Desiccators
Desiccators, also known as dehydrators or dehumidifiers, protect container and enclosure contents from moisture damage
what is desiccant?
Why use desiccant?
TYPES OF DESICCANT
(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?
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.
(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?
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:
- The volume of interior space to be protected.
- The amount and shape of space available for the desiccator.
- The level of moisture protection required.
- The ambient moisture and temperature levels.
- The length of time desired before recharging.
- The anticipated leakage rate of the enclosure.
- Whether a Breather Valve and/or Humidity Indicator are required.
- Dusting protection required, if any.
- RFI/EMI protection required, if any.
- 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
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.
GLOSSARY TERMS
ABSOLUTE (OR SPECIFIC) HUMIDITY
The ratio of the mass of water vapor in a sample of air to the mass of air of the sample.
ABSOLUTE PRESSURE (PSIA)
The pressure above the absolute zero value of pressure that is theoretically obtained in empty space. (Example: The absolute pressure at sea level is 14.7 psia.)
ABSORPTION
The penetration of a substance into the inner structure of a solid or liquid where it is held in place by surface tension effects and is readily displaced.
ADSORPTION
The surface retention of solid, liquid or gas molecules, atoms or ions by a solid or liquid. The adsorbed material is held in place by Van der Waals and other weak inter-molecular forces and is not readily displaced.
ALTITUDE
Height, measured as distance along the extended earth’s radius above a given datum, such as average sea level.
ANGSTROM (Å)
A unit of length, 10-8 cm (10-10 meter). Used to define pore size in molecular sieves.
ASCENT
Motion of an aircraft in which the path is inclined upward with respect to the horizontal.
ATMOSPHERES (ATM)
A unit of pressure equal to the pressure exerted by a column of air one inch square and extending from sea level to infinity. 1 atmosphere = 14.7 psi.
BREATHING
Movement of gas, vapors or air in and out of an enclosure as a result of expansions and contractions induced by temperature or pressure change (such as altitude change).
CONDENSATION
The process by which vapor becomes a liquid and energy is released.
DESCENT
Motion of an aircraft in which the path is inclined downward with respect to the horizontal.
DEW POINT
The temperature at which air becomes saturated when cooled without addition of moisture or change of pressure; any further cooling causes condensation.
DIURNAL
Pertaining to meteorological actions which are completed within 24 hours and which recur every 24 hours.
DRY BULB THERMOMETER
An ordinary thermometer, especially one with an un-moistened bulb; not dependent upon atmospheric humidity.
DUNNAGE
Cushioning material in a container to protect shipped goods from damage.
EQUILIBRIUM
Condition in which no change occurs in the state of a system as long as its surroundings are unaltered.
GAS (VAPOR)
A phase of matter in which the substance expands readily to fill any containing vessel; characterized by relatively low density.
GRAIN (GR)
A unit of mass equal to 1/7,000 of a pound, or to 6.47989 x 10-5 kilogram (kg).
GRAM (G OR GM)
The unit of mass in the centimeter-gram-second system of units, equal to 0.001 kg.
H2O
Water, in any phase, solid (ice), liquid (water), or gas (water vapor).
HYDROPHILIC
Having an affinity for, attracting, adsorbing or absorbing water.
HYDROPHOBIC
Lacking an affinity for, repelling, or failing to adsorb or absorb water.
HUMIDITY
Atmospheric water vapor content.
HYGROSCOPIC
Readily absorbs or adsorbs moisture and retains it.
LEAKAGE
Undesired and gradual escape or entry of a quantity, such as air or water vapor from an enclosure.
MICRON
A unit of measure of length or distance equal to 1 x 10-6 meters, or 0.00003937 inches.
MOISTURE
Water that is dispersed through a gas or liquid in the form of water vapor or small droplets, dispersed through a solid or condensed on the surface of a solid.
PRESSURE
A type of stress which is exerted uniformly in all directions; its measure is the force exerted per unit area.
PSYCHROMETRIC CHART
A graph, each point of which represents a specific condition of a gas-vapor system (such as air and water vapor) with regard to temperature (horizontal scale) and absolute humidity (vertical scale); other characteristics of the system, such as relative humidity, wet-bulb or dew point temperature, specific volume and enthalpy, are indicated by lines on the chart.
PURGING
The act or process of cleaning and purifying by replacing the current contents with a cleaner or purer medium.
REACTIVATION
The restoring (through the application of heat or other processes) of the adsorption capacity of a desiccant.
RELATIVE HUMIDITY (RH)
The ratio of the actual water vapor pressure of the air-water vapor mixture to the saturation water vapor pressure, stated as a percent (dimensionless) and varies based on temperature.
SATURATION
The point where no more of a substance can be absorbed, adsorbed, dissolved or retained. When referred to desiccant, saturation occurs when the desiccant material will no longer take on moisture for the condition it is exposed to. For water vapor in air, when saturation occurs, liquid water precipitates (or rains) out of the air-water vapor mixture.
TEMPERATURE
A measure of the internal energy of a substance, usually referred to as the degree of hotness or coldness as measured on a specific scale.
UNIT OF DESICCANT
Desiccants are usually sold by performance (rate, capacity), not by weight. A standard performance definition is the desiccant unit. One unit of desiccant is defined as having an adsorption capacity of 6 grams of water at 77°F (25°C) and 40% RH. For instance, a unit of silica gel weighs 26 grams, a unit of molecular sieve weighs 32 grams and a unit of clay weighs 33 grams.
VACUUM
A space in which the pressure is far below normal atmospheric pressure.
VAPOR PRESSURE
The partial pressure of water vapor in the atmosphere.
WATER VAPOR
Water in the form of a gas, especially when below the boiling point and diffused.
WATER VAPOR TRANSMISSION RATE (WVTR)
The rate that water permeates through the wall or barrier of an enclosure, usually given in g/(m2 x day)or g/(in2 x day).
WET-BULB THERMOMETER
A thermometer having the bulb covered with a cloth, usually muslin or cambric, saturated with water in which the temperature reading is affected by the amount of humidity in the air.
desiccators F.A.Q.
WHAT IS A BREATHER DESICCATOR?
WHY ARE THEY CALLED BREATHING DESICCATORS?
HOW MUCH desiccant can the desiccator cartridge hold?
does the desiccant cartridge affect the air flow through the valve?
WHAT DESICCANT MATERIALS ARE AVAILABLE IN BREATHING DESICCATORS?
what is the difference between a desiccator and a desiccant basket?
what sizes, shapes and materials are desiccators available in?
what size of desiccant baskets are available?
can i put a breather valve in the cover of the desiccant baskets?
what size mounting holes are required, and, is mounting hardware provided?
AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716
520.881.2130
agmwebsales@agmcontainer.com
