US 3500552 A
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March 11, 1910 D, F, FAR ET AL 3,500,552
METHOD ANDAPPARATUS FOR DRYING FOODS Filed March 7, 1.968 2 Sheets-Sheet 1 MATERIAL TO BE DRIED Fig.1 T
AIR HE ATER ,4
AIR COMPRESSOR f3 D.F. FARKAS 8; M.E. LAZAR INVENTORS ATTORNEYS March 17, 1970 I D. F. FARKAS ET AL 3,500,552
METHOD MD APPARATUS FOR DRYING FOODS Filed March 7, 1968 2 Sheets-Sheet 2 SPENT AIR CYCLONE COLLECTOR l I8 DRY l PRODUCT T HOT AIR CENTRIFUGAL FORCE 23 3K5 /6 DRY I FEED PARTICLES MVJf/lMMlZlVlMf/IVJVJVJMM 1 F 3 HOT AIR D.F. FARKAS a M.E. LAZAR INVENTORS BY fl/ ATTORNEYS S; -Wdcs United States Patent 3 500 552 METHOD AND APPARAIiJs FOR DRYING FooDs Daniel F. Farkas, Berkeley, and Melvin E. Lazar, Oakland, 'Calif., assignors to the United States of America as represented by the Secretary of Agriculture Filed Mar. 7, 1968, Ser. No. 711,315
Int. Cl. F26b 5/08 US. Cl. 34-8 Claims ABSTRACT OF THE DISCLOSURE In dehydrating particulate foods and other heat-sensitive materials by blowing hot air through a bed of the material, centrifugal force is applied to the bed to resist the lifting action of the air stream. This permits, the use of high air velocities without blowing material out of the bed, and results in faster and more uniform dehydration and reduces the size of the equipment needed for a given throughput.
A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sub-licenses for such purposes, is hereby granted to the Government of the United States of America.
This invention relates to and has among its objects the provision of novel apparatus and methods for drying foods and other thermally-sensitive materials, e.g., wool, feathers, kapok, and other organic fibrous materials; biological products, etc. Further objects of the invention will be evident from the following description and the attached drawing.
In the drawing, FIG. 1 is a side view of the apparatus of the invention, partly in cross-section. FIGURE 2 is an end view of the device, parts being broken away to illustrate the internal disposition of the feed pipe. In both FIGS. 1 and 2, auxiliary units of conventional design are depicted by block diagrams. FIGURE 3 is a fragmentary section, on an enlarged scale, of the centrifuge basket illustrating the bed of material undergoing drying therein.
In the dehydration of foods in particulate form-for example, potato dice, carrot dice, potato granules, etc. a known technique involves forming a bed of the particles on a screen or perforated metal plate and blowing hot air up through the bed. In conducting dehydration in this manner the rate of air flow through the bed may be held at a level such that the bed is static. More preferably, the air flow is high enough to cause the individual particles to move about or circulate within the bed. Such movement of the particles generally provides a better rate of moisture evaporation and more uniform dehydration of individual particles. However, there is a limiting factor in that if the air velocity is too high the particles are blown out of the bed before they are properly dried. As a result, one cannot realize the full benefit to be gained from high velocity air flow.
A primary object of the invention is the provision of apparatus and method by which the foregoing problem is obviated. The present invention makes it possible to hold the particles within the bed even though very high rates of air flow are applied. As a net result the invention permits one to take full advantage of the benefits deriving from high air velocities.
Basically, the objects of the invention are attained by counterbalancing the levitating effect of the air with centrifugal force. More particularly, the invention envisages a procedure as follows: A bed of particles of food or other thermally-sensitive material is subjected to rotation, and, concomitantly, drying air is forced inwardly (i.e., toward the axis of rotation) through the bed. The rotation creates forces tending to move the particles outwardly (i.e., away from the axis of rotation) whereby it opposes the tendency of the air to move particles inwardly. Accordingly, by suitable adjustment of the sp ed of rotation, one is enabled to employ air velocities higher than those which can be employed in conventional drying systems. As a result, many advantages are obtained, for example, faster rate of evaporation, and more uniform dehydration-not only of individual particles but also more uniform dehydration of portions of individual particles. With regard to rate of evaporation, our investigations have shown that we can readily achieve a rate of seven times that attained with conventional drying systems. Indeed, the rate of evaporation is so high that one is enabled to used air temperatures lower than with conventional systems, whereby the vital attributes of the products such as color, flavor, odor, etc. are preserved to a greater degree. Another advantage flowing from the high rate of evaporation is that our apparatus is much more compact than conventional drying units of the same capacity. A further item is that our invention is capable of application to low-density materials such as feathers, bulk wool, cotton linters, stable fibers, and the like. The drying of such light and fluffy materials normally requires the use of air suspension dryers which are very bulky pieces of apparatus. By application of the principles herein explained, even these low-density materials may be effectively dried in apparatus having a fraction of the size of conventional air suspension dryers.
