Desiccator Theory & Characteristics

 
 

WHY USE DESICCANT?
Protecting the contents of a package, container or any enclosure from moisture has been a problem ever since people began to collect, store and transport their possessions.  As we have all seen, moisture, whether trapped inside a sealed enclosure or entering it over time, can result in expensive losses due to mold, corrosion, short circuits, etc.  Using Desiccators, which contain desiccant, is the most cost-effective way to remove moisture, both initially and over extended periods of time.

WHAT IS DESICCANT?
Desiccant is an adsorbent (not an absorbent), that is, it attracts water vapor molecules, which adhere to its surface.  Since most desiccants can hold up to a maximum 35% of their weight in moisture before losing their effectiveness, they must trap the moisture while it is still in a water vapor form, before it can condense (become liquid) and cause damage.

Some of the more common desiccants are:
(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.

(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).

(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.

(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.

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 and 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 contents of the enclosure.  Electronic components are well known for the amount of moisture they will hold.  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. 

Trapped Moisture Chart

(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).   

Moisture Leakage Chart

Moisture Leakage Chart

(C) VALVE BREATHING a breather valve's primary function is to protect an enclosure from excessive pressure or vacuum differentials.  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.  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.

WHAT ARE DESICCATORS?
Desiccators (also known as dehydrators or dehumidifiers) are self-contained moisture control systems that use 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 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 chemical desiccant.  They are generally much less expensive, but require periodic re-energizing 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 that are possible 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 three principal chemical desiccants: silica gel, molecular sieve or bagged clay (used on 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 satisfy 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 and packaged in a dry box, and the packaging used has an extremely low water vapor transmission rate.

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.