Loose Bulk Density
The loose bulk density (sometimes called the poured bulk density) of a bulk material is the weight per unit of volume (usually pounds per cubic foot) that has been measured when the sample is in a loose, non-compacted or poured condition. The loose bulk density may be close to the “as conveyed” bulk density and is preferred for the purposes of pneumatic conveying system design.
Packed Bulk Density
The packed bulk density of a bulk material is the weight per unit volume (usually pounds per cubic foot) that has been measured when the sample has been packed or compacted in, for instance, a silo or bin or after containerized transportation. The packed bulk density does not compare to the conditions that would be found in a pneumatic conveying system. It is for this reason that the loose bulk density is preferred for the purposes of conveying system design.
Fluidized Bulk Density
Fluidized bulk density is the apparent bulk density of a material in its fluidized state.
It is generally lower than either the packed or loose bulk density due to the air absorbed into the voids.
The mass of a particle divided by its volume. For a bulk material, average particle density is used, found by dividing the mass of the material by its volume, excluding the voids.
Particle Size Distribution
The particle size distribution of a bulk material is a tabulation of the percentage of particles by mass in each particle size range. The percentage described is either that passing or being retained on a specific screen size. In the United States, the screens used are “U.S. Standard Screens” or “Tyler Test Screens”.
Other methods of size analysis may be used, particularly in the case of very fine and/or cohesive powders. These methods include photo sedimentation, optical microscopy, electrical sensing zone techniques (such as the Coulter counter), and laser diffraction spectrometry.
Maximum Particle Size
Maximum particle size is the maximum dimension in inches (in the case of lumpy materials) or the maximum sieve size (in the case of powders and granules) of the largest lump or particle in the bulk material. Maximum particle size can be critical in the design of pneumatic conveying systems.
Median Particle Size
The median size or mid-point of the particle size distribution.
The shape and form of the particles of a bulk material can vary considerably. The following definitions specifically describe the individual particle shape only and not the bulk material as a whole.
Needle-like – Long, thin, rigid, straight, and pointed
Angular – Sharp-edged or having a multi-faced, irregular shape.
Crystalline – Of geometric shape or multi-faced regular shape.
Dendritic – Having a branched, crystalline shape with the branches extending from the faces of the body.
Fibrous – Regularly or irregularly thread like with a flexible structure.
Flaky – Plate-like.
Spherical – Globe-like.
Out-of-Round – Similar to spherical but being somewhat deformed or elongated.
Agglomerated – Several individual particles bonded together
Bulk Material Composition
The following definitions describe the general compositions that may be found in a bulk material.
Uniform – A single bulk material whose particles possess the same size and shape.
Non-Uniform – A single bulk material whose particle size and shape may vary.
Granular – A bulk material comprised of individual particles which can be visibly discerned.
Powder – A bulk material comprised of individual particles which cannot be visibly discerned.
Mixed – Two or more different bulk materials which have been combined.
Flowability is the ease with which a bulk material flows under the influence of gravity only.
Cohesiveness describes the tendency of a material to adhere to itself. The cohesiveness of a bulk solid material can be caused by any and sometimes by all of several factors. These include electrostatic charging, surface tension effects, and interlocking of certain particle shapes, particularly fibrous types of materials. Cohesiveness in bulk solids causes erratic flow from bins, pipeline feeding problems, and adverse effects in certain kinds of valves.
Adhesiveness can be described as “external cohesiveness” — that is, the ability of a material to adhere to other surfaces.
Fluidized describes the state some bulk materials achieve when a gas has been entrained into the void spaces between the particles of the material. Material in a highly fluidized state tends to behave more like a fluid (as the term implies) than a solid bulk material.
The action of introducing air (or gas) to a bulk material by any means. Aeration may cause the material to become fluidized or agitated.
Angle of Repose
The angle of repose of a bulk material is the angle between the horizontal and the sloping surface of a heap of the material which has been allowed to form naturally without any conditioning, usually by gravity flow from a funnel or other similar device.
A property of a solid material which contributes to its overall abrasiveness. The harder a material is, generally, the greater the erosion for a given velocity on a pipeline. Hardness is difficult to quantify and is somewhat subjective when described. Moh’s Scale of Hardness is used to describe the materials when designing pneumatic conveying systems.
The abrasiveness of a material is determined by its hardness factor and the shape of its particles. A material which has, for instance, a high Moh’s hardness factor and has sharp, angular-shaped particles will be considered highly abrasive.
Generally, most bulk materials are handled at ambient temperature conditions. However, in some cases, the material may be at an elevated temperature. Elevated temperature can affect both the condition of the material itself and its surroundings -particularly the equipment that is being used to convey it. Care should be taken, when considering high temperature materials, that the temperature range is clearly and accurately stated, and any effects on the material (particularly its handling characteristics) should be noted.
The temperature of the bulk material, measured in °F. or °C., for purposes of pneumatic conveying design, is the material temperature taken at the point of entry to the system.
Material Temperature Sensitivity
The temperature at which a bulk material changes its characteristics.
