Abstract:
The invention relates to a dryer for drying fuel materials such as wood bark, wood chips, sludge, garbage, peat moss or the like. In a preferred embodiment the dryer comprises a conveyor, consisting of twin endless belts, which carries the material to be dried along a vertical path defined between parallel runs of the endless belts, and ductwork which serves to direct heated air (received from any appropriate source) across the vertical path to remove moisture from the material as it is being conveyed. The ductwork includes at least one feed duct for use in delivering the heated air to one side of the vertical path, and at least one exhaust duct for use in withdrawing moisture-laden air on another side of the vertical path. Suction is applied at the exhaust ducts to draw drying air through the ductwork, and the feed and exhaust ducts are made to seal against the conveyor to reduce the introduction of ambient air into the dryer ductwork under the suction applied.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a dryer for use in drying fuel materials such as wood bark, wood chips, sludge, peat moss or the like. 
       BACKGROUND OF THE INVENTION 
       [0002]    Dryers may be used to remove moisture from a variety of fuel materials. One example of such fuel materials are peat moss or peat moss pellets that are intended to be burned as a fuel. Such products tend to have considerable moisture content because they are often stored in locations where they are exposed to the elements. When these products are used as a fuel in a burner, a substantial part of the heat energy generated during their consumption tends to be lost to a burner stack, as the moisture contained in the product is evaporated and escapes. Fuel economy can be enhanced by reducing the moisture content of these products prior to combustion. 
         [0003]    Drying apparatuses have been used in which wood by-products have been tumbled in a rotating fashion while being subjected to drying air. This manner of drying tends to separate fine and course materials thereby providing a dried product having non-uniform burning properties. This separation of fine materials from coarse tends also to contribute to dust problems, fine particles tending to be entrained with drying air or otherwise scattered from the dryer. 
       SUMMARY OF THE INVENTION 
       [0004]    According to a first aspect of the invention, a dryer for drying a material to be used as fuel is provided. The dryer comprises means for conveying the material to be dried along a substantially vertical path extending between an upper end of the conveying means, where the material is received, and a lower end of the conveying means, where the material is discharged. The dryer also includes directing means for directing a heated drying gas across the vertical path to remove moisture from the material as it is conveyed. The directing means includes a feed duct means for use in delivering the heated drying gas to the conveying means on one side of the vertical path, and an exhaust duct means for withdrawing moisture-laden drying gas from the conveying means on another side of the vertical path. 
         [0005]    According to a second aspect of the invention, a dryer for drying a fuel material using a drying gas is provided. The dryer comprises at least one endless belt comprising a substantially vertical run. The vertical run defines a vertical path. A plurality of flights are connected to the endless belt, which conveys the material along the vertical path. At least one feed duct is located on one side of the vertical path, and is adapted for directing the drying gas substantially across the vertical path. At least one exhaust duct is located on another side of the vertical path, and is adapted for receiving the drying gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention will be better understood with reference to drawings illustrating a preferred embodiment of the invention. In the drawings: 
           [0007]      FIG. 1  diagrammatically illustrates a steam generating system employing a dryer according to an embodiment of the present invention; 
           [0008]      FIG. 2  is an end view of the dryer; 
           [0009]      FIG. 3  is a side view of the dryer showing inlet and outlet conveyors and their drive motors; 
           [0010]      FIG. 4  is a plan view along lines  4 - 4  of  FIG. 3  with extraneous detail omitted to illustrate dryer ducts and their mounting brackets; 
           [0011]      FIG. 5  is a view along lines  5 - 5  of  FIG. 3  detailing structure of the dryer conveying belts; 
           [0012]      FIG. 6  is a perspective view detailing structure of the chains used to carry conveying belts in the dryer, according to an embodiment of the present invention; 
           [0013]      FIG. 7  is a fragmented view illustrating a sensor switch which regulates operation of an inlet screw conveyor, according to an embodiment of the present invention; and, 
           [0014]      FIG. 8  diagrammatically illustrates control circuitry for use in regulating the operation of the dryer, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Reference is made to  FIG. 1  which illustrates a steam generating system  10  including a dryer  12  constructed according to a preferred embodiment of the invention. Temperatures indicated on or adjacent to components of the steam generating system  10  are temperatures of intake or output air flows, as the case may be. It will be understood by those skilled in the art that the temperatures of intake and output air flows are intended to be exemplary of the typical system and may be varied in any suitable fashion for particular applications. 
