JP5211856B2 - Exhaust air circulation type grain dryer - Google Patents

Description

  The present invention relates to an exhaust air circulation type grain dryer provided with a suction exhaust part for exhaust air circulation in a drying part that receives hot air from a hot air supply part and dries the grain.

Conventionally, as shown in Patent Document 1, a drying unit having a hot-air transmission structure that receives hot air from a hot-air supply unit to dry the grains, and a suction / discharge unit that sucks and discharges exhaust air from the drying unit so as to be recirculated. A wind circulation type grain dryer is known. This exhaust air circulation type grain dryer uses a high-temperature, high-humidity mixed hot air combined with the return exhaust air from the suction discharge section to achieve the desired dryness rate without causing drying problems such as shell cracks. Can be dried at high speed.
JP-A 61-195266

  However, when the grain temperature is low, drying is difficult to proceed due to a decrease in moisture transfer in the grain, so both the combustion amount of the hot air supply unit and the return exhaust air from the suction exhaust unit are increased to ensure the drying speed. As a result, there is a problem that the combustion efficiency is lowered.

  The problem to be solved is to provide a wind circulation type grain dryer that ensures combustion efficiency and enables energy-saving drying even in a low temperature period where moisture transfer in the grain decreases.

  The invention according to claim 1 includes a burner (6a) for generating hot air, a hot air chamber (11) through which the hot air passes, and an exhaust fan (7a) for sucking hot air supplied to the grains and discharging it as exhaust air. ), An exhaust air return passage (15) for returning the exhaust air to the hot air chamber (11), a moisture meter (8) for detecting the moisture content of the grain, a grain temperature detecting means for detecting the grain temperature, In the exhaust air circulation type grain dryer provided with the control part which adjusts and controls the amount of exhaust air returned to the hot air chamber (11), the exhaust air absolute humidity corresponding to the grain moisture value and the minimum corresponding to the grain moisture value The grain temperature is set in advance, and the amount of exhausted air that is returned to the hot air chamber (11) is controlled so as to be the exhausted absolute humidity corresponding to the grain moisture value, and the grain detected by the grain temperature detecting means When the temperature falls below the minimum grain temperature corresponding to the grain moisture value, the burner (6a) burns to increase the grain temperature. Characterized in that to increase the amount.

  Based on the relationship between the grain temperature obtained as a drying characteristic of the grain and the amount of moisture transferred inside the grain, and the relationship between the absolute humidity of the exhaust air and the amount of drying on the surface of the grain, the amount of moisture transferred suitable for the drying process is secured. Grains that meet the minimum temperature condition and the absolute humidity condition after determining the minimum temperature condition of the kernel and the absolute humidity condition of the exhaust air to secure the amount of moisture transfer for each grain moisture value By adjusting the amount of combustion and the amount of circulation according to the moisture content of the grain, the amount of moisture movement inside the grain and the amount of drying on the grain surface are commensurate for the entire period up to the end of drying, including when the outside air temperature is low Maintained in a state.

According to a second aspect of the present invention, in the configuration of the first aspect, the combustion amount of the burner (6a) is controlled by a drying rate control that performs a drying process in accordance with a target drying speed set before the drying operation, and the grain temperature is the grain. It is configured to be controlled by the minimum grain temperature maintenance control that raises the grain temperature when it falls below the minimum grain temperature corresponding to the grain moisture value, and the minimum grain temperature maintenance control is performed in preference to the drying rate control.
The above-mentioned exhaust circulation type grain dryer preferentially secures the minimum temperature condition of the grain when the outside air temperature is low, etc., while assuming the drying rate control of the target drying rate by the drying rate control Therefore, the grain is dried by the minimum grain temperature maintenance control.

  In the invention of claim 1, the grain is set so as to satisfy the minimum temperature condition of the grain and the absolute humidity condition of the exhaust air that can secure the amount of moisture transfer suitable for the drying treatment determined based on the moisture movement characteristics inside the grain. By adjusting the amount of combustion and the amount of circulation according to the moisture value of the water, the amount of moisture transferred inside the grain and the amount of drying on the surface of the grain are commensurate for the entire period up to the end of drying, including when the outside air temperature is low Therefore, even when the outside air temperature is low, the moisture transfer of the grains necessary for the drying process is ensured by the minimum amount of combustion, and energy-saving drying by efficient fuel control becomes possible. .

