Agitated feeder

A drier manufacturer proposed a new system to a user. The user suggested he purchased his own screw feeder to deliver the damp solid to the drier, but the manufacturer insisted the feeder was an integral part of the installation and had to be provided as part of the drying system. 

Tests were conducted at the premises of a feeder manufacturer with representative material , to the satisfaction of all concerned, and the order progressed. A feeder costing about 2000 was incorporated in the system. Upon commissioning, it was found the feeder did not provide reliable flow due to the material arching . Following some acrimonious discussion between the parties it was agreed that a second feeder with twin screws be provided at the shared expense of the feeder and drier manufacturers. However, this also proved inadequate to feed reliably, because the variable moisture content sometimes caused an intransigent flow condition of the product. 

Responsibility for performance could not be agreed. The dispute culminated in litigation, the user suing for around 2 000 000 for damages. 

Firstly, the specification for the material was not properly defined. The Institution of Mechanical Engineers publication Guide to the specification of bulk materials for storage and handling applications sets out the basic handling parameters of bulk material for handling duties and highlights features of performance that should be considered. 

Note It is not practical to expect any screw feeder to handle a bulk material in an undefined range of conditions. For example, what if the material freezes, contains foreign bodies, cakes, or goes off in any way Unless the bounds of variation can be defined, the user is normally best able to assess the limits of product condition that may be presented to the feeder, but this does not mean that he accepts responsibility for performance of the equipment. In practice the user will have less understanding of the sensitive features of screw feeder behaviour. There is a need for an open exchange of information and pre-contract agreement as to the basis on which the feeder is expected to perform. 

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Information given for quicklime also applies to hydrated lime except that bin agitation must be provided. Bulk bin outlets should be provided with nonflooding rotary feeders. Hopper slopes vary from 60 degrees to 66 degrees. 

Volumetric or gravimetric feeders may be used, but volumetric feeders are usually selected only for installations where comparatively low feed rates are required. Dilution does not appear to be important, therefore, control of the amount of water used in the feeding operation is not considered necessary. Inexpensive hydraulic jet agitation may be furnished in the wetting chamber of the feeder as an alternative to mechanical agitation. The jets should be sized for the available water supply pressure to obtain proper mixing. 

Aminoplast is pneumatically transferred from bin (1) through a feeder and an injector to bin (2) provided with a cyclone separator and an agitator. 

Limestone is fed to the system through a variable-speed feeder (F-1) which is mounted on top of the tower slurry tank (TK-2). Slurry from this tank is pumped to the absorber section of the tower where it is contacted countercurrently with the flue gas. The reacted slurry then flows by gravity back to the primary tower slurry tank. An agitator mounted on top of the tank maintains a uniform slurry suspension. To prevent the buildup of reaction products, a small stream is bled from the tower slurry tank (TK-2) and sent to the FDS slurry tank (TK-1). 

The dissolution of sodium silicofluoride in the solution tank of a dry feeder package unit can be accomplished by a jet mixer, but again a mechanical mixer is preferred. Because of the low solubility of sodium silicofluoride, particularly in cold-water applications, and the limited retention time available for dissolution, violent agitation is needed to prevent the slurry discharge. The preferred construction materials for a mechanical mixer are 316 stainless steel, plastic-coated steel, and Hastelloy-C (15). 

Limestone Long-Term Tests with Two Scrubber Loops and Forced Oxidation. The venturi/spray tower system was modified for two-scrubber-loop operation with forced oxidation as shown in Figure 2. Two tanks were used in the oxidation loop (venturi loop) air was injected to the first of these tanks through a simple 3-inch diameter pipe below the agitator. Adipic acid was dry-fed to the spray tower effluent hold tank. This was accomplished by manually adding one-pound increments hourly to maintain specified concentration, usually totaling only a few pounds per hour. A small screw feeder would serve the purpose in a full-scale plant. 

An alternative method to slurry feeding has been proposed for PBX. The new scheme uses an agitated hopper and screw feeder to move water-wet PBX to a feed line with air or other conveying medium. The feeder would be specially designed for use with PBX material. The advantages for this feeding scheme are ... 

Following pretreatment, the seawater is pumped into an agitated reactor vessel where it is contacted with a strong alkali, typically either lime or dolime. The dolime is withdrawn from storage by means of a weigh-belt feeder so that precise control over the rate of addition can be maintained. 

Useful as mixer, flow meter, force feeder, or agitator. 

D-5) Concentrated HCI receiver (D-6) Concentrated HCI pump (D-7) Concentrated HCI storage (D-8) Acid recycle pump (E) Crude product pump (F-1) Caustic make-up tank (F-la) Caustic make-up agitator (F-2) Caustic transfer,pump (F-3) Caustic storage tank (F-4) Caustic feeder (F-6) Neutralizer (F-6) Spent caustic separator (G-1) Flash still feed pump... 

PET chips are fed into the fluid bed by a variable-speed vibro feeder and a fixed-speed rotary valve that acts as a gas seal. They meet an oncoming stream of heated gas (e.g., nitrogen or air) and become partially suspended in the flow. As more chips are fed in, the fluid bed becomes established as a deep agitated mass of material exhibiting many properties of a fluid. 

Shows the need for feeder accessories such as agitators, vibrators, rotors, special hoppers, and the like. 

Supplementary equipment, such as agitators, vibrators, air cannons, wall-activated devices, and sundry methods of injecting air, are commonly used to promote flow. While these can overcome bridging problems, and in some cases act to de-aerate fluidized powders, they introduce uncertainty about the form of flow channel developed in the hopper serving the feeder, and must be proven empirically. When a mass flow pattern is a requirement of the supply hopper, the use of agitators, air injection, or fluidizing pads to promote flow should be avoided, as the pattern of flow they generate cannot easily be predicted. 

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