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Fiber drying is a very important part in the production of medium density fiberboard. The quality of fiber drying will directly affect the quality
The quality of medium density fiberboard. It not only requires the moisture content of the fiber to be dried to 8%-12% for a certain period of time, but also requires that the resinous aldehyde attached to the fiber is not cured during this time. In addition, the heat energy consumption of the dryer accounts for more than 1/2 of the entire production line, and energy conservation is a crucial part of reducing product costs. Therefore, in the design process of medium density fiberboard production line, the choice and design of the dryer is particularly important. This paper will discuss the design, structure and energy saving of the dryer.
1 dryer works
During the MDF production process, the chips are ground into a slender fiber by a fiber separator under steam pressure. The ground fiber is mixed with glue and paraffin through a spray valve under the pressure of steam in the hot mill and sent to the drying pipe. In the pipeline, it is mixed with hot air for heat exchange, and is sent to the cyclone through a certain wind pressure and wind speed. In the cyclone separator, the fiber is separated and falls to the blanking rotary valve, and then to the fire belt conveyor, and finally Get dry fiber.
The hot air for drying is obtained by an air heater and blown into the drying duct by a drying fan. The dry gas stream and moisture are discharged at the top of the cyclone. The drying principle is shown in Figure 1.
2 Determination of drying parameters
2.1 Determination of the length of the drying pipe (L)
The determination of the drying parameters is very important in the process of drying the fibers. Because the fiber has the characteristics of large specific surface area, easy evaporation and drying, low density, etc., and is flammable and explosive. At the same time, it is required to not cure the adhesive attached to the surface during the drying process. Therefore, the fiber should be dried, and the drying time should not be too long. According to the characteristics of the venous rubber, the effective drying time is generally between 3 and 4 s.
Since the moisture content of the fiber entering the dryer is about 50%-60%, it is easy to adhere to the drying pipe, so there is a certain requirement for the wind speed of the dry hot air, which can be determined according to parameters such as material characteristics, suspension speed and density. Generally, it is calculated according to the minimum speed required for the heaviest fiber, because the fibers with different specifications have different time in the gas flow. The light fiber specific fiber can pass through the drying pipe faster, and the heavy fiber also needs to be dried for a longer period of time. . Therefore, the airflow speed is about 26m/s. Because the length of the drying pipe is L=T*V, where T is the drying time and V is the airflow speed, it is generally about L=100m.
2.2 Determination of dry air volume (Q)
To determine the air volume of the drying fan, first determine the pipe diameter (D) of the drying pipe, and the determination of the diameter of the drying pipe is determined by the amount of dry fiber. The determination of the amount of dry fiber is determined by the output of the MDF. Since most of the above is beyond what we are talking about, we only start by introducing dry pipes. The principle of determining the drying pipe (D) is to ensure that the hot air and the fiber are thoroughly mixed, and secondly, it is convenient to manually clean it. Because once the wrong operation or long time work, the fibers of the sedimentation tube wall are easily carbonized and fired, it must be manually cleaned. The diameter of the drying pipe of the medium density fiberboard production line of 15000m3/a is about 800mm; the medium density fiberboard production line of 30000m3/a Take about 1000mm; and the 50000m3/a production line takes about 1200mm; the 100000m3/a production line takes about 1700mm.
Therefore, the fan air volume Q = 3600 * 10 -4 * π / 4 · D2 · V (m3 / h)
The wind pressure of the fan is generally determined according to the length of the pipe, the number of pipe bends, the conveying height, the friction coefficient of the pipe, etc. to calculate the pressure loss.
2.3 Determination of the inlet and outlet temperature of the dryer
Since the dried fiber is separated from the moisture, the water is vaporized and discharged from the top of the cyclone. The outlet temperature (T2) must be such that it does not condense twice before the steam is discharged. Therefore, it is generally between 60 and 70 °C. .
The inlet temperature of the dryer is determined based on the amount of water vaporized. Moreover, because the moisture content of the fiber entering the dryer is between 50% and 60%, and the moisture content of the fiber after drying is between 8% and 12%, the vaporized water content is generally about 1.1 times that of the hot-milled dry fiber. . Since the fiber temperature of the dryer is between 160-170 ° C, the heat (Q) required to dry the fiber is:
Q=1.1k·t
Where: k—vaporization heat, 539kca1/kg;
t—The quality of the dry fiber.
