US2764819A - Method for drying granular material - Google Patents

Description

Get. 2, 1956 N. J. HALLMAN 2.764 819 METHOD FOR DRYING GRANULAR MATERIAL Filed Sept. 4, 1952 INVENTOR NILS JOHAN HALLMAN ATTORNEYS NIETHUD FUR DRYINGGRANULAR MATERIAL Nils Johan Hallman, Stockholm, Sweden, assignor to AktiebolagetZander 8; Ingestrom, Stockholm, Sweden Application September 4, 1952, Serial No. 307,734

2 (Ilaitns. (CI. 34-13) The presentinvention relates to the art of drying granular material, especially grain;

More particularly, the invention refers to a method in the drying of granular material, especially grain, according to which the material is preheated, for instance in'a separate preheating zone, by means of a passing current of hot air and is thereafterbrought to pass through a drying zone in which it is traversed by air of suitable temperature. which such a'method is performed, are previously known, and such apparatus may be provided below the drying zone with a cooling zone to reducethe temperature of the grain before it leaves the apparatus. Itis, furthermore, known to let the vapours comingfrom the drying zone indirectly heat the grain in the preheating zone.

It is an object ofthe present invention to improve in such apparatus the heat economy and reduce the time required for drying the grain. Withthis object in view there has been provided according to the invention a drying method according to which the material, after having been preheated, is brought to pass through a sweating zone where it is traversed by air of such a temperature that the temperaturein the outer or surface layer of the grain is reduced and the moisture inthe interior ofthe granules penetrates to the said outer layer, the material being only thereafter introduced into the drying zone. A particularly good economy is obtained if hereunderthere is supplied'to the sweating zone, air which has passed through and has had its temperature reduced in the drying, zone. To facili tate regulation of the flow of air and the percentage of moisture in the exhausted air, the air which has been used in the different zones can be dischargedthrough a common outlet, substantially the entire amount ofthe discharged air being passed'through the sweating zoneibefore its discharge.

The invention is of interest also in connection with the known method in which the material, after its passage through the drying zone, is introduced into a cooling zone, where it is traversed by air of such a temperature as to be cooled thereby and in which fresh air is taken in through an inlet which is common to the zones, substantially the entire amount of the fresh air taken in being then conducted through the cooling zone. In case a separate preheating zone is being used, it may be suitable, according to the invention, to circulate air in a closed circuit through the preheating zone and an external circuit, in which external circuit the circulating air together With additional air from the cooling zone is heated, the excess of heated air thus obtained being conducted to the drying zone and from the latter to the sweating zone and finally out to the atmosphere.

One form of application of the invention in a shaft type drying apparatus for grain is illustrated in diagram form in the accompanying drawing.

According to the example illustrated in the drawing" the grain 1 is fed into a preheating zone 2 through which air at a temperature of for example 60 C. can be con- Shaft or column type drying apparatus, in

ite States Patent "ice 2. ducted in a manner known per se, so that the grain is heated for instance to 40 C. In case the grain has already begun to be heated by itself on account of internal chemical changes, it need not be separately heated in the preheating zone. The grain then sinks under its own weight down through one or more constricted openings 3 and enters a sweating zone 4, Where'the grain is traversed by an air current having a lower temperature, so thatthe temperature in the. surface layer of the granules is reduced for example to 30 C., which requires that the temperature of the air is lower than the surface temperature of the preheated granules. This results in that moisture from the interior ofthe granules will pass towards the surface, so that the granules will sweat. The

major portion of the moisture concentrated in this manner to the surface layer is evaporated and carried away by the traversing air which thus becomes comparatively moist. Thereafter the grain is conducted through a restricted opening 5 and enters the drying zone proper 6, in which it istraversed by comparatively dry and warm air of a temperature of for'instance 60 C. In said zone further moisture, which mayremain after the sweating, is removed and the grain is dried down to the desired percentage of moisture.

Particularly in cases when the material sinks down through a vertical shaft'or column the distribution of the material is generally maintained in vertically extending layers as indicated at A1 and A2, Blland B2, and C1 and C2. The material in the vertically extending A layer at Alin the sweatingzone will be traversed by air coming infrom the duct21, whereasthe same material in the A layer at A2 in the drying zone will be traversed by air passing out to the duct 21. This means that the ma terial in the A layer at A1 in the sweating zone will be traversed by air that has been cooled by passage through the three layers C2, B2 and A2'in the drying zone, and thatthe materialin the A layer at A2 in the drying zone will be traversed by air that has been cooled in zones C2 and B2 in the drying zone. Material in the C layer at C2 in the drying zone is traversed by air passing directly from the duct 18, whereas material at C1 in the sweating zone is traversed by air which has been heated by its passage through thezonesAl and B1. Thus there is involved a countercurrent flow of air in which the material which has been subject to the lesser amount of cooling in the sweatingzoneis subjected to the greater amount ofheating in the drying zone and the material which was subjected to the greater amount of cooling in the sweating zone is subjected to the lesser amount of heating in the drying zone.

