Causes and countermeasures of dry defects in difficult hardwood
2019-08-13 23:02:54
1.1 Dry defects common to difficult wood
Common drying defects include end cracking, cracking, honeycomb cracking, warping, deformation, and shrinkage.
1.2 The main cause of wood drying defects
The occurrence of drying defects is caused by the unreasonable combination of many drying factors in the furnace. There are mainly the following factors:
a, the sealing property of the furnace body b, the circulation speed of the gas flow in the furnace c, the medium temperature d, the relative temperature
2. Countermeasures
There are no absolute guidelines for dry benchmarks that can be referenced. The furnace type is different, the performance of the furnace body is different, and the drying standards are not the same. Simultaneously. The drying standard is relatively stable, but the drying process is dynamic. Therefore, it is necessary to accumulate and summarize, carefully observe and compare.
The guiding ideology for determining the drying process is: to ensure that the surface moisture of the material evaporates into the air and the speed of the deep moisture movement of the material to the surface is basically balanced, and the drying speed is increased as much as possible to obtain the desired economic benefits under the premise of ensuring the dry quality. And social benefits.
In order to obtain the dry quality of the customer's satisfaction, it is necessary to understand the interaction of the drying factor in the drying process, and comprehensively analyze and judge the parameters in the drying process and the actual conditions of the material in the furnace to make a correct decision. Ensure that the entire drying process is always under control.
2.1 Stacking of materials
First of all, it is necessary to ensure that the wood properties in the same furnace are the same or similar, and the thickness difference is not large; the thickness of the spacers should vary depending on the thickness of the material. Generally, materials with a thickness of 40 mm or less can be separated by 25 mm thick spacers. If the circulation speed of the gas in the furnace is slow and the final moisture content of the material is low, it is necessary to use a thick spacer of more than 3Omm in order to meet the requirements in quality and speed. For materials with a thickness of more than 5Omm, a spacer of 3Omm thickness is generally used to ensure a reasonable drying speed and uniformity of drying. The spacing between the spacers should be reasonable, the spacers of each layer should be arranged neatly in height, and the materials placed in the same layer should have the same thickness.
2.2 Preheat treatment
The pre-heat treatment time should be determined according to the tree species, thickness and moisture content of the wood. The material with a thickness of 5Omm is taken as an example. Under normal circumstances, the initial spray is 12-15h. In order to obtain a reasonable temperature gradient, the cooked material also needs to be treated for 6-8 hours. For Cyclobalanopsis glauca and Southern Elm. These two materials are very thin cells, ray parenchyma cells, mahogany, batti, gambourine, etc., which are usually densely sprayed for 24 hours to completely soften the wood, dredge the cell cavity, and partially dissolve the cell cavity. Filling in preparation for the subsequent drying process.
The initial spray temperature should not be too low, otherwise the steam flow into the furnace is too small, so that the relative humidity in the furnace reaches the benchmark requirement, and the effect of preheat treatment is not good. Generally, the initial treatment temperature is 95-10O degrees.
Whether the fan is continuously activated during the initial processing depends on the tightness of the furnace body. If the furnace body has good sealing performance, it can be continuously started after 3-4 hours of initial spraying; if the sealing performance of the furnace body is poor, continuous operation for a long time will increase the temperature inside the furnace and lower the humidity, which will lead to drying defects during the initial treatment. produce. In this case. Generally, it is turned on once every 2~3h. The steam is evenly dispersed on the surface of the material. A better treatment effect can be obtained.
After the initial treatment is completed, it is not allowed to enter the drying stage immediately. The temperature should be kept for 4~5h. When the moisture inside the material begins to vaporize and move, the surface of the material will absorb moisture sufficiently and uniformly, resulting in the correct temperature gradient and water content gradient before starting to dry.
2.3 Drying process
Some people think that wood drying is not technical, to ensure that the quality of the wood is difficult to dry, as long as the temperature is low, this statement is extremely wrong. Thick wood with a low temperature of more than 5Omm is too long and difficult. Secondly, if the low temperature is not combined with the suitable humidity, it is easy to crack. This is the same as the cracked wood, even if it is air-dried in a cool place, the degree of cracking is far more serious than the drying of the oven using a reasonable drying method. reason. Therefore, the drying process should be carried out according to the column mode: high and low temperature combined, the relative humidity wavewise changes slowly from high to low, and the circulation speed of the airflow is selected according to the severity of cracking of the material.
