Title of Invention

"AN IMPROVED NATURAL CONVECTION DRIER USEFUL FOR PROCESSING AGRICULTURAL PRODUCTS"

Abstract

An improved natural convection drier useful for processing agricultural products, which comprises an enclosed drying chamber (1) having a frontal door (9) and bottom (11) openings (6) characterized in that the said chamber being provided with conventional means for horizontal stacking of a plurality of removable perforated trays (4) for holding material (5) to be dried, the said chamber being also provided with centrally placed top and bottom (7) open longitudinal vertical duct (3), the said duct (3) being provided with heating means (8) such as herein described, at the said bottom (7) and resistance / mixing baffles at the top.

Full Text

This invention relates to an improved natural convection drier for processing agricultural products. The improved drier of the present invention particularly relates to the upward flow of heated air due to the decrease in air density caused by the increase in air temperature with reference to !he ambient air temperature and downward movement of the said hot air thus produced due to the increase in density of the air caused by the decrease in air temperature. The simultaneous self initiated upward flow of hot air and downward movement of the hot air through the bed of wet material to be dried makes the flow of hot air continuous through the bed of material. The contact of the hot air with the bed of wet material results in the drying of the wet material due to the simultaneous heat and mass transfer between the material and the hot air. Some of the agricultural and plantation products require slow drying at lower temperatures due to mainly structural characteristics, retention of flavour components, prevention of denaturation of some of the essential constituents, case hardening, non-availability of the necessary electrical/mechanical energy to make the system forced flow drying and factors related to cost of drying. The conventional natural convection driers are designed and operated as follows. The drier chamber has essentially two compartments, bottom chamber and top chamber. The two chambers are separated by perforated plates, slats etc depending upon the design. In the bottom chamber, the gas heated pipes, steam heated pipes or electrical heaters, as the case may be, is placed to cover the entire cross section of the chamber for heating the air contained in the lower chamber. Ventilation ports are provided at different points for the admission of fresh air. When the pipes get heated up the air surrounding the pipes also gets heated up. With increase in the temperature of the air the air density decreases and starts rising up to the upper chamber of the drier. Generally perforated racks are built in the upper chamber one above the other. The wet material to be dried is spread on these racks and through the wet material the hot air rises and in the process heat is transferred to the material and moisture is transferred from the material to the bulk of the rising air stream. The hot humid air finally escapes through the ventilation holes provided at the top of the chamber. References to the natural convection driers working on the above mentioned principle and procedure for the drying of different commodities are given in the literature and is outlined below. Reference may be made to the book Coconut Palm Products by Brian E. Grimwood, Published by Food & Agricultural Organisation (FAO) of the UN (1975), wherein details of (i) natural convection driers like Samoa drier, Comoro drier, the New College Copra drier (Philippines); (ii) the iron hot-table drier (Indonesia), Tonga hot-air drier, modified Tonga hot-air drier, NDO drier are given. The Samoa drier is made out of brick and mortar. The flue pipes (2 Nos. interconnected) made from sheet iron is placed in the bottom chamber. The gas unit is connected to a chimney outside the drier. The hot air rises through the bed of material kept on trays and racks in the upper compartment. The other types of driers as listed above also work on the same principle, but varies with the design features such as using iron plates instead of pipes, layout of the heating pipes/plates. The natural convection driers as mentioned above have certain drawbacks. Quite often from the trays kept above the heat source, the material to be dried falls on the hot pipes/plates. This leads to fire and may eventually destroy the complete dryer and material inside. The heated pipes heats up the surrounding air and such heated air rises up. Since the surface area of the pipes cover only a portion of the total drier. Cross sectional area, wide non-uniformity of air temperature inside the drier can be observed. This leads to overdrying in some parts and underdrying in some other parts. Another disadvantage is the non-uniform air flow through the bed of material due to shrinkage of the wet particles on drying. Most of the agricultural products shrink as they give up moisture during the drying process, thus reducing the resistance to flow of hot air. This will also lead to increase in airflow velocity, leaving cold spots in the product. This will result in the non-uniformity of drying in the product. This defect will also cause