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Desiccant Dehumidification

Dehumidification is the removal of moisture from air. Thought of in another way, dehumidification is the drying of air. The degree of dehumidification varies with the application requirements and greatly influences the type of equipment utilized. Most engineers are familiar with mechanical dehumidification. A process of cooling an air-stream to below it's dew point temperature causing moisture to condense from the air. This process frequently requires re-heating of the air to avoid supplying saturated air to a space. Desiccant dehumidification is becoming more familiar. Many engineers are just becoming knowledgeable concerning the use of desiccants for dehumidification. Desiccant dehumidifiers utilize a "sorption" material to attract and hold moisture from air. Once the sorption material, called a desiccant, is "saturated" with moisture, it can be reactivated or regenerated. Reactivation is usually accomplished by thermal means and restores the desiccant's dehumidification capacity. The mass exchange of the moisture from and to an air-stream occurs in the vapor phase.

Desiccant dehumidifiers are required for use below the frost point where mechanical refrigeration type dehumidifiers experience freezing on the coil surface or when dehumidification is required, but cooling is not, such as for dry goods storage or preservation requirements. Desiccant dehumidification is also utilized to provide for humidity control independent of temperature control in occupied spaces.

Desiccant dehumidifiers are available with either dry or liquid desiccants. Dry desiccants are available with either adsorption or absorption desiccants. AMC ELITE desiccant dehumidifiers utilize a dry adsorption type desiccant.

Adsorption type desiccants do not change state during the dehumidification process. The action of adsorption can be selective. An advanced form of silica gel that has a high capacity to remove water vapor from an air-stream is becoming the standard desiccant throughout industry. In desiccant dehumidifiers there is no liquid condensate to be concerned with. The advanced silica gel is also readily reactivated and is chemically inert, non-corrosive and non-toxic. Other dry type desiccants such as molecular sieves or activated alumina are also available. Absorbent desiccants such as lithium chloride experience deliquescent problems when exposed to high relative humidity. Absorbent desiccants become liquid if not reactivated in a timely manner. The velocity of the air-stream passing through a lithium chloride wheel can cause the liquid solution to migrate within the flute or cause it to become air borne. Absorption type desiccants can continue to attract moisture and over-absorb causing loss of desiccant or softening of the rotor structure.

Advanced Desiccant Rotor



The desiccant dehumidifiers used by AMC ELITE are a rotary type of dehumidifier using a humidity adsorbing rotor. This rotor is highly advanced and is the result of over thirty years of research and development and is completely unique. The structure is made with a ceramic matrix meeting the International Agency for Research on Cancer (IARC) standards for non-respirable fibers. The ceramic matrix is combined with synthesized silica gel to form the most advanced dehumidifier rotor. The ceramic matrix gives structural support for the thin layer of advanced silica gel.

The synthesized advanced silica gel is self-bonding to the ceramic matrix and to itself through the matrix, filling in all the voids in the matrix and totally encapsulating the ceramic. Only the desiccant is in contact with the air-stream! This advanced form of silica gel has an extremely high "affinity" for water vapor. The affinity or "attractive" forces are a combination of several factors such as Van der Waals forces and other electrical, magnetic and/or physical factors that are not yet fully understood. This combination of attractive forces is known as adsorption. The advanced silica gel has an active surface with numerous macro-pores of uniform size and shape. The many internal micro-pores quickly adsorb humidity and are readily de-sorbed for convenient and continuous dehumidification. The synergistic form of the desiccant and ceramic matrix allows for distinct advantages over both granular adsorbents and impregnated desiccants. With the advanced silica gel rotor there are no granules and no dusting. Silica gel stops adsorbing moisture when the adsorbed moisture is in equilibrium with the moisture in the passing or surrounding air-stream. There are never any deliquescent problems associated with this system.