2.3.1 Temperature selection Drying starts with a lower temperature, generally 50-55 degrees is appropriate, the whole drying process needs to be divided into 6-8 temperature segments, the temperature difference of each temperature zone is about 3 degrees, when the whole material When the water content of the section is above the fiber saturation point, the temperature in the furnace is preferably below 60 °C. When the water content of the whole section of the material is below the fiber saturation point, it is dried at a higher temperature. The thicker the material, the final water content The lower the rate requirement, the higher the temperature in the later stage. 5Omm thick sheet, the late temperature is generally 68-70 degrees; the sheet above 6Omm, the final temperature of 73-75 degrees, can be dry, but this temperature can not last for a long time, to maintain 2 ~ 3d is appropriate.
2.3.2 Selection of gas circulation speed in the furnace Difficult to dry hard wood is not suitable for drying with a strong air circulation. Stronger wet and heat exchange will cause a lot of cracks in the wood, resulting in material degradation. scrapped. Therefore, for a short-shaft forced circulation drying furnace, it is preferable to use a variable-speed axial flow fan to adjust the air flow speed. For the centrifugal forced circulation drying furnace, a method of placing the reversing plate of the upper air passage at different positions may be used. Deliberately destroy the airflow speed to achieve the purpose of reducing the speed of heat and heat exchange.
2.3.3 Maintain high relative humidity to maintain high relative humidity in the furnace, which can be adjusted in two aspects: First, multi-spray, we usually spray once every 5-6d, time is sprayed every 1cm Steaming for 1h, maintaining 1h meter; second, stopping heat preservation, the purpose of this is: because after a few hours of drying, the moisture content gradient inside and outside the material is large, the humidity in the furnace is low, and there is no condensed water on the furnace wall. In order to delay the wet heat exchange on the surface of the material, the fan is stopped for several hours, so that the deep water of the material slowly flows out, moistens the surface of the material and improves the relative humidity in the furnace. Open and close intervals (but the drying temperature is still low to high according to the reference), which can effectively prevent cracking and warpage. The interval time varies depending on the tree species and thickness. Generally speaking, the harder the hardness, the more material the thin-walled cells and the rays, the longer the heat preservation time is, such as Shennongjia eucalyptus, red sandalwood, Patty, Qinggangdong, etc. Difficult to dry materials, the holding time and the drying time is basically 1:1o
2.3.4 Handling of special conditions In the case that the boiler stops supplying steam for some reason, the equipment stops running due to failure, etc., and the drying interruption time is long (usually 8~10h), when the drying furnace is re-run, it is best not to start and dry immediately. Because the furnace is shut down for a long time, the temperature inside the furnace is far from the reference temperature, and the moisture inside the material has basically stopped evaporating outward, and the relative humidity in the furnace is low. If you turn it on at this time, observe the thermometer reading, only the dry bulb temperature rises rapidly, the dry and wet bulb readings are getting bigger and bigger. When the dry bulb reading no longer changes, the wet bulb reading will slowly follow up. After 2-3h, the relative humidity tends to be stable. The reason is that the temperature inside the furnace rises too fast, and the inside of the material is not yet heat-permeable, so the internal moisture does not start to evaporate and move, and the moisture in the furnace is quickly blown away. The relative humidity drops rapidly in a short time, and the internal crack of the material is inevitable.
The correct treatment method is: open the steaming tube at the same time as the fan is turned on, so that the temperature and humidity rise synchronously. When the temperature rises to the reference temperature, the fan and the steaming tube are turned off, and the reference temperature is maintained for 2 hours, so that the surface of the material is sufficient. Moist, the interior is heated, the moisture begins to move outwards, and then turns on and dry.
This method is only suitable for short-term drying interruptions. If the gas-stopping time is long, it is best to treat it by pre-heating.
The drying condition in the furnace should be observed frequently, but the furnace door should not be opened normally. It is easy to make the relative humidity and temperature in the furnace drop suddenly, causing drying defects. In general, it is advisable to open the door for 2 to 3 days.
2.3.5 Evaluation of drying time The drying time of hardwoods will vary greatly depending on the material thickness, origin, and species. For example, materials with a thickness of 5Omm and a moisture content of 40% or more are used. The materials of ash, pineapple, boiled beech, eucalyptus, and eucalyptus in the northeast can be baked after drying for 2Od, while mahogany and croton. Materials such as Batty, Southern Elm, Qinggangdong, and Oak need 28 days to be baked; if the material thickness is between 60-80mm, it takes 35-40d, or even longer.
