Metering screw feeders

The equipment system scheme is essentially the same as that shown in Fig. 6.14 but with two differences (1) The orifice plates are used for metering airflow rates and (2) Since the equipment is much larger than that used in the model investigation and therefore the feeding rate is much larger, the screw feeder for wet material feeding in the quasi industrial test is not as complex as that shown in Fig. 6.14 in structure, but is a common one. 

Starch slurry make-down systems are designed to prepare a uniform suspension, using batch or continuous systems. In modem paper mills, automated batch or continuous systems are used. Water content and bulk density will affect the flow of dry starch.90 In the batch slurry system, weighed increments of starch are drawn from the silo and periodically added to a measured quantity of water in the make-up tank. The suspension is screened and automatically pumped to a larger storage tank on demand by a level transmitter. Continuous make-down of starch slurry is accomplished by simultaneously feeding starch and water at controlled rates into the slurry make-down tank. The dry starch is metered with a volumetric screw feeder. The slurry is screened before dispersion by batch or continuous cooking. 

The intake behavior is also substantially influenced by the resistance of the downstream plastification section. If discharged at atmospheric pressure, e.g., via screw feeders, the throughput rate is considerably higher than if the same feed section is succeeded by a plastification section (Fig. 11.4). For instance, if metering a product with a bulk density of 0.35 g/cm3 and a melt phase density of 0.7 g/cm3, it is clear that considerable quantities of gas will have to be extracted upstream through the intake hopper. 

The test rig is equipped with a feedstock hopper suitable to low-bulk-density biofuels and the biomass is fed continuously into the sand bed of the reactor via an injector screw which can feed fuel either into the bottom of the bed or to about 1 meter above the air distributor. The maximum fuel mass flow rate is 90 kg/h. The feeding system consists of two screw feeders in series separated by a rotary valve. The second screw has a higher feeding rate thus it will remain almost empty and therefore is not likely to be blocked by pyrolysis products. Steam can be added to the primary air as a gasification agent. 

The dried and crushed wood fuel is pressurised in a lock-hopper system to a level which basically is determined by the pressure ratio of the gas turbine, and is fed by screw feeders into the gasifier a few meters above the bottom. The operating ten erature of the gasifier is 950 - 1000 C and the pressure is approximately 17 bar, The gasifier is of a circulating fluidized bed type and consists of the gasifier itself, cyclone and cyclone return leg. The three parts are totally refractory lined. 

Solids are pulverized in a Wiley laboratory knife-blade mill to less than 1 mm in size. These solids are then metered into a 1.3-cm OD stainless steel injection tube by a screw feeder. The speed of the screw feeder is variable between 0-400 rpm. The solids are mixed with process air in the injection tube. The solids-air mixture comes out of the end of the 1.3-cm OD tube and into the annulus of a 3.7-cm ID alumina tube. This alumina tube extends to within 1.3 cm of the bottom of the 15-cm ID alumina containment vessel. During normal operation of the reactor, the depth of the molten salt expands from a quiescent level of 15 cm to an expanded depth of about 30 cm. 

PI) Pressure indication device (PIC) automatic pressure control (TI) temperature indicating device (TIC) automatic temperature control (FI) flow-meter (LC) level indicator (R) reactor (M) motor (WT) heat exchanger (ZR) bucket wheel lock (KM) membrane compressor (KR) cryostat (K) cooler (S) screw feeder (G) container (F) flare (TG) dip pipe (Z) cyclone (P) pump (EF) electric separator (DK) packed distillation column... 

Coal fines stored in bins (2) are discharged through several pantlegs (3) into a metering hopper (4). Variable speed discharge belts (8) under the bin (7) and hopper (4) meter the correct amount of each feed into the screw conveyor (9) the binder is pulverized in the hammer mill (10). Both components are premixed in a double screw mixer (11) and elevated (12) into another screw feeder (13). The latter discharges into two vertical pug mills (14) excess feed overflows at the end of the conveyor through a chute (30). 

These problems are resolved by briquetting. The DMT flakes are readily densified and bonded under moderate pressure in a roller press that is equipped with a screw feeder and pillow shaped pockets. The briquettes have high density and are well formed. During a screening operation they are separated into singles and leakage and land areas that surround the briquettes and are rubbed-off on the screen, are recirculated to the press. The briquettes have a volume of about 4cm, are about 20 mm square and 14 mm thick, pack, store, and handle well, and can be easily metered and remelted. 

FIGURE 17.2 Typical feed systems for pneumatic flash dryers (a) metering, mixing and rotary feeder (b) metering and screw feeder and (c) disk feeder (From Flash dryer, Deutsche Babcoock, Babcook-BSH GMBH, 1998.) 1, Wet product bin 2, metering 3, recirculated product 4, mixer 5, sling 6, flash dryer tube 7, disk feeder. 

Dedicated screw feeders may be classified according to the primary function they undertake. These duties may be broadly broken down between bin discharge feeders and metering feeders. There is no sharp... 

The general type of screw feeder required will be clear from the nature of the duty. The choice of proprietary equipment for metering duties is often influenced by the support offered by suppliers for integrated control systems, examples of pre-proved applications, or the facility to carry out full-scale representative trials with like or similar units. For duties of discharging bins, which is usually a less onerous task by virtue of scale helping to secure reliable flow, some suppliers offer standard dimensions or recommended sizes. 

Hopper ha-par (13c) n. In extrusion injection molding, the bin mounted over the feed opening that holds a supply of molding material. The hopper may be intermittently filled or continuously fed (See hopper loader). Feeding from the hopper is ordinarily by gravity, but it may be aided by vibrators, stirrers, or screw feeders. In some setups the hopper feeds into a metering device such as a gravimetric feeder or vibratory... 

Usually, production quantities are made up automatically. The standard automatic make-up plant will consist of a mixing vessel, into which water is admitted at a constant rate. The solid polymer is metered out from a hygroscopically secure hopper, using a screw feeder, into the incoming stream of make-up water. Some automatic systems use an air blower to convey the polymer, entering the air stream via a venturi, to a mixer, where the water enters with a cyclone action to keep the powder away from the mixer walls. From the mixer the product falls into a stirred ageing vessel. 

Screw feeders provide a convenient way to deliver solids to a gas-solid transport system or process, and they can also be used as metering devices. These units permit accurate delivery of precise amounts of solids. Generally, these units do not work on high-pressure systems but can work adequately up to about 2 atm pressure. 

---