Therefore, the dryer inlet temperature (T1) is:
T1=Q/P·C·Ql+TZ
Where: Q1--the required air volume for drying;
C--air specific heat, 0.24;
Q--drying heat;
T2--outlet temperature;
P--air density, 1.4kg/m3
3 hot air dryer structure and its characteristics
3.1 characteristics of hot air dryer
The dryer for the production of medium density boards can be classified into a vertical dryer and a horizontal dryer according to the form of the drying pipe. That is, when the fiber starts to enter the drying pipe, the pipe is a vertical type vertical dryer (see Fig. 1); when the fiber is first entered into the drying pipe, the horizontal pipe is a horizontal dryer. Both have advantages and disadvantages. They are all used and will not be discussed here. The dryers that are distinguished by the way of obtaining hot air are also divided into two types: the first type is a steam dryer (Fig. 1), that is, the cold air is blown to the heat sink by the fan. After the heat exchange, it becomes hot air, and then the dry pipe is mixed with the wet fiber, and the heat sink form is shown in Fig. 2.
The heat dissipation of the heat sink is obtained by heating with internal steam or hot oil. This dryer is simple and safe to operate. At present, most domestic MDF manufacturers use this form. However, due to its low thermal efficiency, a large amount of steam will be consumed each month, increasing production costs. In today's increasingly competitive wood-based panel market, cost has become the biggest concern for manufacturers. Especially in winter, the temperature difference between the temperature of the first heat sink and the temperature of the cold air is the largest, and the heat sink is often in a state of “suddenly rising and shrinking†due to equipment failure or improper production management. , greatly shortening its life. Therefore, such a heat sink generally has a phenomenon of steam leakage within half a year, and must be completely replaced in three years. It takes about 250,000 yuan to replace the heat sink. In view of the above reasons, we have proposed the use of a hot air oven dryer. That is, the air passing through the "hot air stove" furnace is heated and directly enters the dryer. This dryer has high efficiency and long life, which greatly reduces the production cost.
3.2 Structure of the hot air dryer
The hot blast stove dryer consists of a hot blast stove, a drying pipe, a cyclone separator, a spark detection and fire protection device, and an electronic control system, as shown in Figure 3.
The drying pipe, cyclone separator, spark detection and fire-fighting device of the hot-blast stove dryer are the same as the steam-type dryer, and will not be discussed here. This article focuses on the GRL360 type used in the 30000m3/a medium-density fiberboard production line of Fuyang Wood-based Panel Co., Ltd. Hot air stove. The hot blast stove is a chain machine type, which is composed of a furnace body, a blast system, an air intake system, a coal loading device, a slag discharging device, a dust collector, a chimney, etc., as shown in Fig. 4.
The hot air generated by the hot blast stove is obtained by indirect heat exchange, which can make the physical properties of the hot air consistent with the ambient air, and adopts the method of positive pressure operation, the danger of the smokeless gas mixing and the flashing point mixed. It can fully meet the heat source requirements and safety requirements of MDF production materials in airflow drying mode. Its upper limit of wind temperature is 250 °C, and it can achieve relatively stable hot air output by adjusting the relatively constant combustion state.
The high-temperature heating part of the furnace adopts special 25Gr20Ni precision casting components, and the furnace is made of high-quality stainless steel, so it can adapt to the continuous operation of the unit for a long time without any operation error.
