US 3460265 A
Description (OCR text may contain errors)
Aug. 12, 1969 H. L. SMITH, JR
METHODS OF DRYING 5 Sheets-Sheet 1 Original Filed Feb. 14, 1967 INVENTOR HORACE L. SM/ TH, JR. BY M wN mJ 3 wt ATTORNEYS Aug. 12., 1969 H. L. SMITH, JR
METHODS OF DRYING 5 Sheets-Sheet z INVENTOR TTORNE S Original Filed Feb. 14, 1967 Aug. 12, 1969 Original Filed Feb. 14, 1967 H. L. SMITH, JR 3,460,265
METHODS OF DRYING 5 Sheets-Sheet 4 'soo- 25 I; "-1 In I :1: I43b o RADIANT SOURCE O1 700 F. I420 I 400 I 20 2 MEL J -J IJ w o I; RADIANT SOURCE III 600 F. 1300" l5" 2 S 2 o I; L; RADIANT SOURCE m 700 F.
E zoo- 3 10- I RADIANT LB./ HP HR. w w souRcE III 600 F.
o 0 6,450 9,Ioo ll,l50 I2,9oo
VELOCITY OF IMPINGING AIR (PPM) F IG. 5
TEST RATE WITH 0 N I RADIANT SOURCOE m 600 F.
E I E THEORETICAL RATE 9 WITH ROADIANT SOURCE at 600 E nd ii AIR m 450%? 33 I48 EtI o F I 6. 7 2 Z O T INVENTOR 0 vi 0c TY SR I ZP N6 N6 AIR IFPM) QJOOHORACE M L l I I JM M M flaw ATTORNEYS Aug. 12, 1969 Original Filed Feb. 14. 1967 EVAPORATION RATE, LB. HZO/ HR. FT
H. 1.. SMITH, JR 3,460,265
METHODS OF DRYING 5 Sheets-Sheet 5 RADIANT SOURCE at 600F.
RADIANT SOURCE at 600F WITH COVERS OVER THE RADIATORS 0F lMPlNGING AIR (FPM) VELOCITY FIG. 6
INVENTOR HORACE L. SMITH, JR.
BY/M, fl'rw M,
ATTORNEYS United States Patent US. Cl. 34-1 11 Claims ABSTRACT OF THE DISCLOSURE Methods of drying web, sheet, and similarly configured material by the impingement of a fluid medium and, if desired, the combination of radiant heating with fluid impingement.
Cross reference to related application This application is a division of application No. 615,966 filed Feb. 14, 1967, which is now US. Patent No. 3,403,456.
Background and summary of the invention The present invention relates to drying and, more specifically, to methods of drying material of sheet, web, and similar configurations.
One method of drying materials related to that which I have developed is disclosed in US. Patent No. 3,199,213 issued Aug. 10, 1965, to F. H. Milligan et al., for Method of Changing the Moisture Content of Wood. In one aspect, my novel method differs from Milligans in the use of radiant energy plus fluid impingement instead of impingement alone. I have found that the combination of radiation and fluid impingement produces significant increases in drying rates in comparison to those obtained with impingement alone. In fact the increase is appreciably greater than would be obtained if the effects of impingeemnt and radiant heating were additive. This indicates that a combination of radiation and fluid impingement in accord with the present invention has a synergistic effect on the drying of web and sheet materials, particularly those which are difficult to dry.
In conjunction with the foregoing the increased drying rates provided by the present invention produce a decrease in the time for which the product being dried is retained in the dryer. This permits a reduction in the size of the dryer needed, resulting in a saving in initial equipment costs as well as savings in installation costs and in the costs of a structure for sheltering the drying apparatus. Accordingly, the increase in drying rates provided by the present invention is accomplished by a considerable decrease in initial costs.
