Abstract:
A heat exchanger for use on existing grain dryers or integral to new grain dryers that recovers heat from the moist hot exhaust air leaving the grain columns and uses this recovered heat to preheat ambient air being drawn into the grain dryer via a blower. The heat exchanger utilizes heat exchange columns separated from ambient air columns by heat conductive walls. The heat conductive walls allow for a transfer of heat from the moist hot exhaust air through the heat conductive walls and into the ambient air being drawn into the grain dryer through the ambient air columns. The heat conductive walls do not allow for the mixing nor contact of the moist hot exhaust air with the ambient air. Therefore, heat is transferred to the ambient air being drawn into the grain dryer without the addition of moisture to the ambient air and the resulting decrease in moisture holding capacity that would result. The heat exchanger also has a cleansing mode whereby the grain flow can be directed to flow through the heat exchange columns thereby scrubbing or cleansing the walls of the heat exchange columns as the grain flows through the heat exchange columns and out to the bottom grain auger.

Description:
BACKGROUND OF THE INVENTION 
     (i) Field of the Invention 
     The present invention relates generally to grain drying equipment. In particular, the present invention relates to a heat exchanger that utilizes the moist hot exhaust air from a grain dryer to pre-heat ambient air prior to entering the blower of the grain dryer. 
     (ii) Description of the Related Art 
     Grain dryers are generally well known in the art. A typical prior art grain dryer is shown in FIG.  1 . which is a cut-away view of a single blower and single plenum grain dryer. Most grain dryers in use today utilize this design or variations on it. 
     Grain dryers of the type shown in FIG. 1 are known in the prior art and therefore its construction and operation will only be generally described herein. The typical grain dryer is constructed from steel and sheet metal. The major components of a typical grain dryer  11  include: a blower  10 , a top conveying auger  12 , grain columns  14 , an air plenum  16 , inner porous walls  18 , outer porous walls  20 , a lower conveying auger  22 , and an air heater and an air mixing chamber  24 . 
     A typical grain dryer  11  operates by receiving a supply of wet or moist grain from a separate conveyor (not shown) at a top opening of the dryer and transporting the wet or moist grain across the top of the grain dryer  11  via the top conveying auger  12 . The grain then flows by gravity downward from the top conveying auger  12 , down through the grain columns  14  where it is dried and then to the lower conveying auger  22 . The dried grain is then transported via the lower conveying auger  22  to a bottom opening of the dryer and out of the dryer  11 . 
     The grain is dried in the grain dryer  11  via the flow of hot dry air through the grain columns  14 . The grain columns  14  are separated from the air plenum  16  by the inner porous walls  18 , are separated from the exterior environment of the grain dryer  11  by the outer porous walls  20 , and are separated from other adjacent grain columns  14  by inner panels  26 . Ambient air is drawn directly from the exterior environment of the dryer and into the grain dryer  11  via the blower  10 . The blower  10  blows the ambient air through a heating chamber  24  where the ambient air is heated as it flows through the heating chamber. The heated air is then blown through an air mixing chamber  24  to ensure that the air is evenly heated. The thoroughly heated and mixed air hen flows into the air plenum  16 . The action of the lower  10  blowing air into the air plenum  16  causes the air pressure in the air plenum  16  to rise to a pressure higher than that of the atmospheric pressure of the exterior environment of the grain dryer  11 . The higher pressure in the air plenum  16  causes the heated air to flow through the inner porous walls  18  of the grain columns and into the columns  14 . The heated air then flows through the grain that is moving downwardly through the grain columns  14  and then out through the outer porous walls  20  to the exterior environment of the grain dryer  11 . When the heated air flows through the grain passing through the grain columns  14 , moisture is extracted from the grain by the heated air and is conveyed to the exterior environment of the grain dryer  11 . Therefore, as the grain flows downwardly through the grain columns  14  and hot air is passed from the air plenum through the inner porous walls  18 , around the grain and through the grain columns  14  and out through the outer porous walls  20  to the exterior environment of the grain dryer  11 , the grain is heated and moisture is removed. The desired moisture content of the grain exiting the grain dryer  11  via the lower conveying auger  22  can be controlled by altering the flow rate of grain leaving the grain dryer  11  which controls the rate that grain passes downwardly through the grain columns or by changing the temperature of the heated air being blown into the air plenum  16 . 
