GLOSSARY
DRYING & MIXING TERMINOLOGY
ABSOLUTE VACUUM (or pressure)
Refers to a pressure or vacuum gauge that compensates for (subtract) the actual barometric reading. PSIA = pounds per square inch absolute - PSIG = pounds per square inch gauge (more common)
ABSOLUTE HUMIDITY
The water vapor present measured by weight per unit volume of air (e.g., pounds of water per cubic foot of dry air).
AGGLOMERATION
Adherence of particles into a small mass due to moisture, static charge or chemical or mechanical binding.
ANGLE OF REPOSE
The angle at which a divided solid will stop flowing - measured from the horizontal. A steeper angle indicated less flowabilty. In tumble blenders it raises the center of gravity of the mass and increases the horsepower required for rotation. Typically between 30° to 60°- we assume 45° if unknown.
BOUND MOISTURE
This is the liquid which exerts a vapor pressure less than that of a pure liquid at a given temperature due to containment in pores or capillaries, by chemical or physical absorption, by solution in the material or by chemical adsorption on a solid surface.
BTU
British thermal unit - energy required to raise one pound of water 1°C - there are 3,412 Btu's per kilowatt
BULK DENSITY
The weight per unit volume for a solid materials. Expressed as pounds per cubic foot or grams per cubic centimeter. Untapped bulk density is the lowest attainable density - Tapped bulk density is the highest attainable density for a sample without compression.
COEFFICIENT OF VARIANCE
The standard deviation of a mixture times 100 divided by the sample mean. Expressed as a percentage.
CONDUCTION
Transfer of heat from one part of a body to another part or to another body in which it is in contact.
CONDUCTIVE DRYER
(indirect contact) - Heat is transferred by conduction through a solid (the steel shell) separating the heating medium (steam/oil) and the material being heated. Vapor is usually removed by a combination of condenser and vacuum pump.
CONSTANT-RATE DRYING PERIOD
The drying period during which the rate of water removal is constant. Indicates the presence of unbound moisture.
CONTACTIVE DRYER
(direct contact) - Also known as a (forced) convection dryer - the heating medium (air) is in direct contact with the material being dried. Vapor is removed by the circulating air. The air provides both heat and mass (vapor removal) transfer.
CAPILLARY FLOW
The flow of liquid by liquid-solid molecular attraction through the material. As liquid is removed from the exterior of the particle the equilibrium is shifted and the interior liquid is induced to move to the surface.
CONVECTION
The transfer of heat from one point to another in a fluid by mixing one portion of the fluid with another. Natural convection is caused by density variations due to temperature differences. Forced convection is the motion produced by mechanical means.
CRITICAL-MOISTURE CONTENT
The moisture content of the material when the constant-rate drying period ends. Indicates that the unbound moisture has been removed (higher vapor pressure) and only bound moisture (lower vapor pressure), which is less easily removed, remains. Evidence of the critical moisture content being reached in drying is an increase in product temperature and deepening of the vacuum level.
DEW POINT
The temperature below which condensation of vapor begins to occur as the atmosphere is cooled (pressure dependent).
DIFFUSION
Movement of internal liquid or vapor from the interior of a solid to the surface
DRY-BULB TEMPERATURE
Temperature obtained with a dry measuring element. Used in conjunction with wet-bulb temperature to measure air moisture saturation level.
EQUILIBRIUM MOISTURE CONTENT
The limiting moisture to which a given material can be dried under specific conditions of temperature, humidity and pressure. Additional moisture removal requires increased temperature, reduced pressure or reduced humidity.
EVAPORATIVE-COOLING
Heat transfer occurs as a result of mass transfer of moisture due to removal of moisture in the surrounding atmosphere.
EXPANSION
All gases expand and contract at the same rate of 1/273 per degree Celsius. (ref. gas law)
FALLING-RATE DRYING PERIOD
The drying period during which the drying rate continually decreases. This usually occurs as unbound moisture is depleted and only bound moisture remains.
FREE MOISTURE CONTENT
This is the liquid that is removable at a given temperature and pressure. It may be bound or unbound moisture.
FRIABILITY
The tendency for a materials to break down into smaller sizes due to handling.
HEAT-TRANSFER COEFFICIENT (U)
An empirical number that describes the rate of heat transfer from one body to another in Btu's/(°F×ft²×hr). Inverse of the resistance to heat flow or insulative quality of a material (K). For a Rota-Cone Vacuum Dryer "U" is normally between 4 and 20. For agitated dryers it's higher - between 8 and 30. Lower values of "U" often indicate less contact area such as plastic chips while higher numbers indicate finer particles and more contact points. U is strictly an empirical number and cannot be determined exactly for a particular substance without experimentation.
HEAT CAPACITY (C)
The amount of energy a mass can store for each degree of temperature difference with its surroundings. Defines the quantity of energy required to increase the average temperature of a mass. Common units are Btu's/lb. or Joules/Kg.
HYGROSCOPIC MATERIAL
A material that contains bound moisture or readily absorbs moisture.