Moisture Removal

The moisture removal capacity of a desiccant dehumidifier is related to several parameters. One parameter is the amount of "surface" area of the desiccant that is exposed to the air-stream. Each rotor contains hundreds of square feet of sheet area per cubic foot of rotor volume. This surface area multiplied by the "internal" pore surface area results in thousands of square feet of area of desiccant available for adsorption. This tremendously high ratio of surface area to volume is one of the significant reasons for the excellent performance of the desiccant dehumidifier. The rotor speed is also optimized such that a maximum amount of desiccant is "rotated" through the process air-stream without causing over heating of the desiccant. By selecting the optimum speed of rotation, the adsorption cycle is carefully balanced against the de-sorption cycle. All desiccant dehumidification rotors are bearing supported for long life and dependable mechanical support. Rotors with a diameter larger than 20-inches are shaft mounted and bearing supported. The full weight of the rotor and the adsorbed moisture are supported by the shaft and bearing arrangement.

Adsorption

Adsorption occurs when the attractive forces of a desiccant capture water vapor. The vapor is drawn to and adheres to the surface of the desiccant. The vapor is then drawn into the macro-pores and then the micro-pores by capillary action. In the process the moisture converts adiabatically from vapor to a quasiliquid and is stored within the desiccant. (An adiabatic process occurs without the external addition or removal of heat.) It is important to distinguish between the vapor quasi-phase change as opposed to a desiccant phase change. Desiccants, such as lithium chloride, which undergo a phase change, are known as absorbents. Desiccants like silica gel do not undergo a phase change during the adsorptionprocess. Heat, equivalent to the "heat-of-vaporization" and the "heat-of-wetting" - taken together as the "heat of adsorption" is released to the air-stream as vapor is adsorbed by the desiccant. This is similar to a mechanical process such that when vapor is condensed to liquid, the heat of vaporization is given up in the process. This explains why latent cooling requires more "tons of refrigeration" than does sensible cooling. The greater the amount of humidity adsorbed the higher the temperature rise of the dehumidified air-stream. This increase in sensible temperature is due to the conversion of latent heat to sensible heat by the action of the desiccant. The more efficient the dehumidification process, the warmer the temperature of the dehumidified air-stream will be.

Moisture Gradient

The moisture content and the temperature, which is a measure of molecular activity of an air-stream, effects the ability of a desiccant to dehumidify a given air-stream. For an air-stream with a constant vapor pressure or dew point temperature (humidity content), the dehumidification process becomes more effective as the relative humidity is increased. This can be explained by thinking in terms of "moisture concentration" or "moisture gradient" as explained by Fick's law for mass diffusion (see ASHRAE Handbook of Fundamentals for a complete discussion of Fick's law). The greater the concentration gradient between the moisture in an air-stream and that within a reactivated desiccant, the more effective the adsorption process becomes. Hence, the effectiveness of a dehumidifier may be improved by increasing the relative humidity of the air-stream to be treated. This is usually accomplished by precooling the air stream. The cooling reduces the internal molecular energy of the air-vapor mixture. It becomes apparent, that for an air-stream of lower molecular energy, water vapor is less resistant to the desiccant's "affinity" forces as compared to an air-stream of higher molecular energy having an equal humidity ratio.

Reactivation

Reactivation or regeneration of a desiccant is the process of removing the adsorbed water vapor from the desiccant. In systems that are applied for ambient humidity control applications, reactivation is accomplished by thermal means. The de-sorption rate is directly related to the heat input rate of the reactivation cycle to the limit of the desiccant's residual moisture content temperature. For each desiccant type there exists a threshold temperature at which adsorption ceases and de-sorption occurs for a given air-stream with given dew point temperature. For silica gel, this threshold temperature is relatively low. For reactivation, the higher the dew point temperature of the air-steam, the higher the temperature needed to supply the de-sorption energy for complete de-sorption. The thermal energy is required to heat the desiccant which in turn transmits heat to the adsorbed moisture and to expand the reactivation air, which will "receive" the de-sorbed moisture. The expansion of the reactivation air associated with heating acts to reduce the relativity humidity of the air thus changing it's concentration gradient as described by Fick and allowing the air to hold more moisture.