     A disadvantage of this typical grain dryer  11  is that the moist hot exhaust air leaving the grain columns  14  via the outer porous walls  20  exits to the exterior environment of the grain dryer  11  while it is still at a temperature substantially above that of the ambient air being drawn into the grain dryer  11 . Therefore, significant amounts of energy can be wasted by the exhausting to the exterior environment of the grain dryer  11  moist hot air that is still capable of heating grain and possibly absorbing additional moisture. 
     Another type of grain dryer is shown in the Noyes et al. U.S. Pat. No. 4,268,971. This grain dryer has both a heating and a cooling mode. This is accomplished by dividing the typical single air plenum into two vertically separated chambers. The grain is heated and moisture is removed in the upper half of the dryer by blowing heated air through the upper chamber of the plenum, through the inner porous walls of the upper half of the grain dryer, around the grain and through the grain columns and out through the outer porous walls of the upper half of the dryer. The grain is cooled in the lower portions of the grain columns by the blower drawing ambient air through the outer porous walls of the lower half of the grain dryer, around the hot grain and through the grain column and through the inner porous walls of the lower half of the grain dryer and into the lower chamber of the air plenum. This cooling air, which is now partially heated and containing moisture removed from the grain, is then drawn from the lower chamber of the air plenum into the blower and mixed with ambient intake air being drawn into the grain dryer to be heated and used to dry grain. This grain dryer also includes a structure for recycling the moist hot exhaust air exiting the grain drying section of the grain dryer along with partially heated air from grain cooling. The moist hot exhaust air is directed to the intake of the blower. The moist hot exhaust air is mixed with the ambient air and with the partially heated air from the second chamber of the air plenum that was utilized to cool the grain. Thereby, the air entering the heater is significantly warmer than that of the ambient air and requires less energy input from the heater section to effectuate the drying of the grain. A significant disadvantage of this grain dryer is that the moist hot exhaust air that is being mixed with the ambient air and with the partially heated air from the lower chamber of the air plenum has a high moisture content. The high moisture content is a direct result of the heating of and moisture extraction from the grain. Since moist hot exhaust air already has a higher moisture content, its additional moisture holding capability is less than that of the cooler and dryer ambient air. Likewise, the partially heated air from grain cooling also has an elevated moisture content above that of the ambient air. Therefore, while requiring less energy input to obtain a desired air temperature, the moisture removing capability of the heated air being used to dry grain is reduced. Therefore, the grain flow rate through the dryer will need to be reduced or the amount of air flowing through the grain dryer will need to be increased to compensate for the reduced moisture holding capacity of the heated air resulting in limited overall savings of energy or time, and possibly reducing grain throughput. 
     Some grain dryers utilize two blowers in conjunction with a two plenum grain dryer. These grain dryers also have a heating mode and a cooling mode. One blower is connected with a heating element in order to blow heated air into the upper heating air plenum and through the grain column to heat the grain and remove moisture. The other blower is utilized in conjunction with a lower cooling air plenum. The cooling mode takes ambient air and blows it into the cool air plenum and through the lower half of the grain column, thereby cooling the heated grain. 
