Paddles

In a gas flotation unit, air is bubbled through oily water to capture oil particles which then rise with the bubble to form a scum at the surface of the flotation unit. The scum can be removed by rotating paddles. Chemicals are often added to destabilise the inlet stream and enhance performance. 

One can also do work by stirring, e.g. by driving a paddle wheel as in the Joule experiment above. If tire paddle is taken as part of the system, the energy input (as work) is detemiined by appropriate measurements on the electric motor, falling weights or whatever drives the paddle. 

Let s just suppose, hypothetically, that the underground chemist is too stupid or unwilling to get a stir plate. In this case a single unit portable hotplate can work, but controlled heat using these is best accomplished by water or oil baths. Stirring can be accomplished by attaching a shaft and paddle to a power drill or any gear driven motor. 

The natural process of bringing particles and polyelectrolytes together by Brownian motion, ie, perikinetic flocculation, often is assisted by orthokinetic flocculation which increases particle coUisions through the motion of the fluid and velocity gradients in the flow. This is the idea behind the use of in-line mixers or paddle-type flocculators in front of some separation equipment like gravity clarifiers. The rate of flocculation in clarifiers is also increased by recycling the floes to increase the rate of particle—particle coUisions through the increase in soUds concentration. 

A plate-type filter, the PDF filter (18), uses a paddle wheel with radial, longitudinal plates coveted with filter cloth and manifolded to the filter valve at one end of the vessel, instead of a dmm. This filter uses a horizontal pressure vessel, was built to have only 0.75 or 1.5 m area, and operates at 25 kPa. A central screw conveyor collects the cake blown off the plates and conveys it to the discharge end of the vessel. 

Historically, internal scrapers or paddles were used in some designs, and loose rails were used in others to break up any caking material which formed. The nature of these designs mechanically limited the furnaces to relatively small sizes producing about 3000 t/yr. High maintenance costs were also involved. 

In California, Spirulina sp. grown in paddle-wheel-agitated open ponds with CO2 is harvested through stainless steel screens, with recycling of the nutrient-rich water to the ponds. The wet Spirulina is spray-dried at 60°C for a few seconds to yield a food-grade product (47). 

Paddle Type. This type is similar to the ribbon type except that interrupted flight blades or paddles transfer materials from one end to the other, or from both ends to the center for discharge. The paddle-type mixer can be used for dry materials or pastes of heavy consistency. It can be jacketed for heating or cooling. 

Planeta-iy Type. Paddles or whips of various configurations are mounted in an off-center head that moves around the central axis of a bowl or vessel. Material is mixed locally and moved inward from the bowl side, causing intermixing. This mixer handles dry materials or pastes. 

Fig. 2. Internieshing co-iotating twin-sciew extrudei A, motoi B, geai box C, feed poit D, clam shell baiiel E, vent poit F, sciew shafts G, conveying screws H, kneading paddles 1, baiiel valve and J, bhstei rings. Courtesy of APV Chemical Machinery Inc.

Sohd—sohd blending can be accompHshed by a number of techniques. Some of the most common iaclude mechanical agitatioa which iacludes devices such as ribboa Headers, impellers, paddle mixers, orbiting screws, etc a rotary fixed container which iacludes twia-sheU (Vee) and double-cone blenders and fluidization, ia which air is used to Head some fine powders. 

OtherRota.tiona.1 Viscometers. Some rotational viscometers employ a disk as the inner member or bob, eg, the Brookfield and Mooney viscometers others use paddles (a geometry of the Stormer). These nonstandard geometries are difficult to analy2e, particularly for an infinite bath, as is usually employed with the Brookfield and the Stormer. The Brookfield disk has been analy2ed for Newtonian and non-Newtonian fluids and shear rate corrections have been developed (22). Other nonstandard geometries are best handled by determining iastmment constants by caUbration with standard fluids. 

Orthokinetic flocculation is induced by the motion of the Hquid obtained, for example, by paddle stirring or any other means that produces shear within the suspension. Orthokinetic flocculation leads to exponential growth which is a function of shear rate and particle concentration. Large-scale one-pass clarifiers used in water installations employ orthokinetic flocculators before introducing the suspension into the settling tank (see Water,... 

MUNICIPAL WATER treatment). Scale-up of orthokinetic flocculators, generally in the form of paddle devices, is based on the product of mean velocity gradient and time, for a constant volume concentration of the flocculating particles. 

Clarifiers. The largest user of clarifiers is probably the water-treatment industry. The conventional one-pass clarifier uses horizontal flow in circular or rectangular vessels (Eig. 2) with feed at one end and overflow at the other. The feed is preflocculated in an orthokinetic (paddle) flocculator... 

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