  In addition to the effect of the invention of claim 1, the invention of claim 2 performs a drying process by the drying rate control only within a range in which the minimum temperature condition of the grain is satisfied, thereby achieving a high speed by the drying rate control. In drying, energy-saving drying with controlled combustion can be ensured by controlling the minimum amount of combustion required when the outside air temperature is low.

FIG. 1 is a longitudinal side view showing a main internal structure of a grain dryer.
The grain dryer stacks the storage part 2 that receives the stretched grain and the drying part 3 that dries the grain from the storage part 2 and dry with hot air, and the lower part of the drying part 3 stores the grain. It is set as the grain collection part 4 for returning to the part 2, and the elevator part 5 which reaches the storage part 2 from this grain collection part 4 through the front part of a body is provided. The drying unit 3 has a hot air supply unit 6 provided with a burner 6a at the front position, and a suction discharge unit 7 that discharges the exhausted air by a wind exhaust fan 7a and a wind control valve 7b at the rear position. It is configured to be dried while circulating the grains. A moisture meter 8 is attached to the elevator unit 5. A controller 9 has a built-in control unit.

  As shown in FIG. 2, the drying unit 3 includes a hot air chamber 11 in which introduction ports W and W for receiving hot air from the hot air supply unit 6 are opened on the front surface, and a suction discharge unit. The air is exhausted by a perforated plate that guides the grains received from the storage section 2 while receiving hot air by forming a hot air between the hot air chamber 11 and the exhaust air chamber 12 so that the hot air can pass therethrough. The drying net path 13 is provided with a feeding valve 14 at the lower end of the drying net path 13 so that the grains flowing down the drying net path 13 are fed to the grain collecting unit 4 by a predetermined amount.

The drying net passage 13 is configured to extend in the front-rear direction of the drying unit 3 and is arranged in parallel in the left-right direction of the drying unit 3, and the lower end portions of the two adjacent drying net passages 13, 13 are configured to merge, A feeding valve 14 that rotates in the forward and reverse directions is provided at the lower end portion so that the grains of the two drying net passages 13 and 13 are fed out alternately.
The exhaust air chamber 12 is disposed inside the both drying net passages 13 and 13, and the hot air chambers 11 and 11 are respectively formed outside the both drying net passages 13 and 13. In addition, on the inlet side of both the drying net passages 13, the flow dividing portions 13 a and 13 a each having a rhombus shape and a perforated plate are provided to ensure a long drying path while receiving a wide range of grains.

  Inside the hot air chamber 11, an exhaust air return passage 15 that communicates with the exhaust air branching duct 7 d of the suction exhaust unit 7, which will be described later, and communicates with the hot air supply unit 6 is provided in order to dry at high speed using exhaust air. In addition, a far-infrared radiator 23 (described later) that promotes moisture transfer inside the grain by far-infrared rays is provided at the front end portion of the exhaust chamber 12, and far-infrared rays are provided to the two drying net passages 13 and 13 facing both sides thereof. Radiate.

  The grain collecting unit 4 includes left and right inclined plates 17 and 17 for collecting discharged grains below the feeding valve 14 and a transfer spiral 18 for transferring the collected grains to the elevator unit 5.

  As shown in FIG. 3, the hot air supply unit 6 has an enlarged longitudinal sectional view of a main part thereof, a burner 6 a that takes in the outside air and generates hot air while heating, a burner case 6 b that covers the periphery of the burner 6 a, and its generation A hot air passage 22a for guiding the hot air to the hot air chamber 11, and a guide duct 22 that communicates with the exhaust air return passage 15 and forms an exhaust air passage 22b that joins the hot air passage 22a. Provided.

  The guide duct 22 is provided adjacent to the front sides of the two hot air chambers 11, 11 and the exhaust air chamber 12, and serves as a partition wall that partitions the hot air passage 22 a and the exhaust air chamber 12. Far-infrared radiation by projecting the partition wall portion partitioning between the hot air passage 22a and the exhaust air chamber 12, that is, the partition wall portion of the front portion facing the combustion flame of the burner 6a, into the cup shape toward the exhaust air chamber 12 A body 23 is formed.

  The far-infrared radiator 23 is formed in a deformed polygonal shape both in front view and side view. The concave surface viewed from the front side of the machine body is a hot air receiving surface 23e that receives the hot air generated by the burner 6a, and the convex surface viewed from the rear side of the machine body is far infrared. An irradiation surface 23b is used, and a far-infrared irradiation surface 23b is coated with a paint that generates far-infrared rays.