         [0016]    The steam generating system  10  includes a solid fuel burner  14  which receives peat moss, wood bark or other similar product at a fuel inlet  16 , and air for combustion at air inlets  18  and air inlet  20  which is coupled to an air pump  22 . The solid fuel burner  14  has a burner outlet  23  from which air heated to a temperature of about 1,800 degrees Fahrenheit is released. 
         [0017]    The heated air generated at the burner outlet  23  is received by a steam generator  24 . The steam generator  24  uses the heat received with the air at the burner outlet  23  to generate steam, which is then made available at a steam outlet  28 . The air originally received by the steam generator  24  is then exhausted at an air outlet port  30 , where it is at a temperature in the order of 850 degrees fahrenheit. 
         [0018]    The air escaping from the steam generator  24  at the outlet port  30  is received by a heat exchanger  32 . The heat exchanger  32  also receives air at room temperature (approximately 70 degrees fahrenheit) from an air pump  34 . The air so received from the air pump  34  is heated by the air escaping from the steam generator  24  to a temperature of about 450 degrees fahrenheit and leaves at an outlet port  36 . 
         [0019]    The air heated by the heat exchanger  32  is received at an inlet port  38  of the dryer  12 , and used to dry wet peat moss or other product received at a wet fuel inlet  40 . (Alternatively, the dryer  12  can be made to receive heated air directly from the outlet port  30  of the steam generator  24 ). The peat moss or other product, once dried, is delivered by a conveyor (not illustrated) to the fuel inlet  16  of the solid fuel burner  14 . Water vapor (at a temperature of about 220 degrees fahrenheit) is removed from the dryer  12  at an exhaust port  42  and delivered to an exhaust stack  44 , together with exhaust air (at a temperature of about 550 degrees fahrenheit) from the heat exchanger  32 . The mean temperature of the stack  44  is in the order of 350 degrees fahrenheit. 
         [0020]    The preferred embodiment of the steam generating system  10  is intended to be illustrative of a particular use of the dryer  12 , and it is not to be construed as limiting the types of application for which a dryer constructed according to the invention is intended. 
         [0021]    The dryer  12  according to a preferred embodiment of the invention is better illustrated in the views of  FIGS. 2-3 . 
         [0022]    The dryer  12  has a support frame  50  (constructed of steel I-beams) which supports a dual conveyor  52  suited to the conveying of wood bark, peat moss, sludge, or the like. 
         [0023]    The conveyor  52  comprises first and second endless steel belts  54 , 56 . The belts  54 , 56  are carried by sprockets  58 , and driven by a ¾ horsepower electric motor  60  mechanically coupled to one of the sprockets  58  by means of a reduction gear assembly  62 . The motion and speed of the belts  54 , 56  is synchronized by means of a synchronizing chain  64  which moves about synchronizing gears  66  (best illustrated in the view of  FIG. 3 ) two of which are mounted on the axles shown on each of the sprockets  58 . Because of this arrangement, the second belt  56  is effectively driven by the first belt  54 . 
         [0024]    The belts  54 , 56  have two substantially parallel runs which define down the centre of the conveyor  52  a substantially vertical path (not specifically indicated) having a depth of about three inches, and a width of about 9 feet. The material being conveyed is dried along this vertical path. 
