Patent Publication Number: US-3879857-A

Title: Spiral moisture equaliser and method of using same

Description:
[ 51 Apr. 29, 1975 United States Patent [1 1 Neville 3.527.643 9/l970 Drexler et 34/12 SPIRAL MOISTURE EQUALISER AND METHOD OF USING SAME 75 Inventor: Richard Ernest Gartside Neville, Camby Salisbury, England AIIOIHF), Agent, or Firm-George W. Price; Charles 1. Worth [73] Assignee: AMF Incorporated, White Plains,  
 ABSTRACT The temperature an d moisture content of particulate [22] Filed: Feb. 27, 1974 rotatory motion so that the particulate material pro gresses up the spiral to an outlet. Means are provided References Cited UNITED STATES PATENTS for circulating air through the spiral and for adjusting the temperature and relative humidity thereof.  
 18 Claims, 3 Drawing Figures m m m &#34;n &#34;on c k C wd o m LAF 77-5 746 899 H 5-J-I- 097. U 34 90a 7-3 SPIRAL MOISTURE EQUALISER AND METHOD OF USING SAME BACKGROUND TO THE INVENTION This invention concerns improvements to the standard deviation of moisture content in tobacco or other particulate materials, but especially those with a large surface area to volume ratio as for example, laminate materials such as cut tobacco leaf.  
  There is a need for uniform moisture contents throughout tobacco leaf processing. but particularly in the dried and cooled cut tobacco (rag) fed to the cigarette making machines. The need here is for every gram of rag which goes to make a cigarette to have the same particular moisture content as the next.  
  There is a scatter in the moisture content of tobacco fed to the cigarette maker and the minimum cigarette weight and therefore, target weight are aimed at giving acceptable firmness allowing for this scatter. If the moisture content scatter can be reduced the cigarette minimum weight and target weight can be reduced, again increasing the yield of cigarettes per pound of tobacco.  
  The most uniform moisture content body of tobacco that can be achieved is that in which all the pieces of tobacco are in equilibrium, i.e., have the same temperature and the same vapour pressure and all the grades are evenly blended throughout the mass. The moisture content of each piece of tobacco will not be the same. Grade, crop, position on the plant, curing etc., will all affect the vapour pressure for a particular moisture content, so equilibrium is reached with all the pieces at varying moisture contents. If the pieces are evenly mixed or blended any sample taken for moisture testing will consist of an equal mixture of moisture contents and hence given an equal moisture content reading. So a uniform moisture content depends on blending as much as equilibrium temperatures and vapour pressures.  
  Blending at present is achieved firstly by lamina blending bins which coarse blend the various grades throughout an operation by layering, so that each 100 to 200 lbs discharged contains equal proportions of each grade. Secondly, by the stem add-back system which by measuring the flow of cut lamina on a weighing conveyor, to lb at a time, adds back the correct weight of cut stem. And thirdly, by a rotary drier which by tumbling about 20 lbs of rag per foot of cylinder aims at fine blending the cut strands, so that each cigarette weight contains the same blend.  
  Moderately accurate moisture contents are obtained at present by measuring the moisture content of tobacco before drying and after cooling by electrical means and using the measurement to control the heat to the drier to obtain the desired output. But at the best, this method only corrects the long term variations or the mean moisture content of very large masses of tobacco compared with the weight in one cigarette.  
  Using the best apparatus available at present tobacco can be cooled after drying to within 5 of ambient, but generally cooling is not as good as that.  
  Final equalising of moisture and temperature are at present attempted by storing the tobacco in storage silos. Whilst some equalising of adjacent pieces of to bacco occurs it is not possible in a reasonable time and using a reasonable space for remote pieces to equalise since the effectiveness of bulking is limited by the rate of diffusion of vapour in a relatively high bulk density mass of tobacco.  
  The object of the present invention is to provide a means for continuous high speed bulking to smooth the short term fluctuations in moisture content and to correct the moisture content and tobacco temperature to pre-set values with high accuracy.  
 SUMMARY OF THE lNVENTlON According to the invention there is therefore provided a method for adjusting the temperature and moisture content of particulate material to desired levels, comprising passing a flow of air through the particulate material, the air having a temperature and relative humidity which would be in equilibrium with the particulate material at said desired moisture content and temperature prior to passing through the particulate material.  
