Abstract:
An vertical dryer ( 10 ) is provided having a plurality of individual dryer decks ( 12-18 ) including alternating fan and heater decks ( 14, 16 ), an air circulation assembly ( 11 ), and an upright, common plenum chamber ( 26 ) in communication with the decks ( 12-18 ). The assembly ( 11 ) is operable to create a continuous drying air stream which passes upwardly in countercurrent flow relationship to product on the decks ( 12-18 ). Also, the assembly ( 11 ) serves to pass the drying air stream into, through and out of the plenum ( 26 ) at the level of each heater deck ( 16 ); in the plenum ( 26 ), the air stream velocity is decreased, causing particulate fines to fall out of the stream for collection. The use of the common plenum ( 26 ) also allows independent control of the decks ( 12-18 ) in terms of air flows therethrough and recirculation characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is broadly concerned with vertical, multiple-deck continuous batch dryers designed for drying of pellets and other similar products. More particularly, the invention pertains to such dryers and drying methods wherein the dryers have a series of superposed, air-pervious drying decks and an air circulation assembly operable to generate and direct a continuous drying air stream through the decks; a common upright plenum chamber is provided adjacent and in communication with the dryer decks, and the air circulation assembly is designed to pass the air stream into, through and out of the plenum chamber as the air stream passes through respective decks. This allows easy removal of entrained fines within the drying air stream and also permits the user to individually control both the air flow and the percentage of air recirculation through each deck, independently of the other decks. 
     2. Description of the Prior Art 
     Multiple-deck vertical continuous batch dryers have been used in the past for drying of pellets and other agricultural and food products. A vertical design allows product transfer between decks with good product separation. Moreover, a higher degree of moisture uniformity is achieved owing to multiple turning of the product as it passes between the vertically spaced decks. The countercurrent design of these dryers (product descends and air flows move upwardly) also gives higher energy efficiencies. 
     Several different design approaches have been tried in the past with vertical dryers. In one system, each deck assembly is provided with a separate fines collection unit in the form of a cyclone separator, fan and heater. This approach does have the virtue of removing fines at each deck level, thus minimizing the tendency for fines to accumulate on internal dryer components. However, this is a very expensive expedient, with the multiple fines collection units greatly increasing equipment costs and necessary plant space. In another system, only a single collection device is provided at the upper outlet of the dryer. This significantly reduces costs, but does not remove fines at each dryer stage. Thus, it is necessary to have increasing air velocities from bottom to top of the dryer in order to insure that the fines remain entrained in the drying air stream for ultimate separation at the final collector. 
     Furthermore, both of these prior art approaches suffer from the inability to effectively and efficiently control dryer operation at each deck, independently of the other decks. This means that the air flows (velocities) through each deck cannot be independently controlled, nor can the amount of air recirculation at each deck be controlled. 
     There is accordingly a real and unsatisfied need in the art for an improved vertical dryer which avoids the high costs associated with multiple collector type dryers, while at the same time giving the same or a better degree of staged fines removal. Also, there is a need for a vertical dryer wherein the individual decks thereof can be independently controlled in terms of airflow velocities and recirculation characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems outlined above, and provides an improved vertical dryer which comprises a plurality of superposed, air-pervious drying decks which support quantities of product to be dried thereon and which are selectively openable to allow the product quantities to descend from deck-to-deck during drying thereof. The dryer also includes an air circulation assembly operable to generate and direct a continuous drying air stream through the respective decks and the product quantities thereon. The vertical dryer of the invention also includes an upright, common plenum chamber adjacent and in communication with the drying decks, such air circulation assembly being operable to pass the continuous air stream into, through and out of the plenum chamber as the air stream passes between respective decks. Such traversal through the plenum chamber facilities fines removal and also allows independent deck control. 
     In preferred forms, the plenum chamber is sized so that when the drying air stream passes into and through the chamber it loses substantial velocity, which facilities dropout of suspended fines. Furthermore, a series of diverters are located within the plenum chamber for redirecting the air flow through the chamber. The plenum preferably has a particle collector adjacent the lower end thereof. 