2.3.6 The final treatment and storage of the finished material is usually 8h, and the temperature is close to or slightly higher than the temperature. In order to reduce the deformation of the finished product, the material after the discharge is preferably placed in a warehouse or work shed without direct sunlight for 14 days in order to eliminate internal stress and stabilize the moisture content gradient inside and outside the material.
Common drying defects include end cracking, cracking, honeycomb cracking, warping, deformation, and shrinkage.
1.2 The main cause of wood drying defects
The occurrence of drying defects is caused by the unreasonable combination of many drying factors in the furnace. There are mainly the following factors:
a, the sealing property of the furnace body b, the circulation speed of the gas flow in the furnace c, the medium temperature d, the relative temperature
2. Countermeasures
There are no absolute guidelines for dry benchmarks that can be referenced. The furnace type is different, the performance of the furnace body is different, and the drying standards are not the same. Simultaneously. The drying standard is relatively stable, but the drying process is dynamic. Therefore, it is necessary to accumulate and summarize, carefully observe and compare.
The guiding ideology for determining the drying process is: to ensure that the surface moisture of the material evaporates into the air and the speed of the deep moisture movement of the material to the surface is basically balanced, and the drying speed is increased as much as possible to obtain the desired economic benefits under the premise of ensuring the dry quality. And social benefits.
In order to obtain the dry quality of the customer's satisfaction, it is necessary to understand the interaction of the drying factor in the drying process, and comprehensively analyze and judge the parameters in the drying process and the actual conditions of the material in the furnace to make a correct decision. Ensure that the entire drying process is always under control.
2.1 Stacking of materials
First of all, it is necessary to ensure that the wood properties in the same furnace are the same or similar, and the thickness difference is not large; the thickness of the spacers should vary depending on the thickness of the material. Generally, materials with a thickness of 40 mm or less can be separated by 25 mm thick spacers. If the circulation speed of the gas in the furnace is slow and the final moisture content of the material is low, it is necessary to use a thick spacer of more than 3Omm in order to meet the requirements in quality and speed. For materials with a thickness of more than 5Omm, a spacer of 3Omm thickness is generally used to ensure a reasonable drying speed and uniformity of drying. The spacing between the spacers should be reasonable, the spacers of each layer should be arranged neatly in height, and the materials placed in the same layer should have the same thickness.
2.2 Preheat treatment
The pre-heat treatment time should be determined according to the tree species, thickness and moisture content of the wood. The material with a thickness of 5Omm is taken as an example. Under normal circumstances, the initial spray is 12-15h. In order to obtain a reasonable temperature gradient, the cooked material also needs to be treated for 6-8 hours. For Cyclobalanopsis glauca and Southern Elm. These two materials are very thin cells, ray parenchyma cells, mahogany, batti, gambourine, etc., which are usually densely sprayed for 24 hours to completely soften the wood, dredge the cell cavity, and partially dissolve the cell cavity. Filling in preparation for the subsequent drying process.
The initial spray temperature should not be too low, otherwise the steam flow into the furnace is too small, so that the relative humidity in the furnace reaches the benchmark requirement, and the effect of preheat treatment is not good. Generally, the initial treatment temperature is 95-10O degrees.
Whether the fan is continuously activated during the initial processing depends on the tightness of the furnace body. If the furnace body has good sealing performance, it can be continuously started after 3-4 hours of initial spraying; if the sealing performance of the furnace body is poor, continuous operation for a long time will increase the temperature inside the furnace and lower the humidity, which will lead to drying defects during the initial treatment. produce. In this case. Generally, it is turned on once every 2~3h. The steam is evenly dispersed on the surface of the material. A better treatment effect can be obtained.
After the initial treatment is completed, it is not allowed to enter the drying stage immediately. The temperature should be kept for 4~5h. When the moisture inside the material begins to vaporize and move, the surface of the material will absorb moisture sufficiently and uniformly, resulting in the correct temperature gradient and water content gradient before starting to dry.
2.3 Drying process
Some people think that wood drying is not technical, to ensure that the quality of the wood is difficult to dry, as long as the temperature is low, this statement is extremely wrong. Thick wood with a low temperature of more than 5Omm is too long and difficult. Secondly, if the low temperature is not combined with the suitable humidity, it is easy to crack. This is the same as the cracked wood, even if it is air-dried in a cool place, the degree of cracking is far more serious than the drying of the oven using a reasonable drying method. reason. Therefore, the drying process should be carried out according to the column mode: high and low temperature combined, the relative humidity wavewise changes slowly from high to low, and the circulation speed of the airflow is selected according to the severity of cracking of the material.
