ACCUAir Cooling Condensing Unit
ACFMActual Cubic Feet Per Minute
Adiabatic A thermodynamic process during which energy (heat) is neither added to nor removed from the system. The process is plotted on a psychometric chart showing constant enthalpy between state points. The energy content of the mass of the air and humidity mixture remains constant: Btu/Lb (Cal/KG). A change in humidity results in a change in temperature such that the total energy is constant. Static dehumidification by use of a desiccant (such as may be used in (packaging) is an adiabatic process in which moisture is removed from air while the temperature is proportionately increased.
AdsorptionAdherence of molecules of a gas (vapor) to the surface of another substance called the adsorbent. Materials that adsorb molecules and can then be reactivated are called desiccants. Finely divided or micro-porous materials presenting large areas of active surfaces are strong adsorbents. Activated carbon, activated alumina, silica gel, and molecular sieves of varying size and shapes are examples of adsorbents. 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. However, micelles of water vapor (aggregates of molecules in chain form) 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.
Air ChangeA rate of measure of the air volume added to or removed from a space divided by the volume of the space. Typically measured in air changes per hour (ACH).
AmorphousNon-solid, non-crystalline substance having no molecular lattice structures that 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. Silica gel desiccant is an amorphous material that we have learned to use as a dependable desiccant.
AngstromA 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. SSCR types M and Ultra rotors utilize an in-situ manufactured advanced form of silica gel that has a macro-pore opening of 20 to 30 angstroms. The water molecule has an effective diameter of 3.2 angstroms.
AnhydrousDescriptive of an inorganic compound that does not contain water either adsorbed on its surface or combined as the water of crystallization. Dry.
ARIThe Air-conditioning & Refrigeration Institute. ARI offices are located in Arlington, Virginia. The scope of the Desiccant Cooling and Dehumidification Equipment Section covers desiccant dehumidification.
ASHRAEThe American Society of Heating, Refrigerating, and Air-Conditioning Engineers. ASHRAE offices are located in Atlanta, GA. The scope of Technical Committee 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.
BBCHPSee CHP for Buildings
BTUBritish Thermal Unit


CDIClimate by Design International
CeramicCeramics 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.
CHP for BuildingsAn acronym for Cooling Heating and Power for Buildings. Formerly, referred to as BCHP. 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.
CFMCubic Feet per Minute
Co-efficient of Performance (COP)The ratio of the rate of net heat output to the total energy input expressed in consistent units and under designated rating conditions. Most commonly associated with the ratio of refrigeration capacity to the work absorbed by the compressor per unit of time. For desiccant dehumidifiers it has become typical to express COP as the ratio of BTU/Hr of latent heat removed from an air stream to the heat input measured at the regeneration heater.
CWChill Water


DehumidificationThe removal of water vapor from air. Dehumidification can be accomplished by cooling an air stream to below its dewpoint temperature causing the condensation of vapor or by desiccant adsorption/absorption resulting in removal of humidity from air in the vapor phase.
DeliquescentTendency 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.
Delta TTemperature difference
DDCDirect Digital Controller
DensityMass per unit of volume
DER / Distributed Energy ResourcesDistributed Energy Resources as fostered by the U.S. Department of Energy. Power generators are located close to or within the point of use rather than at a central generation station with distribution grid. The power generators such as micro-turbines, mini-turbines or ICE/gen-sets are distributed rather than the power itself being distributed over high tension transmission lines.
DesiccantGenerally, 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 CoolingThe 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/ DHPassive 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, e.g. rotary desiccant dehumidifiers such as the ARID-Ice and Critical Process Food Grade Desiccant Dehumidifiers.  Check out Dr. Dry Talks for an explanation from Dr. Dry.
Desiccant VentilationThe 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).
DesorptionThe removal (release) of adsorbed moisture from a desiccant. See Reactivation.
DewpointThe temperature at which an air sample is saturated with water vapor (100% relative humidity). Cooling of the sample below the dewpoint temperature would cause condensation of the water vapor to occur.
DOA Dedicated Outside Air
DOASDedicated Outside Air System
Dry AirAir 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 than 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 TemperatureSee Temperature. Check out Dr. Dry Talks for an explanation from Dr. Dry.
DXDirect Expansion


EncapsulateComplete 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 SG America rotor encapsulates the ceramic fiber substrate. The ceramic fiber gives the rotor its strength and shape.
EnthalpyA 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.
ESPExternal Static Pressure. The measurement of all the resistance in the duct system that a fan has to work against.  Typically measured in inches of water column. 


