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Desiccants - A Powerful Weapon Against Moisture and Humidity
Moisture presents a serious threat to product integrity in a variety of industries,
including electrical engineering and electronics. Sensitive products like circuit
breakers can begin to corrode at a mere 30% relative humidity (RH). Many products
require a constant RH of 10%-15% or less for product stability and extended
life cycles.
Fungus, mildew, odors, discoloration and product deterioration are just a
few of the damaging consequences of moisture contamination. However, not all
products will show visible signs of moisture damage. Instead, they will simply
not work.
Desiccants protect products through a process called "ad"sorbtion. Unlike "ab"sorption
where one substance is chemically integrated into another, during adsorption,
moisture is held inside a desiccant by physical bonds; no chemical change takes
place. Along with protecting products from external elements, a certain amount
of moisture inherent in all hygroscopic (moisture absorbing) items can also
be trapped within chambers. desiccants adsorb this moisture quickly and thoroughly,
preventing condensation during transport and storage.
Desiccants are typically available in one of three materials:
- Silica gel is a non-toxic, non-corrosive form of silicon dioxide synthesized
from sodium silicate and sulfuric acid and processed into granular or beaded
form. Its active interconnected pores from a vast surface area that attracts
and hold water through adsorption and capillary condensation, allowing it
to adsorb up to 38% of its weight in water vapor. While silica gel works most
effectively at room temperature and high humidity levels, its potency is somewhat
diminished at humidity levels below 30%.
- Derived from sodium, potassium, or calcium crystalline hydrated aluminosilicates,
molecular sieve, another synthetic desiccant, contains a uniform network of
crystalline pores and empty adsorption cavities. Because it is synthetic,
its pore size can be controlled in the manufacturing process, allowing for
a large range of adsorptive capabilities in the finished product. As one of
the more aggressive materials, molecular sieve can reduce ambient humidity
to nearly zero percent (below normal detectable levels), but must be handled
in a controlled environment before insertion to prevent loss of capacity.
- Activated Clay (Montmorillonite and Bentonite Clay), a natural occurring,
non-hazardous sulfur-free substance, is the most widely used desiccant material.
The lease expensive of available desiccant materials, the layered structure
of activated clay attracts and adsorbs moisture onto its surface and between
its layers. Activated clay has essentially the same capacity as silica gel
at humidity levels between 25% and 30%, but outperforms it at levels of 20%
humidity and below.
Siemens, a world leader in electrical engineering and electronics, uses desiccants
to help ensure that its circuit breakers provide the highest degree of safety
and reliability. The high voltage circuit breakers produced by the Siemens Berlin
switchgear plant handle voltages up to 800 kV and clear current faults up to
120 kA. The active part of the circuit breaker, the interrupting chamber, must
be sealed from the ambient atmosphere. Furthermore, it is essential that the
moisture and humidity inside the interrupting chamber is minimized.
SF6 molecules decompose when in contact with high thermal
stresses and electrical discharges, which occur in arcs and sparks. The resulting
secondary products are either gaseous or fluid. The gaseous products are uncritical,
whereas the fluids may further decompose into hydrogen fluoride and sulfur dioxide,
both highly corrosive substances that could deteriorate the circuit breaker's
inner parts.
To prevent any formation of corrosive substances, Siemens Berlin production
technicians insert a predetermined quantity of specially developed desiccant
bags from Sud-Chemie Performance Packaging into the circuit breaker chamber
before it is sealed. Air is then removed by a vacuum pump and replaced by SF6
gas. Flanges with O-ring gaskets guarantee extremely low leakage rates.
The molecular sieve desiccant bags adsorb the decomposition products before
they can react further, avoiding the formation of any corrosion that would result
in increased maintenance cost and reduced efficiency. The environmentally friendly
desiccant bags are made from tear-proof GDT material containing molecular sieve
with a pore structure of 1 nanometer.
In weighing the various choices of desiccant materials, it is important to
consider the ambient environment within the product in relation to the desiccant's
individual capabilities. The key parameters are initial humidity, amount of
moisture in the product, volume of both product and container, Moisture Vapor
Transmission Rate (MVTR), length of storage time, activity curve of the desiccant,
and the maximum level of relative humidity permitted within the enclosed product.
Humidity levels and temperatures can fluctuate drastically during shipping,
and, of course, different materials respond differently to these variables.
Using desiccants will improve product stability and extend product life cycles,
but the intangible benefits of using desiccants are even more important. Desiccant
protection allows companies to consistently deliver products of uncompromising
quality and integrity. With the tremendous costs associated with research and
development, manufacturing, packaging and distribution.
Submitted by
Robert Crossno
National Sales Manager
Sud-Chemie Performance Packaging
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