After the drying zone the material passes one or more restricted openings 7 and enters a cooling zone 8 where the material is traversed by a cooling air current which reduces the temperature of the material to, for instance, 20 C. The dried material is discharged in ordinary manner, for instance by means of a rotating shovel wheel 9.

Of interest in this connection is the manner in which the air currents are propelled through the different zones. From a suction fan 10 fresh air is sucked in from the intake 11 which communicates with the atmosphere. The fresh air passes through a duct 12 to the cooling zone 8, in which it is spread out and is therefrom carried off through the duct 13 to the duct 14, the latter being connected to the suction side of a circulation fan 15. From the fan 15 the air is pressed through a heating arrangethem 16, where it obtains a temperature of for example 60 C., and is conducted further through a duct 17.- Said duct divides itself into the branches 18 and 19. The branch 19 conducts the air to the preheating zone 2 where the air heats the grain and is itself cooled down, and

passes through the duct 20 to the duct 14, so that a closed circuit is obtained for said portion of the air. The other branch 18 of the hot air coming from the duct 17 is con- -ducted, while being cooled down, through the drying Zone 6 and from the latter through a duct 21 to the sweating zone 4, through which it passes, and is thereafter discharged with a comparatively high percentage of moisture through the duct 22 to the atmosphere.

With regard to the performance and purpose of the various zones the following can be said.

In the preheating zone 2 the hot air current, on account of its high temperature as compared with the temperature of the material to be dried, efiects only a supply of heat to the material to be dried but does not effect any real drying of the grain.

, zone by a current of air having a temperature higher than that of the. particles to produce evaporation of moisture from the surface of each of the particles, and traversing the dried particles in a fourth zone by a current of air having a temperature lower than that of the particles to cool the particles, and combining and heating the air In the sweating zone the particles of material are; traversed by an air current which is considerably cooler than the particles and asa result of the movement of mois- I ture from the inner portions of the particles to the outerF,

portions of the particles, the particles sweat, the quantity and temperature of the traversing air should be so selected, 7 that the air passing out through the duct 22 is as'near to the different zones a damper 24 can be provided at the lower opening of the storage container 23. Said damper.

can be used, for instance, when there is to be dried a comparatively small quantity of grain which cannot fill up the shaft or column, in which case the entire charge may be placed on top of the damper which is thereafter opened, so that the charge drops down into the lowermost zone or zones 6 and 8 respectively, Where a simple drying is effected. i

The cold air intake at 11 is preferably adjustable, for example by means of a damper 25, whereby the renewal of air in the system is regulated'arid the percentage of moisture in the air discharged through theduct 22 can be controlled. It is of particular interest that the air between the drying zone and the sweating zone passes in countercurrent with respect to the feeding direction of the grain.

In the example shown it has been assumed that the grain sinks down through the zones and the restricted parts on account ofits own weight The movement in the zones and between the zones can, however, be effected in other manner, for instance through mechanical or pneumatic means. I

I claim:

discharged from said first and fourth zones and recirculating the heated air to said first and third zones.

2. A method of drying particles of material comprising passing the v particles through a plurality of successive 1 zones, traversing the particles in a first zone by a current of ,air having a temperature higher than that of the particles topreheat the particles, traversingthe preheated particles. in'a second zoneby a current of'airhaving a temperature lowernthan thatof the. particles to produce a movement of" moisture in an outward direction through each of the particles,traversing the particles in a third zone by a'current of air h avinga temperature higher than that of theparticl esto produce evaporation of moisture from the surface of each of the particles, and traversing ";the dried particles in afourth zone by a current of air having a. temperature lower than'that of the particles to cool the particles, passing said currents of air through a recirculating system in which the air passing through said second zone is the air discharged from said third zone,

.in which the air discharged from said first and fourth zones is combined, heated and recirculated to said first and third zones, and in which relatively cool dry air is admitted into the system through said fourth zone and relatively warm moist air is discharged from the system through said second zone. I

ing passing the particles through a plurality of successive References Cited in the file of this patent UNITED STATES PATENTS 14,588 Appleby Apr. 8, 1856 1,127,974 E111; Feb. 9, 1915 1,303,209 King -1 May 6, 1919 1,949,427 McCornb Mar. 6, 1934 2,245,664 Gronert June 17, 1941 2,439,741 Litty Apr. 13, 1948 I FOREIGN PATENTS 582,012 Great Britain Nov. 1, 1946 712,139 Germany Oct. 17, 1941

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