Desiccant Dehumidifiers

Desiccant dehumidification is an important "air-conditioning" process by which many industrial processes or products are improved or even possible. And now, desiccant dehumidification is being utilized in commercial HVAC applications too.

Improvements in desiccant performance and manufacturing are currently encouraging. With these improvements, desiccant-based equipment holds the promise of successful incorporation into more and more commercial HVAC applications. Such equipment is intended to reduce the adverse affects of untreated humidity contained in ventilation air as well as generated sources of humidity from within commercial buildings. Dehumidified ventilation air allows the HVAC designer another option towards improving Indoor Air Quality (IAQ) which may have been adversely affected by moisture allowing the growth of mold and mildew, etc. Dehumidified air does increase the efficiency of sensible cooling equipment and increases the comfort and productivity of building occupants.

Absorption:

The penetration of one substance into the inner structure of another substance, as distinguished from adsorption in which one substance is attracted to and held on the surface of another substance. Some deliquescent salts such as lithium chloride absorb water vapor from air in the liquid phase. The absorbent may be consumed in the process as absorption may cause a phase change of the absorbent substance (water vapor to liquid).

Adiabatic:

A thermodynamic process during which energy (heat) is neither added to nor removed from the system. The process is plotted on a psychrometric chart showing constant enthalpy between state points. Natural evaporation/condensation is an adiabatic process in which a liquid can change phase to a vapor and back to a liquid without the total energy of the system being changed from outside the system.

Adsorption:

Adherence of molecules of a gas (vapor) to the surface of another substance called the adsorbent (desiccant). Finely divided or micro-porous materials presenting large areas of active surfaces are strong adsorbents. Activated carbon, activated alumina, silica gel, and molecular sieves are examples of adsorbents. The attractive (affinity) force of adsorption is relatively small, on the order of Van der Waal's forces. When vapor molecules of two or more different substances are present, those of one substance may be more readily adsorbed than those of the other substance(s). The priority of adsorption is directly related to the polarity of the molecule being adsorbed. Adsorbents do not change phase due to the adsorption process. Adsorption is thought to occur as a mono-molecular process, i.e. one molecular layer adheres to the surface of the adsorbent. Micelles (aggregates of molecules in chain form) of water vapor are sometimes present in air and may form linked chains to the surface of an adsorbent. These chains are held to the adsorbent surface by one molecule. The effect may add to the capacity of the adsorbent to hold adsorbed molecules.

Amorphous:

Non-crystalline, having no molecular lattice structure which is characteristic of the solid state. All liquids are amorphous. Amorphous materials that are apparently solid, such as glass, silica gel, etc. lack a definite crystal structure and a well defined melting point. These materials are considered high viscosity liquids. Amorphous materials may be strengthened, reinforced or tempered.

Angstrom:

A unit of length of one one-hundred-millionth (10-8) centimeter (10-10 meter or 10-7 mm). Used in stating dimensions of molecules, atoms and atomic particles. AMC ELITE uses an advanced silica gel that has a macro-pore opening of 20 to 30 angstroms. The water molecule has an effective diameter of 3.2 angstroms. Mold spores are measured in microns, many times larger than the pore opening of the rotor. Thus mold spores, or fragments cannot penetrate the rotor and those that gather on the face are removed in the react phase.

Anhydrous:

Descriptive of an inorganic compound that does not contain water either adsorbed on its surface or combined as the water of crystallization. Dry.

ARI:

The air-conditioning & Refrigeration Institute. ARI offices are located in Arlington, Virginia. The scope of the Desiccant Cooling and Dehumidification Equipment Section covers desiccant dehumidification.

ASHRAE:

The American Society of Heating, Refrigerating and Air-Conditioning Engineers. ASHRAE offices are located in Atlanta, Georgia. The scope of TC 3.5 Desiccant and Sorption Technology covers desiccant dehumidification.

Attrition (Desiccant):

The loss of desiccant material due to chemical, physical or thermal breakdown. The cause of the breakdown may be due to mechanical agitation, thermal cycling or chemical action such as ion exchange or deliquescent properties.