     Heat recovery systems are available on these dual fan/dual plenum grain dryers. These heat recovery systems typically direct the air exiting the cooling portion of the grain dryer into the intake of the blower used in the heating section of the grain dryer The disadvantage to this type of heat recover system is that the cooling air has been partially heated by the hot grain that it is cooling and also removes additional moisture from the grain, thereby decreasing this air&#39;s moisture holding capacity below that of the ambient air in the exterior environment of the grain dryer. Therefore, while requiring less energy to heat the air mixture to the desired temperature, the moisture removing capacity of the heated air drying the grain is reduced and the grain flow rate through the dryer will need to be reduced or the amount of air flowing through the dryer increased to compensate for the lower moisture removal capability, resulting in a limited overall savings of energy or time and possibly reducing grain throughput. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an improved grain dryer which overcomes the disadvantages associated with existing grain dryers by utilizing heat exchange columns to extract heat from the exhaust air. The extracted heat is transferred to the ambient air being drawn into the grain dryer by the blower via heat conductive walls that do not allow for the mixing of the exhaust air with the ambient air being drawn into the grain dryer. The heat exchanger is also suitable for use with existing grain dryers. 
     The grain dryer has a central air plenum and a plurality of grain columns that extend vertically across opposite sides of the air plenum. Each grain column has an inner porous wall separating the grain column from the air plenum, and an outer porous wall that is spaced outwardly from the inner porous wall and the air plenum. A plurality of heat exchange columns extend vertically along the outer porous walls of the grain columns. The heat exchange columns have heat conductive walls mounted over the outer porous walls of the grain columns and enclose interior volumes of the heat exchange columns that communicate with the grain columns through the outer porous walls. There is also a plurality of ambient air columns extending vertically along the heat exchange columns. The ambient air columns have external walls mounted over the heat conductive walls of the heat exchange columns and enclose interior volumes of the ambient air columns that receive heat from the heat exchange columns through the heat conductive walls. Thus, the grain dryer has an integral heat exchanger that recovers heat from the moist hot exhaust air that exits the grain dryer through the outer porous walls of the grain columns, and transfers the recovered heat via the heat conductive walls to ambient air being drawn into the grain dryer through the ambient air columns without mixing the moist hot exhaust air with the ambient air being drawn into the grain dryer. 
     The grain dryer heat exchanger is designed for assembly to an existing grain dryer, where the grain dryer has a plurality of grain columns extending across opposite sides of an air plenum, where each grain column has an inner porous wall separating the grain column from the air plenum and an outer porous wall spaced outwardly from the inner porous wall. The heat exchanger has a plurality of heat conductive walls mountable over the outer porous walls of the grain columns to form the plurality of heat exchange columns extending along the grain columns. The heat exchange columns have interior volumes that communicate with the grain columns through the outer porous walls. A plurality of external walls are mountable over the heat conductive walls of the heat exchange columns to form a plurality of ambient air columns extending along the heat exchange columns. The ambient air columns have interior volumes that receive heat from the heat exchange columns through the heat conductive walls. Thus, the grain dryer heat exchanger can be utilized on existing and new grain dryers to recover heat from the moist hot exhaust air that would otherwise go to waste. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention are set forth in the following detailed description of the preferred embodiment of the invention and in the drawing figures wherein: 
     FIG. 1 is a perspective, partially cut-away view of a prior art grain dryer; 
     FIG. 2 is a perspective, partially cut-away schematic representation of a grain dryer with a heat exchanger of the present invention; 
     FIG. 3 is a cross-sectional end view of the schematic representation of the grain dryer of FIG. 2; 
     FIG. 4 is a side view of the schematic representation of the of a grain dryer of FIG. 2 with the heat exchanger of the present invention; 
     FIG. 5 is a partial, cross-sectional view of the grain dryer of FIG. 3 in a plane along the line  5 — 5  of FIG. 3; 