FDHFood Processing Desiccant Dehumidification
FiberglassA 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 psychometric 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).  Check out Dr. Dry Talks for an explanation from Dr. Dry.


Heat of VaporizationThe 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.
HumidityWater 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.
Humidity RatioThe ratio of water vapor mass to the mass of dry air for the same volume of air. 
HVACHeat, Ventilation, and Air Conditioning
HWHot Water
HygroscopicDescriptive 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.


IAQIndoor air quality.
ImpregnateA 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.
Ion-Exchange ResinIon-exchange resins are reactive solids, which have the property of exchanging ions from both liquid and gaseous solutions. Properties include: complete insolubility in water and solvents to prevent imparting tastes, odors, or color to solutions being treated; high exchange capacity per volumetric unit; rapid and complete exchange with counter-ions; chemical stability to prevent degradation by oxidizing and reducing agents; resistance to osmotic shock to prevent breakdown. Ion-exchange resins have been used for years in industrial processes to remove selected gases from air streams. Seibu Giken is the first to apply ion-exchange resins to the removal of water vapor from air streams in place of a desiccant in an enthalpy exchange "Heat" wheel. Heat wheels are commonly used to treat ventilation air for buildings. The highly polarized water molecule acts like an ion and has a higher order of attraction to the resin as compared to gases with lower polarization or those that are non-ionized.


Lithium ChlorideWhite 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.


MatrixA 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.
MDHMake-up Air Desiccant Dehumidification
MicronUnit of length. A micrometer is one millionth (10-6) of a meter or 10,000 Angstroms.

Moisture Removal 

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 SieveA group of synthetic adsorptive desiccants that are crystalline aluminosilicate materials, chemically similar to a class of minerals known as zeolites. 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 internal cavity surface and volume determine the 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).


NTM NoThruMetal


OA/OSAOutside Air


Partial Pressure

The vapor pressure that a single component contributes to the total pressure in a system. E.g. Water vapor mixed with other gases such as N2, O2, CO2 that make up air.

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.

Passive DesiccantSee Desiccant Dehumidification
PermAn 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.
PLC Programmable Logic Controller 
Psychrometric ChartA 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.
PsychrometricsThe branch of science (physics), concerned with the measurement or determination of atmospheric conditions, particularly of moist air.


RA / Return AirAir that is returning to the air handling unit or dehumidifier from the space or other area.
ReactivationReactivation 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.
RegenerationSee Reactivation.
Relative Humidity/RHThe 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.
RotorA 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).
RRCReactivation Rate Controller
SSCFMStandard Cubic Feet per Minute
Silica gelA 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.

Specific VolumeThe ratio of a substance’s volume to its mass. 
Standard AirStandard air is dry air (no moisture content - see Dry Air) with its 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.
SynthesizedManmade material designed and manufactured for specific properties similar or identical to naturally occurring elements or for enhanced performance or economy.



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. 

Check out Dr. Dry Talks for an explanation from Dr. Dry.

TonsThe cooling capacity of an air conditioning system. One ton is equal to the amount of heat required (288,000 Btu) to melt one ton of ice in a 24-hour period. This results in 12,000 Btu/hr per ton.


UVGIUltraviolet Germicidal Irradition


Vapor PressureThe pressure exerted by a vapor in thermodynamic equilibrium with its liquid phases at a given temperature. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. 
VaporizationThe change of state (phase) of a substance from liquid to vapor. Requires and input or energy.


WeepingTendency of a deliquescent material to become a liquid solution and "weep", drip, leak or run from a dehumidification wheel typically using lithium chloride as the absorbent.
Wet Bulb Temperature

See Temperature

Check out Dr. Dry Talks for an explanation from Dr. Dry.

WheelSee Rotor


ZeoliteA 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.