         [0025]    The belts  54 , 56  carry a plurality of rectangular, steel flights  68  (two specifically indicated in end view in  FIG. 5 ) which serve to drive material through the conveyor  52  in a controlled fashion. The motion of the belts  54 , 56  is so timed that the flights  68  proceed along the vertical path in a paired fashion (see  FIG. 5 ) effectively closing the vertical path and preventing the free-fall of material through the conveyor  52 . 
         [0026]    The arrangement described above has three principal advantages. First, as the material to be dried moves vertically through the conveyor  52 , the motion is assisted by gravity and consequently an electric motor of relatively small horse power can be used to drive the conveyor  52 . Second, the vertical arrangement permits conservation of floor space in a plant where the dryer  12  is to be used. Third, fine material is suspended together with coarse material during drying, and consequently a relatively homogeneous dried product is made available, and dust problems are reduced. 
         [0027]    The belts  54 , 56  are preferably constructed of a plurality of flat steel plates which articulate with one another for movement around the sprockets  58 . The plates are perforated to permit passage of drying gas into or out of the vertical path during conveyance of a material to be dried. 
         [0028]    A plate  70  is typical of those found on the belts  54 ,  56 , and is illustrated in end view in  FIG. 5 . The plate  70  is provided with upper and lower flanges  72 ,  74 , respectively. A downwardly inclined baffle  76  is preferably integrally formed with the lower flange  74 , and serves a function which will be described more fully below. 
         [0029]    The plate  70  has punched from its surface a plurality of baffles  78  (only one being specifically indicated in  FIG. 5 ). The baffles  78  incline downwardly when the plate  70  is moving along the vertical path defined between the belts  54 , 56 . As apparent in  FIG. 3  (in which the outwardly facing surface of the endless belt  54  is visible) the baffles  78  are arranged in a staggered fashion, which is preferred in order to prevent formation of relatively stagnant or dead pockets of air in the vertical path. It will be appreciated that all plates of the belt  54  are formed with such baffles (which have not been completely illustrated owing to the excessive detail). 
         [0030]    The baffles  78  and the apertures provided beneath them permit a drying gas (typically heated air) to be delivered to the material being conveyed and thereafter exhausted in a substantially unobstructed fashion. Because the baffles  78  are downwardly inclined (when they are moving through the vertical path) they tend to prevent the material being conveyed from clogging the openings beneath the baffles  78 . Also, because of their downward orientation, the baffles  78  deflect the drying gas downwardly as it enters the vertical path, and then deflect the moisture-laden drying gas upwardly as it is removed. Because the baffles  78  force the drying gas to move in such a fashion, there is less tendency for dust particles to be entrained with the drying gas and thereby removed from the conveyor  52 . Additionally, it will be appreciated that the baffles  78  function as flights, which are sufficient for conveying course materials such as peat moss pellets or bark, but that the flights  68  which extend more fully across the vertical path are better suited to conveying materials such as sludge in a controlled fashion. 
         [0031]    A plate  80  immediately above the plate  70  has a lower flange  82  (similar to the flange  74  of plate  70 ). A baffle  84  depends downwardly from the flange  82  (when the plate  80  is moving along the vertical path), and covers the space between the adjacent flanges  72 ,  82  of the plates  70 ,  80 . The baffle  84  thus serves to prevent lodging of the material being conveyed between the plates  70 ,  80 , and reduces the escape of dust between the flanges  72 ,  82 . 
         [0032]    The plates are secured to endless chains  88 ,  90  which are preferably constructed of flat links (as illustrated in  FIG. 6 ) suited to travel along the teeth of the sprockets  58 .  FIG. 6  shows the connecting structure of the chain links which is used in a conventional manner to secure the plates to the chain links. 
         [0033]    A feed conveyor  92 , located at an upper end of the conveyor  52 , and secured to the support frame  50  in any suitable manner serves to distribute the material to be dried across the vertical path between the belts  54 , 56 . The feed conveyor  92  comprises a hopper  94  with an open upper face where the material to be dried can be received, as from a conventional conveyor. Preferably, a worm gear  98  contained within a steel housing  100  serves to distribute the material received in the hopper  94  across the vertical path. 