Another important difference between the drying process disclosed herein and that disclosed in the Milligan patent is in the impingement of the air or other treating fluid on the material being treated at an angle having a component in a direction opposite to the direction of movement of the material rather than normally as in the Milligan process. The inclination of the flow apertures at an angle to the material being treated and the inclination of the apertures in a direction opposite to that of material movement increase the capability of the fluid for scouring evolved volatiles away from the surface of the material being treated. Accordingly, this flow aperture arrangement also produces increased efliciency.
There is yet another advantage of inclined jets over those oriented normally to the material being dried. Specifically, the latter produce a pattern of pressure distribution in the fluid treating medium which tends to make the web or sheet being dried flutter or drift. This Patented Aug. 12, 1969 "ice problem can be at least substantially eliminated by the use of properly inclined flow apertures.
In conjunction with the foregoing, part of the treating fluid is so directed toward the material being treated that it has a velocity component directed toward the inlet to the exhaust structure for removing spent fluid, evolved volatiles, etc.
This is an important practical feature of the present invention since the fluid will assist in effecting a flow of spent fluid, evolved volatiles, etc. into the exhaust structure, increasing the operating efliciency of the apparatus involved.
From the foregoing it will be apparent that the primary object of the present invention resides in the provision of novel improved methods of drying materials with sheet, web, and similar configurations.
Other objects, additional important features, and further advantages of the present invention will become apparent from the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing.
Detailed description of the drawing In the drawing:
FIGURES 1A and 1B together constitute a partially diagrammatic longitudinal section through a dryer adapted to dry web, sheet, and other materials in accord with the principles of the present invention;
FIGURE 2 is a partial and partly broken away isometric projection of a fluid impingement and radiant heating unit employed in the dryer of FIGURE 1;
FIGURE 3 is a section through a flow plate incorporated in the unit of FIGURE 2;
FIGURE 4 is a diagrammatic illustration of a system for supplying fluid to and exhausting it from the impingement and radiant heating units in the dryer of FIGURE 1 and of a system for heating the fluid and the radiant heaters in the foregoing units;
FIGURE 5 is a chart showing the relationship for an exemplary material being dried between the velocity of the fluid supplied to the drying apparatus of FIGURE 1 on one hand and the amount of water evaporated per blower horsepower hour and per square foot of radiant energy emitting surface on the other;
FIGURE 6 is a chart showing the contribution made by radiant energy in drying materials in accord with the principles of the present invention; and
FIGURE 7 is a chart showing the synergistic effect of combined impinging fluid and radiant energy on drying rates.
Description of an exemplary preferred embodiment Referring now to the drawing, FIGURES 1A and 1B depict a single pass dryer 20 having fluid impingement and radiant heating units 22 for drying the product to be treated in accord with the principles of the present invention. Dryer 20 also includes a casing 24, a series of parallel, spaced apart, rotatably mounted rolls 26 establishing a path 28 for the product or material being dried, which is in the form of a web 3% and endless, open mesh belts or felts 32 which confine and prevent warpage of web 30. Conventional idler rolls 34 and drive rolls 36 are also provided to support and effect movement of belts 32. Except as discussed below the dryer components are of conventional construction and will not be described further herein.
As shown in FIGURES 1A and 1B, units 22 are located between adjacent rolls 26 and on both sides of path 28 to dry web 30 by simultaneously applying radiant heat to both sides of the web to evolve volatiles from it. Units 22 also direct air or other treating fluid at high velocity into contact with the sides of the web to assist in evolving the volatiles and to scour away evolved volatiles from adjacent its surfaces. The high velocity fluid thus prevents retardation of the drying process by eliminating the formation of a stagnant layer adjacent the Web and, in addition, causes evaporative cooling adjacent the surfaces of the material being treated and thereby prevents it from being overheated. It is preferred that the treating fluid also be heated, preferably to a temperature typically on the order of 250 to 700 F., since a hot, high velocity fluid such as air is a highly effective drying agent.
Referring now to FIGURE 2, each of the novel fluid impingement and radiant heating units 22 incorporated in dryer 20 includes a main duct pair consisting of elongated main supply and return ducts 38 and 40 separated by a dividing wall 42. The main ducts 38 and 40 extend lengthwise of dryer 20.