INCHES OF MERCURY ("Hg)
Vacuum level with a scale of 0" Hg to 29.92" Hg (gauge) with 0" being equal to atmospheric pressure at sea level. The absolute vacuum levels scale runs reverse with 29.92" Hg equaling atmospheric pressure. Often used to describe rough vacuum whereas torr is used for deeper vacuum level (i.e, use 20 torr rather than 29.13"Hg)
INDIRECT CONTACT
The heating medium and product do not intermingle. Heat transfer is by conduction though a body such as a steel shell.
INITIAL MOISTURE CONTENT
The percentage of liquid in the solids prior to drying. May be expressed on a dry or wet weight basis.
INTERNAL DIFFUSION
The movement of liquid or vapor through a solid as a result of reduced moisture at the surface of the solid (see also capillary action).
LATENT HEAT (latent, Latin for hidden)
The energy required to change a substance from one state to another. Heat of vaporization (Hv)= liquid to gas or gas to liquid. Heat of fusion (Hf) = solid to liquid or liquid to solid.
NATURAL REGAIN
The moisture that a material acquires from the surrounding air.
NON-HYGROSCOPIC MATERIAL
A material that contains only unbound moisture.
Q
Total Heat Flow - Q is the total amount of heat required to perform a function such as heating a substance or vaporizing a liquid.
For Heating a Single Mass Q = mass (lbs.) x specific heat x DT
For Heating a System Qtotal = Qdryer + Qsolids + Qliquids
For Vaporization of a Liquid Q = mass (lbs.) x latent heat of vaporization x DT
For calculating maximum heat flow such as through a jacket vessel:
Q = U x A x DT (used for drying when temperatures are relatively constant)
Q = U x A x LMDT (used for heating or cooling when temperatures are changing)
RADIATION
Heat transfer by infrared radiation through space (air or vacuum) from a warmer mass to a cooler mass.
RELATIVE HUMIDITY
Ratio of actual water vapor present to the amount contained in saturated air - expressed as a percentage.
ROUGH VACUUM
Vacuum level between 760 and 1 torr. Medium vacuum = 1 - 0.001. High vacuum <0.001.
SATURATION TEMPERATURE
same as dew point
SEGREGATION
Separation of particles differing in size, shape and density. Small, smooth and dense particle will tend to sink through large, rough light particles.
SENSIBLE HEAT
The heat that can be felt or sensed. The energy input which causes an increase or decrease in temperature (converse of latent heat of vaporization or fusion).
SPECIFIC HUMIDITY
The water vapor content measured as weight of vapor per unit weight of air.
SPECIFIC HEAT
Ratio of the heat capacity of a substance to the heat capacity of water. The quantity of heat required for a 1°F temperature change per unit weight of material. Water = 1.0 Btu's/lb/°F. Most organic substances are less than 0.5, most organic solvents are between 0.4 and 0.7 and steel is 0.12.
STANDARD DEVIATION (mixing)
The square root of the sum of the differences of each data point divided by the number of data points minus one.
TEMPERATURE DIFFERENCE
When the heating source and mass being heated are both at constant temperatures (e.g., addition of heat for vaporization) the temperature difference is simply the (average) difference in temperature between the two masses or Dt. When the temperatures of one or both masses (as in a dryer that is being heated from ambient temperature to an elevate temperature) then the log mean temperature difference, LMTD, is used. where Dt1 = greater temperature difference and Dt2 = least temperature difference (T and t are often used in place of t1 & t2) and ln is the natural log or 2.3 x log
THERMAL CONDUCTIVITY (K)
This is a factor used to express the ability of the body to transfer heat by conduction. Units are Btu's/(ft×hr×°F)
TIP SPEED
Refers to the speed of a point (as on an agitator) on the outermost radius in feet per minute. Used for expressing and comparing the speed of various diameter rotating bodies including agitator elements. Diameter" x p x rpm)/12 = feet per minute.
TON
unit of refrigeration - equal to 12,000 Btu's/hour
TORR
Vacuum level with a scale of 0 to 760 torr with 760 being equal to atmospheric pressure at sea level - often used to refer to vacuum level deeper than 28" - same as mm Hg
U
see heat transfer coefficient
UNBOUND MOISTURE
The liquid on the exterior of the material or on exterior cells or fibers. Unbound moisture has the same vapor pressure as the liquid in the absence of the solid material at a given temperature. All water in a non-hygroscopic material is unbound.
VACUUM
Negative pressure commonly expressed in inches of mercury ("Hg), millimeters of mercury (mmHg) which is equal to torr. One atmosphere equals 14.7 psia (0 psig), 29.92"Hg (0"Hg absolute), 760mm Hg, 760 torr or 1,013 mbar.
VAPOR PRESSURE
The pressure exerted by a liquid in equilibrium with itself at a given temperature. The vapor pressure of water is 14.7 psi at 212°F and increases with temperature. At reduced pressure liquids will vaporize at lower temperatures which increases the temperature differential during drying.
WET-BULB TEMPERATURE
The depressed temperature obtained due to the effect of evaporative cooling. Used in determining relative humidity.
Copyright© 2003 by Paul O. Abbé. All rights reserved. INV 1/23/03
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