3.3 GRL360 hot blast stove technical parameters
Output total heat 3.6 million kcal;
Heating standard air volume 65000m3 / h;
The highest wind temperature is 250 ° C;
Drying fan model 9-26 centrifugal fan,
Speed ​​960r/min,
Full pressure 6.3-6.9kPa,
Power 164KW
Air volume 52392-69856m3/h;
Blower model 4-72 centrifuge,
Speed ​​2900r/min,
Full pressure 2.0-2.5kPa,
The air volume is 5712-9194m3/h,
Power 7.5kW;
Induced draft fan model Y5-17-6C boiler induced draft fan,
Speed ​​2620r/min,
Full pressure 3.0-3.8kPa,
The air volume is 9110-15670m3/h,
Power 22kW
4 dryer technical guarantee measures
4.1 Fiber moisture content adjustment measures
Due to the change of tree species, the moisture content of the fiber discharged from the hot mill also changes, so the moisture content of the dried fiber is also unstable, which makes the product defective rate increase or even impossible to produce. In addition, although the amount of fiber discharged from the hot mill has a theoretical value corresponding to the drying temperature, it is difficult to accurately correspond in actual production. The moisture content of the dried fiber is not too high or low, the high water content will affect the production yield, and the low quality will affect the product quality, which is not desired by the producer. In the steam dryer, the air is adjusted by a side air passage (see Figure 1). By adjusting the size of the damper on the side air passage, the cold air volume entering the dry pipe is controlled to adjust the final moisture content of the dry fiber. . However, because the adjustment range of the above method is too small and difficult to control, a water injection type adjustment method is adopted in the hot air oven type dryer, that is, a spray with a maximum amount of 2 t/h is set in the material of the dryer material.
Water pipes to increase or decrease the amount of dry water for ultimate adjustment. This method also effectively offsets the defect of the hot air furnace exhaust humidity difference of 110 ° C.
4.2 fine screen shielding measures
In order to prevent a large amount of combustibles from entering the heating zone of the hot blast stove with cold air, a fine mesh screen is added at the cold air inlet of the fan, and is manufactured in a replaceable manner so that the screen is damaged or dusted. When the leaves are blocked, they will not be shut down.
5 hot air dryer use and precautions
Since the wind-drying machine is the first fiber drying in China for MDF, it has insufficient experience, so it must be strictly required and used correctly. Otherwise, it will affect the life of the stove and the danger. The procedures and precautions for use are as follows.
1 Before starting the hot blast stove, first check whether the dryer parts are normal, such as checking whether the door is closed, whether the motor fan is normal, etc. The furnace can be ignited after all is determined to be normal. When the drying fan is started at the same time as the ignition, the air inlet must be closed when the drying fan is started, so as to prevent the starting current from being too large to burn the electrical components. After determining that the fan is started, open the fan air inlet to make it half-open. At the same time, the dry pipe gate valve is closed, and the hot air release valve is opened to allow the initial hot air to be exhausted.
2 After the fire is on, start to heat up, and slowly heat up, the speed is generally not more than 100 melon, so as not to affect the life of the furnace body and the furnace. When the wind temperature reaches about 40 °C, open the dry pipe gate valve, close the release valve, and adjust the drying fan inlet to the maximum position.
3 When the wind temperature reaches 100 °C, the heating rate can be increased appropriately, but the time to rise to 130 °C must be controlled at 15-20 min.
between. The purpose is to dry the moisture remaining in the drying pipe to prevent condensation when the fiber enters the drying pipe, causing fiber deposition.
4 When the wind temperature reaches about 130 °C, the hot mill starts and discharges into the drying pipe. As the wind temperature rises gradually, the moisture content of the dried fiber gradually reaches the required value.
5 Under normal circumstances, when the wind temperature is around 160 °C, the production will enter the normal state, and will not continue to heat up. The moisture content of the fiber at this time should be slightly lower than the water content required for production, and finally the spray water pipe is opened to increase the moisture content of the fiber to meet the production requirements, and the purpose of this is to facilitate the adjustment.
6 When the moisture content of the dry fiber differs from the moisture content of the fiber required for production, it can be controlled by adjusting the air temperature and the amount of water sprayed. However, it must be noted that when the moisture content of the dry fiber is too low, the water volume is first opened to the maximum position, and then the air temperature is lowered, and the water spray amount is lowered while the air temperature is lowered to achieve the purpose of saving energy.
6 Conclusion
The application of GRL360 hot blast stove in the 3000Um3/a medium density fiberboard production line of Fuyang Artificial Board Co., Ltd. shows that the total cost can be saved by more than 70,000 yuan per month, and the output and product quality can be improved. This proves that the application of the hot air oven dryer is theoretically feasible and technically reliable, and is an ideal device for drying medium-density fiber.