     FIG. 6 is a partial, cross-sectional view of the grain dryer of FIG. 3 in a plane along line  6 — 6  of FIG. 3; 
     FIG. 7 is an exploded view of a partial cross section of the plenum, grain columns, heat exchange columns, and ambient air columns showing how they are arranged and connected relative to one another on an existing grain dryer; and 
     FIG. 8 is a partial cross section view of a pivoting panel at the top most sections of the heat exchange columns. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows the heat exchanger of the invention assembled to the exterior of a grain dryer  30 . The grain dryer  30 , apart from the heat exchanger of the invention to be described, is similar in construction and operation to the prior art grain dryers of the types described earlier. The grain dryer  30  of FIG. 2 is shown with a single blower  32  that blows air through a heater and mixer (not shown) into the air plenum chamber  34  at the center of the dryer. Although only the single blower  32  is shown, it should be understood that the heat exchanger of the invention may be employed with any of the other types of grain dryers that are commercially available. As in the prior art grain dryers, the grain dryer  30  has an upper auger  36  that receives moist grain from a supply exterior to the grain dryer and conveys the moist grain across the top of the grain dryer. The moist grain conveyed across the top of the dryer by the auger  36  then falls by gravity downwardly across opposite sides of a divider plate  38  toward the grain columns  40 . The grain columns  40  extend vertically downward across opposite sides of the air plenum chamber  34  and are defined by inner porous walls  42  that surround the air plenum chamber  34 , outer porous walls  44  and divider plates  46  between adjacent grain columns. The grain columns  40  extend downwardly around opposite sides of the air plenum chamber  34  directing the moist grain across the opposite sides of the air plenum chamber to a lower channel containing the lower auger  48 . The lower auger  48  conveys the dried grain across the bottom of the grain dryer to an exterior conveyor that transports the dried grain away from the dryer. 
     In the preferred embodiment of the invention the heat exchanger is assembled directly to the exterior of the grain dryer. The heat exchanger can be retrofit to an existing grain dryer or can be included as a component part of a grain dryer as it is manufactured. 
     The heat exchanger is comprised of a plurality of heat exchange columns  50  that extend vertically downward across opposite sides of the grain dryer. The heat exchange columns  50  are formed from pairs of heat conducting walls  52  that are mounted over the outer porous wall  44  of each grain column. In the preferred embodiment the heat conductive walls  52  are narrow panels of sheet metal that are assembled over the grain columns  40  in the same manner in which the grain columns are constructed. The heat conductive walls  52  can be secured to the component parts of the grain columns  40  in the same manner as the component parts of the grain columns are assembled to each other. 
     An example of the assembly of the heat conductive panels  52  over the outer porous wall  44  of a grain column  40  is illustrated in FIG.  7 . FIG. 7 shows inner porous walls  42  of a pair of adjacent grain columns  40 , their outer porous walls  44  and their divider plates  46 . The inner porous walls  42  have opposite bent edges  54  that are connected between adjacent divider plates  46  and the outer porous walls  44  also have opposite bent edges  56  that are connected between the adjacent divider plates  46 . In the construction of the grain dryer the bent edges of the walls can be connected to the divider plates in any conventional manner such as by sheet metal screws, by nut and bolt fasteners, by spot welds, etc. In a like manner the heat conductive walls  52  of the heat exchange column can also be connected to the divider plates. The heat conductive walls  54  have bent inner edges  58  and bent outer edges  60 . In assembling the heat conductive walls  52  to the grain dryer, the inner edges  58  of the walls are secured between the outer edges  56  of the outer porous wall and the adjacent divider plates  46  of the grain columns. Again, these connections can be made by the same attachment method employed in constructing the grain columns of the grain dryer. The outer edges  60  of the conductive walls are also connected together using the same method of attachment. The opposite bent edges  58 , 60  of the heat conductive walls  42  form the heat exchange columns  50  between the conductive walls  52  and the outer porous wall  44  of the grain column. As seen in FIG. 7, the heat exchange column  50  has an interior column  62  with a generally triangular configuration due to the shapes of the edges of the conductive walls. 
