US3785765A

US3785765A - Rotary tobacco dryer

Info

Publication number: US3785765A
Application number: US00227693A
Authority: US
Inventors: L Rowell; J Cartmell
Original assignee: Imasco Ltd
Current assignee: Imperial Tobacco Canada Ltd
Priority date: 1972-02-17
Filing date: 1972-02-17
Publication date: 1974-01-15
Anticipated expiration: 1991-01-15
Also published as: SE7511038L; JPS4887096A; BR7301057D0; SE389003B; DE2305948A1; IT977192B; DE2366052A1; CA1007447A; SE7511039L; DE2305948B2; JPS506556B2; GB1419151A; DE2366052C3; SE416872B; DE2366052B2; GB1419152A; FR2172652A5

Abstract

A tobacco dryer is disclosed which comprises a rotating substantially cylindrical drum in which the tobacco is dried. The drum has a first end and a second end and the tobacco which is to be dried enters the first end of the drum and leaves at the second end of the drum. Process air which has a low absolute humidity is introduced into the drum at the second end and flows through the drum in the opposite direction to the flow of tobacco, so as to pick up and remove from the drum, moisture released from the tobacco. A gas fired burner located within the dryer housing produces heating air. This heating air is forced over the exterior surfaces of the rotating drum at a velocity between 500 feet per minute and 3,500 feet per minute so as to increase the heat conductance through the rotating drum. In addition, the dryer is also provided with a controlled water spray which is introduced into the second end of the rotating drum and thereby controls the moisture content of the tobacco leaving the dryer.

Description

. United States Patent 1191 Rowell et al.

[451 Jan. 15,1974

1 1 ROTARY TOBACCO DRYER [75] Inventors: Lorne A. Rowell, Lachine, Quebec;

James H. Cartmell, Preville, Quebec, both of Canada [73] Assignee: lmasco Limited, Montreal, Quebec,

Canada 22 Filed: Feb. 17,1972

[21] App1.No.: 227,693

Primary Examiner-John J. Camby Attorney-Christopher Robinson et a1.

[5 7] ABSTRACT A tobacco dryer is disclosed which comprises a rotating substantially cylindrical drum in which the tobacco is dried. The drum has a first end and a second end and the tobacco which is to be dried enters the first end of the drum and leaves at the second end of the drum. Process air which has a low absolute humidity is introduced into the drum at the second end and flows through the drum in the opposite direction to the flow of tobacco, so as to pick up and remove from the drum, moisture released from the tobacco. A gas fired burner located within the dryer housing produces heating air. This heating air is forced over the exterior surfaces of the rotating drum at a velocity between 500 feet per minute and 3,500 feet per minute so as to increase the heat conductance through the rotating drum. In addition, the dryer is also provided with a controlled water spray which is introduced into the second end of the rotating drum and thereby controls the moisture content of the tobacco leaving the dryer.

9 Claims, 3 Drawing Figures [52] US. Cl 432/107, 34/60, 34/139 [51] Int. Cl. F271) 7/08 [58] Field of Search 432/107; 34/60, 135, 34/137,139-,131/134,137

[56] References Cited UNITED STATES PATENTS 3,013,785 12/1961 King 34/135 X 2,746,170 5/1956 Wilson etal.... 34/135 X 2,238,161 4/1941 Drew et a]. 34/135 X 2,980,117 .4/1961 Saedeleer 131/134 Win55 CONTROL PATENTEDJAN 1 SIGN SHEET 1 [IF 2 PATENTEU 3 785 755 SHEET 2 [IF 2 ROTARY TOBACCO DRYER GENERAL DESCRIPTION, DISCUSSION OF PRIOR ART AND OBJECTS OF THE INVENTION The present invention relates to a rotary drum type tobacco drying device and more particularly to a device for drying tobacco while maintaining the moisture content of the outgoing tobacco substantially constant within close tolerances. This novel drying device also exhibits a dynamic operating range which is wider than many known devices by increasing the quantity of heat transferred through the dryer drum surface by moving the heating air over the drums exterior surface at high relative velocities. In addition, the dryer of the present invention further increases the quantity of heat transferred to the tobacco by forcing heating air at high velocities through hollow paddles which are arranged on the interior of the drying drum.

