CA2289267A1 - Rotary dryer - Google Patents

Abstract

A rotatable dryer for drying pieces of material includes a generally cylindrical shell having an inner surface which defines a cavity for receiving the material. The cavity is open at each end. A heating element is arranged to force hot gas through the cavity. Shell flights are mounted on the inner surface of the shell. A plurality of shafts are mounted to extend lengthwise in the cavity. Dwell flights are mounted on each of the plurality of shafts. The dwell flights and shell , flights are constructed and positioned relative to one another to promote uniform distribution of the material within the cavity so that substantially each piece of material is dried without being burned.

Description

NOU-10-99 14:21 314 231 4342 P.03 R-094 Job-210 NOU-10-99 WED 01.25 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. 03 ROTARY DRYER

2 Back round:

4 The present invention relates to rotary drum type dryers used in the wood drying industries ofi partide hoard, oriented strand board manufact<rrine (OS6), pulp 6 mill sledges and the like. Previous inventions covered in U.S. Patent numbers:
7 x,860,462, 5,454,176 4,964,266 4,741,fi22 4,628,6'14 8 4,549,699 3,861,055 3,798,789 3,717,937 3,593,430 9 among others, allow hot gasses to penetrate down the length of the dryer drum 7 Q where partially dried material Mmes d~aggd due to thermal damage, dry some 11 material more than other, caused med~anicai and thermal damage to the material 12 and produced at low opacity, or if operated at normally required production rates 13 require very high inlet temperatures and operate at very low material dwell. Most 14 other righting systems all~nr hot gas to penehate well down the length of the drum where partiaNy dried strands become damaged or at le~asvt over~ried while other 16 strands are under~lried. Uneven and slightly populated spaces promote the 17 penetration of hot gasses down the length of the dryer which would cause then»al 18 damage to the material being dried.
19 Some of these designs were required to operate at high inlet temperari.xes, over 1000° F which resulted in a high amount of VOC pollution requiring e~nsive 21 gee dean~rp systems arxi high operating costs. Lost production and poor quality 22 titer often resulted, causing lost prnftts. The most common reason for plants not 23 2maining their deign potential is dere to the limited capacity of their drying system.
24 This invention addresses those problems.
26 Summary a~f the Invention:
27 Among the several objectives and feahu~s of the present invention may be 28 noted the prorrtsion of a drying process wherein strands are dispersed evenly across 29 the dryer cross sections as well as along the longitudinal length to promote ur'tirtorm drying. This invention promotes even dispersion and inhibits areas or sections of 31 utleven loading.