Reference is now made to FIGS. 1 and 2 which depict a form of apparatus in accordance with the invention.
The apparatus includes plenum chamber 1 communicating with hot air inlet 2. In operation, air is forced by compressor 3 through heater 4 and into plenum 1 whereby this chamber is kept full of hot air under pressure higher than atmospheric.
Mounted within plenum 1 is a rotable centrifuge basket, generally designated as 5, which serves to support a bed of material under dehydration. Basket 5 includes a base member 6, a throat 7, and a circular wall 8, the last being of screening or perforated sheet metal. The openings in wall 8 are so selected that the particles of material are retained within basket 5 whereas hot air (from plenum 1) can pass therethrough.
Basket 5 is journalled in bearings 9 and 10, these including conventional gas seals to prevent escape of the hot air under pressure contained in plenum 1. For rotation, basket 5 is keyed to shaft 11 which is driven by a variable speed motor (not illustrated), or the like.
For feeding material to be dried into the system there is provided hopper 12, communicating via tube 13 with feed pipe 14. Compressed air introduced from inlet 15 impels the material into feed pipe 14 and into the interior of basket 5. By suitable adjustment of valve 16, the material may be introduced into the system at a desired rate. As shown in FIG. 2, feed pipe 14 extends into basket 5 with a slight twist so that material is not fed directly downward, but about 3545 counter to the direction of rotation. This disposition of the feed pipe is preferred so that the entering material quickly forms a part of the bed.
Dried material discharged from basket 5 passes to :hamber 17, provided with an outlet duct 18. For separation of the dried material from the accompanying flow of air, the stream exiting from duct 18 is conducted to a :onventional cyclone separator, represented by block 19.
A sight glass 20 is provided in the top of chamber 17. This permits the observer to see the bed of material within basket 5, so that he can make proper adjustments of speed of rotation, rate of hot air flow into plenum 1, :tc., as may be required.
Generally, it is preferred. to conduct dehydrations on a continuous basis (as opposed to batch operation) and the apparatus of the invention is particularly useful in such applications. In this type of operation, material to be dried is fed at a predetermined rate through feed pipe 14 into basket 5 while the latter is rotated and hot air forced into plenum 1. The speed of rotation and rate of air introduction are so correlated that the material forms a bed about the inside of perforated wall 8, and that the particles in the bed are fluidized or at least tumble about so that there is a circulation of the particles within the I formly, and with apparatus of small size relative to its capacity. Typical applications of the invention, in addition to those noted above, are provided below by way of illustration but not limitation.
The invention can be used for the puffing of various food products. In these applications the air stream is maintained at a temperature well above those conventionally used for dehydration so that the evaporation takes place so rapidly as to expand or pulf the material I under treatment. Depending on the type of product and bed. Also, the aforesaid variables are so correlated that i If it is desired to operate the system batchwise, a suitable quantity of material is fed into basket 5, which is then set into rotation and hot air applied to plenum 1. The speed of rotation and the rate of hot air introduction are again correlated to keep the particles in the form of a bed with circulation of individual particles within the bed. These conditions are continued until the product is dry, then the rate of air flow is increased (or the speed of rotation decreased) so that the product is blown out into chamber 17 Reference is now made to FIG. 3 which illustrates the bed, the flow of material, etc. during continuous dehydration. Reference numeral 22 designates the bed of particles, held in place about the inner face of wall 8 by the cen trifugal force generated by rotation of basket 5. The centrifugal force is represented by arrow 23. Hot air from plenum 1 is concomitantly forced through the bed counter to the direction of the centrifugal force. This flow of hot air, represented by arrow 24, counteracts the compacting effect of the centrifugal force and causes individual particles in the bed to move about randomly or circulate within the bed, as indicated by arrows 25. At the same time, there is a movement of particles from right to left caused by the continuous addition of fresh material adjacent to base plate 6. Thus, in all, the individual particles not only move about in random circulatory paths within the bed, they also move toward throat 7. As the particles approach the throat-end of bed 22 they are in a dehydrated condition and have a low density so that they are entrained by the exiting air flow and pass to discharge chamber 17.