         [0034]    The housing  100  is illustrated in the views of  FIGS. 2 ,  3  and  7 . The housing  100  comprises a trough  102  of generally U-shaped cross-section (see  FIG. 2 ) a capping plate  104 , and an end plate  106 , which can be bolted together in any suitable manner to provide an enclosure along which the worm gear  98  can move material to be dried. 
         [0035]    The trough  102  has a longitudinally-directed opening  108  through which the material to be dried can escape into the conveyor  52  (in a substantially controlled fashion) while being moved horizontally by the worm gear  98 . The opening  108  has a length corresponding substantially to the width of the belts  54 ,  56  so that material can be distributed across the full width of the vertical path. 
         [0036]    A pair of guide plates  114  extend downwardly from the trough  102 , one on either side of the opening  108 , substantially parallel to one another, to direct the material to be dried into the conveyor  52 . The guide plates  114  incline towards one another slightly, and lower-most edge portions are so spaced that the guide plates  114  can in practice extend substantially into the conveyor  52  (as will be apparent from the view of  FIG. 2 ). Preferably, a certain amount of clearance is provided between the belts  54 ,  56  and the guide plates  114  to avoid contact between the guide plates  114  and flights  68  during operation. 
         [0037]    In practice, the trough  102  need not be provided with a U-shaped cross-section, and a generally rectangular shape may be preferred for ease of construction. If desired, the longitudinal opening provided in the bottom of such a trough can be constructed as several aligned openings, each of which is provided with a sliding gate to regulate aperture size. If the bottom of the trough is flat (as with a rectangular trough), each gate can be constructed of a steel plate with a flange bent from one end portion thereof (for use in sliding the steel plate across one of the openings), and two overhanging lips can be provided in the bottom of the housing to receive oppositely disposed side edge portions of the steel plate to retain the plate and also to guide its sliding motion. The gates so constructed can be used to restrict the rate at which material is delivered to the conveyor  52 , and to vary the distribution of material being delivered to the conveyor  52 . 
         [0038]    The operation of the feed conveyor  92  is preferably regulated by a feed sensor end switch  116  which is detailed in the view of  FIG. 7 . The function of the feed sensor end switch  116  is to ensure that an excessive amount of material is not delivered to the conveyor  52 . To this end, the feed sensor end switch  116  is electrically coupled to and controls the operation of an electric motor  118  (shown in  FIG. 3 ) which drives the worm gear  98 . 
         [0039]    The feed sensor end switch  116  is mounted on the end plate  106  of the housing  100 . 
         [0040]    The feed sensor end switch  116  includes a micro-switch  120  activated by a plunger  122 , and a plate  124  which pivots about a hinge  126  attached to the end plate  106 . The plate  124  is deflected by material delivered through the opening  108  by the worm gear  98 , and when so deflected depresses the plunger  122  of the micro-switch  120 . A lever arm  128  extends through an opening  130  in the end plate  106  and supports a counterweight  132 . The counterweight  132  ensures that the plunger  122  is not depressed by the plate  124  until some predetermined build-up of material occurs at the upper end of the conveyor  52 . In practice the appropriate choice of a weight for the counterweight  132  will depend principally on the type of material which is being dried, generally increasing with the density of the material. Alternatively, a spring can be mounted between the plate  124  and the end plate  106  to bias the plate  124  away from the micro-switch  120 . 
         [0041]    When the plunger  122  is depressed, the motion of the electric motor  118  is stopped. Consequently no further material is delivered to the conveyor  52  until any backlog which has occurred at the upper end of the conveyor  52  is cleared. The feed sensor end switch  116  is preferably coupled as well to the conveyor which feeds the feed conveyor  92  so that no further material is delivered to the hopper  94 . 
         [0042]    A discharge conveyor  134  (shown in  FIGS. 2 and 3 ) is attached to the support frame  50  at a lower end of the conveyor  52 . The discharge conveyor  134  is positioned directly beneath the vertical path to receive and carry away material dried by the dryer  12 . 