It is advantageous that tobacco dryers be capable of operating over a wide range of loads. For example, one week a dryer might be required which is capable of drying tobacco-at the rate of 10,000 pounds per hour, and that same dryer the next week, might be required to dry tobacco at the rate of 2,500 pounds per hour, representing a loading change on the dryer of approximately 400 percent. This type of load change can be thought of as a long term load change. A tobacco dryer capable of handling this type of long term load change must necessarily have a wide dynamic range of efficient operation. r

A second type of load change, defined herein as short term load change, is also found in tobacco drying. Short term load change is caused by drying tobacco generally having varying initial characteristics, as for example, tobacco having a varying initial moisture content. A tobacco dryer capable of handling short term variations must be capable of quickly changing its operating conditions so that it can follow the varying tobacco conditions to yield a final product which exhibits constant characteristics. a

Steam drying units are one type of known device for drying tobacco. A steam dryer consists generally of a long rotating jacketed steam .heated tubular shaped drum through which is passed the tobacco to be dried. Steam pipes are arranged longitudinally within the steam heated tubular drum to form paddles. Steam is fed through rotary joints and radially extending steam pipes to the steam heated drum and to the pipes making up the paddles, and in this manner head energy is transferred via the pipes and drum walls to the tobacco tumbling within the drum. The quantity of heat transferred to the tobacco is directly dependent on the surface temperature of the steam system which, of course, is dependent on the temperature of the steam circulating within the system. If the long term load on the dryer changes, the amount of heat energy released by the steam must vary accordingly, to compensate for this load change. In order to change the heat energy output of the steam, its temperature must be varied and this is accomplished by changing the pressure of the steam. The rotating drum and rotating joints must be considered as pressure vessels, carrying with them all of the limitations of pressure vessel design. Present steam drying Units normally operate between 227 F and 324 P which constitutes a dynamic temperature operating range of approximately 100 F. This dynamic temperature operating range represents a steam pressure variation from 5 to psi. In order to increase the dynamic temperature range of the steam tobacco drying unit beyond F it would be necessary to build pressure vessel parts capable of withstanding pressures in excess of 80 psi. Pressures below 5 psi are not practicable.

There are several other disadvantages of the present steam type dryers which are overcome by the forced hot air drum dryer of the present invention. The steam drying system requires a boiler room and pipes interconnecting the boiler room with the drying unit. The boiler room is costly and less efficient compared with the present inventionwhich generates all of the tobacco drying heat within the drying unit itself.

The tobacco dryer of the present invention has a rotating drum which is free of steam pipes, rotating joints and radially oriented steam pipes. These obstacles tend to afford places within the dryer where tobacco can hang up, producing gum and overvdrying the tobacco.

The second most common form of rotary tobacco dryer is known as the Gas Roaster type. The gas roaster type dryer is comprised of a rotating drum having a flue mounted directly above it and a fire situated directly below it. The fire, usually a gas fire, runs the entire length of the drum. The fire heats up the air surrounding the surface of the rotating metal drum thereby transferring heat into the tobacco. The heated air is circulated around the surface of the drum by convection only and as a result, in order to transfer sufficient heat energy through the rotating drum, the air temperature surrounding the drum must be very high. Indeed, in order to obtain a wide dynamic range in the roaster type dryer necessary to efficiently handle wide variations in long term dryer loading, the air temperature surrounding the rotating drum becomes extremely high.

The present invention utilizes the fact that the quantity of heat transferred through a surface is dependent on the velocity of the heated air passing that surface in addition to the temperature of that air. For example, under similar loading conditions, air at a temperature of 455 F moving at a velocity of 3,000 feet per minute past a surface, transfers the same quantity of heat through that surface as air at a temperature over 800 F travelling at a velocity of 400 feet per minute over the surface. As a result, a tobacco dryer in accordance with the present invention yields an increased dynamic operating range without the necessity of raising the air temperature surrounding the rotating drum into excessive temperature ranges.

I It is well known that the main heating load for any dryer is that required to evaporate water. It is also well known that the amount of water removed from the material being dried can be altered in a rotary drum type dryer by changing the temperature of the heated surfaces. However, the response time to such change is quite long due to the large thermal inertia involved and the length of time that the material takes to travel throughthe drying drum.

Within its controllable range, a dryer in accordance with the present invention is provided with a constant evaporating load. This is accomplished by adding a water spraying system which injects a fine water spray into the tobacco exit end of thedrying drum, thereby providing a variable evaporating load. The sum of water evaporated from the tobacco and from the fine water spray remains substantially constant.

