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Chapter VIIIPneumatic Conveying Terms CEMA Standard No. 805

MATERIAL CHARACTERIZATION

Loose Bulk Density

The loose bilk density (sometimes called the poured bilk density) of a bulkmaterial 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 pouredcondition. The loose bulk density may be close to the as conveyed bulk densityand 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 (usuallypounds per cubic foot) that has been measured when the sample has beenpacked or compacted in, for instance, a silo or bin or after containerisedtransportation. 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 theloose bulk density is preferred for the purposes of conveying system design.

Fluidised Bulk Density

Fluidised bulk density is the apparent bulk density of a material in its fluidised

state.

Fluidised Bulk Density (Cont.)

It is generally lower than either the packed or loose bulk density due to the airabsorbed into the voids.

Particle Density

The mass of a particle divided by its volume. For a bulk material, average particledensity 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 percentageof 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 UnitedStates, the screens used are U.S. Standard Screens or Tyler Test Screens.


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Other methods of size analysis may be used, particularly in the case of very fineand/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 lumpymaterials) 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 becritical in the design of pneumatic conveying systems.

Median Particle Size

The median size of mid-point of the particle distribution.

Particle Shape

The shape and form of the particles of a bulk material can very considerably. The

following table specifically describes the individual particle shape only and notthe bulk material as a whole.

Term Definition

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 shapewith the branches extending from thefaces of the body.

Fibrous Regularly or irregularly thread like witha flexible structure.


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Term DefinitionFlaky Plate-like

Spherical Globe-likeOut-of-Round Similar to Spherical but being

somewhat deformed or elongated.Cylindrical Cylinder-shaped

Agglomerated Several individual particles bendedtogether.

Bulk Material Composition

The following table describes the general compositions that may be found andshape.

Term Definition

Uniform A single bulk material whose particlespossess the same size and shape.

Non-uniform A single bulk material whose particlesize : and shape may vary.

Granular A bulk material comprised of individualparticles which can be visiblydiscerned.

Powder A bulk material comprised of individual

particles which cannot be visiblydiscerned

Mixed Two or more different bulk materials

which have been combined.

Flow ability

Flow ability is the case with which a bulk material flows under the influence of

gravity only.

Cohesiveness

Cohesiveness described the tendency of a material to adhere to itself. Thecohesiveness of a bulk solid material can be caused by any and sometimes by allof 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 feedingproblems and adverse effects in certain kinds of valves.


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Adhesiveness

Adhesiveness can be described as external cohesiveness --- that is, the abilityof a material to adhere to other surfaces.

Fluidised

Fluidised 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 ahighly fluidised state ends to behave more like a fluid (as the term implies) thana solid bulk material.

Aeration

The action of introducing air (or gas) to a bulk material by any meansAeration may cause the material to become fluidised or agitated.

Angle of Repose

The angle of repose of a bulk material is the angle between the horizontal andthe sloping surface of a heap of the material which has been allowed to formnaturally without any conditioning, usually by gravity flow from a funnel or othersimilar device.

Hardness

A property of a solid material which contributes to its overall abrasiveness. Theharder a material is, generally, the greater the erosion for a given velocity on apipeline. Hardness is difficult to quantity and is somewhat subjective whendescribed. Mohs scale of Hardness is used to describe the materials whendesigning pneumatic conveying systems.

Abrasiveness

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 Mohs hardnessfactor and his sharp, angular-shaped particles will be considered highly abrasive.

Material Temperature

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 it sell and its

surroundings --- particularly the equipment that is being used to convey it. Careshould be taken, when considering high temp.


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Erature materials, that the temperature range is clearly and accurately stated,and any effects on the material (particularly its handling characteristics) shouldbe noted.

The temperature of the bulk material, measured inoF. or

oC., for purposes of

pneumatic conveying design, is the material temperature taken at the point ofentry to the system.

Material Temperature Sensitivity

The temperature at which a bulk material changes its characteristics.