Each of the units 22 also includes a number of generally T-sectioned branch supply and return ducts 44 and 46 extending transversely across the associated main supply and return ducts 38 and 40. The main and branch ducts are in part integral in that the side walls 48 of the main ducts form the end walls for the branch ducts.
The branch ducts are also defined by co-operating, generally Z-sectioned side wall members 50 disposed in parallel, spaced apart relationship and so arranged that each side wall member is disposed in mirror image relationship to the side wall members 50 on either side of it. This results in an array of adjoining, internested ducts with branch supply and return ducts being alternated. At the ends of the units the open sides of ducts thus formed are closed by wall members 51 (see FIGURES 1A and 1B).
Branch supply and return ducts 44 and 46 also include top walls 52 and 54. As shown in FIGURE 2, the top walls 52 of the branch supply ducts span only those parts of the ducts adjacent main return duct 40, and the top walls 54 of branch return ducts 46 similarly span only those portions of the return ducts overlaid by main supply duct 38. Thus, the branch duct top walls prevent fluid from flowing between the branch supply ducts and main return duct and between the branch return ducts and main supply duct. The elimination of the top wall members from the remaining portions of the branch ducts provides communication between the branch and main supply ducts 44 and 38 through openings 56 which are generally equal in length to the width of the main supply duct and similar openings 58 provide communication between the branch and main return ducts.
Preferably, air vanes or other air distributors are employed in branch supply ducts 44 to provide a generally uniform distribution of the treating fluid over the span of the impingement units and, accordingly, over the span of web 30. One exemplary (but by no means the only) distribution arrangement which may be employed is illustrated in FIGURE 2. This arrangement, which is capable of equalizing distribution of the treating fluid with only a relatively low pressure drop, includes a set of four vanes 59a-d in each branch supply duct (only one set of vanes is shown). Vanes 59a-d divide the flow of incoming fluid into segments which have a predetermined relation to the areas over which they are to be distributed and divert the segments of flow to the points at which the fluid is discharged against web 30. By dimensioning and positioning vanes 59a-d in accord with available techniques, a generally uniform distribution of fluid across the span of the supply duct can be provided. Equalization of fluid distribution can be obtained to any degree required, generally speaking, by increasing the number of distributor vanes.
The fluid supply arrangement just described minimizes resistance to fluid flow and produces a discharge of fluid from the branch supply ducts which is substantially uniform over the entire width of web 30 and a similar uniform flow of spent treating fluid and/r evolved volatiles from adjacent the web into the branch return ducts. This 4 results in uniform treatment of web 30 across its entire width and, consequently, the production of a highly uniform treated product.
The minimization of flow resistance in the impingement units by the structure described above also reduces to an appreciable extent the power required to circulate the fluid treating medium through the units 22. This is, of course, a significant advantage in that it reduces the size of the blower required with a consequent reduction in both initial and operating costs. In fact, the fan horsepower requirements of drying apparatus in accord with the present invention are only 10-20% of those of the wood veneer dryers most widely used at the present time.
Air or other treating fluid is accerelated and directed at high velocity (typically on the order of 2000-l5,000 feet per minute) against web 30 by flow plates 60 which form the outer or exposed walls of branch supply ducts 44. Flow apertures 62 (see FIGURES 2, 3, and 6A) are drilled in a predetermined pattern through each of the flow plates 60 with the inlets 64 to apertures 62 preferably being chamfered as shown in FIGURE 3 to prevent the formation of vena contracta in the apertures.
One of the important features of the present invention resides in drilling flow apertures 62 so that the fluid exiting through them impinges upon web 30 at an angle rather than normally as in the conventional arrangement. In the latter the normal impingement of the treating fluid on the web results in instability, drift, and flutter as the web moves at high speed past the flow plate.
By inclining the flow apertures at an angle to the web a velocity and pressure distribution pattern which will result in minimization of flutter can be produced. Also, the arrangement just described virtually eliminates areas in which the flow velocities are too low to produce the desired scouring effect. Thus there is more uniform treatment of the material in the present invention than in the conventional arrangement described above.