If it is wished to evaporate more moisture from the tobacco, the quantity of water injected into the dryer is reduced. Altemately, if it is wished to evaporate less water from the tobacco, the amount of water injected into the dryer is increased. The total evaporating load on the dryer remains constant as is illustrated by the following table:

Tobacco Tobacco Moisture Moisture Content Content Moisture Evaporated Entering Leaving by Dryer Dryer Dryer From From Total Tobacco Spray 20.0% 16.5% 3.5% 0.1% 3.6% NORMAL 19.5% 16.5% 3.0% 0.6% 3.6%

With this method of control, the reaction time to short term evaporating load changes is very short and thus lends itself to automatic control.

A further advantage of the water spray technique according to the present invention shows itself in minimizing or eliminating over drying of the tobacco at the beginning and end of the drying operation. The water spray acts as a dummy evaporating load.

Canadian Pat. No. 596,376 which issued to John A. Maul on Apr. 19, 1960, discloses a conveyor drying device which varies the moisture content of the tobacco by injecting steam into the dryer. This technique however, does not maintain a constant evaporating load on the dryer because the water introduced has already been evaporated and is accompanied by the heat of vaporization. This extra heat increases the exit temperature of the tobacco which in turn causes more drying to take place in the cooler and automatic control of moisture very difficult.

It is therefore an object of the present invention to provide a tobacco dryer of the rotary drum type which has a wide useful dynamic range of operation capable of being economically used to dry tobacco under a wide range of long term loading conditions.

It is a further object of the present invention to provide a tobacco dryer of the rotary drum type which is capable of maintaining the moisture content of the exiting tobacco to within close tolerances.

It is yet another object of the present invention to provide a rotary drum type tobacco dryer which has a wide dynamic range of efficient operation while maintaining relatively low heating air temperatures by achieving high heat energy conductance through the drum surface by imparting a high velocity to the heating air passing over the heated metal surfaces in contact with the tobacco.

In accordance with the present invention there is provided a tobacco dryer, comprising a rotating substantially cylindrical drum in which the tobacco is dried; said drum having a first end and a second end; means for introducing tobacco at said first end and means for removing tobacco at said second end; means for introducing process air into said drum at said second end and extracting said process air from said drum at said first end so that said process air travels in a direction within said drum opposite to that of said tobacco; heating means for maintaining the temperature of heating air; and means for circulating said heating air at velocity between 500 feet per minute and 3,500 feet per minute over the outer surface of said drum to thereby achieve high conductance through said drum.

DESCRIPTION OF THE DRAWINGS I The invention will be described hereinbelow with the aid of the following drawings, in which:

FIG. 1 is a schematic sectional side view of one embodiment of a tobacco dryer according to the present invention;

FIG. 2 is a schematic sectional end view of the embodiment shown in FIG. 1 taken along 22 of FIG. 1;

and

FIG. 3 is a schematic sectional end view of the embodiment shown in FIG. 1 taken along 3--3 of FIG. 1.

DETAILED DESCRIPTION Referring now to FIG. 1, a tobacco drying drum 40, situated within an insulated housing generally indicated at 41 is mounted for rotation on two sets of

trunnion wheels

42 and 44 via trunnion rings 46 and 48. An electric motor (not shown) is arranged to drive the drum at a rotational speed'of approximately 10 R.P.M. The tobacco 50, which is to be dried, is fed by any conventional conveyor 51 into the tobacco input hopper 52 so that it falls on the screw type conveyor 54 which feeds the tobacco into the drying drum 40. The drum 40 is slightly tilted upwardly at its tobacco input hopper end so that the tobacco entering at that end will eventually move along the drum and exit at the tobacco output hopper end generally indicated at 56. A fan 58 provides process air pressure at the tobacco hopper output end 56. The process air enters the drying drum 40 at its exit end and travels within the drum in a direction as indicated by arrows 60 opposite the direction of travel of the tobacco being dried.

The process air is extracted via the extractor pump 62 and is exhausted to the atmosphere via exhaust pipe 64. A rotary air outlet screen 68 is situated in the tobacco input hopper 52 to remove small particles of tobacco which may be suspended in the process air.

