Table 2-5 shows characteristics
of some of the most common heat transfer fluids.
2.1.7.1 Silicone fluids.  Silicone heat transfer fluids have many favorable
properties which make them prime candidates for collector fluids.  They do
not freeze, boil, or degrade.  They do not corrode common metals, including
aluminum.  They have excellent stability in solar systems stagnating under
400 deg. F.  Silicone fluids are also virtually nontoxic and have high flash
and fire points.  Current evidence indicates that silicone fluids should last
the life of a closed-loop collector system with stagnation temperatures under
350 deg. - 400 deg. F.  The flash point is fairly high, 450 deg. F, but since
the HUD standards state that heat transfer fluids must not be used in systems
whose maximum stagnation temperature is less than 100 deg. F lower than the
fluid's flash point, this limits most silicone oils to systems with a maximum
temperature of 350 deg. F or less.  Also silicones do not form sludge or
scale, so system performance does not decrease with time.
The main drawback of silicone fluids is their cost.  Currently silicone fluid
costs about $25-$35 per gallon.  Thus the cost of the 20 to 30 gallons of
collector fluid required for a typical 500 ft2 collector system becomes
considerable.  As with hydrocarbon oils, the lower heat capacity and higher
viscosity of silicone fluid requires larger diameter and more expensive
piping.  Due to the higher viscosity, larger pumps will be required and
subsequent higher pumping costs.  One other problem with silicone fluids is
the seepage of fluid at pipe joints.  This problem can be prevented by proper
piping installation and by pressurizing the system with air to test for
leaks.  There have also been reports of seepage past the mechanical seals of
circulating pumps.  The use of magnetic drive or canned wet rotor pumps when
available in the proper size is a method of avoiding mechanical seal leakage.
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