The upper part 23c of the far-infrared radiator 23 is formed in a mountain shape in front view in order to avoid dust accumulation, and the lower part 23d is inclined so as to follow the left and right drying net passages 13.
The lower part 23d is configured to incline forward and lower, so that the scattered fuel received in the event of an ignition mistake or misfire is returned to the hot air supply unit 6 side, and the scattered fuel evaporates without stagnating to prevent a fire. Can do.
The upper portion 23c is configured to be inclined downward and rearward so that the far infrared rays irradiated from the far infrared irradiation surface 23b can be irradiated obliquely upward, and the drying net passage 13 is efficiently spread from the front portion to the entire rear portion. Can you irradiate far infrared rays? .

  The far-infrared radiation surface 23b of the far-infrared radiator 23 is provided with a steel material 23a coated with a radiation coating substantially horizontally over the entire front-rear direction, and preferably with an angle material or the like to avoid accumulation of dust on the upper surface. The upper surface is configured in a mountain shape like a roof. By comprising in this way, the far-infrared effect can be extended to the back of the exhaust chamber 12 using the heat-transfer effect of the temperature rise of the steel material 23a extended to the back.

  If the far-infrared radiator is extended to the back of the exhaust chamber 12, the space through which the exhaust air passes in the exhaust chamber 12 is reduced, and the suction exhaust wind is reduced. However, as in the present embodiment, the far-infrared radiation is reduced. Since the radiator 23 is disposed on the front side of the exhaust chamber 12, the suction exhaust function is prevented from being lowered, and the back of the exhaust chamber 12 is radiated by the heat transfer effect of the steel material 23a. Far-infrared rays can be irradiated over the entire front-rear direction. In addition, it is desirable that the front-rear length of the far-infrared radiator 23 is not more than one-fourth the length of the exhaust chamber 12 in the front-rear direction.

  Further, the guide duct portion 22 is formed with an outside air introduction hole 22c having a slit-like opening on the outer side surface of the hot air passage 22a also serving as a burner inspection port. By providing the outside air introduction hole 22c on the outer surface of the hot air passage 22a in this way, the air volume can be increased by lowering the ventilation resistance, and the high-temperature guide duct portion 22 that receives the hot air for drying from the burner 6a and the cooling of the burner 6a. Flame inspection is possible.

  Moreover, outside air introduction holes 24a and 25a by slit-like openings are formed in the outer side surface portion of the hot air chamber 11 so as to serve as a burner inspection port on the side hopper door 24 for grain input and the lid 25 on the opposite side. As a result, the air volume can be increased by reducing the ventilation resistance, and the high-temperature portion in the vicinity of the inlet W receiving the hot air for drying from the burner 6a can be cooled and the flame of the burner 6a can be inspected.

Explaining the suction / exhaust unit 7, an exhaust fan 7 a is provided at the rear end of the exhaust chamber 12, an exhaust exhaust duct 7 c is connected to the rear of the exhaust fan 7 a, and the exhaust exhaust duct 7 c is provided inside. Is provided with a wind exhaust control valve 7b that rotates about the horizontal axis, and the exhaust wind guided by the exhaust wind control valve 7b is branched into the left and right exhaust guide ducts 15 and supplied to the exhaust wind exhaust duct 7c. The branch duct 7d is connected.
Reference numeral 7e denotes a second exhaust air control valve. The amount of exhaust air exhausted by the exhaust air fan 7 by the control of the exhaust air control valve 7b and the second exhaust air control valve 7e is returned to a desired amount. It is trying to become.

Next, the operation of the exhaust air return dryer with a far-infrared radiator of the present embodiment will be described.
The grains in the storage chamber 2 flow down to the drying net passage 13, are fed to the grain collection chamber 4 by the feeding valve 14, transferred to the elevator unit 5 by the transfer spiral 18, and lifted toward the storage chamber 2 again by the elevator unit 5. It is cerealed and circulated in the same way.

  Hot air generated by the combustion flame generated in the burner 6 a passes through the left and right hot air passages 22 a in the guide duct 22 and is supplied from the hot air inlet W to the hot air chamber 11. The hot air supplied to the hot air chamber 11 passes through the drying net passage 13, exposes the hot air to the flowing down grain, deprives the grain of moisture, and is supplied to the exhaust air chamber 12.