Source: Tai Fook Evergreen (Yichun) Group Co., Ltd. Lexus Harbin Electrical and Mechanical Technology Co., Ltd. Harbin Municipal Engineering Design Institute Author: Zhao Yue Shi Kui
The quality of medium density fiberboard. It not only requires the moisture content of the fiber to be dried to 8%-12% for a certain period of time, but also requires that the resinous aldehyde attached to the fiber is not cured during this time. In addition, the heat energy consumption of the dryer accounts for more than 1/2 of the entire production line, and energy conservation is a crucial part of reducing product costs. Therefore, in the design process of medium density fiberboard production line, the choice and design of the dryer is particularly important. This paper will discuss the design, structure and energy saving of the dryer.
1 dryer works
During the MDF production process, the chips are ground into a slender fiber by a fiber separator under steam pressure. The ground fiber is mixed with glue and paraffin through a spray valve under the pressure of steam in the hot mill and sent to the drying pipe. In the pipeline, it is mixed with hot air for heat exchange, and is sent to the cyclone through a certain wind pressure and wind speed. In the cyclone separator, the fiber is separated and falls to the blanking rotary valve, and then to the fire belt conveyor, and finally Get dry fiber.
The hot air for drying is obtained by an air heater and blown into the drying duct by a drying fan. The dry gas stream and moisture are discharged at the top of the cyclone. The drying principle is shown in Figure 1.
2 Determination of drying parameters
2.1 Determination of the length of the drying pipe (L)
The determination of the drying parameters is very important in the process of drying the fibers. Because the fiber has the characteristics of large specific surface area, easy evaporation and drying, low density, etc., and is flammable and explosive. At the same time, it is required to not cure the adhesive attached to the surface during the drying process. Therefore, the fiber should be dried, and the drying time should not be too long. According to the characteristics of the venous rubber, the effective drying time is generally between 3 and 4 s.
Since the moisture content of the fiber entering the dryer is about 50%-60%, it is easy to adhere to the drying pipe, so there is a certain requirement for the wind speed of the dry hot air, which can be determined according to parameters such as material characteristics, suspension speed and density. Generally, it is calculated according to the minimum speed required for the heaviest fiber, because the fibers with different specifications have different time in the gas flow. The light fiber specific fiber can pass through the drying pipe faster, and the heavy fiber also needs to be dried for a longer period of time. . Therefore, the airflow speed is about 26m/s. Because the length of the drying pipe is L=T*V, where T is the drying time and V is the airflow speed, it is generally about L=100m.
2.2 Determination of dry air volume (Q)
To determine the air volume of the drying fan, first determine the pipe diameter (D) of the drying pipe, and the determination of the diameter of the drying pipe is determined by the amount of dry fiber. The determination of the amount of dry fiber is determined by the output of the MDF. Since most of the above is beyond what we are talking about, we only start by introducing dry pipes. The principle of determining the drying pipe (D) is to ensure that the hot air and the fiber are thoroughly mixed, and secondly, it is convenient to manually clean it. Because once the wrong operation or long time work, the fibers of the sedimentation tube wall are easily carbonized and fired, it must be manually cleaned. The diameter of the drying pipe of the medium density fiberboard production line of 15000m3/a is about 800mm; the medium density fiberboard production line of 30000m3/a Take about 1000mm; and the 50000m3/a production line takes about 1200mm; the 100000m3/a production line takes about 1700mm.
Therefore, the fan air volume Q = 3600 * 10 -4 * π / 4 · D2 · V (m3 / h)
The wind pressure of the fan is generally determined according to the length of the pipe, the number of pipe bends, the conveying height, the friction coefficient of the pipe, etc. to calculate the pressure loss.
2.3 Determination of the inlet and outlet temperature of the dryer
Since the dried fiber is separated from the moisture, the water is vaporized and discharged from the top of the cyclone. The outlet temperature (T2) must be such that it does not condense twice before the steam is discharged. Therefore, it is generally between 60 and 70 °C. .
The inlet temperature of the dryer is determined based on the amount of water vaporized. Moreover, because the moisture content of the fiber entering the dryer is between 50% and 60%, and the moisture content of the fiber after drying is between 8% and 12%, the vaporized water content is generally about 1.1 times that of the hot-milled dry fiber. . Since the fiber temperature of the dryer is between 160-170 ° C, the heat (Q) required to dry the fiber is:
Q=1.1k·t
Where: k—vaporization heat, 539kca1/kg;
t—The quality of the dry fiber.