         [0043]    The discharge conveyor  134  has a structure similar to that of the feed conveyor  92 . The discharge conveyor  134  comprises a worm gear  136  disposed in a trough-like housing  138  (an upper face of which is open to receive material from the dryer  12 ). An electric motor  140  (indicated in  FIG. 3 ) rotates the worm gear  136  to advance the dried material towards a discharge hopper where it can be carried away by any of a variety of means. 
         [0044]    The operation of the discharge conveyor  134  need not be regulated by any type of feed sensor switch; the worm gear  136  need simply be made to rotate at a speed sufficient to ensure that all material possibly delivered to the trough-like housing  138  is carried away. 
         [0045]    The construction, mounting and operation of dryer ductwork will now be described with reference primarily to  FIGS. 2 ,  3  and  4 . As will be apparent from  FIG. 2 , the dryer  12  comprises four substantially identical intake ducts  144 ,  146 ,  148 ,  150 , and four substantially identical exhaust ducts  152 ,  154 ,  156 ,  158 , paired as shown. 
         [0046]    These ducts are mounted in the interior of the endless belts, as apparent in  FIG. 2 , with substantially only intake and exhaust ports extending from within the belts. The motion of drying air in and out of two typical ducts is indicated by arrows in the view of  FIG. 4 . Preferably, the particular arrangement of ducts is such that two pairs of intake-exhaust ducts (pair  144 , 158  and pair  148 , 154 ) direct drying air in a first direction across the vertical path, and the remaining two pairs (pair  146 , 156  and pair  150 , 152 ) direct drying air in an opposite direction, thereby preferably ensuring that the material conveyed tends to dry equally on either side of the path. It will be understood by those skilled in the art that the word “across”, as used in reference to the flow of the drying gas in relation to the vertical path, means any direction except other than vertical. 
         [0047]    The pair feed and exhaust ducts  150 ,  152  (whose construction and relative orientation are typical of all the ducts) are better illustrated in the plan view of  FIG. 4 . The ducts  150 , 152  may be constructed primarily of sheet metal, and are preferably substantially identical in structure. Preferably, the intake port  160  of the feed duct  150  is about 50% larger than the exhaust port  162  of the exhaust duct  152  (with attendant changes in the dimensioning of the body of the ducts) to reflect the fact that hot air delivered to the conveyor  52  will cool and contract considerably before being exhausted from the dryer  12 . 
         [0048]    Only the exhaust duct  152  will be described in detail, as the remaining ducts preferably have substantially identical structure. The exhaust duct  152  has two openings. One such opening is in the exhaust port  162 , and the second is an open face (not specifically indicated) which extends substantially from top to bottom of the exhaust duct  152 . When the dryer  12  is assembled, the open face is preferably positioned immediately adjacent to one side of the vertical path, that is, substantially parallel and adjacent to the vertical run of the endless belt  54  defining one side of the vertical path. A corresponding face of the feed duct  150  is similarly positioned adjacent to a vertical run of the endless belt  56 , opposite the feed duct  150 . In this manner the feed duct  150  can deliver heated drying air to one side of the vertical path, and the exhaust duct  152  can exhaust moisture-laden drying air on the opposite side. 
         [0049]    The open face of the exhaust duct  152  is placed in substantially sealing engagement against the vertical run of the endless belt  54 . To this end, a sealing strip  166  (which may be constructed in four lengths) is secured by means of a metal retaining strip (together with pop rivet or bolts) to inside surfaces of the exhaust duct  152 . The sealing strip  166  circumscribes the open face, and contacts an inside surface of the endless belt  52 , as illustrated in the view of  FIG. 5 . 