NOV-10-99 14:21 314 231 4342 P-04 R-094 Job-210 NOU-10-99 WED 01.25 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P, 04 32 Ead~ strand is ads~rded the acme opportunity for air Gassficetion, separation, 33 and exposure to the drying gases, p:omo~ng swan drying of each strand according 34 td its pautlaalar size, weight, and moisture content. This iS sieved with the unique 35 l9ighhng system. The flighting system contained herein provides a short and 38 oxt~sistant fall distauxa promatinp high me~t~riei dwell while else providing a dense 37 shower of material to evenly cool the gases while alio~ng the s~arat:on of thin and 38 fine material to mood through the dryer in less time than ttx larger, wetter material 39 which requites longer dwe~il time. This is espeaalty difficult when drying bulky 40 materials tike OSB. This invention aprcficalry addressee that type of matoriai, but is 41 applicable to other materials.
42 TheaB goals are met while operating the dryer at a slow rolationa! speed 43 adding to the dryer life and capaatyr since the slower rotational speed rrsufts in less 44 tame that the material spends failing and flying thro~.~h the dryer $rid more time laying 45 on the hot steel plates with the gag flowing by. Old designs would operate at 5-t2 46 rpm while this design will operato at 2~ rpm due to the fighting e~fectevenese. 'ibis 47 design is effective at gas velocities of up to 600 feet per minute versus old designs 48 which wane limited to 300 feet or 4pU fravt pot minute. This Invention provides 49 material dwell at those higher gas vdocibes of 15 to 20 minutes versus 4 to S
50 minutes for older designs. This design will produce strand-to-strand moisturre 51 variations of one or two per-cer~tage pvirtts versus four or more for alder designs. This 52 invention will produce volumetric dryer loadings of 10% to 20% an large, bulky 53 materials suds as OS6 strands in the 6 indi to 12 inch lerx"~ range versus 5°~ on 54 older designs. This fighting system will accept a wide variation in strand lengths from 55 3 inches or lass to 12 inches in lerx~th. Other objectives and features of the pnasent 56 Invention will be in part apparent end in pert pointrd out henaafter.
5T Res~erring now to Sks~tct~es l, II. III and N, but in partia.rlar to Sketch I which 58 depicxs the dryer drum assembly. Hot gas and material enter the inlet cone (#2), 59 which is special in its design to provide a restriction that eiimirletes mafierial retumlng 60 upstream during operation. The inks oone including a substantially smaller inlet and 61 larger discharge with sufficient length and angle of flair to become an a~edive block NOV-10-88 14:21 314 231 4342 P.05 R-084 Job-210 NOU-10-99 WED 01:26 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. 05 G2 against material migrating upstroam, also has a pkxelity a! sa~ewfotvverd fllghtinp (~
63 for the purpose of moving the mslertal into the shell fligh4ng (6~ fob on the irlalde 84 of the major diem~ter of the drum (3). Because the upstream ratn~ory lined ducting 6b is hot enough to cause wood tn bum, thus restriction creates an important feature.
G6 The shell is aoratruded of a plurality of sections (3), which are »pptoximately 6'7 ten feet long for shipping. There are 8 to 20 seciians as required to 2lchieve sedfons fib of a shippable size. Eectt appr~dmately 10 foot section is made from three or more 69 segments of 90° to 120° each. This is true in larger sizes such as 20 foot diameter TO design, but would net be used txlow 12 foot diameter. The entire drum rotatbs on 71 two tracks (4) and (5) about a generally harimoMal axis which ride on trunnion wheels, 72 not Shown, such as described in U.S. Patent Numbers 4,86Q.462 and 5,4,54,17fi, 73 which are incorporated herein by reference. The drum rotates at Iow speeds, about 1 74 td 6 rpm, prvferabty 2 to 4, driven through sprodcert teach {8) which are bolted to the T5 shell. 'The slower speeds are more desirable since lower speed results in higher 7S dwell time for the materiel. More etfeCtive fliphting will operate at slamar rotational 7T speed 2nd resuR in longer equipment tife_ 78 Tfie material and gee east the dryer out its discharge fittlnp (9) which is s 79 scat design to promote even feed and smooth acceleration of tt~e gasses and 80 material Into the discharge pipe (1 t)). Tht3 discharge fitting (9) includes ftightlng (11 81 {Sketch I and ll), which lift arid distribute the material into the accelerated gas stream 82 and into the d'~arge ducting (10). The shape ottt~ di9charge fitting (9) is 83 comprised of segmented canes or a parabolic cone designed to efficier~iy acoeler~ate 84 the gas and material up to conveying velocity for the material txlng driod.
95 Now refer to Sketch II, which shows the drum c~.rt-away. Note for clarity thg 86 dwell fllghting that mounts ir~ick la net shown. See Sicatch III, which is a typitai dwell 87 (lighting t5undla_ There are dwell fligMing bundles mounted cnnoer~tricsZlty inside most 88 or all of the drum ~ectlans.