     The air circulation assembly includes a plurality of individually controllable fan units which can be adjusted to provide independent control of the velocity of the drying air stream as the latter passes through individual decks. Such fan units typically comprise a fan and an adjacent, selectively openable and closeable damper. Alternately, variable speed fans can be employed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a prior art vertical dryer wherein each drying zone includes an individual cyclone collector for fines removal; 
     FIG. 2 is a schematic side view of another prior art vertical dryer employing only a single exhaust air cyclone collector for fines removal; 
     FIG. 3 is a rear schematic view with parts broken away of the improved vertical dryer of the invention; 
     FIG. 4 is a vertical sectional side view taken along line  4 — 4  of FIG.  3  and showing air flows and air-directing structure associated with the preferred dryer decks and upright plenum of the invention; 
     FIG. 5 is a vertical sectional rear view taken along line  5 — 5  of FIG.  4  and illustrating the construction of the preferred upright plenum chamber; 
     FIG. 6 is a vertical sectional rear view taken along line  6 — 6  of FIG.  4  and depicting certain of the air-conveying passageways associated with the vertical dryer decks; 
     FIG. 7 is a vertical sectional central view taken along line  7 — 7  of FIG.  4  and illustrating other air-conveying passageways of the vertical dryer decks; and 
     FIG. 8 is a schematic side view of the improved vertical dryer of the invention, shown with exemplary air flows throughout the height of the dryer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings, and particularly FIGS. 4-7, a vertical dryer  10  in accordance with the invention is illustrated. The exemplary dryer  10  is made up of a total of eight superposed, vertically spaced apart decks (although a less or greater number of decks could be used), and a multiple-fan air circulation assembly broadly referred to by the numeral  11  associated with the dryer decks. The dryer  10  has an uppermost inlet deck  12  six alternating fan and heater decks  14  and  16 , and an optional lower cooling deck  18 . Each set of decks  14 ,  16  provides a drying zone, and thus the depicted dryer  10  has three such zones. The inlet deck  12  is surmounted by a product inlet housing  20  having a product inlet  22 , while product collection hopper  24  is located beneath cooler deck  18 . The overall dryer  10  also includes an upright, vertically oriented, common plenum chamber  26  supported by and communicating with the decks  12 - 18 . The dryer  10  is designed to receive quantities of product through inlet  22  and to dry such product by successive passage of quantities thereof through the decks  12 - 18  for ultimate collection in hopper  24  and delivery to other downstream equipment (not shown). 
     Inasmuch as all of the fan decks  14  are identical to each other, a description of a single fan deck will suffice for all; likewise, given that all of the heater decks  16  are identical to each other, only a single description thereof is provided. Each fan deck  14  includes a substantially flat, air-pervious floor  28  (FIG. 7) as well as upright outboard sidewalls  30 ,  32 , front wall  34  and rear wall  36 . In addition, a pair of upright inboard walls  38  and  40  are located adjacent the sidewalls  30 ,  32 . As illustrated in FIGS. 7, the walls  38 ,  40  extend from sidewall  34  for a majority of the length of the deck  14 , but are shortened to leave a rear space  41  which is important for purposes to be described. A depending wall  41   a  extends from the top of the deck downwardly and is affixed to the butt ends of the walls  20 ,  32  and  38 ,  40 . Each wall  38 ,  40  includes a vertical segment  42 , an outwardly extending segment  44 , and a perforated inturned return segment  46 . 
     As best seen in FIGS. 4 and 7, the spaces between the walls  30 ,  38  and  32 ,  40  beneath the segments  44  houses a plurality of elongated arcuate vane members  48 ,  50 ,  52 . The vane members cooperatively define a total of four air passageways which are open along the bottom margin of the deck and at the rear ends thereof. 
     The floor  28  is made up of a plurality of elongated, side-by-side, pivotally moveable slats  54 . The floor  28  is selectively openable via a conventional drive  56  (FIG. 4) coupled with the floor slats  28 . When the floor  28  is in its closed position depicted in FIG. 7, it is operable to support a quantity of product  58  thereon but is nevertheless air-pervious. However, when the drive  56  is actuated the floor slats  54  are pivoted to an open position, thereby allowing the product  58  to descend under the influence of gravity onto the next lower deck. 
     In order to provide access to the internal components of the fan deck, front and rear access doors  60 ,  62  are provided. 
     Referring next to FIG. 6, it will be seen that a double fan assembly  64  is housed within the space  41 . In particular, the assemble  64  includes a pair of powered fans  66 ,  68  respectively mounted in the sidewalls  30 ,  32 . A somewhat Y-shaped duct  70  extends from the base of deck  14  upwardly to the inlet sides of the fan  66 ,  68 . The outlet sides of the fans in turn communicate with the passageways defined by the vanes  48 - 52  extending along each side of the deck. 