         [0050]    In  FIG. 5 , end walls of the ductwork have been broken away to reveal chains supporting the endless belts  54 ,  56 , and consequently only an upper run of the sealing strip  166  is illustrated therein. It will be appreciated that in the context of a mechanical device such as the dryer  12  perfect sealing engagement will be difficult if not impossible to achieve, and that where sealing engagement is mentioned in this specification leakage of air can be tolerated provided that a greater part of the drying air delivered by a feed duct to the vertical path is exhausted through a corresponding exhaust duct. 
         [0051]    The manner of mounting of the feed and exhaust ducts  150 ,  152  is typical of all ducts of the dryer  12 . The ducts  150 ,  152  are supported from the framework  50  by means of oppositely disposed mounting assemblies generally indicated by the reference numerals  172 ,  174 . The mounting assemblies  172 , 174  are substantially identical in structure, and consequently only the mounting assembly  172  will be described in detail. 
         [0052]    The mounting assembly  172  comprises an elongate, rectangular backing plate  176  which is secured by bolts to the support frame  50 . The backing plate  176  is substantially vertically disposed in the support frame  50 , is shown (fragmented) in the view of  FIG. 2 . 
         [0053]    A channeled guide member  178  is bolted to the backing plate  176 . The guide member  178  has a substantially uniform cross-section (shown in the plane of  FIG. 4 ) defining two channels  180  which serve to guide the chains carrying the endless belts  52 ,  54 . 
         [0054]    A number of connecting flanges are welded to the guide member, and corresponding connecting flanges are secured to the feed and exhaust ducts  150 ,  152 . The paired connecting flanges have holes which can be placed in registration and through which a bolt can be passed in order to secure the ducts  150 , 152  to the guide member  178  and backing plate  176 . Three pairs of connecting flanges support each duct, one pair located towards the top of each duct, one pair, toward the bottom of each duct, and one pair disposed substantially midway between the two other pairs. 
         [0055]    The basic operation of the dryer  12  according to a preferred embodiment of the present invention is as follows. The material to be dried is distributed by the feed conveyor  92  across the vertical path defined through the conveyor by the endless belt  54 ,  56 . The material is then conveyed through the conveyor  52  by the flights  68  of the belts  54 ,  56  (which flights prevent the free-fall of material through the conveyor  52  under gravity). With coarse materials, it will be apparent that the baffles of the plates constituting the endless belts  54 ,  56  serve also as flights conveying the materials. 
         [0056]    Heated drying air is delivered from any appropriate source (for example, the heat exchanger  32  of  FIG. 1 ) to the feed ducts, is then delivered by the feed ducts to the material being conveyed, and is then removed by the exhaust ducts. The exhaust ducts are preferably coupled by ductwork to an air pump (not shown) which serves to draw the moisture-laden drying air into the exhaust ducts; and the scattering of dust from the dryer  12  can be significantly reduced by utilizing suction as the means by which the drying air is drawn from the feed ducts into the vertical path. The particular arrangement of feed and exhaust ducts illustrated, that is, one which allows for the flow of drying gas in opposite directions across the vertical path, is preferable because it causes the material being conveyed to be dried more evenly on both sides of the conveyor  52 , as mentioned above. 
         [0057]    Dust loss from the dryer  12  may be reduced in several ways. First, drying air is preferably drawn through the dryer  12  by means of suction applied at the exhaust ducts, rather than being forced under positive pressure into the intake ducts. The tendency for dust to be scattered from the conveyor  52  is thereby significantly reduced. In practice, the volume and rate at which air is to be drawn from the exhaust ducts (by an air pump or the like) will be determined principally by the moisture content of the material being dried, the rate at which the material is being conveyed, and the temperature of the incoming drying air. 
         [0058]    Second, the channeled guide member  178  may be provided with an elongate surface  192  (indicated in  FIG. 4 ) which is positioned immediately adjacent the side edge of the chains carrying the endless belts  54 ,  56  to close off one side of the vertical path, thereby reducing dust scattering. (A similar surface will be found on the corresponding guide member on the opposite side of the dryer  12 ). Consequently, the surface  192  is preferably positioned as close to the chains of the endless belts  54 ,  56  as possible without interfering with their motion. To this end the backing plate  176  which supports the guide member  182  is preferably bolted to the support frame in such a manner that the spacing between the surface  192  and the endless belts  54 ,  56  can be adjusted by appropriate insertion or deletion of washers or shins. 