This arrangement is particularly important in dryers of the type shown in FIGURES 1A and 1B in which impingement units are located on both sides of the web since, in this type of arrangement, the instability caused by one flow plate is apt to reinforce that caused by the other, materially aggravating flutter and drift and lack of stability in the web.
Also, as shown in FIGURE 3, the majority of the flow apertures 62 in flow plate 60 are so oriented that the treating fluid discharged from them impinges upon web 30 in a direction opposite to the direction of movement of the web. As a result, the velocity of the Web and the velocity component of the treating fluid in a direction parallel to the web are additive. Thus, for a given velocity of fluid flow through apertures 62, a greater effective velocity can be obtained by the preferred orientation than by using the conventional arrangement.
Also, the flow arrangement just described produces greater turbulence than would be obtained if the treating flued impinged on the web at right angles to the direction of movement of the web. The net result of the foregoing factors is that treating fluid impinging on the web in the manner just discussed has a significantly greater scouring action than could be obtained if the fluid impinged upon the web at right angles or in the direction of web movement.
Referring again to FIGURE 2, the spent treating fluid, together with its burden of evolved volatiles, flows into branch return ducts 46 through inlet apertures 74. These extend the length of the branch return ducts and accordingly minimize the resistance to the flow of the treating fluid and evolved volatiles in to the return ducts. Moreover, since the exhaust openings span the material there is virtually no lateral flow of the spent fluid and evolved volatiles. This is important since, as indicated above, lateral flow of the fluid and volatiles can produce variations in drying conditions across the sheet and, accordingly, a nonuniform product.
Referring now to FIG. 2, each of the flow plates 60 (except for those on the ends of impingement units 22) is located between the inlets 74 to two successive branch ducts; and the rows of flow apertures 62 adjacent each of the branch duct inlets are so oriented that the treating fluid discharged from them is inclined toward the branch duct inlets. This is another important feature of the present invention since the fluid discharging toward the inlet ducts tends to force the spent fluid and evolved volatiles adjacent web 30 through inlets 74 into branch return ducts 46. As shown in FIGURE 3, these nozzles are preferably inclined at a somewhat greater angle to the surfaces of the flow plates than the remaining apertures to increase the effect just discussed.
While a uniform distribution of the treating fluid across the span of web 30 is normally provided by uniformly spacing flow apertures 62 the length of flow plate 60, controlled nonuniform distribution of the treating fluid can be provided by an appropriate nonuniform spacing of the flow apertures, where desired.
The flow apertures will typically be drilled through the flow plate at an angle of The total area of the openings will normally be from 1.25 to 4% of the total area of the plate and may even be a lower percentage of the total plate area.
It will be apparent, from the foregoing, that volatiles may be evolved from web 30 merely by effecting a flow of air or other treating fluid at high velocity and high temperatures through flow plate 60 into contact with the material being dried. However, in the preferred embodiment of the present invention, volatiles are evolved much more rapidly, thereby materially increasing the efliciency of units 22, by heating flow plates 60 to temperatures above that of the treating fluid so that emission of radiant energy in the infrared portion of the electromagnetic spectrum from the plates will be enhanced. Thus, the material being treated will also be dried by radiant energy as well as impinging fluid.
In the fluid impingement and radiant heating unit 22 illustrated in FIGURE 2, this is readily and simply accomplished by welding or otherwise fixing tubular type heaters 76 in heat conductive relationship to the inner or back sides of the flow plates as shown in FIGURE 3. Each heater 76 consists of a plurality of parallel, spaced apart, straight legs 78 extending longitudinally of the associated flow plate 60. The legs 78 are connected by end bends 80 alternately located at opposite ends of the associated flow plate.
If they are to be employed as sources of radiant energy, the flow plates will preferably be fabricated of a material which is a good thermal conductor and has a relatively high coeflicient of emission yet can be drilled and welded without undue difliculty. One satisfactory material is sheet steel.
