Some years ago I was involved in simulating a feed system for wet fine coal into a fluid bed boiler. We had a screw conveyor with conventional full flight on central tube. While running, the coal filleted in between the flight and tube. We replaced it with a centreless screw which was a significant improvement.

The chute to the screw was inclined at 70 degrees to the horizontal, the coal being discharged from a hopper by a variable speed conveyor. We tried both a UHMW surface and a polished stainless steel surface at high an low rates. At low rates the coal tended to stick to both surfaces, while at high rates it would slide off. Also, after a period of running the surface of the UHMW became "furry" at the impact point giving a greater tendency for the coal to stick. This confirmed some old handling lore that dribble feed chutes are more likely to block than those with a significant flow of material.

Just a couple of memories that may be of assistance. ■

I have used centreless screws on greasy sewage screenings and fine gypsum powder. Both worked well.

Your existing ribbon screw looks to have a good size to the flighting. The centreless screw will likely have a simlar size flight and maybe a wear bar. But I don't think they will wear any better than what you have. And if you wear the centreless screw out it will collapse on you. Centreless screws usually have a UHMW liner which will also require frequent replacement with your material.

Round or rectangular with round end chute is the only way to go with a sticky material. Corners are trouble. Polished stainless might work better than UHMW if there is any impact. Your chute looks to taper in at the top as it goes down, try to go straight down if you can.

Only down side to centreless screws is the price of the replacement flight. If you want to eliminate the discharge bearing on your existing screw arrangement, connect the screw to the drive shaft via a drive disk and make the drive shaft rigid with 2 bearings. ■

Briefly, it is presumed that the feed chute is flooded so that the screw can control the feed rate to the drier, in which case it would be better for the chute to diverge slightly and have radius corners andf the screw to have a progressive pitch and expand after the inlet section to relax the associated pressures of confinement. The screw is short enough to cantilever from double support bearings or a hollow shaft gearbox to dispense with the tail end bearing and avoid the deflection and extra tip wear of a centreless screw.

The screw shown has flat bar backing the flights, which will induce high shear in the conveyed material due to the spiral motion of the product sliding on the screw shaft. 'Lynfow' screws, by Ajax Equipment, have helical support ribs tangential to the shaft, radius corners and chamferred leading edges to prevent build up.

Flight face wear is minimised by radial projections of hard weld deposits that capture 'wear boxes' of the abrasive product. Tip wear is minimised by a leading edge tip of hard weld deposit and a relief chamfer to avoid wide face contact and high confinement pressures.

On demanding applicationfs such as this, fine attention to detail, intigrating the chute and screw construction, optimising screw design based on wall friction values and hopper interfacing good practice and reducing contact pressures pays large dividends. ■