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Chapter 19. The Dravo Process for Non-Compacting Feed


G. M. Pigott
University of Washington
Seattle, Washington


In the early stages of development, fish are too small to eat anything other than finely ground particles. As the fish grow, it is necessary to feed larger particles containing a complete diet. Hence the development of pelleted feeds increases the availability of a balanced diet and at the same time reduces the tremendous loss that occurs when powdered or 'soupy' materials are fed to fish. The pellet size must be increased with the development of the fish.

The conventional method for manufacturing pellets involves forcing a feed mixture under pressure through a metal plate called a die. This die contains holes of the desired pellet size. As the mixture is extruded through the die holes, a rotating blade cuts the feed to the desired length. Although there are many variations in equipment design, all pelleting machines involve the extrusion principle. There are many disadvantages to this type of pelletizing operation. A large amount of pressure is required to force feed through the dies, which results in friction and, consequently, heat. Often pellets coming off the dies are so warm that they cannot be held in one's hand. The nutritive value of diets subjected to these conditions obviously is reduced through oxidation of heat-sensitive components. In addition, excessive heat and pressure frequently cause material to clog the dies, requiring stoppage of the process until the die plate is removed and cleaned. The operating cost is high, since the extruding operation requires considerable maintenance labour beyond that needed for production.

The dense, hard pelleted product from this operation sinks rapidly when placed in water. This minimizes the opportunity for a fish to consume the pellet before it reaches the bottom of the pond or raceway. With those species. Of fish that will not eat off the bottom, the feed is lost. With those species that will eat off the bottom, the chances for contracting diseases are greatly increased.

A final disadvantage of the conventional pelletizing processes is that a limitation is placed upon the number of pellet sizes that may be produced. A 1/32 in (0.079 cm) pellet is about the smallest that can be manufactured by this technique, since it is most difficult to force these feeds through a smaller hole. All in all, these disadvantages can be overcome by a pelleting technique that produces a light, non-compacted pellet that will not sink rapidly in water and that has not been heated to the extent of destroying nutritional components in the product.


2.1 Process
2.2 Pelleted Product

2.1 Process

It was discovered that dry fish feed forms a ball shape when sprayed with a water mist. on an oscillating table. The pellets were physically more desirable than those prepared by the more conventional means of manufacture. It was also discovered that this technique had been used in the fertilizer and chemical industries and that equipment was commercially available for production of the basic pellet.

A commercial Dravo pelletizing disc (originally developed for producing ceramic beads) is well suited for applying the non-compacting technique to making fish feed. Structurally, the Dravo pelletizer is simple. It consists of a disc that rotates at approximately a 45° angle. As the feed is tossed about on the disc, a fine spray mist causes the feed to form a ball shape of finely controlled particle sizes. Three metal projections aid in separating the particle sizes. The exact way in which pellets are produced can best be understood by using the number system that one finds on a clock and applying it to the rotating disc (Figure 1).

Fig. 1. Palletizing disc and attachments

The operation is started by feeding a dry mixture onto the disc bed at approximately the six o'clock position. A fine spray of water is introduced at approximately the eleven o'clock position. Speed is first regulated so that the fine, dry mixture will be stopped about half-way up on the first disc projection. With continuing spraying and introduction of feed, pellets will form. The speed is then regulated so that the smaller pellets will collect about half-way up on the first disc projection. A flow pattern, as indicated by the arrows in the diagram, is achieved. Eventually uniform pellets will come off at the seven to eight o'clock position. This operation does not become efficient until a size concentration gradient of pellets builds up in the bed. The following may be done to decrease pellet size:

(i) increase the rate at which the mixture is fed into the disc; this reduces the time that the pellets may roll in the fines;

(ii) increase the disc angle;

(iii) feed closer to the three or four o'clock position; or

(iv) increase the speed of the disc.

Similarly, the following may be done to increase the size of the pellets:

(i) decrease the rate at which the dry mixture is fed into the disc;
(ii) decrease the disc angle;
(iii) feed closer to the 7 o'clock position; or
(iv) decrease the speed of the disc.

Using various combinations of controlling factors, any pellet size used in fish feeding may be obtained.

Pellets prepared by introducing the atomized droplets of water increases moisture content by 10-15 percent. Thus, pellets must be dried to remove this excess moisture. A conventional tray drier, utilizing counter-current flow of warm air is normally used for drying the finished pellets.

2.2 Pelleted Product

A major advantage with the described type of pelleting technique is that much of the cost associated with maintenance is eliminated. There is no die to clog and remove for cleaning, so that the operation can run continually. The pelleting operation does not involve the heat or pressure associated with conventional methods of pellet manufacture. The retention of nutrients during pelleting is improved. The final product is soft, but hard enough to withstand normal handling procedures. The pellets usually float for a short period and then sink slowly. This gives the fish greater opportunity to consume the pellet before it reaches the bottom of a pond.

Another advantage is that many different sizes of pellets can be produced. As mentioned earlier, pellets are not made smaller than 1/32 in by conventional methods of manufacture. This means that the fish must go from a diet with flour-like consistency to a 1/32 in pellet or larger. This transition occurs at the most crucial, time in the animal's development, during the first weeks of life.

There are several disadvantages to overcome in using a pelletizing disc. Although a yield of over 90 percent is common, fines are produced as a result of pelleting and drying. It is necessary to recycle these fines for economic reasons. Further, differences in the specific weight of dietary ingredients can cause separation during the pelleting operation. This may be minimized by using raw material that is ground extra fine. In fact, fine uniform particle size is important to the efficiency of the pelletizing process.


A generalized flow diagram of a non-compacting pelleting process is shown in Figure 2. There are several items of equipment necessary for this process and they are enumerated below.

(a) Meal storage bins, the size and number of the bins depend on the amount of material and the number of ingredients to be stored.

(b) A metering system for compounding the diet directly from storage bins.

(c) A ribbon blender, or other means of completely mixing compounded diet.

(d) Conveyors for moving the blended diet to the processing area.

(e) A storage tank for the blended meal; again, the size of this tank would depend upon the amount of meal being processed.

(f) Small tanks for storing additives such as vitamin premix and oil, to be blended just prior to pelletizing.

(g) Blenders for blending the final additives prior to pelletizing.

(h) A surge hopper for holding the blended feed.

(i) A metering feeder for supplying the diet to the pelletizing disc.

(j) A rotating, pelletizing disc such as that manufactured by the Dravo Corporation (Engineering Works Division, Neville Island, Pittsburgh, Pennsylvania 15225, USA).

(k) A conveyor and surge hopper to drier.

(l) A drier for reducing the water content from approximately 20-30 percent to below 10 percent. This drier should be so constructed as to cause minimum attrition of the damp particles.

(m) Screens for classifying pellets.

(n) A blower and lines for returning the screened fines to process.

(o) Bagging or packaging facilities.

Fig. 2. Flow diagram of a non-compacting pelleting process

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