     In referring to FIG. 2, it can be seen that each heat exchange column  50  is constructed from four pairs of heat conductive walls  52 , three pair of which are assembled to sections of the outer porous wall  44  of the grain columns in the manner described above. The top most pair of walls  52 T of each column is constructed in the manner shown in FIG.  8 . The inner edges  58 T of the top pair of walls of each column are secured to each other adjacent a pivoting panel  110  that is yet to be described. The outer edges  60 T of the top most walls are secured together in the same manner as the other three pairs of conductive walls. The pivoting panels  110  are separate from the top walls  52 T of the heat exchange columns  50 . The four pairs of heat conductive walls  52  are joined together end to end at miter joints  62  to form each heat exchange column  50 . Each column extends vertically along and outside of each outer porous wall  44  of the grain columns  40 . A triangular drain opening  64  is left at the bottom of each heat exchange column  50  to allow the removal of condensate and fines and a triangular exhaust opening  66  is left at the top of each heat exchange column to exhaust moisture laden air. 
     Ambient air columns  70  of the heat exchanger are formed between adjacent heat exchange columns  50  below the top most walls  52 T of the heat exchange columns. The ambient air columns  70  are formed by external walls  72  mounted over the adjacent heat conductive walls  52  of two heat exchange columns  50  between which each ambient air column  70  is formed. As seen in FIG. 7, the external walls  72  have opposite bent edges  74  that are attached to the outer edges  60  of the adjacent heat exchange columns  50 . This forms each ambient air column  70  with an internal volume having a triangular cross-section. 
     At the opposite front and back of the heat exchanger, angled external walls  78  form partial ambient air columns. The angled exterior walls have opposite bent  80  and straight  82  edges. The bent edges  80  of the angled walls are connected to the outer edges  60  of the adjacent conductive walls  52 T of the adjacent heat exchange columns. The straight edges  82  of the angled walls are connected to the inner edges  58  of the conductive walls of the heat exchange columns and to an edge of the back wall  84  or front wall  86  of the grain dryer. The cross-section configurations of the grain columns  40 , the adjacent heat exchange columns  50  and the alternating ambient air columns  70  is shown in FIG.  6 . 
     In viewing FIG. 2 it can be seen that each ambient air column  70  is made up of three sections of external walls  72  connected together end-to-end and connected between the outer edges  60  of adjacent heat exchange columns  50 . The topmost external wall  72  of each ambient air column  70  ends at the mitered connection between the top two pairs of conductive walls  52 T that make up each heat exchange column  50 . This leaves an inlet opening  88  into each ambient air column  70  at the top of each ambient air column. Each ambient air column is also provided with a triangular bottom wall  89  closing the bottom of each column and an adjacent outlet opening  90  at its bottom end. The outlet opening  90  of each ambient air column communicates with an air manifold  92  that extends across the bottom of the grain dryer on opposite sides of the grain dryer. The air manifolds  92  communicate with an enclosed air directing conduit  94  that in turn communicates with the housing of the blower  32 . In the preferred embodiment of the invention, the air manifold  92  and the air directing conduits  94  are all constructed of the same materials as the grain dryer and the heat exchange columns and ambient air columns of the heat exchanger. 
     Roof panels  96  cover over the heat exchanger on opposite sides of the grain dryer. As best seen in FIGS. 2-3, the roof panels  96  have top edges  98  that are attached to the top edges of the heat exchange columns  50  giving the roof panel top edges a serrated appearance. The triangular exhaust opening  66  of the heat exchange columns  50  border the top edges  98  of the roof panels. The roof panels  96  extend downwardly to their bottom edges  100  that are spaced above the sides of the heat exchanger. The roof panels  96  are supported above the top sections of the ambient air columns  70  by struts (not shown) that extend between the undersides of the roof panels  96  and the divider plates  46  of the grain columns. Thus, the configurations of the roof panels  96  do not prevent rain from the exterior environment of the grain dryer from entering the heat exchange columns  50  through the exhaust openings  66 , but do prevent rain from entering the ambient air columns  70  through their inlet openings  88 . 
