Dense Phase Conveying in Building Materials Plants

Building materials plants move very fine powders over long distances: from a mill to a silo battery, from a silo to a load out, from a precipitator to an ash silo. These materials retain air, which means they fluidise readily and move as a dense bed at low velocity, and that is what makes conveying runs of hundreds of metres practical.

The price is pressure. A long dense phase line needs a high conveying pressure, and the pressure case drives the vessel design, the compressor selection and the energy cost of the whole system. A design that was done on rate and distance without the pressure profile will either not convey or will convey at a velocity nobody intended.

Moisture is the destroyer. Cement and lime react with water, and a line that admits condensation, rain through a vent, or damp compressed air develops a deposit that narrows the bore and eventually sets. Compressed air quality is therefore part of the conveying specification, not a utility detail.

And the far end wears. Even at modest design velocities the expansion of air towards the receiver raises the speed, and fly ash is abrasive enough that the final bends are the ones that fail.

A system designed from a pressure profile along the whole line, with the compressor and the vessel matched to it and the velocity known at every point rather than at the inlet only.

Dry air and a dry line, with air treatment specified to a dew point that keeps cement and lime from hydrating inside the pipe.

Protected and monitored bends at the receiving end, where velocity and therefore erosion are highest.

In chemical production the same line is decided by the exposure limit and by oxygen sensitivity first, and dilute phase retained for a harmful product only moves the problem into the filter.

Design starts from material characterisation: permeability, de aeration rate and the pressure required for the rate over the distance. For fine powders these are measured properties and they are what make a dense phase design calculable rather than speculative.

The pipeline is then stepped. For a long line this is not an optimisation; it is the difference between a line that runs at four metres a second throughout and one that runs at twenty at the receiver. The step positions follow from the pressure profile.

Compressed air is specified with a dew point. Aftercooling, drying and drain management keep water out of a line where water is the one thing that cannot be allowed in, and a plant that has had a cement line set solid once never argues about it again.

Bends and the receiving end are then detailed: long radius hardened or lined bends at the fast end, fixed wall thickness measurement positions, and a receiving vessel vent and filter sized for the air volume at the end of a conveying cycle, including the surge when a pressure vessel blows down.

• Permeability, de aeration rate and pressure requirement established by measurement.
• Pressure profile calculated along the line, with velocity known at every point.
• Stepped pipeline positioned from the pressure profile.
• Compressed air specified to a dew point, with drying and drain management.
• Bends at the receiving end hardened or lined and measured on fixed positions.
• Receiving filter and vent sized for the end of cycle surge, not the average.

• Distance and rate, which together set the pressure case and therefore the whole system cost.
• Permeability and de aeration rate of the powder, measured, which decide whether fluidised dense phase works.
• Available air pressure and volume, including whether a high pressure compressor is justified.
• Moisture control, air dew point and condensation risk along the route.
• Abrasiveness, which at these velocities concentrates at the receiving end.
• Receiving vessel venting, sized for the blow down surge.