To increase the absorptivity of heat from tubular heaters 76 and the emissivity of the flow plates, the surfaces of the latter may be covered with a high emissivity coating. A number of satisfactory coatings are described in my Patent No. 3,262,494 issued July 26, 1966, for Heat Exchangers.
Referring next to FIGURES 1A, 1B, 2, and 4, the treating fluid is supplied to the main supply duct 38 in each unit 22 by a blower 82 connected through a duct 84 to a fluid heater '86. As it flows through the fluid heater, the treating fluid is heated by a heat exchanger 88. From heater 86, the fluid flows through supply trunks 90 into the main supply ducts 38 of units 22 and then into branch supply ducts 44. Similarly, the spent treating fluid and its burden of evolved volatiles flows from the branch return ducts 46 of the various units 22 through ports 58 into the associated main return ducts 40, which are displaced from their normal position adjacent supply ducts 38 in FIGURE 4 for explanatory purposes. From these ducts, the spent fluid and its burden flows into a schematically illustrated return trunk identified by reference character 92 in FIGURE 4.
In the present invention, return trunk 92 is preferably connected to the inlet of circulating blower 82 so that the spent treating fluid may be recirculated through the system. This eliminates the loss of sensible heat which would result if the spent fluid were discharged from the system.
In many applications of the present invention, such as in the drying of paper or other cellulosic products, for example, the percentage of moisture or other volatiles in the treating fluid must be closely controlled to produce the desired characteristics in the treated product. To permit such control, return trunk 92 is provided with a make-up duct 94; and a vent duct 96 branches from the duct 84 between blower 82 and fluid heater 86. Valves 98 and 100 control the flow through makeup and vent ducts 94 and 96, respectively. By adjusting valves 98 and 100, recirculated fluid can be discharged from the system and replaced with fluid having a lower content of volatiles to maintain tthe volatile content at the desired level, Valves 98 and 100 may be adjusted manually or, if desired, may be automatically adjusted by diagrammatically illustrated controllers 102 of any suitable type or by a system of the type described in my Patent No. 3,208,158 issued Sept. 28, 1965 for Dryers.
An economizer 104 of conventional construction may be connected between the vent and make-up ducts to extract sensible heat from the vented fluid and add it to the make-up fluid, if desired. This recovers otherwise wasted heat, increasing the efliciency of the drying system.
Referring still to FIGURE 4, the heat exchanger 88 in fluid heating unit 86 and heaters 76 are all of the type through which a heated fluid heat transfer medium is circulated to elevate them to the desired temperature. The preferred heat transfer mediums are high boiling point organic liquids and eutectic mixtures of inorganic salts, which can be circulated at extremely high temperature in liquid form. Suitable media of this type are discussed in detail in Patent No. 3,262,494.
The advantage of employing liquid heat transfer media is that the radiant treating fluid may be heated to temperatures of several hundred degrees Fahrenheit without the problems appurtenant to the extreme pressures associated with high temperature steam.
The system illustrated in FIGURE 4 for heating and circulating the liquid heat transfer medium includes a storage tank (not shown) from which the liquid can be pumped to the main circulation system. This includes main supply and return conduits 106 and 108 and is a closed loop through which the liquid is circulated by a pump 110.
From main return conduit 108 the heat transfer liquid flows into a liquid heating unit 112 where it is heated to the desired temperature. This unit may be of any desired construction and preferably includes a temperature responsive controller 114 which so regulates the flow of fuel to the heating unit through valve 116 and conduit 118 as to maintain the temperature of the heated liquid flowing into main supply conduit 106 substantially constant.
From main supply conduit 106, part of the heated liquid flows through branch supply conduit 120 to the heat exchanger 88 in fluid heater 86. The volume of flow through conduit 120 is controlled by temperature responsive controller 122, which is connected to a valve 124 in conduit 120 and has a sensor (not shown) in supply trunk 90. Controller 122 regulates valve 124 so as to maintain the temperature of the treating fluid flowing into the supply trunk substantially constant. From fluid heating unit 86, the heat transfer liquid flows through return conduits 126 and 128 back into main return conduit 108.