The ability of a material to absorb moisture from its surroundings. Moisture may be absorbed from either the ambient air (especially during high humidity conditions) or the conveying air of the pneumatic system.
In certain conditions, some bulk materials can form potentially explosive mixtures when combined with air. These conditions depend on (a) the nature of the material itself, which would include its ignition temperature, its chemical reaction with oxygen, its particle size distribution, and so on; and (b) the nature of the operation involving the material. Details of explosion risk, reactivity, and fire hazard are now required by law in most states in the U. S. to be listed on the Material Safety Data Sheet (MSDS). The MSDS must accompany any material which is transported, stored or tested.
In all cases involving explosive materials, reference should be made to NFPA classifications.
Some materials have chemical properties which will, when combined with other materials such as moisture and air, cause chemical deterioration to materials of construction.
Friability describes a bulk material where particles are easily crumbled or pulverized.
The permeability of a bulk material is the degree to which air (or other gas) may be passed through the void spaces between the particles of the material.
Air retention is the ability of a material to retain air (or other gas) in the void spaces of the material after the air (or gas) supply to it has been terminated. Air retention capability can vary between almost zero and several days, depending upon the material’s other physical characteristics.
BASIC TERMS AND DEFINITIONS
Material Mass Flow Rate
The mass of material conveyed over a specified period of time, usually expressed in tons/hour or lbs./minute. Material mass flow rate is also called conveying rate or system capacity.
Actual Gas Velocity
Actual gas velocity is the volume flow rate at pressure and temperature conditions per unit cross-sectional area of the empty pipe, normally expressed in distance/time. Actual gas velocity varies throughout the entire length of the pipeline.
The saltation velocity of a material is the actual gas velocity in a horizontal pipeline at which particles in a homogeneous mixture with the conveying gas will begin to fall out of the gas stream.
The choking velocity of a material is the actual gas velocity in a vertical pipeline at which particles in a homogeneous mixture with the conveying gas settle out of the gas stream.
Minimum Conveying Velocity
The minimum conveying velocity is the lowest gas velocity that can be used to insure stable conveying conditions. Since the minimum conveying velocity occurs at the material feed point in the system, it is also known as the “pickup” velocity. These terms are generally applied to dilute phase systems.
Terminal Gas Velocity
The terminal gas velocity in a pneumatic conveying system is the velocity of the gas as it exits the system. It is also known as the ending gas velocity and conveying line exit velocity.
Average Gas Velocity
The average (also called mean) gas velocity of a system is usually defined as the mean of the beginning (or pickup) gas velocity and the terminal gas velocity.
The material velocity is the velocity of the material itself, which is somewhat lower than the gas velocity. Material velocity is usually specified as either average (or mean) velocity or terminal velocity.
There are no reliable means, at the present time, for measuring the actual material velocity, and only an estimate can be made.
Volumetric Gas Flow
The user should be aware that there are several different terms used when considering volumetric gas flow. The volumetric gas rate during conveying is expressed as “free air delivered” or FAD. Most air movers, such as blowers and compressors, are specified in terms of FAD, measured in standard cubic feet per minute (SCFM).
FAD is the volumetric gas flow at the suction port of a positive pressure blower or compressor or at the discharge port of a vacuum blower or vacuum pump. SCFM is the volumetric gas flow rate at standard atmospheric conditions (i.e., barometric pressure at sea level, 68°F. and 36% relative humidity).
Actual cubic feet per minute (ACFM) or inlet cubic feet per minute (ICFM) is the volumetric gas flow at the actual conditions that will be experienced where compressor or blower is located. The ACFM or ICFM must be calculated from the SCFM, taking into account elevation of the location and maximum summertime ambient conditions.
In the case of vacuum systems, the pressure drop of the system must also be taken into account when calculating the gas flow at the inlet of the blower.
The conveying pressure for any system is that required to overcome resistances in the system caused by interactions between the conveying gas, the material being conveyed, the pipeline, and other system components. It is also referred to as “pressure drop”.
The conveying pressure is the difference measured between the beginning and the end of the pneumatic system and is applicable to both positive pressure and vacuum (negative pressure) systems.
All bulk solid materials pneumatic conveying systems operate on a two-phase flow principle. That is, a solid phase (the materials being conveyed) and the gaseous phase (the conveying gas).
Dilute Phase Conveying
A dilute phase system is any pneumatic conveying system for which the conveying gas velocity is generally equal to or above the saltation velocity of the material being conveyed.
Dense Phase Conveying
A dense phase system is any pneumatic conveying system for which the conveying gas velocity is generally below the saltation velocity of the material being conveyed.
Material To Air Ratio
A parameter used by pneumatic system designers. It is the ratio of the mass of material conveyed to mass of conveying gas used. It is also referred to as “phase density”, “solids loading ratio”, and “mass flow ratio”.
The flotation velocity is the velocity at which material will be suspended in air. Knowing flotation velocity is critical to determining “enclosure velocity”, which is the upward velocity of gas in a filter receiver or bin vent. This term is typically used in the design of bag-houses and dust collection systems.