     Each heater deck  16  is in many respects similar to the fan decks  14 , and thus the same reference numerals are applied to like parts. Each fan deck  14  includes a floor  28 , outer sidewalls  30 ,  32 , front wall  34 , rear wall  36 , inner sidewalls  38 ,  40 , internal space  41 , short wall  41   a , and vanes  48 - 52 . Moreover, the deck  28  is made up of slats  54  moveable via drive  56 . However, the heater decks  16  differ from the fan decks  14  (see FIG. 6) by provision of dampers  72 ,  74  at the outlets of the air passageways, as well as box-like ducts  75  extending from the dampers  72 ,  74  and communicating with outboard side openings  76  provided in rear wall  36 . In addition, the wall  36  includes a pair of inboard openings  78 . The deck  16  also includes a bifurcated duct  80  which includes a pair of lower, arcuate, converging segments  82 ,  84  and side segments  86 ,  88 . The duct  80  communicates with the inboard openings  78  and is open at the upper margin of the deck  16 . A heater  90  is situated within the duct  80 , and is typically gas fired. The heater  90  is coupled to a combustion air inlet conduit  92  which leads to atmosphere. 
     The inlet deck  12  is identical with each of the heater decks  16 , except that the inlet deck has no duct  80  or heater  90 . While this deck is equipped with the outboard openings  76 , it does not have the corresponding inboard openings  78  of the decks  16 . 
     The cooler deck  18  is identical with each of the fan decks  14  with the exception that the rear wall  36  thereof has a central fresh air inlet opening  94  formed therein. As noted previously, provision of a cooler deck is optional. 
     The product inlet housing  22  is located atop inlet deck  12  and is designed to house a conventional rake or other spreader device (not shown) serving to level incoming product delivered via inlet  22 . The housing  20  includes upright sidewalls  96 ,  98 , front wall  100 , rear wall  102  and top wall  104 . In addition (see FIG. 7) a pair of inner walls  96   a ,  98   a  are provided adjacent corresponding sidewalls  96 ,  98 . A pair of uppermost air inlet openings  106  are provided in rear wall  102  and communicate with the regions between the walls  96 ,  96   a  and  98 ,  98   a.    
     The plenum  26  is located adjacent the rear walls of the decks  12  and  14 - 16 . The plenum has rearwardly projecting sidewalls  108 ,  110  (FIG. 3) as well as a rear wall  112  and top wall  113 , the latter having an outlet  113   a  formed therein. The rear wall  112  is equipped with a pair of access doors  114  at the level of each heater deck  16 . Moreover, the rear wall  112  has an elongated, central, vertically extending recess  116  therein allowing access to the central doors  62  associated with the fan decks  14 . Inwardly extending walls  118  define the recess  116  and are connected with plenum rear wall  112  and the rear walls  36  of the decks. A powered combustion air fan  120  is operatively coupled with each of the conduits  92  within. 
     The plenum  26  is equipped with a series of diverters  122  which are located adjacent each of the inboard openings  78  associated with the heater decks  16 . Referring to FIG. 5, it will be observed that each of the diverters includes an oblique segment  124  as well as a depending wall segment  126 . 
     Finally, the plenum  26  includes a lowermost collection hopper  128  presenting a fines outlet  130  as well as an elongated, axially rotatable fines conveying auger  132 . 
     The outlet  113   a  of the plenum  26  is coupled to a conventional exhaust fan  134 ; if desired, an additional cyclone separator may be employed to insure the separation of any fines or dust entrained within the outlet air from the plenum. 
     In operation, product (e.g., pellets) are delivered to the dryer  10  via inlet  22  and are initially leveled on the floor of inlet deck  12 . During the drying process, individual quantities of the product are delivered to each of the decks in serial order so that, during continuous operation, individual quantities are supported on each of the decks  12 - 18 . This condition is illustrated in FIG.  7 . 
     Considering the operation of the dryer  10  during such continuous drying, it will be seen that fresh air is drawn into cooler deck  18  by the associated fans  66 ,  68 , this air being directed by the vanes  48 - 50  to a point below the floor  28 . This air is then directed upwardly through the deck floor and the quantity of product situated thereon (see arrows  136 ,  138 ). When the fresh air passes through this product, it is drawn upwardly through the perforated return segments  46  of the deck  18  whereupon it enters the vane-defined passageways of the next above heater deck  16  (arrows  140 , FIG.  7 ). This air is then drawn rearwardly by the fans  66 ,  68  of the next above fan deck  14  through the dampers  72 ,  74 , along the box ducts  75  and through the openings  76  to enter the plenum  26  (arrows  142 , FIG.  5 ). Given that the plenum presents a much greater volume, the velocity of the air traversing the plenum is greatly reduced, thereby facilitating dropout of fines from the air stream. Moreover, the air from the openings  76  is forced to traverse a tortuous path owing to the presence of the diverters  122 . The air from the plenum chamber passes back into the deck  16  through the inboard openings  78 , where it is conveyed by the duct  80  through the heater  90  and, in a heated condition, to the duct  70  of the next-above fan deck  14 . Also, additional combustion air as needed is delivered by the fan  120  through conduit  92  into the heater  90 , which combustion air thus joins the air stream. 