     Referring to FIGS. 3 and 8, a pair of pivoting panels  110  are mounted at the tops of each of the grain columns  14  on opposite sides of the air plenum  16 . The pivoting panels  110  are mounted along the top edges of the outer porous walls  44  of each of the grain columns by pivot connections  112 . Each of the panels  110  extends along the entire length of the grain dryer and is spaced outwardly from the divider plate  38  that also extends along the length of the grain dryer at the center of the grain dryer. The pivoting panels  110  are selectively controlled by a mechanical actuator (not shown)to pivot between closed positions shown in solid lines in FIG.  3  and opened positions where they engage against the top edge of the divider plate  38  shown in dashed lines in FIG.  3 . The mechanical actuator employed to control the pivoting movement of the panels  110  may be of any commercially available type of pivoting actuator. As seen in FIGS. 3 and 8, when the pivoting panels  110  are in their closed positions shown in solid lines in FIG. 3 they separate the top of the interior volume of the grain dryer  114  from upper portions of each of the heat exchange columns  50  adjacent the exhaust openings  66  of the heat exchange columns. When the actuators of the pivoting panels  110  are operated to cause the panels to move to their opened positions shown in dashed lines in FIG. 3 they communicate the top of the interior volume of the grain dryer  114  with the interior volumes of the heat exchange columns  50  at the tops of the columns. The tops of the ambient air columns  70  remain isolated from the top interior volume of the grain dryer  114  regardless of the position of the pivoting panels  110  as can be seen in FIG.  8 . With the pivoting panels  110  moved to their opened positions shown by dashed lines in FIG. 3, the panels channel a flow of grain distributed through the top interior volume of the grain dryer  114  by the top auger  12  downwardly through the interiors of the heat exchange columns  50 . Channeling a flow of grain through the heat exchange columns  50  is done to scrub the column interiors of grain fines and condensed moisture that will collect in the interiors of the columns during operation of the grain dryer. 
     A pair of pivoting doors  116  is provided at the bottom of the grain dryer where the grain columns  14  come together and channel grain to the bottom auger  48 . The pivoting doors  116  extend the entire length of the grain dryer. The pair of pivoting doors  116  separate the bottom of the grain columns  14  from the bottoms of the heat exchange columns  50 . The pivoting doors  116  are selectively operated by mechanical actuators to move between their closed positions shown in solid lines in FIG.  3  and their opened positions shown in dashed lines in FIG.  3 . Mechanical actuators employed to control the pivoting movements of the doors  116  are any type of commercially available actuators. In the closed positions of the doors  116  shown in solid lines in FIG. 3 they direct the grain pouring downwardly through the grain columns  14  to the bottom auger  48 . In this position of the pivoting doors  116  any condensation in the heat exchange columns  50  or any rain water that collects in the heat exchange columns  50  drains downwardly through the columns and out of the grain dryer through the bottom openings  64  of the heat exchange columns. When the pivoting doors  116  are moved to their opened positions shown in dashed lines in FIG. 3 the interiors of the heat exchange columns  50  are communicated with the bottoms of the grain columns  14  and the lower auger  48 . In the opened positions of the pivoting doors  116  any grain that is channeled downwardly through the heat exchange columns  50  by the pivoting panels  110  being moved to their opened positions shown in dashed lines in FIG. 3 will be directed by the pivoting doors  116  to the lower auger  48  where the grain employed in scrubbing the interior of the heat exchange columns  50  will be conveyed out of the grain dryer. When the scrubbing operation of the heat exchange columns  50  is completed, the pivoting panels  110  and the pivoting doors  116  are moved to their closed positions shown in solid lines in FIG.  3 . 