89 Sketches t and II shrnnr the inlet c5one (2), which acts as a material channeling 90 ctevics. The eonlcai shape of this inlet (2) combined with tl-sa saewfvrward fiiflhtinp 91 (7) channel the material into the shelf Righting (6) ratter than the dwell !fighting. 'lire NOU-10-99 14:21 314 231 4342 P.06 R-094 Job-210 NOV-10-99 WED 0126 PM SENNIGER, POWERS, ET AL FAX N0. 314 231 4342 P. 06 92 screw tllghtlng (7) are sized to handle the required volume of malaria) and are 93 shaped and located to a~,t as aggressive movers, imparting forward motwn vin the 94 ditlre assembly is rotated. The spaang and size of these Righting must 95 accommodate the bulk o~f the material and coincide with the shelf fligtzting so as Do 96 provide a smooth transition into the shelf flighting (6) v~hout alknMng bunching or 97 ptudging. The shell (lighting (8) is spaced and shaped to accommodate the bulk 98 density of the material and often is spaced along its longitudinal soda to ellmlnals 99 pinching or hang-ups. Material whidi is more granular with a greater bulk density 100 would require smaller, more closely spaced t9ightirtg while material suet eis OSB
101 (wAOd stands), which has a Iow bulk density, would requlro larger rights with bigger 102 spaces. This must be achieved while mairrtsining a corfiguration which ma~omizes 103 the lifting and showering rrt the material.
104 There are mercy dirt shapes which will achieve this effect. For the diffia~ft 1 OS materials, such as TSB, a series of plates oriented radlaiy are bent end notched to 108 indude at least two angles along with one radial. There would be a forward anpie as 107 welt as a I~ckv~ard angle shape. All stapes would inGude a formed innenrbst edge 148 of at least 1 inch and not over 4 inches, which would be bent about 90° from the 109 major portion of the flight and notched at a space proportionate with the strand 110 IenQlh. Four to B inch Long strands would use a notch length of about 4 inche~6 111 inches. Longer strands would use longer nvtd~s, These flights are arranged in a 112 pattern, which repeats throughout the length of the dn,on.
113 In sorry cases, these frights are shaped to return the material n for 114 the purpose of bunching herd to dry materials. These shapes would include twisted 115 pieces of plate, which, resembling a propeller or shovel and would serve to scoop or 116 Isy the material further upstream nether than 9lmply lifting end showering. Such flights 117 would not have a formed inner lip, taut vHOUId allow materiel to slide off in bunches for 118 the purpose of scooping the materiel. Thane shapes vwuid be usod for materials 119 such as sawdust or sludges, whid~ require exb'emety long dwell timers to achieve the 120 dryness requtrod. The disd~erge (9) is sized to prov'~de good pneumatic conveying 121 velocity and requires special flights (11 ) to pick up, distribute, and Present the NOU.-10-99 14:21 314 231 4342 P-OT R-094 Job-210 NOU-10-99 WED 01.26 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. 07 722 material Into the gas stream allowing even acxeleration end conveying out of the 123 dryer. The discharge size is based on tt~e tvlat gas voiurne and density so that the 124 gas stream Idnetic ertergyr will accelerate and convey the material.
Slrnply providing f 25 adequate veloaty is insufficient fior a srrtooth operation. Ther~efors, the discharge 126 flightir~g (11 ) is designed also to pick up the volume of mafierial ae the dram rotaries 127 and with that material sliding on its xaface, present it substantially into the 128 accelerated gas stream in a showered fashion. The shoV~ar o! material is again 129 obtained by providing bent inner edges with natd~es, not shown in Sketch 11, so that 130 material is dropped off in smell bunches rather than large clumps, 131 Sketd~ III illustrates the dwelt fllghttng txmdle. All the above aspects aro 132 aitical in combination with tho dwell flightir~ design, as the results msufd not ocarr 133 vvithout most or all of these teatcmes. Some drying would oaaarthe dwell 134 fligMing, however, it would tx a fraction of the capacity and quality of that which t 35 oars with it. Etch dwell tlighting module, Sketch ttl, is oor~s6ructed in a like manner 13fi but cxuld have d'iffen~nt quantity of elements (1~1) and (15) at various point along the 137 length of the dryer dnarn d~aper~ding vn the material being dried and the results 138 require, There is no dwelt flightinq In the fast 10 foot 5octfon, for instance, to promote i 39 the separaflon o! fines and thins and allow thedn to move out faster than the coar9er, 140 wetter material.
141 The absence of dwell (lighting bandies in the fast 10 fast of the dryer allows 142 the material to fall the er~ire distance tram the top to tho bottom of the drum. In That 143 fail distance the gas flowing through the dn~m has longer to work on the material and 14d tftersfore eKectively moves the ~r~es and thin materiel, more easily diaptaced, further 145 down the length of the drum reusing f! to become separated from the larger, thicker 146 a wetter material which i9 less atfvcted by the gas kine4c energy.
147 The dwelt flighbng bundles are located cvrxxrxrically to the drtrm, centered 14B along the length of each segment and designed to allow showering fiom the ou6sr f49 shell flighting to the outermost circle comprised of (12), (14), and (17).
150 The distance is based on the material charactariatics, OS8 in the 4 ind-s to F
151 irx~~ lengths wau(d require 24 in~es tn 36 inches of firm shell )night to dw~ett NOV-10-99 14:21 314 231 4342 P.06 R-094 Job-210 NOU-10-99 WED 01:26 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. OS