     It will thus be appreciated that the continuous air stream created in the dryer  10  passes upwardly from deck-to-deck, being successively heated as required in the heater decks  16  and with supplemental combustion air being added. Air drawn into each deck as leakage through the deck structure is also added to the continuous air stream. At the upper end of the dryer  10 , at the level in inlet deck  12 , fresh inlet air is drawn through the openings  106  by the fans  66 ,  68  of the highest fan deck  14 , such air passing downwardly through the product on the inlet deck. Also at the inlet deck  12 , the drying air stream passes through the uppermost outboard opening  76  into the plenum  26  for ultimate passage through outlet  113   a.    
     The described circulation of air through the dryer  10  creates a situation where air is drawn in opposite directions through adjacent decks. Thus, air is drawn upwardly through the cooler deck  18 , while air is draw downwardly through the next-above heater deck  16 . This alternating pattern continues throughout the full height of the vertical dryer  10 . 
     Attention is next directed to schematic FIG. 8 which illustrates exemplary air flows during the operation of the dryer  10 . In this example, all air flows are in cubic meters per hour (m 3 /h). As shown, fresh air at the rate of 10,000 m 3 /h is draw into the bottom of the dryer  10  by the cooler deck fans; this air passes upwardly through the cooler deck to a point above the product thereon. The fan in the next higher fan deck  14  is set to draw air at the rate of 20,100 m 3 /h from the region above cooler deck  18 . This is to accommodate 100 m 3 /h leakage at the cooler deck, and also to achieve a 10,000 m 3 /h air flow downwardly through the next-above heater deck  16 . The 20,100 m 3 /h air flow then passes through the plenum  26  and thence through the heater  90  of the deck  16 . At this point, the needed combustion air, in this example 1,000 m 3 /h, is drawn by the fan  120  of the deck  16  into the heater. 
     Given that in this example the user wishes to maintain a 10,000 m 3 /h air flow through each of the decks, it is necessary for the fans  66 ,  68  of the next-above deck  14  to deliver 20,000 m 3 /h, i.e., this air flow is split 10,000/10,000 m 3 /h between the two adjacent decks. This being the case, the fan is set to draw 19,000 m 3 /h of air from the plenum  26 , which with the 1,000 m 3 /h of combustion air provides the necessary 20,000 m 3 /h. The excess air (1,100 m 3 /h) simply passes upwardly through the plenum for ultimate exhaust through outlet  113   a.    
     This same pattern is thus repeated throughout each of the deck pairs throughout the height of the dryer  10 , so that, at each deck a 10,000 m 3 /h air flow is maintained and excess air is exhausted through the plenum outlet. This is an important advantage provided by the present invention. That is, by selective fan and/or damper control, it is possible to individually regulate the air flow and recirculation through respective decks. Such precise control has heretofore not been obtainable in vertical dryers. Moreover, the ability to economically remove fines and other particulates from the drying air stream also represents a significant advance in the art. 
     These important differences can best be understand by a consideration of the prior art designs depicted in FIGS. 1 and 2. In FIG. 1, a multiple-deck vertical dryer  144  is provided which has an individual cyclone dust collector assembly  146 ,  148  and  150  associated with corresponding dryer decks. While this approach does remove fines at each deck level, it is disadvantage for a number of reasons. Provision of separate collectors greatly increases costs and requires more plant space. In addition, and again referring to the exemplary air flows given in FIG. 1, it will be seen that air flows generally increase from top to bottom, with a 10,000 m 3 /h air flow at lower levels and culminating in a 16,600 m 3 /h air flow at the dryer outlet. Thus, larger collection equipment is needed from bottom to top of the dryer because greater quantities of air are being handled at the upper decks. 
     In the FIG. 2 prior art system, use of individual dust collection assemblies is avoided, there being only a single assembly  136  to treat the exhaust air from the dryer. While this design is less costly than that shown in FIG. 1, there is no fines removal at each deck, which may result in fines accumulation in internal components unless the system is carefully designed and maintained. Further, this system suffers from the same increasing air flow from bottom to top described in connection with the FIG. 1 dryer. 
     It will thus be seen that the present invention provides cost and operational advantages which cannot be duplicated in prior art systems. These advantages are derived from the use of a multiple-deck vertical dryer having a common upright plenum and an air circulation assembly whereby the air circulation assembly operates to pass the continuous drying air stream into, through and out of the plenum chamber as the air stream passes between respective decks.