As noted herein, the invention may be employed for the dehydration of all kinds of thermo-sensitive materials in particulate form. Typical examples of such materials are peas; beans; grains; berries; grapes; minced or diced fruits, vegetables, or meats, for example, diced apples, diced carrots, diced chicken flesh, minced onions, chopped water cress; granulated, powdery, or crystalline materials-such as potato granules, sugar, starch, flour, etc. Other examples are fibrous materials such as feathers, bulk wool, cotton linters, staple rayon fibers, and the like. The invention may also be applied to biological materials such as botanicals, yeast and other microbial preparations, bone,- organs, etc. Other examples are oil seeds, and residues from oil extraction such as the meals remaining from the extraction of cotton seed, safilower seed, etc.
Although the invention finds its greatest field of utility in dehydration, it may be employed to effectuate diverse the degree of puff desired, air temperatures of 350 to 1000 F. may be used. Typical of the materials which may be puffer are: potatoes, carrots, apples, sweet potatoes, or other fruits or vegetables in particulate form such as dice, slices, chips or the like, or in whole form with smaller products such as peas, beans, lentils, wheat, barley, rice, corn, milo, etc. Other products which can be puffed are processed grains, as for example: parboiled rice, bulgur in whole or cracked form, and peeled wheat products in raw or pre-cooked forms such as those described in Patents 3,264,113 and 3,358,723.
The invention can also be used for the roasting of all kinds of food products, for example, coffee beans, soy beans an other legume seeds, cereal coffee substitutes, etc.
The invention can also be used for blanching or cooking all kinds of particulate foods. In such applications, steam or water sprays may be introduced into plenum 1 'so that the gas stream contacting the food will effect the desired blanching or cooking effect without concomitant dehydration, or, with a desired degree of dehydration by suitable regulation of the amount of moisture fed into chamber 1. Also, during the use of the invention in such applications as dehydration, putling, blanching, cooking, etc. the gas stream can be used as a convenient means for contacting the food with a desired agent. Thus, one may introduce into plenum 1 such agents as wood smoke to provide the products with a smoky flavor, or preservative agents such as sulphur dioxide to prevent darkening of the products.
The invention can also be used for cooling, freezing, or pre-freezing particulate foods. In such applications it will be obvious that heater 4 would be replaced by a refrigerating unit so that the gas stream entering plenum 1 is at the appropriate low temperature required for treating the material.
The invention is further demonstrated by the following illustrative examples.
The dryer used in these runs was as described above, and wherein basket 5 had the following dimensions: diameter 6", depth 4" (excluding the throat), thus providing a drying surface (wall 8) of 75 sq. in.
EXAMPLE 1 Continuous dehydration of diced carrots The starting material was inch diced carrots, having a moisture content of It was fed into the system at the rate of 13.2 lbs. per hour. Basket 5 was rotated at 200 rpm. and air at 205 F. was supplied into plenum chamber 1 at a rate of 216 cu. ft. per min. (measured at standard conditions). The pressure in plenum chamber 1 was 5 in. of H 0 above atmospheric. The product, having a moisture content of 79.5% was produced at the rate of 6.6 lbs. per hour. (The piece residence time in the dryer was about 5 min.) The product in this partially dehydrated condition, representing a weight loss of 50% by evaporation of water, is eminently suitable for preparing dehydrofrozen carrots (Patent 2,477,605). It was observed that the diced form of the starting material was retained in the product, and taste tests indicated that there was no damage to color or flavor.
It was calculated that the drying efliciency was 12 lbs. of water evaporated per hour per square foot of drying surface. This is 2 to 3 times the efficiency obtained in conventional belt dryers. Moreover, the residence time in the dryer of the invention was only A to /s that required for a conventional belt dryer, indicating a 4- to 5-fold increase in evaporative capacity.
EXAMPLE 2 Continuous dehydration of diced apples The starting material in this case was A inch diced apples. The apple dice were fed into the system at the rate of 13 lbs. per hour. Basket 5 was rotated at 250 r.p.m. (providing an acceleration of 5 times gravity), and air at 220 F. was supplied into chamber 1 at the rate of about 500 cu. ft. per min. (measured at 220 F.). The pressure in plenum chamber 1 was about 5 in. of H above atmospheric. The product was discharged at the rate of 6.5 lbs. per hour. The piece residence time in the dryer was about min. The product in this partially dehydrated condition, representing a weight loss of 50% by evaporation of water, is eminently suited for preparing dehydrofrozen apples. It was observed that the diced form of the starting material was retained in the product, and organoleptic tests indicated that there was no damage to color or flavor.
It was calculated that the drying efiiciency was 13.7 lbs. of water evaporated per hour per sq. ft. of drying surface.
EXAMPLE 3 In this case the invention was applied for the continuous production of popcorn. Kernels of raw corn (popping variety) were fed into the system at 6 lbs. per hour. Various other conditions of operation were: Speed of rotation of basket 5, 200 r.p.m.; air temperature, 380 F.; air flow, 230 cu. ft. per min. (measured at 380 F.). The run resulted in continuous production of popcorn of excellent quality and large size.