The remainder of the heat transfer liquid flowing through main supply conduit 106 is diverted through supply conduits 130 and 132 to the heaters 76 in fluid im- 7 pingement and radiant heating units 22 of dryer 20. From heaters 76 the heat transfer liquid flows through conduits 134 and 136 and the forementioned return conduits 128 and 108 back to heating unit 112.
The heat transfer liquid circulation system also includes a bypass conduit 138 connected between supply conduit 106 and return conduit 128. A pressure relief valve 140 in bypass conduit 138 insures continued flow under abnormal conditions such as conduit blockage and therefore prevents such abnormal conditions from causing damage to the system.
Further details and the advantages of the foregoing heating and circulating system are described in U.S. Patent No. 3,236,292 issued Feb. 22, 1966, for Dryers and copending application No. 537,132 filed Mar. 24, 1966, for Apparatus and System (now U.S. Patent No. 3,395,487), which are incorporated herein by reference.
As mentioned briefly above, the dryers of the present invention represent an advance in the art in that they are capable of efliciently drying materials which have heretofore been recognized as difficult to dry. Among these are wood veneers which can be dried only with difliculty because of their thickness and because of the tenaciousness with which moisture is retained in the wood fibers. However even wood veneers can be readily and uniformly dried by the combination of radiant heat and high velocity impinging air provided by the dryers of the present invention. For example, pine veneers as thick as fi have been successfully dried in accord with the present invention in approximately 6% minutes. In constrast, in present commercially used dryers, as much as 25-30 minutes may be required to satisfactorily dry veneers.
The foregoing is not to be taken as meaning that no benefits are realized by employing the principles of the present invention in the drying of materials which are more readily handled by conventional techniques. On the contrary, the following exemplary tabulations shows that significant increases in drying rates and corresponding decreases in retention time can also be obtained in employing the principles of the present invention to dry materials which, in comparison to wood veneers, are less difficult to dry.
Drying rates for kraft carton linerboard Industry average: 1
Water evaporated=2.3 lb./hr./sq. ft. of dryer surface Best reported performance: 1
Water evaporated=3.8 lb./hr./sq. ft. of dryer surface Present invention:
Water evaporated=13.2 lb./hr./sq. ft. of dryer surface Drying rates for pulp, unbleached kraft Industry average: 2
Water evaporated=1.2 lb./hr./sq. ft. of dryer surface Best reported performance: 2
Water evaporated=2.1 lb./hr./sq. ft. of dryer surface Present invention:
Water evaporated= 8 lb./hr./sq. ft. of dryer surface For drying wood veneers and similar materials radiator temperatures of 300700 F. are preferably employed to provide radiant energy peaking in the range of about 6.9 to 4.5 microns. The impinging air will typically be heated to temperatures of 400 700 F.
iThe source of these figures is TAPPI Data Sheet 155D. -The source of these figures is TAPPI Data Sheet 155A.
For other materials different radiator and air temperatures may prove more satisfactory. Also, the final moisture to which the product is to be dried will influence the temperature of the air. For example, if cellulosic materials such as cotton linters are to be dried to moisture contents such as 5%, which is very low, air temperatures may have to be reduced to as low as 345 F. to prevent heat damage to the product. On the other hand, for a residual moisture content of 25%, air heated to temperatures as high as 525 'F. may be used without danger of overheating. The most appropriate air temperature in a given set of circumstances can be readily ascertained by test drying samples of the material involved. In any event radiator temperatures will normally be in the range of 400-700 F. or higher, and air temperatures will normally be in the range of 250 700 F.
Air having a low moisture content (less than 0.4 pound of water pound of dry air) or dry air is preferably employed in the practice of the present invention. Contrary to what might be expected from the numerous statements in the literature regarding the increase in drying capacity and thermal efliciency that can be obtained by increasing the moisture content of air, it has been found that increases in drying rates are not obtained by increasing the moisture content of the drying air in the practice of the present invention.
