Heavy Duty Belt Feeder
This is an interesting duty due to the weight of material being transferred. Belt conveying is not my main field, but certain features as a dynamic hopper, of sorts, stand out as of a basic nature. UHMDPE is a prospective choice for the side skirts as it is usually a low frictional contact surface with damp products due to its hydrophobic nature, but it would be prudent to compare its sliding friction value with 2B finish stainless steel to select the optimum material to use. The side skirts should also have as much divergence as practical over the conveyor length to minimise the frictional drag as, even with close pitched rollers and heavy duty belt, the deflection between the rollers will impose a heavy load on the drive to overcome the corrugated shape of the bed underside. The longitudinal divergence will also relieve the vertical resistance of the walls and should allow a minor vertical convergence of the skirts to reduce both the magnitude of wall frictional resistance and minimise the overpressure on the belt, (and the belt deflection between rollers). Measuring the contact friction and the belt and the skirt surface will show the relative contact pressures and, taken with their relative contact areas, indicate the force necessary to ensure movement of the bed. Rubber skirt seals can be fixed on the outside of the skirts and folded under the skirt clearance, preferably as a single length at each side. Disc idlers on the return belt are a must, as is a good belt scraper, because material will inevitably stick to the belt due to its surface tension and the difficulty of air penetrating the mass to satisfy the void demand of separation of the contact layer.
A slowly advancing cohesive bed up to one metre deep will be too stiff to slump evenly as a viscous paste and not fall away as a steady avalanche so consideration should be given to fitting picking rods, not overlapping the belt, but slightly displaced to pull the material away from the mass in smaller sections. It could be useful to fit a transverse collecting screw under the width of the outlet to focus the discharge to a narrow, central region. If so, the screw and casing demands experienced design to avoid clogging and build-up of product.
Overall, it is not an exceptional duty and a specialist manufacturer should present a solution, but I hope these notes are useful. ■
Bunker Found
Your application simply requires a moving bed bunker. Try FSW,Horbury Bridge, Wakefield, if they still exist. There is no other easy way to regulate the flow you describe. If you can't contact FSW, contact me. ■
Very Slow Presscake Storage Bin Conveyor
Having seen such an installation (for a different filtercake), we're considering the use of a short belt conveyor (less than 15 meters long) to accept (in 5-10 minutes) and store an entire 15 tonne filter press load (from a few meters directly above the conveyor) of moist, soft, cohesive clay-like material, and then very slowly (less than 0.3 meters per minute belt speed, over about 2 hours) dump the load into a tall vertical chute to another piece of equipment. The pile on the belt might be a meter tall or so, between vertical skirtboards along virtually the entire loaded length; I envision these to be roughly 2 meters apart, so a belt somewhat wider than this is required. (The skirtboards will be the loading chute, along with maybe a door on the back.) The cake will drop onto the belt as pea gravel-sized pieces (several mm) up to about 300 x 300 x 40 mm chunks (through a 300 x 300 mm grizzly), and is too soft to puncture the belt. Primary particle size is 100% less than 44 microns, but the material should stay moist and cohesive. This material will not come out of a converging chute of our fairly small size requirements due to interlocking of the randomly-piled chunks; furthermore, shear will make the material less friable and very sticky. To be conservative, we envision that the discharge of the conveyor (on/off or speed control) must be managed to keep the level in the tall chute quite low to avoid bridging.
The installation we saw had very closely-spaced idlers to accept the impact of the soft material, what looked like home-made vertical skirt seals (and a single belt wiper) made of stiff plastic, and 20 degree picking idlers.
The CEMA book, the Martin Engineering book, and this forum are wonderfully illuminating on belt conveyor technology, but our application seems to be somewhat unusual. It's not really a conventional belt feeder drawing free-flowing material from storage above. It is, itself, a storage vessel. What items in the design of this slow-moving conveyor are more and less important than others?
Flat vs picking idlers? I imagine that attention shoud be paid to belt cleaners because of how sticky the material can get, maybe disc idlers on the return to help more against buildup and mistracking? The installation we saw had very closely spaced steel idlers under the belt to accept the impact, not rubber impact idlers or impact cradles. Is belt tracking likely to be a problem on such a short and slow conveyor? If not, are the CEMA structural straightness and squareness requirements overkill here? The skirtboards will have filter cake maybe a meter tall dragged along them, albeit very slowly. Low friction lining (UHMWPE) suggested, or just make sure the motor and gearbox can handle the (guesstimated) torque required to drag the cake along stainless steel? (Actually, if we open the skirtboards out slightly along their length this should help a lot, I imagine.) Someone had suggested hanging rubber skirtboads inside an angle iron frame to avoid sticking or bridging, instead of using rigid material. Any comments are welcome. ■