I.5. Smoking kilns

Most traditional kilns used for smoke drying are very simple in design and construction. They range from the simplest type, which is an open fire above which the fish are laid on a grill, to a mud or rush hut in which the fish are placed on racks above a fire. The main disadvantages from which most traditional types of smoke drying kilns suffer are a lack of control over the fire temperature and smoke production, inefficient use of fuel and a low throughput of material.

The following sections describe a number of traditional kilns which have been improved in various ways.

I.5.1. Oil drum kilns

One type of traditional kiln consists of a round or rectangular mud wall about 1 m in height and 1.5 m in diameter or side. The fish are laid on wooden racks placed over the top of the walls, the fire being lit and tended within the kiln through an access hole in one side. An adaptation of this kiln is the simple oil drum kiln, a number of designs of which are found in the tropics. These kilns are both light and portable and are well suited to cases where fishermen migrate along the coast as, for example, in West Africa. The simplest drum kilns, however, are not much more appropriate than the traditional mud type (i.e. a fire is built at the base of the drum, and the fish are laid on racks placed over the open top). The capacity of the drum is often very low, and modifications to allow more fish to be smoked at one time have been suggested.

In one improved kiln, an oil drum with the top removed and the base perforated is placed over a stone or brick-built hearth in which a fire is built. The fish are placed on a series of galvanised wire trays hung within the drum and a loose fitting lid of corrugated iron is fitted to the top. The advantages of this method over the simple drum kiln are an increased capacity, more control over the fire and hence less chance of charring the fish, and an easier maintenance of high temperatures within the kiln. A further modification is obtained by cutting oil drums into three sections and fitting them with handles and rods on which the fish can be hung. An example of a kiln of this type made from two oil drums is shown in Fig. III.1. In the lowest section (i.e. the fire box), the oil drum lid is retained and perforated to allow a well distributed upward passage of smoke, and to prevent charring of the fish being smoked in the lowest sections which can be stacked up to 6 m high. The advantages of this improved kiln include the increased fish smoking capacity and easy portability and flexibility as it is possible to interchange sections to allow even smoking.

Figure III.1. Oven constructed from two drums (Additional drums may be used to build a 6 m high smoking kiln)

Figure III.2. Drum smoker

Another type of modified oil drum kiln is the Watanabe type shown in Fig. III.2. It consists of a smoke box which is connected to a separate fire box via a stove pipe of variable length. This facility allows fish to be smoked at different temperatures unlike the other oil drum kilns described above which are used solely for hot smoking.

I.5.2. Altona type kiln

The simplest version of this kiln is shown in Fig. III.3. It consists of a wooden smoke unit placed above a fire box. The fire box is built from clay or sun dried clay blocks. Typically, the inside dimensions of the fire box are 100 × 110 × 90 cm high. Three of the walls are 30 cm thick whereas the fourth front wall is 20 cm thick. This front wall has a 45 × 45 cm hole near the bottom for inserting the fuel and controlling the fire.

The smoke box is constructed from a wooden batten framework covered with corrugated iron sheets. Typical inside dimensions of the framework of such a unit are 120 × 120 cm with a height of 102 cm in the front and 90 cm at the rear. It is fitted with 7 pairs of parallel supports on which the trays carrying the fish slide. The front of the smoke unit can be closed by two corrugated iron doors, hinged onto the framework, in such a manner that small openings are left in the front and the back between the frame and the top cover. These openings allow a relatively small quantity of air and smoke to pass, thus keeping a high temperature and pressure inside the oven which results in a more uniform distribution of heat and smoke throughout the smoke box. A detailed specification of material needed to construct this kiln is given by FAO (1971). Similar types of kiln have been described by Beatty (1964) and Rogers (1970).

I.5.3. Ivory Coast type kiln

A further development of traditional smoking techniques is the adoption of the West African banda kiln known as the “Ivory Coast kiln”. This kiln is fairly easy to construct from limited amounts of expensive materials and, because the fire is contained within an oil drum fire box, the rest of the kiln can be constructed of wood without the risk of fire. In comparison to the Altona type kiln, the Ivory Coast kiln has a number of advantages in that its operation is similar to that of the traditional type oven, the fuel costs are low, the construction is very simple and more uniform smoking is achieved. The Altona type oven on the other hand is more suitable for large quantities of fish and has a superior racking system for holding the fish.

Figure III.3(a). Simple version of Altona-type oven with fire box built from clay: Overall view

Figure III.3(b). Simple version of Altona-type oven with fire box built from clay: Ground plan of fire box

Notes: 1. If the fire-box is constructed from sun-dried clay, blocks; the inside dimensions should be 120 × 110 cm, and the walls 90 cm high and 20 cm thick.

2. The materials needed to construct a simple Altona-type oven include (for fire-box built from dried clay blocks):

The Ivory Coast kiln, as shown in Fig. III.4, consists of a walled enclosure which can be constructed from a variety of materials such as locally available stone cemented together with mud, or poles and mud, or wooden posts and corrugated metal sheets. The structure must be air tight and the top rim must be smooth and flat. A fire box made from an oil drum is set into one side of the enclosure. A square baffle plate, made from a steel sheet perforated with 1 cm holes, is suspended above a hole cut in the oil drum by wires attached to each of the four corner posts of the enclosure. This ensures a good distribution of smoke and heat. The fish are laid on wooden framed chicken wire racks which rest on top of the walls of the enclosure, one on top of another. Four or five racks of fish can be smoked at once. During smoking, the upper rack is covered with a sheet metal roof for protection against rain.

I.5.4. Mechanical smoking kilns

The traditional and improved smoking kilns mentioned above rely for their operation on heat convection for air and smoke circulation. As with mechanical drying methods, forced convection is also possible for smoking operations. In most designs, it is possible to regulate the relative humidity, temperature, smoke density and air velocity so as to guarantee product uniformity, a prerequisite for the sophisticated smoked fish market. Mechanical smoking kilns are used extensively in Europe and North America where the production of products such as kippered herring and buckling requires a high degree of control. The main advantage of mechanical smoking is the uniform quality product. However, the equipment is generally very expensive and the extra expense may not be worth considering for most Third World situations.