  The hot air containing moisture supplied to the exhaust chamber 12 is sucked by the exhaust fan 7a and discharged as exhaust air into the exhaust air exhaust duct 7c. The exhaust air is divided into exhaust air discharged to the outside by the exhaust air control valve 7b and exhaust air supplied to the hot air chamber 11 again, and the second exhaust air control valve is connected to the exhaust air control valve 7b. A desired amount of exhausted air is returned to the hot air chamber 11 by dynamic control. Further, when the second exhaust air adjusting valve 7e is set to the fully closed position, the exhaust air is prevented from flowing into the exhaust air branch duct 7e, and the entire amount is discharged outside the machine.

  The exhaust air returning to the hot air chamber 11 side is guided to the exhaust air branch duct 7d by the exhaust air regulating valve 7b, and passes from the exhaust air branch duct 7d to the exhaust air return duct 15 and the exhaust air passage 22b and merges with the hot air passage 22a. . The exhaust air supplied to the hot air passage 22 a is mixed with the hot air generated by the burner 6 a and supplied to the hot air chamber 11.

  The hot air generated by the burner 6a and supplied to the hot air passage 22a acts on the hot air receiving surface 23e of the far-infrared radiator 23 provided at a position facing the combustion surface of the burner 6a. And if the temperature of the far-infrared radiator 23 becomes high, the far-infrared rays are irradiated on the dry net passage 13 adjacent to the left and right from the surface on the side of the exhaust chamber 12 and the wall surface on the rear side of the machine body and act on the grains.

  With the configuration of the present embodiment, since the exhaust air return passage is provided in the hot air chamber, the hot air from the hot air passage is mixed with the hot air from the burner by mixing the exhaust air with the hot air from the burner while preventing internal condensation. When the mixed hot air is supplied to the chamber and acts on the grain in the dry net passage, moisture transfer inside the kernel is facilitated by high-humidity hot air, and at the same time, moisture molecules are removed by far infrared rays from the exhaust chamber. By oscillating and accelerating moisture transfer, it is possible to accelerate the moisture homogenization inside the grain and to dry the grain at high speed and high quality. Further, by providing the far-infrared radiator 23 in the exhaust chamber 12, the temperature of the exhaust air can be raised and the high-temperature exhaust air can be returned to the hot air passage 22a, so that the combustion efficiency of the drying operation can be improved. it can.

(Exhaust air circulation operation)
Next, the exhaust air circulation operation will be described.
In a conventional dryer having no exhaust air circulation function, the pressure from the atmosphere is only behind the exhaust air fan 7a of the suction / discharge unit 7, and the main body of the dryer is decompressed from the atmosphere everywhere, In the exhaust circulation type dryer, in order to return the pressurized exhaust air, a part of the inside of the machine is more pressurized than the atmospheric pressure. Since dust is ejected from this portion, it is necessary to prevent pressure from being applied at least to the vicinity of the operation unit of the dryer.

  Therefore, in the setting of the maximum exhaust air circulation amount in the practical use range of the dryer that performs exhaust air circulation by the tempering type, the exhaust part and the peripheral part of the combustion part are exhausted so that the pressure is lower than the atmosphere. Set the air circulation rate. Thus, by reducing the pressure around the operation unit, dust can be prevented from being ejected to the atmosphere. In this case, the configuration is such that the return air volume is larger than the inlet air volume, and the practical use range should be within this range even when the periphery of the operation unit is pressurized.

  In addition, if the exhaust air circulation volume is almost equal to or greater than the inlet air flow of the dryer, the exhaust air circulation air is not sucked into the dryer, so it blows out of the machine and dusts the atmosphere. Therefore, in the practical use range of the dryer that performs exhaust air circulation by the tempering type, “exhaust air circulation amount ≦ inlet air amount <outlet air amount” so that the exhaust air circulation amount does not become larger than the total inlet air amount to the drying net passage 13. , And the amount of air that comes back is always less than the amount of air that enters. In addition, by inhaling the outside air even a little, it is possible to dry the internal grain without stuffiness and to stabilize combustion. In this case as well, even if the return air volume is larger than the inlet air volume, the practical use range only needs to be within this range.