decrease in thermal efficiency. Drying of heat sensitive materials using natural convection driers are very difficult due to the afore mentioned drawbacks. To overcome the disadvantage of the conventionally used natural convection driers, research work was carried out by us for the development of an improved natural convection drier which resulted in the development of the present invention. The main object of the present invention is to provide an improved natural convection drier which obviates the above noted drawbacks. Another object of the present invention is to make the heated air to percolate through the bed of material in a downward direction without the use of any blower or fan as against the conventional upward motion of hot air through the bed of material. Yet another objective of the present invention is to avoid the dried material falling on any hot surface and thereby preventing any fire or explosion. Still another objective of the invention is to admit uniformly heated air to the entry point in the drying chamber and make it to flow downwards, keeping the temperature at the horizontal cross section of the drier constant. The principle on which the development of the improved natural convection drier of the present invention is based on is as follows: Heated air rises up due to decrease in air density. The hot air rising through a duct or pipe is directed to the top portion of the dryer. The hottest air gets collected in the topmost zone of the dryer. When further hot air enters the zone the existing hot air is pushed down. When the hot air comes into contact with the wet material to be dried, the temperature of the air drops and moisture enter into the air steam. This causes an increase in air density with a tendency for the air to flow downwards. The cold humid air finally leaves at the bottom of the drier. There is a steady and uniform movement of the air first upwards and then downwards. Since the flow of air is downward through the material to be dried temperature will be uniform throughout the horizontal cross section. In the drawings accompanying this specification an embodiment of the improved natural convection drier of the present invention is shown. In the drawings, figure 1 represents the front sectional view of the drier of the present invention. Figure 2 represents the sectional top view at XX of figure 1. Accordingly the present invention provides an improved natural convection drier useful for processing agricultural products, which comprises an enclosed drying chamber (1) having a frontal door (9) and bottom (11) openings (6) characterized in that the said chamber being provided with conventional means for horizontal stacking of a plurality of removable perforated trays (4) for holding material (5) to be dried, the said chamber being also provided with centrally placed top and bottom (7) open longitudinal vertical duct (3), the said duct (3) being provided with heating means (8) such as herein described, at the said bottom (7) and resistance / mixing baffles at the top. A feature of the present invention is that the drying chamber may be provided with insulation (2).Another feature of the present invention is that the heating means used may be such as electrical, thermal, solar either alone or in combination. The improved natural convection drier of the present invention useful for the processing of agricultural products comprises a drying chamber (1) in which means for horizontal stacking of a plurality of perforated removable metal trays (4) are provided for keeping the fresh raw material (5) to be dried and a frontal door (9) for putting in and taking out the dried product are provided. The hot air needed for the drying is generated by heating through means (8). The heating means (8) may be one or more metal pipes (8) in the case of use of flue gases, electrical heating elements in the case of electrical heating system or could be the air inlet port from a solar flat plate collector, either used singly or in combination. The hot air surrounding the heating means (8) gets heated up and rises through a top and bottom open longitudinal vertical duct (3) and enters into the top part of drying chamber (1). As some air is lifted up fresh air from atmosphere enters at duct (3) bottom (7) and gets heated up and the process continues. As hot air enters and occupies the top most layer in the drying chamber an equal amount of cold air is pushed down through the perforated trays containing the material to be dried and leaves at the bottom (11) of drying chamber (1) and escapes into the atmosphere through the openings of the weld mesh or perforated plates (6). This process continues and as the hot air surrounds the material to be dried heat is transferred to the material and moisture to the air stream. This results in a decrease in air temperature and increase in density of the air. The decrease in density of the air in the duct (3) and increase in density of the air in the drying chamber overcomes the frictional resistance and ensures a steady flow of hot air through the bed of material (5) kept on perforated trays (4) in the downward direction. The temperature is controlled at desired points by manipulating the heating system (8). To ensure uniformity of