Therefore, the dryer inlet temperature (T1) is:
T1=Q/P·C·Ql+TZ
Where: Q1--the required air volume for drying;
C--air specific heat, 0.24;
Q--drying heat;
T2--outlet temperature;
P--air density, 1.4kg/m3
3 hot air dryer structure and its characteristics
3.1 characteristics of hot air dryer
The dryer for the production of medium density boards can be classified into a vertical dryer and a horizontal dryer according to the form of the drying pipe. That is, when the fiber starts to enter the drying pipe, the pipe is a vertical type vertical dryer (see Fig. 1); when the fiber is first entered into the drying pipe, the horizontal pipe is a horizontal dryer. Both have advantages and disadvantages. They are all used and will not be discussed here. The dryers that are distinguished by the way of obtaining hot air are also divided into two types: the first type is a steam dryer (Fig. 1), that is, the cold air is blown to the heat sink by the fan. After the heat exchange, it becomes hot air, and then the dry pipe is mixed with the wet fiber, and the heat sink form is shown in Fig. 2.
The heat dissipation of the heat sink is obtained by heating with internal steam or hot oil. This dryer is simple and safe to operate. At present, most domestic MDF manufacturers use this form. However, due to its low thermal efficiency, a large amount of steam will be consumed each month, increasing production costs. In today's increasingly competitive wood-based panel market, cost has become the biggest concern for manufacturers. Especially in winter, the temperature difference between the temperature of the first heat sink and the temperature of the cold air is the largest, and the heat sink is often in a state of “suddenly rising and shrinking†due to equipment failure or improper production management. , greatly shortening its life. Therefore, such a heat sink generally has a phenomenon of steam leakage within half a year, and must be completely replaced in three years. It takes about 250,000 yuan to replace the heat sink. In view of the above reasons, we have proposed the use of a hot air oven dryer. That is, the air passing through the "hot air stove" furnace is heated and directly enters the dryer. This dryer has high efficiency and long life, which greatly reduces the production cost.
3.2 Structure of the hot air dryer
The hot blast stove dryer consists of a hot blast stove, a drying pipe, a cyclone separator, a spark detection and fire protection device, and an electronic control system, as shown in Figure 3.
The drying pipe, cyclone separator, spark detection and fire-fighting device of the hot-blast stove dryer are the same as the steam-type dryer, and will not be discussed here. This article focuses on the GRL360 type used in the 30000m3/a medium-density fiberboard production line of Fuyang Wood-based Panel Co., Ltd. Hot air stove. The hot blast stove is a chain machine type, which is composed of a furnace body, a blast system, an air intake system, a coal loading device, a slag discharging device, a dust collector, a chimney, etc., as shown in Fig. 4.
The hot air generated by the hot blast stove is obtained by indirect heat exchange, which can make the physical properties of the hot air consistent with the ambient air, and adopts the method of positive pressure operation, the danger of the smokeless gas mixing and the flashing point mixed. It can fully meet the heat source requirements and safety requirements of MDF production materials in airflow drying mode. Its upper limit of wind temperature is 250 °C, and it can achieve relatively stable hot air output by adjusting the relatively constant combustion state.
The high-temperature heating part of the furnace adopts special 25Gr20Ni precision casting components, and the furnace is made of high-quality stainless steel, so it can adapt to the continuous operation of the unit for a long time without any operation error.
