Bulk material handling glossary

Every discipline has words that carry a design decision inside them. In bulk handling, "angle of repose" and "surcharge angle" are not synonyms, "capacity" means three different things, and "mass flow" is a specific pattern rather than a compliment. These are the terms we use in our notes and proposals, defined the way we mean them.

Material properties

Bulk density

The mass of a volume of material including the voids between particles, usually in t/m³ or kg/m³. Loose (poured) and packed (tapped or consolidated) values differ, and moisture changes both. It converts tonnes into cubic metres and sizes everything — see the properties that decide the design.

Particle size distribution

The proportion of material in each size range, from a sieve analysis. The top size (largest lump) limits chute openings, belt width and bucket choice; the fines fraction drives dust and cohesion.

Angle of repose

The angle a freely poured heap makes with the horizontal. A rough index of flowability and the basis of stockpile geometry. Not the same as the surcharge angle.

Surcharge angle

The flatter angle the load takes on a moving belt, after the belt has shaken the heap down. Used with belt width and troughing angle to calculate the cross-section a belt carries.

Flowability

How readily a material flows under its own weight. Measured properly by shear testing as a flow function; described loosely by angle of repose. Cohesive materials have poor flowability and arch in hoppers.

Cohesion

The strength a bulk material develops from attraction between particles — moisture bridges, fines, electrostatics. Cohesion is what lets an arch or a rathole stand.

Time consolidation

The additional strength a material gains when it sits under load without moving, for example over a weekend. The reason a silo that flowed on Friday will not flow on Monday.

Wall friction

The friction between the material and the hopper wall surface. Low wall friction allows mass flow at a shallower hopper angle; liners exist to reduce it.

Abrasiveness

The tendency of a material to wear the surfaces it slides on. Drives liner, belt cover and screw flight choices and the maintenance interval.

Hygroscopic

Absorbing moisture from the air. Hygroscopic materials such as many fertilisers and soda ash cake in storage above their critical relative humidity — see fertiliser handling.

Fluidisation

The state of a fine powder whose particles are separated by air so that it flows like a liquid. Deliberately produced in an airslide; accidentally produced when a silo flushes.

Storage and flow

Silo, bin, hopper

A silo is a tall storage vessel; a bin a squatter one; the hopper is the converging section at the bottom of either that funnels material to the outlet. The hopper is where flow problems are designed in or out.

Mass flow

A flow pattern in which the whole contents move whenever material is drawn: first in, first out, no stagnant zones. Requires a hopper steep and smooth enough for the material and a feeder that draws across the whole outlet — see why material stops flowing.

Funnel flow

A flow pattern in which only a channel above the outlet moves and the material against the walls stays put. Cheaper per cubic metre; prone to ratholing, segregation and stagnant old material.

Arching (bridging)

A stable arch of cohesive material across the outlet, so that nothing discharges although the outlet is open. Prevented by an outlet larger than the critical arching dimension.

Ratholing (piping)

A stable vertical channel through stagnant material in a funnel-flow silo. The channel empties and the silo reads empty while most of its contents remain.

Flushing

Uncontrolled flow of an aerated fine powder through the outlet and past the feeder, like a liquid. A rate-control failure rather than a no-flow failure.

Segregation

Separation of a mixed material by size or density during filling, flow or transfer — coarse to the outside of a pile, fines to the centre. Undone partly by mass flow, made worse by drops.

Live capacity

The part of a silo's contents that actually discharges without intervention. In funnel flow it can be much less than the geometric volume.

Feeder

The device under a hopper outlet that controls the discharge rate: screw, belt, vibratory, rotary valve. A feeder meters; a conveyor merely transports. A feeder must draw across the whole outlet to keep the hopper live.

Rotary airlock (rotary valve)

A vaned rotor in a housing that passes material from one pressure environment to another — under a filter or into a pneumatic line — while limiting air leakage. Also used as a volumetric feeder for powders.

Slide gate

A flat plate that closes an outlet for isolation and maintenance. Not a rate-control device: throttling with it reduces the live outlet and invites arching.

Flow aids

Devices that restore flow after it has stopped or prevent consolidation: vibrators, air cannons, fluidising pads, bin activators. Recovery tools, not a substitute for hopper design — see what flow aids do.

Belt conveyors

Idler

A set of rollers on a frame that supports the belt. Troughed carrying idlers shape the loaded belt; flat return idlers carry it back. See troughing angle, spacing and selection.

Troughing angle

The angle of the wing rollers of a carrying idler from horizontal. A deeper trough carries more material for the same belt width, if the belt is flexible enough.

Head and tail pulley

The pulleys at the discharge (head) and loading (tail) ends. The head pulley usually drives; the tail pulley often carries the take-up.

