Drying implies the partial removal of water from the material. But when fuel-fired equipment is used in the process then the term dehydration is often applied. Condition essential to both drying and dehydration include a supply of heat energy to evaporate the water and a supply of air to carry away the water vapor produced
In open air Solar drying, the heat is supplied by direct absorption of solar radiation by the material being dried. The vapor produced is carried away by air moving past the material, the air motion being due either to natural convection resulting from contact with the heated material or to winds. In dehydration using fuel heat, the material being dried is placed in an enclosure and heated air is blown past the surface of the surface of the material to remove moisture. Solar dryer makes use of solar radiation, ambient temperature, relative humidity. Heated air is passed naturally or mechanically circulated to remove moisture from the material placed inside the enclosure.
Many variations of solar dryers offer the processor a wide range of opportunity to select the various type of solar dryers depending on the degree of control over the drying process and handling of the material. The principal types of solar dryers are described briefly below.
Solar Cabinet dryer mainly consists of a drying cabinet. One side of the cabinet is glazed to admit solar radiation, which is converted into low-grade thermal heat thus raising the temperature of the air, the drying chamber, and the produce. Usually, the sunlight shines directly on the material being dried. The moisture evaporated by solar heat is removed by air circulation. This is accomplished either by designing to encourage natural convective air flow or by forcing circulation with fans or blowers. The material to be dried is placed in shallow layers on trays inside the drying cabinet. Proper air vents are provided for displacement of hot air. Solar Cabinet drivers are,
Solar greenhouse dryers are characterized by having extensive glazing on their south facing side while the other sides are well insulated. Inside the enclosed area, some means are provided to store the daytime excess heat. Vents are strategically sized and positioned to control air flow. A well-designed greenhouse dryer permits a greater degree of control over the drying process than the solar cabinet dryers and should be used where a relatively large quantity of product is to be dried.
In this type of dryer, the produce is placed on trays inside an opaque drying chamber to which is attached an air type solar collector. The sun does not shine directly on the material to be dried, instead the air heated in solar collector is ducted to the drying chamber for dehydration. Air circulation can be by natural convection, however, it is often forced by blowers. These dryers result in higher temperature than the cabinet dryers or sun drying and can produce a higher quality product.
In many large-scale commercial drying operations, air type solar collectors are being combined with fuel-fired dehydrators in order to reduce the fuel consumption and yet maintained fine control of the drying conditions. The solar heated air is used directly if it is hot enough, otherwise, the fossil fuel system the boost the air temperature to the required level Thus the effect of fluctuations in energy output from the collector is less serious since the fuel-fired system is automatically controlled to provide specific optimum temperature.
Design and specifications of the different type of dryers vary from case to case and it should be worked out individually.
Performance of a solar dryer changes for the different type of dryer operated at a different temperature. A basic requirement for installation of a solar dryer is the open space free from any obstacle to the solar radiation. For indoor drying/dehydration processes by solar heated air, air heating collectors are to be installed in open space on the ground, on a roof or on a terrace and ducted to the drying chamber.
Operation of solar dryers is very simple and it is considered to be maintenance free. However for efficient performance general maintenance of cleaning of the glazing, regular coating of absorbing surface and instruments and controls.
The basic principle of a solar dryer is that air is heated by the Sun in a collector and then passed over the produce, which is to be dried. Many fruits can be dried in Solar Dryers that use the Sun’s heat to evaporate the moisture in food items. When the moisture content is low, the food does not rot or go bad. this way we can preserve food and use it for a long time. In the Solar Fruit Dryer, you can dry fruits, such as grapes, quite easily. And they taste especially good. If fresh grapes are Rs. 20 a kilo in season, and dried grapes are Rs. 200 a kilo, which is a better buy?
Find a box with dimension that will leave about 20 cm in front when a 45o angle is made from the top corner. Cut asection out of the box as shown. The entire inside of the box needs to be black. You can glue black poster paper to the inside with fevicol. If you choose to paint it, first brush on a coat of shellac, let it dry for 5-7 hours, then paint it with a black enamel paint.
Cut a door in the back of the box large enough to let your hand through, plus whatever you will be putting in to dry. Reinforce the uncut, hinging part of the door with a piece of duct tape. Poke holes in the bottom and sides, as shown, with a sharp pencil (about 30 holes). Evaporated moisture will be ventilated out through these holes.
Make 3 or 4 screen support wires out of a thick wire such as a coat hanger wire, and stick them through the box near the bottom as shown and bend down the ends.
Get some nylon fly screen material or mosquito netting and cut it larger than the bottom of the box, so that the edge can be turned up and laced to the sides of the box.
Use “Clear Vinyl Wrap” to cover the open top of the box. Fasten the edges down securely with tape. Make two legs out of cardboard and fasten them to the bottom with duct tape as shown. If you are worried that insects may get in, cover the vent holes with more nylon screen and fasten it with tape.