  The process employed for equalising or smoothing and correcting of, for example, tobacco moisture content is adiabatic absorbtion or desorbtion carried out at room temperatures. The heat of absorbtion or desorb tion is passed to, or provided by, the air. The process is a slow one because the only temperature difference which is available to transfer the heat involved is the modified wet bulb or reverse wet bulb temperature which the tobacco assumes. Modified because the water in the tobacco is not free moisture so the vapour pressure is reduced, and reversed, i.e., hotter than the air when the tobacco is absorbing moisture rather than desorbing or evaporating. The rate of absorbtion or desorbtion is proportional to the difference between the vapour pressure of the tobacco and the air, i.e., proportional to the difference in moisture content of the tobacco from the equilibrium moisture content. So tobacco which deviates from the equilibrium moisture content adjusts exponentially towards the equilibrium, whilst tobacco which is at or near the equilibrium is substantially unaffected.  
  Air of extremely accurate relative humidity is required. For rag tobacco at 14% moisture content after drying and cooling a typical equilibrium condition is 60% relative humidity at F. A change in relative humidity of 2 /2 to 5%, depending on the blend, is equivalent to l% tobacco moisture content. So, relative humidity should be controlled to within i 0.25%. A change in air temperature of 1F alters the relative humidity of air with a fixed vapour content, by approximately 3.3%. so air temperature must also be controlled very accurately to within i 0.08F. in order to generate a continuous supply of fresh air with a relative humidity and temperature of a suitable accuracy, it is preferred to fully recirculate the air.  
  The thermal capacity of the air circulated is small compared with the thermal capacity of the tobacco on the conveyor, similarly the weight of water vapour in the air is small compared with the weight of water in the tobacco. So the air leaving the tobacco reflects the mean temperature of the tobacco and assumes a relative humidity in equilibrium with the mean moisture content of the tobacco.  
  The temperature of the air returned to the tobacco may be controlled thermostatically, for example, by heating or cooling coils and this corrects the tobacco to the desired temperature. This temperature control need not be of very high accuracy as the relative humidity of the air is self-adjusting to the equilibrium condition by the removal or addition of a negligible amount of moisture from the tobacco.  
  The moisture content of the tobacco is smoothed by the air recirculation, but the mean is unaltered. To correct the mean moisture content, water vapour is added or removed from the recirculating air at a rate which is in accordance with the removal or addition required by the tobacco.  
  The water vapour can be added evaporatively by me tered water spray which may be atomised by pressure, compressed air or spinning disc humidifier in which case the addition is accompanied by adiabatic cooling of the air. It can also be added as vapour using a metered steam flow through one or more nozzles or by heating a wick or water surface with a metered water and heat supply, in which case the air temperature is substantially unaltered.  
  Water vapour can be removed by by-passing an adjustable proportion of air through a bed of dessicant such as silica gel or through refrigerated coils to con dense the vapour.  
  The control of the amount of water vapour added or substracted from the recirculating air does not need to be of particularly high orderv The correction to the tobacco would typically not exceed l% moisture content, so that a larger error of in the vapour substracted from or added to the air would only cause a 0.1% moisture content error in the correction.  
  The control can be carried out in two ways. In the first, control is based on the measurement of tobacco moisture content by a continuous moisture meter. In its simplest form this would be a single measurement after the moisture equalising conveyor, the moisture signal being fed to a feed back process controller where the signal is compared with the desired moisture content and control signals generated to adjust the vapour addition or removal.  
  In a more comprehensive system additional measurements of moisture content and tobacco flow rate are made before the moisture equalising conveyor and a water vapour addition or subtraction computed. The computed signal is used as a feed-forward control to set the water vapour addition or substraction. The feedback control can be combined with the feed-forward to correct any errors resulting from the feed-forward control.  
  The above method depends on the accuracy of continuous moisture measurement. There are various known electrical methods available, which give acceptable results, but none of which give the accuracy desired by the industry.  
  In the second method of controlling the addition or removal of water vapour from the air and hence the mean moisture content of the tobacco, the relative humidity of the air is measured and held constant by adjusting the vapour removal or addition. For a well blended mixture of tobacco the equilibrium relative humidity is a measure of the tobacco moisture content. The relative humidity of the air can be measured by various known methods and the measurement signal then compared with a desired value in a process controller which generates control signal to adjust the amount of vapour added or removed so as to maintain a constant relative humidity.  