Take-up

The device that keeps the belt tensioned as it stretches and as load changes: a screw at the tail on short belts, a gravity counterweight or a winch on longer ones.

Transition distance

The length of belt between the last troughed idler and a flat pulley, over which the belt changes from troughed to flat. Too short a transition over-stresses the belt edges.

Tracking (training)

Keeping the belt running centred on its idlers and pulleys. Mistracking spills material and damages edges — see belt tracking problems.

Carry-back

Material that stays on the belt past the discharge and drops off along the return strand. Removed by belt cleaners — see carry-back and cleaner selection.

Belt cleaner (scraper)

A blade held against the belt to remove carry-back: a primary pre-cleaner at the head pulley face, a secondary cleaner just after, sometimes a tertiary or a wash box.

Skirtboard

Vertical plates along both edges of the belt at the loading zone that contain the material while it settles, with a flexible seal along their lower edge.

Belt sag

The dip of the belt between idlers under load, usually limited to a small percentage of the idler spacing so that skirts seal and material stays put.

Transfer points and dust

Transfer point

Any place where material leaves one piece of equipment and enters another. The most intense few metres of a line, and the origin of most dust, spillage and belt damage — see transfer point design.

Trajectory

The path material follows after leaving the head pulley, calculated from belt speed and pulley geometry. The first line drawn in any chute design.

Hood and spoon

The two curved sections of an engineered chute: the hood receives the stream at a glancing angle and keeps it together; the spoon lays it onto the receiving belt in the direction of travel at close to belt speed.

Rock box

A ledge or pocket in a chute where material builds up on itself so that the incoming stream lands on its own kind rather than on steel. Used for abrasive materials.

Impact idlers / impact bed

Rubber-disc idlers or a slider bed under the loading zone that absorb impact and keep the belt flat so the skirt seal works.

Induced air

The air dragged along by a falling stream of material. When the stream stops, the air escapes carrying fines: the mechanism behind most transfer-point dust.

Settling zone

The enclosed length of belt after the loading point in which induced air slows and drops its dust before the belt leaves the enclosure.

Plugged-chute detector

A tilt switch or probe in a chute that signals a blockage and stops the feeding conveyor before the transfer is buried.

Conveyor types

Belt conveyor

An endless belt on idlers, driven by a pulley, carrying material on its top strand. The widest range and the lowest energy per tonne of the mechanical types — see conveyor types compared.

Pipe conveyor

A belt conveyor whose belt is rolled into a closed tube after loading, containing the material and allowing curves — see when a closed belt earns its cost.

Screw conveyor

A rotating helical flight in a trough or tube that pushes material along. Simple and enclosed; sensitive to sticky, fibrous and abrasive materials — see screws for difficult materials.

Airslide (air-gravity conveyor)

A covered channel with a porous membrane through which low-pressure air fluidises a fine powder so that it flows down a slight slope.

Bucket elevator

Buckets on a belt or chain lifting material vertically, discharged at the head centrifugally (fast, spaced buckets) or continuously (slow, close buckets) — see centrifugal vs continuous discharge.

Shuttle conveyor

A reversible belt conveyor on a travelling carriage that moves its discharge point along a row of silos or bays.

Tripper

A travelling pair of pulleys that lifts a belt and discharges it sideways at a chosen point along its length.

Apron / pan conveyor

Overlapping steel pans on chains, used for hot, heavy or very lumpy material such as clinker straight from the cooler.

Engineering and project terms

Nominal, design and peak capacity

The average rate the process needs; the rate the equipment is sized for, including a margin; and the highest short-term rate it must survive. Three numbers, not one — see how to specify a conveying system.

Duty

The complete description of what equipment must do: material, rate, hours, environment, route. "Heavy duty" is a marketing term; a duty statement is an engineering one.

Design basis

The document that fixes the inputs a design rests on — material data, duty, site conditions, standards, interfaces — so that every later decision refers to the same facts. The main output of a feasibility study.

General arrangement (GA)

The drawing that shows where everything is and how it connects — plan, elevations, levels, tags — without fabrication detail — see how to read a GA.

Load list

The forces and moments equipment imposes on the structure at each support, given by the supplier so that the structure can be checked — see new conveyor on existing steel.

Interface

Any boundary where one party's scope meets another's — a chute mouth, a flange, a terminal strip, a foundation bolt. Agreed early in a project or argued over at the end.

Commissioning

Proving that the installed equipment does what it was designed to do: cold (without material), hot (with material), then a performance run — see how a project runs.

Retrofit

Modifying existing equipment or structure to change its performance, rather than replacing it — see retrofit or replace.

Terms we have not covered, or that you use differently in your plant, are worth a message: the enquiry checklist explains how a shared vocabulary makes a first proposal faster.