Furthermore, it has been found that air having a high moisture content will damage the product being dried even while the product still has a high moisture content. In fact, in one actual test in which the air had a relatively high moisture content but a moderate 400 F. temperature, heat damage occurred at a point where the moisture content of the sample was still 200% by weight of the dry weight of the sample. In contrast, material can be dried to moisture contents as low as 5%- 10% without damage in dryers embodying the present invention if dry or relatively dry air and the moderate temperature mentioned above are utilized.
Typically, the impingement velocity of the drying air employed in accord with the present invention will be on the order of 2,00015,000 feet per minute as mentioned above. Even higher velocities may be beneficially employed in certain applications, however, to increase the scouring and evaporative cooling effect. The only significant limitations on this velocity are the ability of the material being treated to withstand the impact of the fluid and the expense of providing and operating equipment capable of delivering fluid at extremely high velocities.
In conjunction with the foregoing, curves 142a and 142!) of FIGURE 5 illustrate that drying rates do increase significantly with increases in impingement velocities. However curves 143a and 143b of the same figure show that the increase in impingement velocity is also accompanied by a marked decrease in the pounds of water evaporated per horsepower hour required to circulate the high velocity air as impingement velocities reach high levels. This of course means that, as the air velocity is increased, there is a corresponding increase in operating costs. For any given application, therefore, a compromise must be made between drying rates and the cost of drying the product involved.
The test conditions for producing the results charted in FIGURE 5 were as follows:
Material dried /s" thick green pine veneer. Radiant energy source 600 F. (4.9a) 700 F.
All temperature 450 F.
Specific humidity of air 0.4 pound of water per pound of dry air.
Air velocity 0+ 10,000+ f.p.m.
Drying time 1 /2-3 minutes.
Initial moisture content--- by weight of the dry wood.
Final moisture content 5% by weight of the dry wood.
As mentioned previously, the drying rates produced by the combination of radiant heat and high volecity drying air are substantially higher than those obtainable by using high velocity air alone. To demonstrate this a cellulosic material was dried with a combination of dry air at a temperature of 450 F. and radiant energy from a source at a temperature of 600 F. The test results are shown by curve 144 in FIGURE 6.
The test was then repeated under the same conditions except with the radiation emitting flow plates covered to substantially block the transmission of radiant heat from them to the material being dried. The results of the second series of tests are shown by curve 146 in FIGURE 6.
A comparison of the two curves shows the substantial contribution made by radiant energy when employing it in combination with high velocity air in accord with the present invention.
In conjunction with the foregoing, the driving rates produced by the combination of radiant heat and high velocity impingement air preferably utilized in the dryers of the present invention are, surpisingly, significantly greater than would be expected from adding the theorectical drying rates of radiant heat alone and high velocity air alone. This is apparent from FIGURE 7 in which curve 148 shows the calculated drying rates for a 600 F. radiant source and 450 F. air over a velocity range of to approximately 8000 f.p.m. This was computed by measuring the drying rate with both the air and the radiant source at a temperature of 450 F. and then adding the increment of radiation derived from subtracting the radiation corresponding to a 450 F. temperature from that associated with a 600 temperature. Curve 150 shows the results actually obtained. Over substantially the entire velocity range the actual drying rates are considerably higher than the calculated rates.
For example, at an impingement velocity of approximately 4000 f.p.m., the actual drying rate is about 9% greater than the theoretical rate predicted by adding the calculated drying effects of the radiant energy and the impingement air. Accordingly, the combined use of radiant heat and fluid impingement in accord with the present invention is highly advantageous, especially where maximum rates are desired or the product is difiicult to dry.
Many modifications may of course be made in the exemplary embodiment of the invention described above without exceeding its scope.
Probably the most important of these is the drying of the material being treated from one side rather than both sides as in the embodiment of FIGURES 1A and 1B. Unexpectedly, it has been found that drying rates obtained by drying from on side are up to 70% as high as those attained by drying material from both sides. Accordingly, it may in many if not most applications he more economical to dry materials from one side only.
