Tank design
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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