     In operation, the grain dryer blower  32  blows air through a heating source (not shown), that heats the air to the desired temperature, through an air mixer (not shown) and then into the central air plenum  34 . The blower  32  causes air pressure in the central air plenum  34  to exceed the atmospheric pressure of the exterior environment of the grain dryer  30 . As shown schematically in FIG. 3, this forces the hot air outward through the inner porous walls  42  and into the plurality of grain columns  40 . The hot air then passes around the grain falling downwardly through the plurality of grain columns  40 , heating the grain and extracting moisture from the grain. The moisture laden heated air them exits the plurality of grain columns  40  through the outer porous walls  44 . When the now moist and still hot air exits through the outer porous walls  44 , it enters into the plurality of heat exchange columns  50  of the invention. Once in the plurality of heat exchange columns  50 , the moist hot air flows upwardly through these columns to the exhaust openings  66  located adjacent to the top ends of the columns  50  and exits to the exterior environment of the grain dryer. 
     The ambient air being drawn into the grain dryer via the blower  32  originates from the exterior environment of the grain dryer  30 . The ambient air passes beneath the roof panels  96  and enters the inlet openings  88  that are located adjacent to the top ends of the columns. The ambient air is drawn through the inlet openings  88  and into the ambient air columns  70 . The ambient air then flows through the ambient air columns  70  in direct contact with the heat conductive walls  52  of the heat exchange column  50  and exits the ambient air columns  70  through the outlet openings  90  located adjacent to the bottom ends of the columns. The ambient air exiting through the outlet openings  90  then enters the manifold  92  and the air directing conduit  94  which directs the drawn air to the intake of the blower  32 . 
     An exchange of heat occurs between the moist hot exhaust air and the ambient air being drawn into the grain dryer  30  by the blower  32 . The heat exchange occurs via the heat conductive walls  52 . As can best be seen in FIGS. 5 and 6, the heat conductive walls  52  separate the ambient air columns  70  from the heat exchange columns  50 . As can best be seen in FIG. 6, the heat exchanger utilizes alternating columns of ambient air columns  70  and heat exchange columns  50 . While the moist hot exhaust air is flowing through the heat exchange columns  50  the ambient air is flowing in the opposite direction through the ambient air columns  70 . The temperature differential between the moist hot exhaust air and the ambient air being drawn into the grain dryer causes heat to transfer from the moist hot exhaust air in the heat exchange columns  50 , through the heat conductive walls  52  and to the ambient air being drawn through the ambient air columns  70 . This allows the ambient air being drawn into the grain dryer  30  to be preheated prior to being drawn into the blower  32  and blown past the heating element. 
     During the heat exchange process the cooling of the moist hot exhaust air via extracting heat through the heat conductive walls  52  can cause moisture to condense on the heat conductive walls and also possibly on the outer porous walls  44 . To allow for the moisture to drain from the heat exchange columns  50  the bottoms of the heat exchange columns are provided with the triangular openings  64 . Additionally, the presence of condensation draining through the bottom openings  64  serves to indicate that heat transfer is occurring between the heat exchange columns and the ambient air columns. The preheating of the ambient air being drawn into the grain dryer  30  reduces the amount of energy necessary to raise the temperature of the air entering the central air plenum  34  to the desired level. Therefore, the energy consumption of the grain dryer  30  is reduced. Additionally, another key advantage of this invention is that the ambient air being drawn into the grain dryer  30  does not mix with the moist hot exhaust air and therefore does not have its moisture holding capability reduced like the heat recovery systems of the prior art. 
     While the invention has been described with the heat exchange columns  50  and ambient air columns  70  having triangular cross sectional areas, it is to be understood that any configuration of the ambient air columns and the heat exchange columns may be utilized to effectuate efficient heat transfer and still be within the scope of the invention. 
     This modular assembly for the heat exchanger allows for the heat exchanger to be designed for and easily installed on existing grain dryers. Since the grain columns on existing grain dryers come in varying widths, the modular approach can easily be designed and installed on existing drain dryers. 
     The preferred method of attaching the heat exchanger to existing grain dryers is via bolting onto and through the inner panels. While bolts are the preferred method, it is to be understood that any method for attaching or fastening the component pieces together are within the scope of this invention. 
     While the present invention has been described by reference to specific embodiments, it should be understood that modifications and variations of the invention may be constructed without departing from the scope of the invention defined in the following claims.