Furthermore, the foregoing modification permits significant reductions to be made in the complexity of the drying apparatus, considerably reducing the capital investment required for a plant of given capacity.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
What is claimed and desired to be secured by Letters Patent is:
1. A method for drying relatively thin sheet and weblike products in a combination radiation and convection type dryer comprising the steps of moving the product to be dried through said dryer; directing radiant energy having wave lengths peaking in the range of from about 4.5 to about 6.9 microns against the product being dried as it moves through the dryer to evolve moisture therefrom; and simultaneously directing against said product at a velocity of at least about 2000 feet per minute a substantially uniformly distributed flow of a fluid heated to a temperature in the range of 250700 F. to assist in evolving moisture from the product being dried and to scour away from the surface of said product the moistur evolved therefrom.
2. A method for drying relatively thin sheet and weblike products in a combination radiation and convection type dryer comprising the steps of moving the product to be dried through said dryer; directing radiant energy having wave lengths peaking in the range of from about 4.5 to about 6.9 microns against the product being dried as it moves through the dryer to evolve moisture therefrom; and simultaneously directing against said product at an acute angle thereto, and in a direction opposite to the direction of movement of the product a substantially uniformly distributed flow of a fluid heated to a temperature in the range of 250-700 F. to assist in evolving moisture from the product being dried and to scour away from the surface of said product the moisture evolved therefrom.
3. A method for drying relatively thin sheet and weblike products in a combination radiation and convection type dryer comprising the steps of moving the product to be dried through said dryer; directing radiant energy having wave lengths peaking in the range of from about 4.5 to about 6.9 microns against the product being dried as it moves through the dryer to evolve moisture therefrom; and simultaneously directing against said product at an acute angle thereto, in a direction opposite to the direction of movement of the product, and at a velocity of at least about 2000 feet per minute a substantially uniformly distributed flow of a heated fluid to assist in evolving moisture from the product being dried and to scour away from the surface of said product the moisture evolved therefrom.
4. A method for drying relatively thin sheet and weblike products in a combination radiation and convection type dryer comprising the steps of moving the product to be dried through said dryer; directing radiant energy having wave lengths peaking in the range of from about 4.5 to about 6.9 microns against the product being dried as it moves through the dryer to evolve moisture therefrom; and simultaneously directing against said product at an acute angle thereto, in a direction opposite to the direction of movement of the product, and at a velocity of at least about 2000 feet per minute a substantially uniformly distributed flow of a fluid heated to a temperature in the range of 250-700 F. to assist in evolving moisture from the product being dried and to scour away from the surface of said product the moisture evolved therefrom.
5. The method of claim 4, wherein the fluid directed against the product has a moisture content of not greater than about 0.4 pound of water per pound of dry fluid.
6. The method of claim 4, wherein said radiant energy and said heated fluid are directed against both sides of the product being dried as it moves through th dryer.
7. The method of claim 4, wherein the distribution of the heated fluid across the product being dried is substantially uniform.
8. The method of claim 4, together with the step of directing fluid heated as aforesaid against said product at an angle thereto and in the direction of movement of the product to direct spent fluid and evolved volatiles into exhaust ducts extending across the path of the product through the dryer.
9. The method of claim 4, wherein said heated fluid is directed against said product in a series of jets and the total area of said jets is in the range of about 1.25 to about 4 percent of the area encompassing said jets.
10. The method of claim 4, together with the step of recirculating fluid exhausted from adjacent said product as it moves through the dryer to conserve the sensible heat therein.
11. The method of claim 4, wherein said heated fluid is directed against said product as it moves through the dryer at a rate suificient to prevent overheating of the product by the radiant energy directed thereagainst.
References Cited UNITED STATES PATENTS Andrews 34-4 XR Andrews 34-68 XR Flynn 34-68 XR Dungler 34160 XR KENNETH W. SPRAGUE, Primary Examiner