  In practise it is more convenient to slightly over-dry the tobacco in the drying and cooling stage and only add moisture (and remove heat) in the equaliser. There are various reasons for this:  
 I. Tobacco absorbs moisture more readily than it desorbs,  
 2. Cooling coils are required to remove the heat of absorbtion (Desorbtion would require heating coils) 3. Cooling coils are required in any case to remove the heat generated by the fan used to circulate the air,  
 4. The tobacco is generally insufficiently cooled by existing coolers so that further cooling is required in the equaliser. Cooling coils are again required to remove this heat,  
 5. It is easier to add a measured weight of water with metering pump or flow meter, than it is to remove a measured weight.  
 6. There is a slight hysteresis in the absorbtion and desorbtion of moisture into leaf, so it is preferable that the equaliser either adds moisture or removes it rather than both.  
 BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:  
  FIG. 1 shows a spiral tobacco cooler of known con struction in a cut-away isometric view,  
  FIG. 2 shows a schematic sectional elevation of an example of the present invention is which the cooler of FIG. I has been modified with a recirculatory system, and  
 FIG. 3 is a plan view of the modified system.  
 DESCRIPTION OF PREFERRED EMBODIMENTS The arrangement shown in FIG. I is a spiral cooler in which the tobacco enters via a feed conveyor 10 onto a spiral perforated tray 11 supported towards its centre by a cylinder 13 forming a central core. The tray is provided with an upstanding rim 14 at its outer edge and and is formed beneath with an air duct (plenum chamber) 16, which communicates by holes 17 with the interior of the cylinder 13.  
  The spiral tray conveys the tobacco upwardly by oscillation of the tray by means of a drive unit 19 (not shown in detail) which imparts an upward and rotational movement to the tray, the tobacco thereby being thrown up the spiral of the tray, in a continuous series of steps until it is discharged by a vibratory conveyor 20.  
  Cooling air enters through an inlet 22, being drawn in by an infusion fan 23 which drives the air down the cylinder 13, through the holes 17 into the duct 16 and from there through the tobacco on the tray. The air then passes over the rim and down the side of the tray and out through the exhaust air duct 25.  
  For carrying out the process of the invention, the tobacco conditioning arrangement shown in FIGS. 2 and 3 is provided in which a spiral cooler substantially as shown in FIG. 1, suitably adapted. A feed conveyor 26 leads to the lower part of a spiral perforated tray 28 supported towards its centre by a cylinder 29 forming a central core. The tray is provided with an upstanding rim 3] at its outer edge and is formed beneath with an air duct (plenum chamber) 32, which communicates by holes 34 with the interior of the cylinder 29.  
  A drive unit 35 (not shown in detail) is provided for oscillating the spiral tray 28 by imparting an upward and rotational movement to the tray.  
  A spearate linear vibrating conveyor 37 leads away from the top of the spiral tray.  
  The spiral tray 28 is supported on links 38 and is enclosed in a heat insulated stationary casing 40.  
  A circumferential slot 41 in the casing floor connects with a scroll-shaped or circular tapered duct 42 which leads to the inlet of an air conditioning unit 43 comprising an insulated casing 44 having at its lower end a high efficiency fan 46, connected by a divergent cowl 47 to a full flow roughing roll filter 49 situated above the fan. A further filter 50 of the panel type is arranged between the roll filter and a multi-row finned tube air cooler 51, the cooling coils of which are supplied with cooling water from a mains supply 52 via a modulating valve 54. The outlet 53 of the air conditioning unit is connected via a duct 55, having an air turn device 56, to the upper end of the cylinder 29 supporting the tray 28 by way of a flexible connector 59. The duct 55 contains water vapour dispensers in the form of steam nozzles 60, controlled from a relative humidity sensor 62 located within the duct 55 close to the air inlet to the cylinder 29 and associated relative humidity control means (not shown). The nozzles 60 are supplied with steam from a supply pipe 61 via a modulating valve 65. The duct 55 also contains an air temperature sensor 64 and associated temperature control means (not shown), which serves to control the water flow through the tubes of the cooler 51.  