BCHP:

An acronym for Buildings Cooling Heating and Power. A concept of using thermally driven prime movers for HVAC equipment and power generation to allow a building to operate independent of the electric power grid. Recovered thermal energy is used by other equipment such as a desiccant dehumidifier, or for heating a building space or water loop. Together, the COP of the installed equipment is higher than that of the prime mover.

C-Trol:

A method of control that prevents the formation of condensate at the point of measurement (cold water pipe, bulkhead, etc.). Used in conjunction with a desiccant dehumidifier.

Ceramic:

Ceramics are made from molten masses of raw materials usually requiring the application of high temperature under controlled conditions in an oven or furnace in order to produce the final properties required for use as an engineering material. The basis of most ceramics are hydrous aluminosilicates. Other raw materials include anhydrous aluminosilicates containing potassium, sodium and calcium. Ceramic filaments or fibers are used to reinforce other materials and to form structural shapes.

Dehumidification:

The removal of water vapor from air. Dehumidification can be accomplished by cooling an air stream to below its dew point temperature causing the condensation of vapor or by desiccant adsorption/absorption resulting in removal of humidity from air in the vapor phase.

Deliquescent:

Tendency of a material to absorb atmospheric moisture (humidity) and become liquid. The term refers specifically to water-soluble chemical salts (e.g. lithium chloride) in the form of powders that are dissolved by the moisture absorbed from air.

Desiccant:

Generally, a hygroscopic substance such as silica gel, molecular sieve, activated alumina, etc. having the ability to adsorb moisture from air and be reactivated (regenerated) by thermal or other means without loss of physical properties. Desiccants are used to maintain a dry (dehumidified) air stream or environment. Some salts, such as lithium chloride and calcium chloride are utilized as desiccants, these are known as absorbers.

Desiccant Cooling:

The use of desiccant dehumidification systems for latent heat (humidity) removal in air conditioning systems in conjunction with sensible heat transfer or evaporative cooling equipment to perform cooling without the use of or with supplemental use of vapor compression equipment. Desiccant cooling equipment is used to treat outdoor air by reducing both sensible and latent (temperature and humidity) loads during cooling seasons. Some systems also include winter heating and humidification components for all season service.

Desiccant Dehumidification:

Passive or Static: The removal of moisture from air by exposing the humid air to a desiccant without the use of an air moving device. Once the desiccant achieves moisture equilibrium with the ambient air the dehumidification process stops and the desiccant must be replaced with active desiccant or removed and reactivated. E.g. Packaged or "bagged" desiccant placed in shipping containers for the purpose of preventing moisture condensation during transit or storage; laboratory desiccators.

Dynamic or Active: The removal of moisture from air by exposing the humid air to a desiccant with the use of an air moving device and a concurrent reactivation (regeneration) process such as by applying thermal heat or pressure swings. The reactivation process can be either intermittent such as for "dual tower" dehumidifiers or concurrent and continuous such as for "rotary" dehumidifiers.

Desiccant Ventilation:

The removal of moisture from air by exposing the humid air to a desiccant with the use of an air moving device and a concurrent reactivation (regeneration) process which is solely dependent upon the difference in the partial vapor pressure of moisture between the ventilation (outdoor) air and exhaust air streams. E.g. an enthalpy wheel. Enthalpy wheels are used to reduce the impact of ambient temperature and humidity of ventilation air on cooling and heating systems. Increased rates of ventilation air are recommended to maintain good Indoor Air Quality (reference ASHRAE Standard 62-1989 currently under continuous maintenance).

Desorption:

The removal (release) of adsorbed moisture from a desiccant. See Reactivation.

Dew Point:

The temperature at which an air sample is saturated with water vapor (100% relative humidity). Cooling of the sample below the dew point temperature would cause condensation of the water vapor to occur.