3.3 GRL360 hot blast stove technical parameters
Output total heat 3.6 million kcal;
Heating standard air volume 65000m3 / h;
The highest wind temperature is 250 ° C;
Drying fan model 9-26 centrifugal fan,
Speed ​​960r/min,
Full pressure 6.3-6.9kPa,
Power 164KW
Air volume 52392-69856m3/h;
Blower model 4-72 centrifuge,
Speed ​​2900r/min,
Full pressure 2.0-2.5kPa,
The air volume is 5712-9194m3/h,
Power 7.5kW;
Induced draft fan model Y5-17-6C boiler induced draft fan,
Speed ​​2620r/min,
Full pressure 3.0-3.8kPa,
The air volume is 9110-15670m3/h,
Power 22kW
4 dryer technical guarantee measures
4.1 Fiber moisture content adjustment measures
Due to the change of tree species, the moisture content of the fiber discharged from the hot mill also changes, so the moisture content of the dried fiber is also unstable, which makes the product defective rate increase or even impossible to produce. In addition, although the amount of fiber discharged from the hot mill has a theoretical value corresponding to the drying temperature, it is difficult to accurately correspond in actual production. The moisture content of the dried fiber is not too high or low, the high water content will affect the production yield, and the low quality will affect the product quality, which is not desired by the producer. In the steam dryer, the air is adjusted by a side air passage (see Figure 1). By adjusting the size of the damper on the side air passage, the cold air volume entering the dry pipe is controlled to adjust the final moisture content of the dry fiber. . However, because the adjustment range of the above method is too small and difficult to control, a water injection type adjustment method is adopted in the hot air oven type dryer, that is, a spray with a maximum amount of 2 t/h is set in the material of the dryer material.
Water pipes to increase or decrease the amount of dry water for ultimate adjustment. This method also effectively offsets the defect of the hot air furnace exhaust humidity difference of 110 ° C.
4.2 fine screen shielding measures
In order to prevent a large amount of combustibles from entering the heating zone of the hot blast stove with cold air, a fine mesh screen is added at the cold air inlet of the fan, and is manufactured in a replaceable manner so that the screen is damaged or dusted. When the leaves are blocked, they will not be shut down.
5 hot air dryer use and precautions
Since the wind-drying machine is the first fiber drying in China for MDF, it has insufficient experience, so it must be strictly required and used correctly. Otherwise, it will affect the life of the stove and the danger. The procedures and precautions for use are as follows.
1 Before starting the hot blast stove, first check whether the dryer parts are normal, such as checking whether the door is closed, whether the motor fan is normal, etc. The furnace can be ignited after all is determined to be normal. When the drying fan is started at the same time as the ignition, the air inlet must be closed when the drying fan is started, so as to prevent the starting current from being too large to burn the electrical components. After determining that the fan is started, open the fan air inlet to make it half-open. At the same time, the dry pipe gate valve is closed, and the hot air release valve is opened to allow the initial hot air to be exhausted.
2 After the fire is on, start to heat up, and slowly heat up, the speed is generally not more than 100 melon, so as not to affect the life of the furnace body and the furnace. When the wind temperature reaches about 40 °C, open the dry pipe gate valve, close the release valve, and adjust the drying fan inlet to the maximum position.
3 When the wind temperature reaches 100 °C, the heating rate can be increased appropriately, but the time to rise to 130 °C must be controlled at 15-20 min.
between. The purpose is to dry the moisture remaining in the drying pipe to prevent condensation when the fiber enters the drying pipe, causing fiber deposition.
4 When the wind temperature reaches about 130 °C, the hot mill starts and discharges into the drying pipe. As the wind temperature rises gradually, the moisture content of the dried fiber gradually reaches the required value.
5 Under normal circumstances, when the wind temperature is around 160 °C, the production will enter the normal state, and will not continue to heat up. The moisture content of the fiber at this time should be slightly lower than the water content required for production, and finally the spray water pipe is opened to increase the moisture content of the fiber to meet the production requirements, and the purpose of this is to facilitate the adjustment.
6 When the moisture content of the dry fiber differs from the moisture content of the fiber required for production, it can be controlled by adjusting the air temperature and the amount of water sprayed. However, it must be noted that when the moisture content of the dry fiber is too low, the water volume is first opened to the maximum position, and then the air temperature is lowered, and the water spray amount is lowered while the air temperature is lowered to achieve the purpose of saving energy.
6 Conclusion
The application of GRL360 hot blast stove in the 3000Um3/a medium density fiberboard production line of Fuyang Artificial Board Co., Ltd. shows that the total cost can be saved by more than 70,000 yuan per month, and the output and product quality can be improved. This proves that the application of the hot air oven dryer is theoretically feasible and technically reliable, and is an ideal device for drying medium-density fiber.
Source: Tai Fook Evergreen (Yichun) Group Co., Ltd. Lexus Harbin Electrical and Mechanical Technology Co., Ltd. Harbin Municipal Engineering Design Institute Author: Zhao Yue Shi Kui
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