Several designs of tanks with circular water flow are in use. What is important when choosing is that the new water is uniformly distributed throughout the entire tank volume. Round or polygonal (6–8 edges) tanks with a circulating flow pattern are suit-able because they have no dead zones provided that the inlet and outlet are correctly designed. Square tanks will, however, have dead zones in each corner and the effective farming volume is therefore not so large; for this reason square tanks are not recommended. Square and rectangular tanks with cut corners have, however, been shown to be quite good; experience with tanks having a total side
length of a and corners of size a/5 shows they are well suited (Fig. 13.5). For other tank designs such as raceways or earth ponds, it is far more difficult to avoid dead zones, and the effective fish production volume is normally less than the actual tank volume.
When selecting a tank design, it is also important to take into account the utilization of the area; square tanks with cut corners utilize this well, achieving several m3 of farming volume per m2 surface area. Raceways will also utilize the area satisfactorily.
Utilization of the tank construction material is another factor that must be considered when deciding the shape of the tanks. Circular tanks will have the best utilization of the construction materials. The pressure of the water is equally distributed all around the circumference of the tank and therefore a thinner wall may be used than for square tanks.
In square tanks the forces are greatest in the middle of the sides, and there is an accumulation of forces in the corners. The height of the tank will also be important because the pressure on the tank walls and hence the necessary thickness will increase.
The bottom of the tank could be horizontal or have a small slope towards the outlet which is usually in the centre of the tank; however, a part outlet might be in the tank wall, see Section 13.10. There is, however, little benefit from sloping the bottom towards the grating and outlet of the tank when having a correct flow pattern inside the tank. This is because the most important mechanisms for transport of the settled solids (faeces, feed loss) are the water flow and its hydraulic force, not gravity. Even a small upward slope (2–5%) to a centrally placed outlet has been used by the author with good results in tanks with a circulating flow pattern. This also confirms that the most important factor for transport of settled solids to the outlet is the force created by the water flow, rather than the bottom slope and force of gravity. When having non-self-cleaning flow conditions, for instance because fry production requires water flow of low velocity, it is important to have a slope to the outlet grating to be able to utilize graviditational forces. However, this slope must be quite large to really get an effect of gravity. In a filter unit the angle is recommended to be above 55° to utilize the force of gravity to get the settled solids to slide; this is because the density of aquaculture solids is low (1.05–1.2) and almost equal to that of water.
The height of the tank compared to the diameter will also affect the water exchange. For tanks with a circular flow pattern, a tank diameter: height ratio of between 2 and 5 has been successfully used. If the tank diameter is 10 m, the height could there-fore be between 2 and 5 m. For tanks that do not fall within these ratios, special attention must be given to the design and placement of the inlet and outlet. If the ratio is lower, the inlet should be placed some distance away from the tank wall, closer to the centre of the tank. Tanks where the height is greater than the diameter are often called silos; by using such tanks high production can be achieved per unit area. It is, however, difficult to get a proper water exchange throughout the entire volume in such constructions.
Various materials are used to fabricate tanks (Fig. 13.6). It is important that there is a smooth surface inside the tank to reduce problems with fouling, and that the material does not release any toxic substances into the farming water. Glass-reinforced plastic is a commonly used material for tanks, because it can be produced with a very smooth surface; small tanks are also light and easy to move. The tanks are either delivered completely finished or as elements that are screwed together on site. Plastic (polyethylene, PE) may also be used; this is also a light and cheap material. New mater-ial has a very smooth surface; however, is it more prone to ageing and the surface gradually becomes less smooth. The surface also scratches more easily. The tanks are either made of plates welded together into tanks or the tanks are rotation cast (a special casting process). Concrete is much used for larger tanks, where the price is competitive; con-crete may also be mixed on site or prefabricated elements can be joined together. The other material that has been used to some extent is metal; for example steel plates (stainless, acid-proof or coated) or aluminium (special quality). Tanks of tarpaulin with a frame of steel or wood represent a low cost easily movable construction.
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