  The equaliser shown in FlGS. 2 and 3 operates as follows:  
  Tobacco which has been dried to a moisture content below that desired and cooled to a temperature above the desired temperature is fed from conveyor 26 onto the spiral tray 28. Air is introduced into the spiral conveyor via the central core, whence it radiates to the duct (plenum chamber) below the perforated tray via the holes in the cylinder wall, which with a pressure drop across them assist in the air distribution. The air is withdrawn from the casing via the circumferential slot 41 to the scroll-shaped or circular tapered duct 42. Air passes up through the tobacco on the tray whence heat from the tobacco is transferred to the air at a rate dependant upon the temperature difference therebetween and moisture is absorbed from the air at a rate dependant upon the moisture content of the tobacco and the relative humidity of the air. Thus the air, after passing through the tobacco is hotter and drier than previously.  
  The hot dry air then passes by way of the duct 42 to the air conditioning unit 43 where any dust carried in the air is removed by filters 49 and 50. A measured amount of cold water is circulated through the coils of the cooler 51 to cool the air and a measured amount of steam is added to the air from the nozzles 60. As the air then returns to the spiral tray by way of duct 55, it passes the temperature sensor 64 and the relative humidity sensor 62. Temperature control means compare the sensed temperature with a predetermined temperature for the air and in accordance with the comparison adjust the modulating valve 54 to alter the flow of cold water to the cooler 51. Relative humidity control means compare the sensed relative humidity with a predetermined relative humidity and in accordance with the comparison adjust the modulating valve 65 to alter the flow of steam through the nozzles 60. Thus, the recirculating air enters the spiral tray at a controlled temperature and relative humidity.  
  The temperature and relative humidity of the air entering the spiral tray are such as to be in equilibrium with the tobacco leaving the device at the desired temperature and moisture content.  
  The upward and rotational movement of the tray causes the tobacco to be thrown up the spiral of the tray in a continuous series of steps until it is discharged by the conveyor 37.  
  The described system receives heat from three sources, which heat the air, namely, tobacco cooling, heat of absorbtion of moisture, and fan compression of The air conditioning unit is insulated and the equalising conveyor is enclosed in an insulated casing 40 so heat interchange with the ambient, if it differs from the controlled temperature, is negligible. The heat of absorbtion is largely off-set by adiabatic cooling of the air if water sprays are used. The cooling coils are multirow with a large heat transfer rate so that the coil temperature will not be below dew point, typically l0F below the air temperature. Dewing would remove moisture from the system, particularly where the air is stagnant in the coils around the headers.  
  The cooling coils are water-cooled, using mains water, which is either thrown away or recirculated through a cooling tower or refrigerator.  
  In addition to simplicity, the above recirculatory system has the advantage of not requiring very accurate temperature control, and, because the system is selfadjusting for equilibrium relative humidity, not requiring very accurate control of water vapour addition.  
  The equalising conveyor can be fluid bed. gauze band conveyor or perforated tray vibrating conveyor. The main requirements are a long retention time of around 5 to l5 minutes and a large ventilating surface area to bring the maximum air in contact with each pound of tobacco. However, a perforated spiral tray is preferred since a fluid bed is not very suitable for tobacco as the surface area is small and tobacco products are not generally fluidisable. Gauze band conveyors may be used. although the air tends to bypass the deeper or denser parts of the tobacco layer if, as is inevitable, the tobacco carpet is not uniform.  
  The most suitable ventilating means is the vibrating tray conveyor with perforated bottom. A particular advantage is that the air resistance of the perforated tray bottom can be made several times greater than the deepest part of the tobacco layer, by using a perforation which gives only about 2%% free area, e.g., l/l6 inch diameter holes at inch staggered pitch. By this means a uniform air flow can be obtained which is substantially independent of the tobacco layer depth. This is economical in air flow and permits a smaller ventilating area than would be required using a gauze band.  
  A band type ventilating means can also be constructed using a band with perforations having the same characteristic as for the vibrating tray.  
  In a particular form of construction, a spiral tray is used which gives a long length of tray in a short space, e.g., 10 turns of 6 feet-9 inches OD X 4 feet-9 inches lD tray gives I feet-0 inches or 12 inches wide tray, which with a recirculating air flow of 8,000 c.f.m. and a tobacco retention time of 5 to 10 minutes is suitable for equalising 2,000 lb/hour of tobacco, and reducing the standard deviation of moisture content by a factor of approximately 3 x.  
