1. Introduction
Agriculture and other food products are dried for use. The agriculture grains are dried for future use or production process, in order to dry the agriculture products the sun drying has been used for many years. The sun drying dries the products in a open air condition with the help of hot sun rays. In past centuries the sun drying systems has been used. Most of the industries use mechanized drying instead of sun drying system. The mechanized drying uses boilers and fan to heat and force the air to the required part. The time taken for the heating process is very low when compare to the sun drying system. The latest development in the heating process is the solar drying system. The solar drying uses sun’s energy for drying not in the open air condition. The solar drying is cost effective than the mechanized drier, which uses fuel to dry the products. In solar drier air is drawn by natural convection, but in the forced convection solar drier the air is drawn by the forced flow, using the fan the air flow is determined. The drying time is reduced by the factor 3. The area of the collector required is reduced. The solar drying system has high installation cost around 2000€ in average. The electricity required is low. The necessity of solar drier is completely depends on the usage scale. The initial cost will be the main constrain to the farmers. The grid electricity can be taken to run the fan.
2. Principle working of Forced Convection Solar Drying
The forced convection solar drier forces the air; the heating time is reduced by the factor. Since it is a time consuming process it has been used wide over. The forced convection solar drier is composed of flat plate with 2*1m2 of area. The forced convection solar drier is composed of drying chamber. The absorber is made of copper plate of 2mm thick. The copper plate is used to absorb and store the temperature quickly. The cost of absorber plate is less when compared to the other conductor plate. The absorber plate is placed behind the transparent glass cover. The temperature is increased using the green house effect principle. The transparent glass plate is of 5mm thickness has been placed. The gap between glass and the absorber cover of 25mm has been left to maintain the constant flow of air. The collector is connected to the one end of the blower and the other end of the collector plate is connected to the drier. The sand is filled between the gap of absorber plate and the insulation. The drier chamber si made of mild steel. The solar air heater is tilted at the angle of 25º. The whole system is tilted to orient in the direction of south. The south orientation can be used for effective heating. The solar drier is connected to the temperature indicator. In forced convectional solar drier the air is forced to flow in to the absorber plate and the absorber plate is heated up by the sun energy. The green house effected is maintained between the transparent plate and the absorber plate. The hot air from the absorber plate is used to heat the application (products). The main advantage of using solar drier is the time for heating is been reduced to the factor 3. The most time consuming factor for drying applications. The initial cost of installation Is the major constrain of using the solar drying system. The forced convection solar drier does not use fuel for drying purpose.
3. Methods of Forced Convectional Solar Drier
The solar drying system has been classified based on the methods of collecting air. There are different methods of solar drying system. The two main methods of solar drier is direct type solar drying and indirect type solar drying. The direct type solar drying system are the natural convectional type solar drying and the indirect type solar drying are the forced convection type solar drier. In natural convection type solar drier the natural air is allowed to flow in to the absorber plate for drying, but in convectional drying system the air is being forced by the fan to flow in to the absorber plate. The methods of forced convection solar drier are
- Solar Drier with Green House as a collector
- Geodesi Dome Fruit Drier
- Solar Tunnel Drier with integral controller
- Solar Air Plate Collector with obstracles
- Indirect type Solar Fruit and Vegetable Drier
3.1 Solar Drier with Green House as a collector
In solar drier with green house collector uses the fan and the plastic film to produce the green house effect. The trays are mounted in the wooden chamber, in which the grapes are used to spread on the trays. The temperature is maintained about 20ºc by green house effect. This type of drier is used for effective heating.
Advantage
- The constant amount of temperature can be maintained using the green house effect.
- Wide variety of products can be used in drying
- The structure is simple
Disadvantage
- The plastic sheet usage will affect the atmosphere
- The cost of installation is high
3.2 Geodesi Dome Fruit Drier
The Dome fruit drier consists of a black absorber sheet and the transparent sheet. This type of drier has the large wooden plate. The air is forced to pass through the two sheets. The air gets heated up on passing through the two sheets and the hot air is allowed pass through the products at the center dome. The dome is switched on during sunshine period and switched off during sun off period.
Advantage
The time taken for drying is low
The dome structure helps to increase the temperature of the air
The dome can be switched off and on in convenient time
Disadvantage
The cost of installation is high
The structure of the dome design is complex
3.3 Solar Tunnel Drier with the Integral Controller
In this type of drier the collector plate and the tunnel drier arrangement has been made. The grapes are spread in the tunnel drier. The air is heated by the collector (absorber). The absorber and the tunnel drier are covered with the help of transparent foil. The air gets heated up and the hot air is allowed to pass through the products.
Advantage
The collector plate and the tunnel arrangement are used to heat the air fast.
The time consuming process of drying is low
Disadvantage
The cost of installation is high
3.4 Solar Air Plate Collector with the obstacles
The solar collector is made of polycarbonate. The absorber plate of aluminum is coated with the black paint. The solar flat plate collector is made of forced convection mode. The obstacles are made of polystyrene. The presence of obstacles is to reduce the maximum amount of heat from the absorber. The collector is placed at the inclination of 45º facing south. It enhances the system efficiency with the minimum load loss.
Advantage
The special materials are used to increase the heating efficiency of drier
This drier enhances the system efficiency with the minimum load loss
The design of the drier is simple, and it is used to dry the fruits like grapes
Disadvantage
The Cost of installation is high
The obstacles reduces the maximum amount of heat from the absorber
3.5 Indirect type solar fruit and vegetable drier
In this type of drier have two different types of solar collector, in one type the air is sucked using the electrically operated ventilator. The air heater is covered with the polyurethane in order to reduce the conduction and convection loss, the two drying boxes are connected individually which are operated in low and medium temperature. The heated air is allowed to pass through the flat bed tray. The range of low temperature is between 50 to 60ºc and the high temperature can be of 75 to 80ºc.
Advantage
This type of drier are effectively used to dry the vegetables
This drier can be used in the two levels of temperature
Disadvantage
Since it is operated in the two levels of temperature, the installation cost is high.
The drier has the complex design structure
4. Conclusion
The world is facing the heavy impact on the temperature change. The use of oil and fuel for mechanized drying application is not recommended. Now days the drying is done using the solar drier. The solar drier acting as the cost effective method when compare to that of the other drying methods. The solar drying method does not use the fuel to dry the grains. The forced convection method of drying is used for the effective use of drying grains in a short interval of time. By the forced convectional method the time for drying is reduced by the factor 3. The most effective way of drying used world over.
References
1. Natural and solar drying, FAO corporate document Repository.Web. 03 Jan 2014. http://www.fao.org/docrep/t1838e/T1838E0V.HTM#Natural and solar drying
2. Solar Dyer, Cilmatetechwiki. Web. 03 Jan 2014. http://climatetechwiki.org/technology/jiqweb-edf
3. Solar Drying, Appropedia. 25 Dec 2011.Web.03 Jan 2014. http://www.appropedia.org/Original:Solar_drying
4. M Mohan Raj, P Chandrasekhar, Performance of a forced convectional solar drier with gravel as the heat storage material. Web. 03 Jan 2013. http://jestec.taylors.edu.my/Vol%204%20Issue%203%20September%2009/Vol_4_3_305_314_Mohanraj.pdf]
5. Solar Thermal Collector, Wikipedia. 03 Jan 2013. Web. 03 Jan 2014. http://en.wikipedia.org/wiki/Solar_thermal_collector