temperature when the hot air rises through the duct (3), resistance/mixing baffles (10) are provided at the top end of the said duct (3). To minimize the heat loss to the surroundings the dryer is insulated on the outside with insulating materials (2). The material in the top most tray gets dried first. The drying will continue till the equilibrium moisture content is reached. The drying can be continued till the material in the bottom most tray also reaches equilibrium moisture content. Alternately, if so much very low drying corresponding to equilibrium moisture content is not necessary then the trays can be interchanged from top to bottom two, three or four times for uniform drying in all the trays. When the drying is over the dried material can be taken out with or without the trays by opening the door (9) provided for the purpose. The drier can handle a wide range of materials and the drying condition can be suitably selected. The use of the improved drier of the present invention is described in the following examples which are provided by way of illustrations only and therefore should not be construed to limit the scope of the invention. Example 1 Drying of fruit: Seedless variety of grapes was taken for the drying experiment. The grapes after cleaning was subjected to alkali treatment for the removal of wax coating from the surface of the berries. The berries were spread on the wire mesh trays and dried at a temperature from 55 to 60°C. The product was comparable to similar product available in the market. Example 2 Drying of vegetable: Bitterguard was selected washed and the adhering water was removed. The vegetable was sliced into rings with about 4 to 5 mm thick. The slices were cleaned to remove the seeds and other unwanted material. The slices are spread on perforated wire mesh trays and dried. The temperature selected was in the range of 50 to 60°C. The product was crisp at the end of drying. Due to the good temperature control and uniformity in air temperature most of the green chlorophyl was intact. Example 3 Drying of an oil containing material: Matured coconuts were taken, water removed and the coconut kernel was removed by hand using knives. The wet kernel was sliced into slices of thickness of 4 to 5 mm and kept on the perforated trays in the drier. The temperature of the drying was between 60 to 70°C. The dried kernel was white in colour. Example 4 Drying of heat sensitive materials Cardamom after harvest is green in colour and on drying unless very uniform temperature is maintained, the green chlorophyl colour gets denatured. Fresh cardamom after alkali treatment was spread on the trays and dried. The temperature was gradually increased from 50°C to 60°C. The product retained most of the green colour. The main advantages of the improved natural convection drier of the present invention are : 1. Simple construction and ease of operation. 2. Compact unit. 3. Economical fabrication/construction of drier. 4. Drying can be controlled to yield desired characteristics for the material to be dried. 5. Improved uniform distribution of temperature across the horizontal cross section of the drier resulting in better dried products. 6. The heating unit and the hot air carrying duct can be placed suitably to suit the location. 7. Since the passage of hot air is in downward direction, all the particles to be dried will be contacted with hot air ensuring uniformity in drying. 8. Chances of outbreak of fire and explosion are eliminated. 9. The size of the drier can vary to suit domestic use to industrial scale with capacity to dry few kilograms to tonnes per batch. 10. Different fuels can be used to generate hot air. Hot air produced using solar flat plate collector also can be used and integrated with the drier and if necessary in combination with other modes of hot air generation. We Claim: 1. An improved natural convection drier useful for processing agricultural products, which comprises an enclosed drying chamber (1) having a frontal door (9) and bottom (11) openings (6) characterized in that the said chamber being provided with conventional means for horizontal stacking of a plurality of removable perforated trays (4) for holding material (5) to be dried, the said chamber being also provided with centrally placed top and bottom (7) open longitudinal vertical duct (3), the said duct (3) being provided with heating means (8) such as herein described, at the said bottom (7) and resistance / mixing baffles at the top. 2. An improved natural convection drier as claimed in claimed 1 wherein the drying chamber is provided with conventional insulation (2). 3. An improved natural convection drier as claimed in claims 1 and 2 wherein the heating means (8) used is selected from electrical, thermal, solar either alone or in combination. 4. An improved natural convection drier useful for processing agricultural products substantially as herein described with reference to the drawings accompanying this specification.

Documents:

300-del-2000-abstract.pdf

300-del-2000-claims.pdf

300-del-2000-correspondence-others.pdf

300-del-2000-correspondence-po.pdf

300-del-2000-dawings.pdf

300-del-2000-description (complete).pdf

300-del-2000-form-1.pdf

300-del-2000-form-19.pdf

300-del-2000-form-2.pdf