Dew-Trac:

A method of dehumidifier capacity control which assures a constant dew point value at the point of measurement (return air, supply air, room/space) in conjunction with a desiccant dehumidifier.

Dry Air:

Air containing no moisture (humidity). Dry air is a mixture of several gases, the composition of which varies with altitude and other conditions at the point of measurement. Dry air at sea level consists of nitrogen (78% by volume, 75.53% by weight), oxygen (20.95% by volume, 23.16% by weight), argon (0.93% by volume) and carbon dioxide, neon, helium, methane, krypton, nitrous oxide, hydrogen, xenon and ozone (balance). The concentration of ozone by volume at sea level is 0.000,001%. "Standard Air" used in scientific and HVAC calculations is dry air by definition with a density of 0.075 lb/ft3 at 68°F, 29.921" Hg (1.29 g/liter at 0 °C, 760 mm Hg). However, dry air does not exist in nature due to atmospheric moisture. The "air" we breathe is a mixture of air and moisture. The addition of moisture into air displaces the molecules of air and directly affects the density of air. In HVAC work, "dry air" is air referred to as air with a humidity value that is lower then the ambient air. The degree of dryness is determined by measuring and comparing relative humidity or dew point temperature of the "dry" air and the ambient air.

Dry Bulb Temperature: See Temperature.

DryHandler:

A desiccant dehumidifier module designed to be installed with other air handling equipment. The unit includes the desiccant section housed in a cabinet with service plenums, a reactivation heater or heat exchanger, reactivation blower and controls.

Encapsulate:

Complete and total covering or surrounding of one material by another. The encapsulated material can be neither seen nor felt at the surface of the finished product. The advanced silica gel desiccant in the rotor encapsulates the ceramic fiber substrate. The ceramic fiber gives the rotor its strength and shape.

Enthalpy:

A thermodynamic property of a substance defined as its internal energy. Formerly called "total heat" or "heat content". Enthalpy refers to the total value of sensible plus latent heat or energy. Unit of measurement is Btu/lb. In psychrometrics, it is expressed as Btu/lb. of moist air. Symbol: h.

Fiberglass:

A fine amorphous, brittle, usually transparent filament. A material made up of fine filaments.

Grain(s):

A unit of weight. One grain is equivalent to 1/7000 of a pound. The unit is derived from old English measures where a 7000 grains of wheat were used as an equivalent of a pound. By comparison, one gram is equivalent to 1/453.6 of a pound. In psychrometric work, the unit of grain(s) is used in the expression of humidity where it expresses the ratio of weight of moisture per pound of dry air. E.g., 70

GPP: Humidity Ratio (W) = (70 / 7000) = 0.010. Symbol: GPP (Grains Per Pound (of dry air)).

H-Trac:

A method of dehumidifier capacity control which assures a constant relative humidity value at the point of measurement (return air, supply air, room/space)

Heat of Vaporization:

The heat (energy) required for a substance to change phase from liquid to vapor. Expressed as Btu/lb. The heat of vaporization is referenced to pressure and temperature. The heat of vaporization for water at 29.921 inches of mercury (14.7psia) atmospheric pressure and 212 °F is 970 Btu/lb. For many air conditioning calculations (70 °F at 50% RH) the value of 1076 Btu/lb. is used for the heat of vaporization. At saturation (100% RH) and 70 °F, a value of 1112 Btu/lb. would be correct.

Humidity:

Water vapor contained in air. Expressed as specific, relative, or absolute humidity. Common units of measure are: percent relative humidity (RH), partial vapor pressure, grains of moisture per pound of dry air (GPP), dew point or humidity ratio (W). The amount of vapor that air can hold is a function of the air temperature.

Hygroscopic:

Descriptive of a substance that has the property of adsorbing or absorbing moisture from air. The water vapor molecules are held by or bound within the molecules of the hygroscopic material. Desiccants are hygroscopic materials, e.g. silica gel, molecular sieve, lithium chloride or calcium chloride. Some other examples are dry powders as used in pharmaceuticals, baked goods, and confectionery ingredients. Other materials include cardboard including cardboard boxes used as containers for stored goods.