EXAMPLE 4 In this run, the dryer was operated batchwise.
The starting material was inch diced carrots, containing 88.2% H O. The diced carrots (1.2 lbs.) were loaded into the basket which was then rotated at 375 r.p.m. Air at 203 F. was supplied into chamber 1 at the rate of 370 cu. ft. per min. (measured at standard conditions). After a period of 10 minutes, the speed of rotation of the basket was decreased, and the product was blown out and collected. It had a weight of 0.345 1b., representing a weight loss by evaporation of water of 70%. It was observed that the diced form of the starting material was retained in the product, and organoleptic tests indicated that there was no damage to color or flavor.
Having thus described the invention, what is claimed is:
1. A process for dehydrating particulate heat-sensitive material which comprises:
providing a mass of said particulate material,
subjecting said mass to rotation while restraining by a perforated surface, the speed of rotation being sufficiently high to hold the mass against the surface by centrifugal force,
directing a stream of hot air, in a direction counter to said centrifugal force, through said perforated surface and through said mass,
the velocity of the gas stream being sufficiently high to cause the individual particles to actively tumble about and circulate within the mass, but not so high as to eject particles from said mass before they have been dehydrated.
2. In the process of dehydrating particulate heat-sensitive material wherein the material is formed into a bed and wherein hot air is blown through the bed to dehydrate the particles, the improvement which comprises:
blowing hot air through the bed at such a rate that normally it would eject the particles from the bed and thereby disrupt the bed,
applying centrifugal force to the bed in a direction counter to the direction of the air flow,
the centrifugal force being high enough to hold the particles in the bed but not so high as to prevent the individual particles from actively tumbling about and circulating withinthe bed.
3. Apparatus for treating particulate heat-sensitive materials with a gas While being subjected to centrifugal force, comprising, in combination:
(a) a gas-tight enclosure defining a plenum chamber,
(b) means for introducing and maintaining a gas under pressure in said plenum chamber,
(0) a centrifuge basket having an axial throat and a perforated circular wall,
(d) the centrifugal basket being mounted for rotation within the plenum chamber, with the throat thereof extending outside the plenum chamber,
(e) sealing means cooperative with the throat and plenum chamber to prevent leakage of gas,
(f) means for rotating the centrifuge basket,
(g) means for feeding material into the centrifugal basket, and
(h) means for collecting product which is discharged through the throat of the centrifuge basket.
4. The apparatus of claim 3 wherein means b is means for introducing and maintaining hot drying gas under pressure in said plenum chamber.
5. The apparatus of claim 3 wherein said means g includes a tube extending generally axially through said throat into the centrifuge basket and terminating therein at a position about 35-45 removed from the lowest point of the circular wall, said angle taken in the direction counter to the direction of rotation.
6; A process for continuously dehydrating particulate heat-sensitive material which comprises:
continuously feeding said particulate material into a dehydration zone,
in the dehydration zone continuously subjecting the material to rotation while restrained by a perforated surface, the speed of rotation being sufficiently high to :hold the material against the surface by centrifugal force,
in the dehydration zone continuously directing a stream of hot air, in a direction counter to said centrifugal force, through said perforated surface and through the material held thereon,
the velocity of the gas stream being sufliciently high to cause the material to be fluidized in the dehydration zone and to be ejected from the dehydration zone only after the particles have been dehydrated to a predetermined degree, and
continuously collecting the dehydrated material which is ejected from the dehydration zone.
7. A process for treating particulate heat-sensitive material with a gas, which comprises:
providing a mass of said particulate material,
subjecting said mass to rotation while restrained by a perforated surface, the speed of rotation being sufficiently high to hold the mass against the surface by centrifugal force,
concomitantly directing a stream of gas, in a direction counter to said centrifugal force, through said perforated surface and through said mass,
the velocity of the gas stream being sufficiently high to cause the individual particles to actively tumble about and circulate within the mass, but not so high as to eject particles from said mass before they have undergone a predetermined treatment.
. 7 8 8. The process of claim 7 wherein the material is a References Cited food. V UNITED STATES PATENTS 9. The process of 01mm 7 wherem the material is a 275,596 4/1883 Conover 34 135 fiber- 1,751,841 3/1930 Pickens 34 133 10. The process of'claim 7 wherein the velocity of the 5 gas stream is sufliciently high to maintain the mass in a FREDERICK L. MATTESON, JR., Primary Examiner fluidized State- H. B. RAMEY, Assistant Examiner