Hygroscopicity

The ability of a material to absorb moisture from its surroundings. Moisture maybe absorbed from either the ambient air (especially during high humidity

conditions) or the conveying air of the pneumatic system.

Explosiveness

In certain conditions, some bulk materials can form potentially explosive mixtures

when combined with air. These conditions depend on (a) the nature of thematerial itself, which would include its ignition temperature, its chemical reactionwith oxygen, its particle size distribution, and so on; and (b) the nature of theoperation involving the material.

Details of explosion risk, reactivity, and fire hazard are now required by law inmost states in the U.S. to be listed on the Material Safety Data Sheet(MSDS).The MSDS must accompany any material which is transported storedor tested.

In all cases involving explosive materials, reference should be made to NFPAclassifications.

Corrosiveness

Some materials have chemical properties which will, when combined with othermaterials such as moisture and air, cause chemical deterioration to materials ofconstruction.


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Friability

Friability describes a bulk material where particles are crumbled or pulverised.

Permeability

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

Air retention is the ability of a material to retain air (or other gas) in the voidspaces of the material after the air (for gas) supply to it has been terminated. Airretention capability can vary between almost zero and several days, depending

upon the materials other physical characteristics.

2. BASIC TERMS AND DEFINITIONS

Material Mass Flow Rate

The mass of material conveyed over a specified period of time, usuallyexpressed in tons/hour or lbs./minute. Material mass flow rate is also calledconveying rate or system capacity.

Actual Gas Velocity

Actual gas velocity is the volume flow rate at pressure and temperatureconditions per unit cross-sectional area of the empty pipe, normally expressed indistance/time. Actual gas velocity varies throughout the entire length of thepipeline.

Saltation Velocity

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 willbegin to fall out of the gas stream.

Chocking Velocity

The chocking 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 ofthe gas stream.


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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 occursat the material feed point in the system, it is also known as the pick-up 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 thegas as it exists the system. It is also known as the ending gas velocity andconveying line exit velocity.

Average Gas Velocity

The average (also called mean) gas velocity of a system is usually defined as themean of the beginning (or pick-up) gas velocity and the terminal gas velocity.

Material velocity

The material velocity is the velocity of the material itself. Which is somewhatlower than the gas velocity. Material velocity is usually specified as eitheraverage (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 whenconsidering volumetric gas flow. The volumetric gas rate during conveying isexpressed as free air delivered, or FAD. Most air movers, such as blowers andcompressors, are specified in terms of FAD, measured in standard cubic feet perminute (SCFM).

FAD is the volumetric gas flow at the suction port of a positive pressure blower orcompressor or at the discharge port of a vacuum blower or vacuum pump. SCFMis the gas flow rate at standard atmospheric conditions (i.e., Barometric pressure

at sea level, 680F, and 36% relative humidity).

Actual cubic feet per minute (ACFM) or inlet cubic feet per minute (ICFM) is thevolumetric gas flow at the actual conditions that will be experienced wherecompressor or blower is located. The ACFM or ICFM must be calculated from theSCFM, taking into account elevation of the location and maximum summertimeambient conditions.


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In the case of vacuum systems, the pressure drop of the system must also be

taken into account when calculating the gas at the inlet of the blower.

Conveying Pressure

The conveying pressure for any system is that required to overcome resist

ances in the system caused by interactions between the conveying gas, thematerial 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 andthe end of the pneumatic system and is applicable to both positive pressure andvacuum (negative pressure) systems.

Two Phase Flow

All bulk solid materials pneumatic conveying systems operate on a two-phase

flow principle. That is, a solid phase (the materials being conveyed) and thegaseous 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 thematerial being conveyed.

Dense Phase Conveying

A dense phase system is any pneumatic conveying system for which theconveying gas velocity is generally below the saltation velocity of the materialconveyed.

Material To Air Ratio

A parameter used by pneumatic system designers. It is the ratio of the mass ofmaterial conveyed to mass of conveying gas used. It is also referred to as phasedensity, solids loading ratio, and mass flow ratio.

Flotation Velocity

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 typicallyused in the design of bag-houses and dust collection systems.

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