Desiccators Theory & Characteristics

What is Desiccant?

Desiccant is an adsorbent (not an absorbent), that is, it attracts water molecules, which adhere to its surface. Unlike absorbents, which change their volume as they take on the water, desiccants do not change their volume. Commonly available desiccants can hold up to a maximum 35% of their weight in moisture before losing their effectiveness. They work by trapping the moisture while it is still in a water vapor form, before it can condense (become liquid) and cause damage. While desiccants can also absorb water in liquid form, usually this is too late in the corrosion cycle to prevent damage.

Why use Desiccant?

Using desiccant to protect the contents of a package, container or enclosure helps reduce or eliminate the damage caused by moisture, including mold, corrosion and short circuits. Desiccant is usually the most cost-effective way to remove moisture, both initially and over extended periods of time.

Types of Desiccant?

(A) Silica Gel, which can be supplied in bags, bulk or desiccators. Used in moderate to high humidity and/or moderate temperature (less than 125°F / 52°C) environments. Maximum capacity is 35% by weight when exposed at 25C and 80% RH or higher.

(B) Molecular Sieve, crystalline metal aluminosilicate, is also available in bags, bulk or desiccators. Used to create very dry environments or in high temperature (up to 400°F / 204°C) applications. Available with different microscopic pore sizes, the most common being 4Å (4 angstroms). Maximum capacity is 21% by weight over a wide temperature range.

(C) Activated Alumina, available in bulk for high moisture adsorption in high humidity environments. It is an inefficient adsorber in 40% or less relative humidity (RH) environments. Normally used in high pressure or compressed gas dryers. Like silica gel, the capacity is very dependent on conditions of exposure. Most people use a capacity of about 7.5% for compressed gas dryers.

(D) Montmorillonite Clay, commonly known as bentonite, normally supplied in packets, canisters and bags holding anywhere from a fraction of an ounce up to 5.8 lbs. (80 units) of desiccant. Used in moderate to high humidity and/or moderate temperature environments. It is inexpensive and used for general packaging requirements. It is not suitable for bulk desiccator applications because of its tendency to pack and restrict air flow. Similar to silica gel, the capacity of this material is dependent on conditions of exposure with a maximum of 26% when exposed at 25C and 80% RH.

AGM can also supply silica gel, clay or molecular sieve, in either bagged or bulk form.

Where does moisture come from?

Understanding where moisture comes from an how it enters an enclosure will help solve the problem of how to eliminate it. Here is a list of major sources of moisture:

(A) TRAPPED MOISTURE is the amount of moisture trapped in an enclosure when it is sealed. There are two sources: water vapor in the air inside the enclosure when it is sealed and water on and inside the solid material contents of the enclosure commonly referred to as hygroscopic moisture. Electronic components are well known for the amount of moisture they will hold in solid solution (hygroscopically). To calculate the amount of moisture in the air, multiply the volume of the enclosure in cubic ft. by the amount of moisture that a cubic ft. of trapped air will hold at a given temperature and relative humidity.

Chart 1 displays the amount of moisture that a cubic ft. of air will hold at sea level at various temperatures and relative humidities. Note that the absolute amount of moisture in saturated air increases exponentially with increasing temperature.

Desiccator Theory & Characteristics Chart 1

(B) MOISTURE LEAKAGE is the amount of moisture that may enter non-sealed, leaking or porous enclosures as a result of temperature or altitude changes and is directly proportional to the enclosure volume and the differences in temperature or altitude extremes (Chart 2 and Chart 3).

Desiccator Theory & Characteristics Chart 2

Desiccator Theory & Characteristics Chart 3

(C) VALVE BREATHING a breather valve’s primary function is to protect an enclosure from excessive pressure or vacuum differentials. Each time the valve opens inward (or “gulps”) it can take in as much as 0.015 g of water per cubic ft. under worldwide storage conditions. During airlift, it can take in as much as 0.09 g of water per cubic ft. during descent. The higher the valve pressure and vacuum reseal settings, the less often the valve will breathe. For this to be an advantage, however, the enclosure must be structurally strong enough to withstand the pressure and vacuum differentials imposed by the valve.

(D) PERMEATION is the movement of individual molecules through a seemingly solid material. Common permeable materials are most seals and seal materials, plastics and other intentionally porous materials. Permeation constitutes a small amount of moisture but over time it can, and will, build up to levels high enough to cause a problem on the inside of the enclosure.

What are desiccators?

Desiccators (also known as dehydrators or dehumidifiers) are packages to hold desiccant which function as self-contained moisture control systems. They hold the desiccant to protect the contents of enclosures from moisture damage and are used in a wide variety of aerospace and electronic applications.

There are two basic types of desiccators: dynamic and static.

Dynamic desiccators allow a moist gas stream to flow through the desiccant bed to dry the gas. More complicated types of dynamic desiccators are self-regenerating, usually large and expensive, and require an external power source.

Static desiccators, on the other hand, can vary in size from tiny packets or capsules to 55-gallon drum-sized canisters and larger, filled with desiccant. They are generally much less expensive, but require periodic re-generation or replacement.

Over the last 40+ years, AGM has designed and produced hundreds of different static desiccators. The ones shown on this website demonstrate the wide variety of shapes and sizes and manufacturing technologies that are used to fit the required application. They are available in both standard and custom configurations, made from various materials and can be supplied with any of the three principal commonly commercially available desiccants: silica gel, molecular sieve or bagged clay (used with TA476 and TA486 desiccant holders and baskets). Optional features, such as breather valves, humidity indicators, non-dusting filters, and RFI/EMI shielding can also be incorporated.

Design Criteria

In order to enable AGM’s Engineers to more quickly recommend either a standard or existing custom desiccator, or to design a new configuration, we will need the following information:

  1. The volume of interior space to be protected.
  2. The amount and shape of space available for the desiccator.
  3. The level of moisture protection required.
  4. The ambient moisture and temperature levels.
  5. The length of time desired before recharging.
  6. The anticipated leakage rate of the enclosure.
  7. Whether a Breather Valve and/or Humidity Indicator are required.
  8. Dusting protection required, if any.
  9. RFI/EMI protection required, if any.
  10. Any unusual environmental conditions.

Because some of the criteria above are performance requirements and others are time and space limitations, it may be necessary to make compromises that meet realistic objectives. The earlier AGM’s Engineers can be involved in the design process, the better AGM will be able to meet the requirements.

Quality Control Considerations

Because both silica gel and molecular sieve react rapidly to moisture, they must be handled under controlled minimum humidity conditions to retain maximum adsorption capacity. At AGM, all desiccators are filled in a dry box, and packaged in materials with extremely low water vapor transmission rates.

Customers are cautioned to leave the desiccators in their original “as received” packaging (with overpack included) until ready for use to assure maximum adsorbent capacity. In addition, they should establish procedures that will minimize the time between removal of the product from its package and its installation in the sealed enclosure. It is also important that the work be done under optimal ambient conditions. Both silica gel and molecular sieve can lose up to 5% of their adsorbent capacity in one hour when exposed to 60% RH at an ambient temperature of 86°F (30°C). Reducing ambient conditions to 10% RH at 77°F (25°C) will increase this time period for molecular sieve to 12 hours and for silica gel to 35 hours.

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