(Drying control)
Next, a control method for drying using exhaust air circulation will be described.
Conventionally, a hot air supply unit 6 for sending hot air by a burner, a drying unit 3 for receiving the hot air to dry the circulating grains, and a suction discharge unit 7 for returning the exhaust air and mixing it with the hot air In the grain dryer provided, drying control is known in which the amount of exhaust air circulation is sequentially increased as drying progresses, but the drying method approaches the equilibrium moisture content with the atmosphere at the end of drying, It is necessary to lower the absolute humidity of the drying air. At this stage, it is wrong to simply judge that the exhaust wind relative humidity is low and the drying energy is wasted, and unless the balance of the speed of moisture movement inside and outside the grain and the state of the exhaust wind are well controlled and the combustion energy is Energy saving drying is not possible.

  Therefore, the relationship between the grain moisture value measured by the moisture meter 8 and the absolute exhaust wind humidity is preset as shown in FIG. 10, and the grain moisture value measured by the moisture meter 8 and the exhaust wind circulation are used. The relationship with the minimum grain temperature necessary for drying is registered (see FIG. 8). And the set temperature of the hot air for drying, that is, the combustion amount of the burner 6a, is the dry control (drying rate control) based on the amount of tension, the finish water, and the target drying speed. In addition to the exhaust air circulation control for controlling the exhaust air amount based on the relationship, the minimum grain temperature maintenance control for adjusting and controlling the temperature of the hot air so as to satisfy the minimum grain temperature condition is performed in order to efficiently perform the exhaust air circulation control.

  Specifically, for example, a target drying speed is set at 0.8% / h, and this temperature is used to correct the hot air temperature with the minimum grain temperature of the moisture reference value. If this condition is satisfied, the exhausted air always maintains a relationship of “internal / external moisture transfer amount = grain surface dry amount”, and therefore, the exhausted air circulation amount is an appropriate value. As a result, the relationship “internal / external moisture transfer amount = grain surface drying amount” enables drying without shell cracks, and this relationship is the minimum necessary for a high-speed drying method that is performed near the upper limit for maintaining grain quality. Energy-saving drying becomes possible by carrying out at temperature.

  In this way, the control unit that adjusts and controls the combustion amount of the hot air supply unit 6 and the circulation amount of the suction discharge unit 7 is dried to a predetermined moisture value while circulating the grains, and in the whole process, the control unit As for the temperature of the grain and the absolute humidity of the exhausted air, so as to satisfy the minimum temperature condition of the grain that can ensure the amount of moisture transfer suitable for the drying process set separately according to the moisture value of the grain measured by the moisture meter 8 While controlling the said combustion amount, the said circulation amount is controlled so that the absolute humidity conditions of the exhaust wind for ensuring the said moisture movement amount set separately according to the moisture value of the grain may be satisfied.

  The above requirement is based on the relationship between the grain temperature and the amount of moisture transferred inside the grain, and the relationship between the absolute humidity of the exhaust air and the amount of dryness on the surface of the grain, which can ensure the amount of moisture transfer suitable for the drying process. The minimum temperature condition of the grain and the absolute humidity condition of the exhaust air for securing the amount of moisture movement are determined by actual measurement or the like for each grain moisture value. After setting this necessary condition in the control unit, the combustion amount and the circulation amount are adjusted according to the moisture value of the grain so as to satisfy the minimum temperature condition and the absolute humidity condition. This condition control allows adjustment of the amount of moisture transferred inside the grain and the amount of drying on the grain surface for the entire period up to the end of drying, including when the outside air temperature is low, so the outside air temperature is low. Even in such cases, the moisture transfer of the grains necessary for the drying process is ensured by the minimum amount of combustion, and energy saving drying by efficient fuel control becomes possible.

  Further, in the above drying control, regarding the relationship of the required minimum temperature based on the moisture value, the grain temperature is measured after one in-machine grain after the start of drying is circulated, and the minimum grain temperature is increased as the outside air temperature is higher. Correcting in the direction, and correcting in a direction to increase the value of the minimum grain temperature as the amount of squeezing increases, makes it possible to perform appropriate energy saving control.

  Thus, the grain drying is performed by configuring the control unit so as to perform the drying rate control process for performing the drying process along the target drying speed within the range where the minimum temperature condition of the grain is satisfied. The machine is premised on the operation control of the target drying speed by the drying rate control, but when the outside air temperature is low, etc., it is controlled to the minimum required combustion amount preferentially. The energy-saving drying that suppresses combustion is possible.

  Next, the drying speed in energy-saving drying will be explained. Depending on the grain varieties and outside air conditions, the change in the air volume may be large, and the drying speed may decrease due to exhaust air absolute humidity control. If the energy saving drying is performed while maintaining the relationship between the value, the minimum grain temperature, the moisture value, and the absolute wind humidity, the drying speed is sacrificed. Reduce to dry.