         [0059]    As mentioned above, the entrainment of dust particles with drying air is reduced by the provision of air-deflecting baffles on the panels constituting the endless belts  54 , 56 . By upwardly directing the air flow out of the conveyor  52 , the baffles encourage fine particles to remain in the material being conveyed, instead of escaping into the dryer exhaust ducts. 
         [0060]    A dryer control system  194  according to the preferred embodiment is illustrated diagrammatically in  FIG. 8 . The control system  194  comprises a controller, which preferably includes two control circuits  196 ,  198  which provide drive signals respectively to the motor  60  which operates the conveyor  52  and to the motor  118  which operates the feed conveyor  92 . 
         [0061]    The control circuit  196  receives a boiler steam demand signal (from the steam generator  35  in  FIG. 1 , for example) at a terminal  200 . The control circuit  196  generates therefrom a conveyor drive signal which is preferably directly proportional to the boiler steam demand signal and which preferably directly varies the speed of the motor  60 . Preferably, the speed of the conveyor  52  thus varies directly with the boiler steam demand signal. 
         [0062]    In addition, the control circuit  196  receives a temperature signal from a temperature sensor  202  located in the exhaust duct  158 . Preferably, the conveyor drive signal is then reduced in magnitude by a signal proportional to the excess of the temperature signal over a predetermined reference temperature signal generated by the control circuit  196 . Thus, if the material conveyed is excessively damp, the temperature of the moisture-laden drying gas in the exhaust duct  158  will tend to be reduced from some predetermined reference temperature (for example 210° F. when the material being dried is wood bark), and the conveyor  52  will be slowed by the control circuit  106  to permit more thorough drying. 
         [0063]    If desired, a second temperature sensor  204  can be disposed in the feed duct  144  to sense the temperature of the incoming drying air. The control circuit  196  can then generate a temperature differential signal indicative of the temperature drop occurring in the drying air, and consequently more accurately reflecting the moisture content of the material being conveyed and the extent to which heat is being lost to the moisture. The conveyor drive signal can then be reduced in magnitude by a signal proportional to the excess of the temperature differential signal over some predetermined reference temperature differential signal. The conveyor  52  may thus be slowed by the control circuit  196  to increase the extent to which the material conveyed is dried until the predetermined temperature differential signal is established between the feed and exhaust ducts  144 ,  158 . 
         [0064]    Preferably, the control circuit  198  receives from the control circuit  196  the conveyor drive signal, and scales that signal to produce a feed conveyor control signal which varies the speed of operation of the motor  118 . The control circuit  198  also receives pressure signals from a high pressure sensor  206  located in the feed duct  144  and a low pressure sensor  208  in the exhaust duct  158 . The control circuit  198  generates therefrom a pressure differential signal indicative of the pressure difference between the feed and exhaust ducts  144 , 158 . The control circuit  198  then reduces the feed conveyor drive signal by an amount proportional to the excess of the pressure differential signal over some predetermined pressure differential reference signal. Since the pressure differential signal will be indicative of the density of packing of the material to be dried in the conveyor  52 , the operation of the feed conveyor  92  will be slowed when excessive quantities of material, quantities which cannot be adequately dried, are being delivered to the conveyor  52 . 
         [0065]    The operation of the feed sensor switch  116  has been described above. When the feed sensor end switch  116  is activated, indicating that material is backing up at the top of the conveyor  52 , preferably the control circuit  198  merely shuts down the operation of the motor  118  and feed conveyor  92 . 
         [0066]    A preferred embodiment of a dryer constructed according to the invention has been described above, and it will be appreciated that various changes may be made to the preferred embodiment described without departing from the scope or spirit of the invention.