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DRYTECH SMALL SCALE DRYING TECHNOLOGIES Globally, drying is the most widely used method for preserving foods for use in the home or for sale. The most common method involves simply laying the product in the sun on mats, roofs or drying floors. This is known as sun drying. As shown below, simple sun drying has advantages and disadvantages.
- Very little cost
- Ideal for products where little or no value is added
- Food usually dried close to the home
- Open to contamination by dust and microbes
- Totally dependent on good weather
- Very slow drying rates with danger of mould growth
- It may not be possible to dry to a sufficiently low level of moisture to prevent mould growth
- Exposure to oxygen may lead to degradation of nutrients, e.g. vitamins (compare to freeze drying)
- Sun drying remains, however, the only economic choice for poor people when drying foods such as grains in large quantities and where little or no value is added by drying. When drying foods such as fruits, vegetables, spices, herbs and nuts value is added by drying and in such cases an investment in improved drying technologies may be economically viable. This technical brief describes improved systems suitable for use by small scale producers.
Solar dryers use a simple construction to more efficiently make use of the sun's heat. Under the correct climatic conditions, they can provide many advantages over sun drying. These include: higher drying temperatures, which results in shorter drying times and the ability to achieve a lower final moisture content. The apparatus also provides protection from rain showers and from contamination by dust. Solar dryers are low cost and simple to construct in local workshops. They consist of a transparent panel above a chamber or collector that is pained black to absorb the sun's heat. Polyethylene ("Polythene"), which is a very inexpensive material, is commonly used to glaze the panel but it may turn yellow and opaque after a few months and will need to be replaced. Plastic films with UV-protection, not easily damaged by sunlight, are now increasing available and should be used if possible. While more expensive, they may have a lifespan of 5 years or more. It is very important to position the collector at the correct angle to the sun: the angle should be greater than 15
to allow rain water to run off the collector should be angled at 90
Tent solar dryers, as shown in Figure 1, are cheap and simple to build and consist of a frame
of wood poles covered with plastic sheet. Black plastic should be used on the wall facing Drying Technology away from the sun. The food to be dried is placed on a rack above the ground. Tent dryers provide protection against rain, insects and dust. In the case of fish for example, this can reduce drying times by 25%. Tent dryers can also be taken down and stored when not in use. They have the disadvantage of being easily damaged by strong winds . Figure 1: Typical Tent Solar Dryer The Brace Solar Dryer as shown in Figure 2 has been widely used for small scale food drying. It consists of a wooden box with a hinged transparent lid. The inside is painted black and the food supported on a mesh tray above the dryer floor. Air flows into the chamber through holes in the front and exits from vents at the top of the back wall. Brace type dryers achieve higher temperatures, and thus shorter drying times, than tent dryers. Figure 2: Typical Brace type dryer A further design of solar dryer uses of a separate solar collector which supplies heated air to a chamber containing trays of food. This is known as an indirect solar dryer and a typical design is shown in Fig 3. Dryers of this type have several advantages over direct dryers which include: Drying Technology the food is not exposed to the direct rays of the sun which reduces the loss of colour and vitamins. the collector can be large and thus heat greater quantities of air Indirect dryers are, however, more expensive and complex to construct. Figure 3: Indirect Solar Dryer The obvious problem with solar dryers is their inability to work when there is little or no sunlight. The McDowell
dryer shown in Fig 4
attempts to overcome this problem by combining solar heat and heat from wood burning in a fire box. The Mc Dowell dryer consists of a block structure with a fire box at one end. Heat from the fire passes through a metal pipe below the tray of food and exits via a chimney. The whole Drying Technology structure supports a glazed solar roof. Figure 4: McDowell Mixed dryer Small Electric Dryers, as shown in Figure 5, can be constructed at a cost equal to or less than the solar dryers described above. Typically, such a dryer consists of a plywood box one metre cube, with a door to the front. Between 6 and 10 mesh trays are supported on wooden runners on the dryer walls. Heated air is supplied by a domestic 2kw fan heater to the rear base wall. The heater should be re-wired with an external thermostat (placed in the drying chamber) so that the heater elements are controlled by the thermostat while the fan runs all the time. This is a simple job that can be carried out by a competent electrician. The air exits via an opening at the top of the rear wall. The trays should be arranged so as to cause the air to follow a zig-zag path as it passes through the dryer as shown in Figure 5. Figure 5: Small Electric Dryer. The air should follow a zig zag path. Provided sufficient value is added by drying to cover electricity costs, such dryers offer distinct advantages over solar dryers which include:
- greater daily output
- higher drying temperatures and thus considerably more rapid drying
- faster drying due to the forced air flow
- total independence from weather conditions
- good control of the drying process.