  Generally, it is known that when the combustion amount is changed, which is the most direct means for changing the drying speed, a lot of loss is caused by a sudden change in the energy amount. As a countermeasure when the drying speed decreases in energy-saving drying, it is possible to avoid such a situation and obtain a moderate effect by exhausted absolute humidity control by changing and controlling the exhaust air circulation amount as the first. it can.

(Control of moisture spots)
Next, the method of homogenizing the finished moisture in the high-speed drying process will be explained. If there is a large moisture difference in the initial moisture, it is discharged in the “adjustment mode” that increases the wind circulation rate and promotes moisture transfer between grains. By increasing the air circulation rate and increasing the hot air humidity for drying, the finished moisture can be made uniform.

  Conventionally, when drying grains, when the harvested grains with different conditions in the growing field are collectively dried, etc., when it is thought that there is a difference in moisture in the inserted grains, the operator performs circulation insertion In the case of exhaust air circulation drying, which is a means of high-speed drying, hot air is exhausted by exhaust air circulation. Since the grain temperature is raised by this high-humidity hot air, moisture transfer is likely to occur inside the grain, and the moisture state must be made uniform and the drying operation must be suspended. As it disappears, drying is completed quickly, and the finish becomes uniform.

  In this case, in order to detect moisture difference, one cycle of the grain at the start of drying shortens the moisture measurement interval, and periodically measures a small amount of grain moisture when the grain is stretched In addition, it is possible to detect spots in the tension state, and in addition, when there are many immature grains by moisture measurement at the initial stage of drying, when the difference in moisture is large by the determination, the exhaust circulation rate is increased and the moisture between grains By configuring the control unit to automatically shift to the “adjustment mode” that promotes the transfer and perform drying, the finished moisture can be automatically made uniform.

Longitudinal side view showing the main internal structure of a grain dryer Cross section of the main part showing the internal structure of the drying section Main section enlarged vertical section of hot air supply section Perspective view of far-infrared radiator The perspective view which shows a drying part and a grain collection part Side view explaining the inside of the suction exhaust section The perspective view explaining an exhaust wind branch duct and an exhaust wind return duct A graph showing the relationship between the minimum grain temperature value and the moisture value in exhaust circulation drying control Perspective view showing hot air supply section and guide duct Graph showing the relationship between grain moisture value and absolute wind humidity

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 Storage part 3 Drying part 4 Grain collecting part 6 Hot-air supply part 6a Burner 7 Suction discharge part 7a Exhaust fan 7b Exhaust air control valve 11 Hot-air chamber 12 Exhaust room 13 Drying net path 14 Fixed delivery valve 15 Exhaust air return path 22 Guide duct 22a Hot air passage 22b Exhaust passage 23 Far-infrared radiator W Hot air inlet

Claims (2)

A burner (6a) for generating hot air, a hot air chamber (11) through which the hot air passes, an exhaust fan (7a) for sucking the hot air supplied to the grains and discharging it as exhaust air, and the exhaust air into the hot air An exhaust air return passage (15) for returning to the chamber (11), a moisture meter (8) for detecting the moisture value of the grain, a grain temperature detecting means for detecting the grain temperature, and an exhaust for returning to the hot air chamber (11) In the exhaust air circulation type grain dryer comprising a control unit for adjusting and controlling the amount of wind,
Preliminarily set the exhaust wind absolute humidity corresponding to the grain moisture value and the minimum grain temperature corresponding to the grain moisture value,
The exhaust air amount returned to the hot air chamber (11) is controlled to be the absolute exhaust air humidity corresponding to the grain moisture value, and the grain temperature detected by the grain temperature detecting means is set to the grain moisture value. When the temperature falls below the corresponding minimum grain temperature, the amount of combustion of the burner (6a) is increased so as to increase the grain temperature.   The amount of combustion of the burner (6a) is controlled when the drying rate is controlled in accordance with the target drying speed set before the drying operation, and when the grain temperature falls below the minimum grain temperature corresponding to the grain moisture value, the grain temperature is reduced. The exhaust air circulation type grain dryer according to claim 1, wherein the control is performed by the minimum grain temperature maintenance control to be raised, and the minimum grain temperature maintenance control is performed in preference to the drying rate control.

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