1 SZ Bight, while more finely granulated material would be Iess and larger materiel would 153 be more. Twelve-into long strands, for example, woukJ require a 3 float too 5 foot 154 space.
155 The basic aonstruaion InvaHes concentric circles of fligtiting elements.
In this 156 example there are two-dv~ell fiipht rings (12) xtd (13) which affect reapedalve cross 157 sectional areas of the drum. The flighting elements (14) and (15) are of s~ different 158 aizr but similar shape. Each element is built up with a support tube or bar, which has 159 a plurality of plate flights (17) attad~ed_ Two or more to a row, with four being the 180 most eftedive, the successive roNVS (18) and (19) apt 45° rotation or offset to split the 161 previous row if there are more than four plates par row. Some materials or smaller 18.2 diameter drums would roquins only one ring of dw~n lllghting.
1fi3 The curved end ring (12) and (13) are connected to one another and to the 164 drt>om with pinned link arms (16) which allowfor the thermal expansion duo to their 165 offset orientation such that thenna! expansion would rotate the rings relative to orm 1f6 anorther slightly rather than a~us~a bending, therefiore the assombty is free to respond 167 to the thermal changes without induang loads, This promotes Long life.
168 The flights (17) era welded to the shaft whidt has a mounting dip (20) welded ifi9 to each end that is bolted to the curved erxt ring (1~) and (13). The rtlghting element 170 (14) and (15) are equally spaced, d around the curved end ring (12) and (13) 171 to provide a maximum interference to the showering material while allowing sufficient 1 T~ freedom of movement so that material does not plug the module.
173 Sketch N ittusttates the cx~ss section of the dryer with the dwell flig~ng 17a module in place. The shell (3) and the shell flighting (6), which are welded to the 175 drum, seNe a few functions, The most impedarlt is to elevate and distribute the 176 material across the er>bre arc of rotation, fom~ 94° to 270°. Ks the strands or other materrat fait, the untied distance (12 to 48 Inches) to the ocher ring of dw~eil Righb 1 rt3 (12), trot gas passes by, promoting convection heaitirlg and drying.
While the 179 strands, or other material, Is falling the drying gas conveys the material axJally along 180 the dyer. Therefore, the fall drstanrs is critical to the maiterial dwell tirt~.