Impregnate:

A process by which one material is permeated by another substance or material. Frequently, binders are also used to retain a material or substance in the impregnated material or substance.

Lithium chloride:

White deliquescent crystals that are very soluble in water and alcohol. One of the most hygroscopic salts known. Low toxicity. Manufactured by the reaction of lithium ores with chlorides. With proper controls, lithium chloride is used as an absorption material for dehumidification of air.

Matrix:

A structure or configuration of a material which gives shape to the finished product and in which another material or structure develops, takes shape, or is contained.

Micron:

Unit of length. A micrometer is one millionth (10-6) of a meter or 10,000 Angstroms.

Moisture Removal Capacity:

A measure of the capacity of a dehumidifier expressed in Lbs./Hr of moisture removal. Symbol: MRC.

Equation: MRC = (CFM * (60 min/hr / Vs) * (GPPin - GPPout)) / 7000 Grains/Lb. Where CFM is the volumetric rate of air expressed in Ft3/Minute, Vs is the specific volume of air expressed as Ft3/Lb and GPP is Grains of moisture per pound of dry air. GPPin - GPPout represents the grain "depression" or removal across the dehumidifier.

Molecular Sieve:

A group of synthetic adsorptive desiccants which are crystalline aluminosilicate materials, chemically similar to a class of minerals known as zeolite. The outstanding characteristic of these materials is their ability to undergo dehydration with little or no change in crystal structure. The dehydrated crystals are interlaced with regularly spaced channels of molecular dimensions. Internal voids (cavities) comprise almost 50% of the total volume of the crystals. The empty cavities in activated "molecular sieve" crystals have a strong tendency to recapture the water molecules that have been driven off by the reactivation process. This adsorption tendency is so strong that if no water vapor is present they will accept most any vapor that can pass through its aperture (pore) into the cavity. Only molecules that are small enough to pass through the pores of the crystal can enter the cavities and be adsorbed or stored on the interior surface. This sieving or screening action, which makes it possible to separate smaller molecules from larger ones, is a most useful characteristic of molecular sieves. Pore sizes are determined by selection of appropriate cations that partially block the crystalline aperture of the molecular sieve. The remaining open "space" becomes the nominal pore diameter. Two types of molecular sieves are commercially available: Type A with pore diameters of 3, 4 and 5 Angstroms, Type X with pore diameters of 10 or 13 Angstroms. The Its internal cavity surface and volume determine capacity of the molecular sieve. A molecular sieve can store or hold adsorbed moisture up to approximately 20 to 30% of its own weight. Above a threshold relative humidity (10 to 30 % varies by type), molecular sieves have a flat capacity profile (pounds of moisture adsorbed per pound of desiccant).

Particle size:

A particle is a discreet unit of matter. Particle size refers mainly to the solid particles of which materials are composed. The smaller the particle, the greater will be the total exposed surface area compared to its volume. Activity is a direct function of surface area; i.e. the finer a substance is, the more efficiently it will react, both chemically and physically such as in adsorption. The size ranges of particles are described as follows:

Subatomic: Protons, neutrons, electrons, etc.

Molecular: Atoms and molecules with size ranging from a few angstroms to about half a micron.

Colloidal: Macromolecules, micelles (aggregates of molecules), and ultra-fine particles resolved in electron microscope; size ranges from 1 milli-micron up to the lower limit of the optical microscope (1 micron).

Microscopic: Particles that can be resolved by an optical microscope.

Macroscopic: Particles that are visible to the unaided eye.

Perm:

An empirical unit of water vapor permanence of one grain (1/7000 lb.) flowing through one square foot induced by a vapor pressure of one inch of mercury.

Psychrometric Chart:

A graphical representation of the properties of moist (humid) air, usually including dry bulb and wet bulb temperatures, specific and relative humidity, dew point temperature, partial vapor pressure, humidity ratio, enthalpy, and specific volume.

