Fly ash recovered from coal-combustion flue gas can serve as a supplementary cementitious material when it meets the requirements of the intended mixture. Its effect on workability, strength and cement replacement depends on the ash and mix design. Moving dry ash from storage into weighing or mixing equipment requires a separate handling specification: concrete suitability alone does not establish conveyor capacity or reliability.
An enclosed tubular screw conveyor is one possible transfer arrangement. Selection should address the actual ash source, temperature, wear duty and retained air. Do not assume that all fly ash is hotter, finer or more abrasive than Portland cement. A storage silo at a batching plant and a collection hopper near a combustion process can present very different duties. Define normal operation, peaks, stopped conditions and loaded restart before selecting the conveyor.
Understanding fly ash conveying characteristics
FHWA’s archived material description identifies fly ash as a fine powder with predominantly spherical particles, while noting differences between sources and the presence of angular carbonaceous particles. This does not establish a universally smaller particle-size distribution than cement. Obtain the actual distribution, agglomerate size, moisture, loose and operating bulk density, and relevant safety information. Particle specific gravity must not be used as bulk density in a capacity calculation.
Air introduced by pneumatic filling or flow aids may remain in the powder and alter its density and flow. An aerated grade can flush through an unsuitable inlet or continue discharging when the screw stops. Determine the conditions that create this behaviour, the time available for air release and the downstream pressure. A stable test with settled ash does not establish behaviour immediately after silo filling.
Temperature must also be specified rather than inferred from the material name. If a particular hot collection duty exceeds 100°C, that measured condition belongs in the design brief; it is not a normal-temperature claim for every fly ash silo. Record continuous and short-duration maxima, ambient conditions and heating/cooling cycles. Collection method, storage and moisture can also change packing and blockage behaviour. Use representative material and the actual storage duration when assessing flow problems.
Wear-resistant material and trough design
Select the tube and flights from the actual abrasion duty, speed and expected service interval. Mild steel does not necessarily fail rapidly with every ash. Conversely, a previous mild duty does not justify ignoring a change of source or contamination. Establish initial wall and flight thickness measurements and inspect locations exposed to concentrated feed or recurring deposits. Distinguish material wear from direct mechanical contact.
Wear-resistant steel, suitable high-chromium materials, hardfacing or replaceable flight tips and liners may be appropriate where inspection or testing supports them. Check attachment, working clearance, temperature compatibility and the consequences of a loose liner or wear fragment. These options can reduce replacement frequency in a suitable application; none guarantees a particular service life without operating evidence.
A tubular body helps enclose the path and fit restricted layouts. It is not inherently a pressure-rated vessel: allowable pressure, joints, covers and seals require specific verification. Determine diameter and speed from required volume flow, bulk density, inclination and permitted loading. Changing pitch can change extraction and transport behaviour, but variable pitch does not automatically lower wall velocity or abrasion. Locate inspection access where it supports useful measurements, with secure closure and safe isolation before opening.
Managing high temperatures and thermal expansion
For hot service, calculate expansion of the screw, tube and supporting structure from their own temperature changes and lengths. They may heat at different rates and to different temperatures. The relevant issue is their relative movement and restraint, including startup and shutdown, not just the final overall elongation. See the thermal expansion guide for the distinction between calculated growth and a complete support arrangement.
Define fixed and floating points, axial location, bearing fits and permissible movement with the supplier. Expansion joints or sliding supports are design options where required. Do not create arbitrary looseness at a thrust bearing to accommodate growth: axial control and load capacity still have to be maintained. Check the effect of connected equipment and pipework, since an apparently movable support can be restrained by an attached inlet or outlet.
Bearing and seal selection should use the temperatures those components actually experience. Appropriate lubricant, isolation from the hot zone, extended housings or engineered cooling may be needed; not every fly ash conveyor requires all of them. Hanger bearing material and lubrication arrangements also need specific confirmation. If process cooling is necessary, an inlet cooler or jacketed thermal screw is a separate heat-transfer design. Establish the heat balance and consider leaks, moisture contact, thermal stresses and maintenance before selecting water cooling. A standard transfer conveyor must not be assumed to provide that function.
Sealing, air management and inlet control
Shaft packing, labyrinths and controlled purge arrangements can help contain fine ash when matched to temperature, speed, pressure and wear. They do not remove entrained air from the bulk material. Define purge-air quality and quantity where used, and include that air in the ventilation assessment. Excess purge air can worsen the very aeration problem being addressed. Hanger supports must resist the actual material duty without creating unacceptable buildup; an immersed plain bearing cannot simply be treated as a sealed external bearing.
For a conveyor receiving controlled feed, the upstream device must limit delivery to the conveyor’s verified capacity. For direct withdrawal below a silo, design the screw and hopper interface as a feeder. A rotary valve or dedicated feeder may be useful, but neither its name nor a nominal volume per revolution proves constant mass flow or complete shutoff of aerated ash. Check leakage, density variation, pressure difference and the stopped state. Inlet control, ventilation and any required isolation function must work together.
Provide a discharge that can accept the full operating flow without accumulation or backpressure. A flexible connection may accommodate specified movement or isolate a weighing system, but it does not correct unsupported equipment or major misalignment. Define its temperature, pressure and movement limits, and ensure that connections do not impose unintended loads on the conveyor or receiver. Verify the sequence during filling, transfer, stopping and abnormal downstream restriction.
Conclusion
Reliable fly ash conveying depends on the specific material and operating envelope. Confirm density and retained air, evaluate wear from evidence, accommodate thermal movement where relevant, and coordinate inlet control with seals and ventilation. Acceptance should demonstrate the required delivery rate and restart conditions while checking leakage, temperatures, current and product condition.
VOGO’s enclosed cement screw conveyor range includes selection diameters from Ø89 to Ø407 mm with project-specific drive and bearing arrangements. Published ranges are selection references, not a guaranteed ash capacity, pressure rating or dust-free result. Review the product configuration and datasheet, and submit the material and installation data for confirmation.