NOY-10-99 14:21 314 231 4342 P.09 R-094 Job-210 NOU-i0-99 WED 01.27 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. 09 181 As this happens, the larser ring of dwell Righting receives the material faring 182 from the shell ftight9. As the dn~rn retstes, boat is transferred into the strands vta 183 conduction frr~m the plate to the strand and hot gas continues to pass through the 16a material promoting convection drying. While the material is resting on the dwell flight 7 a,95 module it does not travel along the dryer length. Tt~ rotating essamtaly including the 186 outer ring of flights ~strit~ute, in a of material, which is collected try the Irtr'ser 18? ring (13) in a liitg mara~2r. Some material falls from the shell drum flights back to the 188 shell at the 100° to 70° area and the 210° to 300° area. For this reason the multiple 189 shapes in the shell flights are important so that the recd trip delivery the material to 190 tt~re dwell flights.
191 An example would tae the ~ ~e material failing from a shell flight at 100°
992 w4uld miss the outer ring of dwell flighEs and fall the longer distance to a point at T0°
193 on the shell flight but further down the length of the drum. Since there are multiples of 194 different shaped shell flights, the odds are, that same matari8) would then be lifted 195 arid showered onto the dwell flightxng. Since this opportunity exi9ts fcx ail material, 196 the effect is averaged into the overall material dwell. It i9 good to minimize this ePfed 197 by using the minimum space possieie behNeen shell and dwell ftlghting without 198 plugging.
199 This also occurs in the dwell flight module to a lesser degree. Minimizing this 200 is important and is achieved by varying ttie shape of all ftlghts_ Clnce the material 2a1 cascades from the outer ring (12) ao the inner rtr~ (13), it is carfied and showered to 202 the Power side of the inner ring (13), then back out to tire outer ring ('t2) end then 203 either bade to the inner ring again or all tt~c way to ttte shall flights.
In this manner the 204 material is continually 5~hvwered and riding on the hot plates at all times as it ~oumeys 205 through the drum. The fell dist~Ce is relatively consistent 2d aN drop prints.
206 Analyzing the motion of material, one sees that there is a certain probability of 207 strands taking various paths, whid~ are dictated by the oriertta4on of these dwell 208 flighting elements. The dwell flight elements (1~i) and (15), described earlier, contain 2Q9 a plurality of prates which are fiom~ed at 90'° on their outer ends which ere notched to 210 promote art even showering as material glides ofl:

NOy-10-99 14:21 314 231 4342 P.10 R-094 Job-210 NOV-~10-99 WED 01.27 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. 10 211 All the plates used in the dwell flight elements (14) and (15) are bent at 60°
2'12 tol2Q° on the outer edge. Also, plates may have mul~pfe bends. One Do four 213 ind~os is required to prpvide tt~e reter>tion and tha9e bends are oriented such that 214 they retain material lying on the plate and cause it to tumble otf or over the edge.
215 These bent portions one r>IOtched or tapered to promote the br~C.aJp of the material 216 as ii is tumbled aAf the end of ttie flight 217 The size, shape, quantity, and placement of the dwell fight elements limits the 218 lair d~tance of strands, which is the key toot in OSB drying to promote materiel, 2 ~ a dw~n.
22fJ Dvaell here is used to define the tyme the wood matertat spends inside the 229 dryer drum. A minimum dwell of about six minutes is required to achieve prac~Ica!
22x production drying of strand9. Preferred dwell is 95 to 20 minutes, Higher dwell is Z23 better vin the gas is urirforrniy cooled so that hot gas dose not come into aontad 224 wish partially dried wood. If the flighting provides a shvu~ew~g of material 225 asxoae the entire cross sectional area of the drum ere well as akmg the length, the 226 drying gas will be uniformly doted. This prevents streams of hot gas from 227 penetrating along the longitudinal ode of the dnnn, v~ich vwuld cause part7ally dried 228 material to over' bass, bum and cause pvlludon through the evolution eat' resinous 229 compounds from the wood. VtJC.
230 This tlighting design promotes material dwell at high rapecities of 15 to 231 minutes while causing uniform cooling of the drying medium and promoting 232 swpa~tion or individualization of the wood errands or partides so that moist 233 oontr~oi on a strand basis is more uniform than other designs.
234 A 20 foot dryer is capable of drying 5Q wet tons per hour of 6 inch long 235 strands from 120°~6 moisture content dry basis down to 2°~
AACDB white operating at 236 an inlet tempefature of 1000" F while delivering a material dwell of 15 minute cr 237 more. At these rates the drum is loaded to as high as 1 S% of the dryer drum volume.
238 Previous designs could only achieve S°~ to 696 fill. This is especially significant when 239 longer strands are used. previous designs would not tolerate strands over 6 irxh or N0~-10-99 14:21 314 231 4342 P.11 R-094 Job-210 NOU-10-99 WED 01.27 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P, 11 240 would be limited to sign~cantly less capacity suds as 5% or fi% fill which translates to 24i either Iv~rer dwell or less capacity.
24.2 To achieve good strand moisture oontroi the strands cr any other particles 243 must be separated from each other and presented tv the gas stream, constantly 244 fuming over and over for the entire dwell time. Only with this wnd of inspection or 245 aepamtion will each strand be dried to its desired moisture.
246 Uniform moisture, 5drand to strand is stn important goa! of OSB drying as well 247 as other maieriafa. To achieve tf~is, the material must spend adequate time at 248 moderate temperattrre~, 40c7° to 600° F, so that each strand has ample opportunity to 249 give up its moisture white being e~osed to hot gasses arid hot sheets of steel. Note:
250 too hot, however', will do damage tn ttre wood, 'i'his moat be achieved while opetattng 251 at inlet temperatures of near 1000° F and while maintaining exit temperatures of over 252 250° 1= io preveryt condensation of water vapor on the duclind. In the older designs 253 the material is forced through the dn~m by high gas veioaty or is dropped long 254 disbancas causing rapid material movement and short dwell. Also most flighdng 255 systems treat strands in one area of Ehe drum's cross-section ditfierentty than other 256 areas. For example, some s:rands drop S-10 feet white those next to them only drop 257 18 inch. The strands that drop 90 fee! mwe much further through the dmm and 2513 exit sooner. Therefore, the goat is to limit the drop distance and maintain a uniform 259 drop distance.
260 This design provides very homogeneous handling of strands at high and (aw 261 feed rates, very uniform heat sink of drying gasses without allowing hot gaga bypass.
2f2 This is done through the superior distributing action of the dwelt (lighting modules.
263 The fact that strands are individuaii~ed and separated while being c~ntinuougly 264 dt~opped from row to row a aonttollad distance previd~ a very dense shower of 2fi5 material which forces a high degree of slip be~n~en the drying gas and the wood 266 s~randa. 'the higher the variation of gas dwell vars~.~ material, the higher is the slip 26? betvueen them thus promoting greater heat input and there~forg higher evaporation in 268 a given size dryer. This Is evidanoad by the high duuell time the wood has even with 2f'a9 gas vetc~ties, (twice that of older desiAns)_ Older desic,~s with gas velroatles of NaU-10-99 14:21 314 231 4342 P-12 R-094 Job-210 NOU-,10-99 WED 01.28 PM SENNIGER,POWERS, ET AL FAX N0. 314 231 4342 P. 12 270 around 300 feet per minute would yield material dwell tirrie9 of only 3-6 minutes while 2T1 this design wiu tolerarte gas veiodties of twice that amount at rrrateriat dwell times of 272 4var 12 minutes. This design will tolerate B00 feet per mirxate as compared to 300 273 feet per minute with the older designs. T'he fact that temperatures ail as oss the dryer 274 diameter are uniformly cooled evidences the effedivenese of the tliphfing ey9dem.
275 Older desi~,s often operated with some arses of the dryer dis~ge moss 276 section 50° to 100° F higher than se,~rounding areas which proved that hot streams of 2TJ gas were penetrating the entire length of the dryer.
278 Since this tiighting allows hig~r gas velocity while providing preat~er masr<erlal 279 dwelt time, better separation of the thin or dry fiom the thick or wetter ~d9 280 oax~rs. That promotes more uniform drying of all strands. Since the dryer will 8coept 281 higher gas velocity while providing gentle handling of strands due to the short fall 282 distances the dryer will operate at a given caparlty v~ittt lower temperatures or higher 2$3 c~pacfies as the user wlishes_ tn either case the resulting stable operatior>t Is safer 284 due to fewer upsets. Upsets ar9 plugging or flre~ caused by high temperatures 285 penetrating loo far down the dryer length. Upsets also occur when the dryer fails to 286 produce strands of suficient dryness, vH,ich is subsequanly moasur~ed causing the 28T dryer operators tv abort the dried material to a dump. These ail have a prvfaund and 2s8 negative impact on ~e plant prod~ion.
289 The modular design 811aw5 fbr adJt~s~ment to the dryer accommodating a 290 user's changing needs with changing resin technology or strand length end 291 thidmess. This is achieved by changing the dwelt Righting modules.
292 Sfr~ce rt~aterials are more uniformly dried without being over-dried and strands 293 or wood are not over-heated, there is less organic malaria! boiled off from the wood 294 and, ther~efone, (ass pollution from this design.

Claims

Claims WHAT IS CLAIMED IS:

1. A rotatable dryer for drying pieces of material comprising:
a generally cylindrical shell having an inner surface which defines a cavity far receiving the material, the cavity being open at each end;
a heating element arranged to force hot gas through the cavity shell flights mounted on the inner surface of the shell;
a plurality of shafts mounted to extend lengthwise in the cavity:
dwell flights mounted on each of said plurality of shafts;
the dwell flights and shell flights being constructed and positioned relative to one another to promote uniform distribution of the material within the cavity so that substantially each piece of material is dried without being burned.