Psychrometrics:

The branch of science (physics) concerned with the measurement or determination of atmospheric conditions, particularly of moist air.

Reactivation:

Reactivation or regeneration is the process of removing adsorbed moisture held bound by a desiccant. By removing the moisture, the desiccant is again ready to adsorb moisture from an air stream. In rotary dehumidification equipment, the reactivation process is continuous and concurrent with the adsorption cycle and is accomplished by thermal heating of the desiccant. In dual tower packed bed dehumidification equipment, the reactivation process occurs with the dehumidifier "off stream" and is not concurrent with the adsorption cycle being accomplished by either thermal heating or pressure swings.

Regeneration: See Reactivation.

Relative Humidity:

The proportional amount of humidity contained in a sample of air compared to the maximum amount of humidity that the sample could hold at a given temperature and pressure.

Rotor:

A formed structure, sometimes corrugated, containing a series of parallel flute shaped channels that hold and contain a desiccant within the flute wall for use in a dehumidifier. Flutes can be triangular, hexagonal, cylindrical or have other shapes. The rotor includes an outer casement (wrapper) to protect the flute structure, hub and supporting spokes (arms).

Silica gel:

A synthesized (manmade) adsorbent material consisting of amorphous silica. Manufactured by treating sodium silicate with acid (usually sulfuric) and/or other chemicals. Silica gel is chemically inactive and is non-toxic, non-combustible, and non-corrosive. Used as a dehumidifying desiccant. Silica gel is commercially available in granular, powder, spherical, bead or other special forms. Some silica gel desiccants include metallic thermal stabilizers. Silica gel has crystalline-like structure with large surface (macro) pores and smaller interstitial micro-pores with capillary channels. Silica gel has an extremely large "surface" area due to the extensive internal network of micro-pores and capillary channels. A teaspoon full of silica gel powder has as much "surface" area as a football field. This large area serves as the storage site of the adsorbed moisture. Silica gel adsorbs polar molecules that are drawn into the channels by capillary action and stored there until reactivation. Silica gel can store or hold adsorbed moisture up to approximately 40 per cent of its own weight. Silica gel's capacity (lbs. of moisture adsorbed per lb. of desiccant) varies with relative humidity at ambient temperature.

Sodium silicate:

High volume synthetic silicate. Sodium silicate is the simplest form of glass. Gels are formed by treatment with acids, coagulated by brine and precipitated (see Silica Gel). Non-toxic, non-combustible.

Standard Air:

Standard air is dry air (no moisture content - see Dry Air) with it's properties measured at a referenced altitude, temperature, and pressure. IP: 0.075 lb/ft3 at sea level, 68 °F, 29.921 inches Hg. SI: 1.29 g/liter at sea level, 20 °C, 101.325 kPa (760 mm Hg.) Standard air values are used in HVAC calculations and rating of equipment performance to allow for a means of comparison. Correction factors for altitude and temperature are applied to calculations and performance data in order to determine the performance of equipment at "actual" conditions.

Synthesized:

Manmade material designed and manufactured for specific properties similar or identical to naturally occurring elements or for enhanced performance or economy.

Temperature:

Dry bulb: The temperature of an air sample indicated by an ordinary thermometer.

Wet bulb: The temperature of an air sample indicated by a thermometer when the bulb is covered with a water saturated wick over which the air passes at approximately 1000 fpm when the air temperature reaches the equilibrium temperature of the water evaporating into the air. The heat of vaporization is supplied by the sensible heat of the air sample.

Vaporization:

The change of state (phase) of a substance from liquid to vapor. Requires and input or energy.

Weeping:

Tendency of a deliquescent material to become a liquid solution and "weep", drip, leak or run from adehumidification wheel typically using lithium chloride as the absorbent.

Wet Bulb Temperature: See Temperature.

Wheel: See Rotor.

Zeolite:

A natural mineral having desiccant properties. Synthetic zeolites, called molecular sieves, are made in a variety of forms and are used for dehumidification as well as the collection and classification by size (sieving) of gas molecules.