Air dampers

70 and 72 located in the input and output pipes respectively of the process air system control the quantity of process air flowing in the drying drum. The process air entering the tobacco output hopper 56 can optionally be slightly heated to reduce the tendency of condensation on metal parts. However, the degree of heating which the process air receives is only slight and does not upset the loading of the dryer.

The dried tobacco leaving the drying drum enters the tobacco output hopper 56 and is fed via gravity into the metering tube 74. The tobacco upon leaving the metering tube 74 is removed from the dryer assembly via a conveyor 76 which can be of any conventional design. It is important that the speed at which the conveyor 76 removes the dried tobacco be controlled so that the metering tube 74 does not become empty of tobacco. The process air fan 58 provides a positive pressure in the tobacco exit hopper and the tobacco filling the metering tube 74 provides a seal which does not allow the process air to exit via the metering tube. Seals 43 associated with the trunnion rings prevent the escape of process air to the exterior of the drying unit.

Situated on the side of the metering tube 74 is a moisture content sensing head 78 which can be of any conventional design which provides a continuous output signal proportional to the moisture content of the tobacco passing through the metering tube. The output signal of the moisture content sensing head is fed to a moisture content controller 80. The controller 80 ad justs a servo operated water valve 82 so as to control the quantity of water being injected into the output end of the drying drum-40. The water spray nozzle 84 is of a conventional type which provides a fine spray.

When the sensing head 78 senses, for example, a decrease in the moisture content of the tobacco passing through the metering tube 74, a signal is received at the controller 80. The controller 80 then opens the servo.

operated watervalve 82 so that more water is sprayed into the tobacco exit end of the drying drum 40. In this manner, the moisture content of the dried exiting tobacco can be maintained to within very close tolerances. Since the moisture content of the exiting tobacco is controlled by a water spray, changes in its moisture content may be carried out without changing the temperature of the heating air, provided the latter is within the controllable range. As a result, the moisture content of the exiting tobacco can be maintained at a predetermined desired level by variations taking only a few seconds.

The heat for drying thev tobacco is supplied by a burner unit 86 mounted in the housing 41. The burner can be of any heat energy producing source, but a gas or oil fired burner has been found to be most efficient. The burner 86 heats up the air in the outer section of the insulated housing generally indicated at 88. The heated air in region 88 of the dryer assembly is pumped by fan 90 into a pressurized annular plenum 91 which surrounds the drying drum in its centre. A series of hollow triangularly shaped paddles 98, seen in cross section in FIG. 2, are arranged longitudinally along the interior of the drying drum 40. FIG. 2 shows only about four paddles for simplicity, but it can be appreciated that any number of paddles could be employed. Each paddle 98 has a port 100 which communicates the interior of the paddles with the outside of the dryer drum 40 in the region of the plenum 91. The ends of the paddles 98 also have ports 102 and 104 (FIG. 1), which communicate the hollow interior of the paddles with the region 88 of the dryer assembly. Mounted externally of the dryer drum 40 is a long stationary cylindri cal shell 96, which, with the exterior wall of the rotating dryer drum, forms a narrow annular region 94. The extremities of the shell 96 are arranged to communicate with the region 88 of the dryer assembly in the region of

ports

102 and 104 in the paddles 98. A large ringlike port 106 in the shell 96 allows for communication between the plenum 91 and the interior of the annular region 94.

The hot air in the plenum is forced through port 106 by the fan 90. Part of the hot drying air travels at a high velocity in a longitudinal direction relative to the exterior wall of the dryer drum 40 and exits into the region 88 in the region of the

ports

102 and 104. The remaining part of the hot drying air enters ports 100 and travels longitudinally along the interior of the paddles 98 to exit back into region 88 of the dryer assembly via

ports

102 and 104. The circulation of the drying air is generally indicated by the arrows 92 in FIG. 1. The cross sectional area of the annular region 94, the cross sectional area of the hollow triangular paddles 98 and the design of the fan 90, are such that the heating air travelling in the annular region 94 and the hollow paddles 98 is maintained at a high velocity. This arrangement tends to transfer a maximum amount of heat energy into the interior of the drum 40. In addition to the increased heat transfer due to the high velocity of the heating air passing over the drum and interior paddle surfaces, the present invention further increases the quantity of heat transferred by increasing the surface area of the heated surface by virtue of the triangularly shaped hollow paddles.