  The air conditioning unit and connections to the spiral conveyor are carefully sealed against leaks so that only the spiral conveyor casing is open to atmosphere at the tobacco inlet and outlet tunnels, which are sealed from draughts by one or more curtain seals. In this way, there is substantially no interchange of the air in the system with ambient air.  
  It is also possible to provide sensors in the path of the air which is not recirculated. the sensors controlling the temperature and humidity ofa supply of air to the conditioning unit from a separate unit.  
  I claim: 1. A method of providing particulate material with the temperature and moisture content thereof adjusted to desired levels, comprising the steps of providing a flow of particulate material and simultaneously providing a recirculating flow of air,  
 adjusting the temperature and relative humidity of the flow of air to levels to be in equilibrium with the temperature and humidity of the particulate material when the temperature and humidity of such material are at the desired levels,  
 passing the air flow with adjusted temperature and humidity through the particulate material, and recirculating the flow of air that has passed through the particulate material.  
  2. The method of claim 1, wherein the temperature of the recirculating air is controlled thermostatically.  
  3. The method of claim 1 wherein the relative humidity of the recirculating air is controlled by adding water vapour to the recirculating air.  
  4. The method of claim 1, wherein the relative humidity of the recircling air is controlled by removing water vapour from the recirculating air.  
  5. The method of claim 3, wherein the water vapour is added evaporatively by a metered water spray.  
  6. The method of claim 3, wherein the water vapour is added as a metered steam flow.  
  7. The method of claim 4 in which the water vapour is removed by passing an adjustable portion of the recycling air through a dessicant or through refrigerated coils.  
  8. The method of claim 3 wherein the relative humidity of the air is measured before passing through the particulate material and is held constant by adjusting the rates of water vapour addition.  
  9. The method of claim 4 wherein the relative humidity of the air is measured before passing through the particulate material and is held constant by adjusting the rate of water vapour removal.  
  10. The method of claim 1, including the further steps of: supporting the particulate material on a spiral tray; oscillating the tray with an upward and rotatory motion to cause the particulate material to progress upwards along the tray;  
 passing the air upwardly through the particulate material substantially over the length of the tray; cooling and adding water vapour to the air after passing it through the particulate material; measuring the temperature and relative humidity of the air;  
 maintaining the temperature and relative humidity constant by adjustment of the cooling rate and the rate of addition of water vapour; and recycling the air through the particulate material as it progresses along the tray.  
 11. An apparatus comprising;  
 a. a spiral tray;  
 b. means for feeding particulate material to the spiral tray;  
 0. drive means for oscillating the spiral tray with an upward and rotatory motion to cause particulate material to progress up the spiral;  
 d. means for removing particulate material from the top of the spiral tray;  
 e. means for recycling air through the spiral tray;  
 f. means for cooling and adding water vapour to the recycling air;  
 g. sensors positioned for measuring the temperature and relative humidity of the recycling air; and.  
 h. control means for adjusting the cooling rate and the rate of addition of water vapour in accordance with any difference between the measured temperature and relative humidity and the temperature and relative humidity of air which would be in equilibrium with the particulate material at said desired temperature and moisture content.  
  12. The apparatus of claim 11, wherein the means for removing particulate material from the top of the spiral tray comprises a linear vibrating conveyor.  
  13. The apparatus of claim 11, wherein the means for recycling comprises a fan, ducting for conveying air from the spiral tray to the fan and ducting for conveying air from the fan to the spiral tray.  
  14. The apparatus of claim 11 wherein the means for cooling the recycling air comprises cooling coils in the path of the recycling air through which coils cooling fluid passes.  
  15. The apparatus of claim 14, wherein the control means for adjusting the cooling rate comprises a modu lating valve in the supply of cooling fluid to the cooling coils.  
  16. The apparatus of claim It wherein the means for adding water vapour to the recycling air comprises at least one steam spray nozzle in the path ofthe recycling air.  
  17. The apparatus of claim 16, wherein the control means for adjusting the rate of addition of water vapour comprises a modulating valve in a steam supply to the at least one spray nozzle.  
 18. The apparatus of claim ll, further comprising a fiiter in the path of the recycling air.