As is shown in FIG. 1, a portion of the air re-entering the region 88 of the dryer assembly is reheated and recirculated through the annular region 94 and the paddles 98, while some of the air re-entering region 88 is exhausted through the chimney 108. If desired, a heat exchanger (not shown) can be inserted inthe region 88 to slightly heat the processing air.

FIG. 3 is a sectional end view of the dryer assembly taken in the region of the tobacco exit 56 and shows the process air fan 58, the metering tube 74, the moisture content sensing heat 78, the conveyor 76, the water spray nozzle 84 and an access door 110. The water system may be connected to the door with a flexible line (not shown) to allow the door to swing open.

In operation, the heat transferred into the tobacco via the surface of the rotating drum and the paddles drives the water moisture out of the tobacco. The process air entering the drum at the tobacco exit hopper end is relatively free'of moisture, but since it is moving in a direction opposite to that of the tobacco the process air quickly picks up the moisture liberated from the tobacco and carries it away. Because the tobacco is moving through the dryer in one direction and the process air is moving through the dryer in the opposite direction there exits within the dryer drum a region, situated at the exit end of the drum, where the tobacco is in its driest state. It is in this region that the water spray is injected for controlling the moisture content of the tobacco. In this manner, the moisture content of the tobacco may be controlled without seriously affecting the evaporation loading conditions of the dryer.

In a second embodiment (not shown) vanes are arranged on the rotating drum to cause the heating air travelling in the annular region 94 to follow a helical path.

We claim:

1. A tobacco dryer comprising a rotating substantially cylindrical drum in which the tobacco is dried; said drum having a first end and a second end; means for introducing tobacco at said first end and means for removing tobacco at said second end; means for introducing process air into said drum at said second end and extracting said process air from said drum at said first end so that said process air travels in a direction within said drum opposite to that of said tobacco to pick up and remove from the drum, moisture released from the tobacco; heating means for maintaining the temperature of heating air; and means for circulating said heating air at a velocity between 500 fpm and 3,500 fpm over the outer surface of said drum to thereby achieve high heat conductance through said drum, wherein said circulating means includes a fan which forces said heating air into a pressurized plenum which encircles the exterior surface of said drum around its longitudinal centre.

2. A tobacco dryer according to claim 1, further comprising a stationary cylindrical shell surrounding said drum; said shell having its central portion interrupted by and in communication with said pressurized plenum and having its end portions in communication with the suction side of said fan and also in communication with the atmosphere, said stationary shell defining with said drum a narrow annular passage over a substantial portion of said drum for guiding part of the heating air from said plenum over the exterior surface of said drum.

3. A tobacco dryer according to claim 2 wherein a plurality of hollow paddles are arranged longitudinally on the interior surface of the drum, each of said paddles having a port through said drum in communication with said pressurized plenum; and a port being provided in said drum at each end of each of said paddles for communicating the interior of said hollow paddles with the suction side of said fan and with the atmosphere, whereby part of said heating air in said plenum is circulated by said fan through said paddles.

4. A tobacco dryer according to claim 3, wherein said drum is tilted upwardly at said first end to allow said tobacco to pass through said drum under the influence of gravity.

5. A tobacco dryer according to claim 2, wherein said heating means is a gas fired burner.

6. A tobacco dryer according to claim 3, wherein said process air is slightly heated so as to reduce condensation on metal parts of said dryer.

7. A tobacco dryer according to claim 6, wherein a heat exchanger is disposed within an insulated housing which contains said drum, said heating means and said circulating means, said heat exchanger being operatively associated with said process air to slightly heat said process air. 7

8. A tobacco dryer according to claim 2, wherein said means for circulating is an electrically driven fan.

9. A dryer according to claim 2, wherein the velocity of said heating air over the outer surface of said drum is 2,000 fpm.

Claims

3. A tobacco dryer according to claim 2 wherein a plurality of hollow paddles are arranged longitudinally on the interior surface of the drum, each of said paddles having a port through said drum in communication with said pressurized plenum; and a port being provided in said drum at each end of each each of said paddles for communicating the interior of said hollow paddles with the suction side of said fan and with the atmosphere, whereby part of said heating air in said plenum is circulated by said fan through said paddles.

7. A tobacco dryer according to claim 6, wherein a heat exchanger is disposed within an insulated housing which contains said drum, said heating means and said circulating means, said heat exchanger being operatively associated with said process air to slightly heat said process air.