2.3.1 Temperature selection Drying starts with a lower temperature, generally 50-55 degrees is appropriate, the whole drying process needs to be divided into 6-8 temperature segments, the temperature difference of each temperature zone is about 3 degrees, when the whole material When the water content of the section is above the fiber saturation point, the temperature in the furnace is preferably below 60 °C. When the water content of the whole section of the material is below the fiber saturation point, it is dried at a higher temperature. The thicker the material, the final water content The lower the rate requirement, the higher the temperature in the later stage. 5Omm thick sheet, the late temperature is generally 68-70 degrees; the sheet above 6Omm, the final temperature of 73-75 degrees, can be dry, but this temperature can not last for a long time, to maintain 2 ~ 3d is appropriate.
2.3.2 Selection of gas circulation speed in the furnace Difficult to dry hard wood is not suitable for drying with a strong air circulation. Stronger wet and heat exchange will cause a lot of cracks in the wood, resulting in material degradation. scrapped. Therefore, for a short-shaft forced circulation drying furnace, it is preferable to use a variable-speed axial flow fan to adjust the air flow speed. For the centrifugal forced circulation drying furnace, a method of placing the reversing plate of the upper air passage at different positions may be used. Deliberately destroy the airflow speed to achieve the purpose of reducing the speed of heat and heat exchange.
2.3.3 Maintain high relative humidity to maintain high relative humidity in the furnace, which can be adjusted in two aspects: First, multi-spray, we usually spray once every 5-6d, time is sprayed every 1cm Steaming for 1h, maintaining 1h meter; second, stopping heat preservation, the purpose of this is: because after a few hours of drying, the moisture content gradient inside and outside the material is large, the humidity in the furnace is low, and there is no condensed water on the furnace wall. In order to delay the wet heat exchange on the surface of the material, the fan is stopped for several hours, so that the deep water of the material slowly flows out, moistens the surface of the material and improves the relative humidity in the furnace. Open and close intervals (but the drying temperature is still low to high according to the reference), which can effectively prevent cracking and warpage. The interval time varies depending on the tree species and thickness. Generally speaking, the harder the hardness, the more material the thin-walled cells and the rays, the longer the heat preservation time is, such as Shennongjia eucalyptus, red sandalwood, Patty, Qinggangdong, etc. Difficult to dry materials, the holding time and the drying time is basically 1:1o
2.3.4 Handling of special conditions In the case that the boiler stops supplying steam for some reason, the equipment stops running due to failure, etc., and the drying interruption time is long (usually 8~10h), when the drying furnace is re-run, it is best not to start and dry immediately. Because the furnace is shut down for a long time, the temperature inside the furnace is far from the reference temperature, and the moisture inside the material has basically stopped evaporating outward, and the relative humidity in the furnace is low. If you turn it on at this time, observe the thermometer reading, only the dry bulb temperature rises rapidly, the dry and wet bulb readings are getting bigger and bigger. When the dry bulb reading no longer changes, the wet bulb reading will slowly follow up. After 2-3h, the relative humidity tends to be stable. The reason is that the temperature inside the furnace rises too fast, and the inside of the material is not yet heat-permeable, so the internal moisture does not start to evaporate and move, and the moisture in the furnace is quickly blown away. The relative humidity drops rapidly in a short time, and the internal crack of the material is inevitable.
The correct treatment method is: open the steaming tube at the same time as the fan is turned on, so that the temperature and humidity rise synchronously. When the temperature rises to the reference temperature, the fan and the steaming tube are turned off, and the reference temperature is maintained for 2 hours, so that the surface of the material is sufficient. Moist, the interior is heated, the moisture begins to move outwards, and then turns on and dry.
This method is only suitable for short-term drying interruptions. If the gas-stopping time is long, it is best to treat it by pre-heating.
The drying condition in the furnace should be observed frequently, but the furnace door should not be opened normally. It is easy to make the relative humidity and temperature in the furnace drop suddenly, causing drying defects. In general, it is advisable to open the door for 2 to 3 days.
2.3.5 Evaluation of drying time The drying time of hardwoods will vary greatly depending on the material thickness, origin, and species. For example, materials with a thickness of 5Omm and a moisture content of 40% or more are used. The materials of ash, pineapple, boiled beech, eucalyptus, and eucalyptus in the northeast can be baked after drying for 2Od, while mahogany and croton. Materials such as Batty, Southern Elm, Qinggangdong, and Oak need 28 days to be baked; if the material thickness is between 60-80mm, it takes 35-40d, or even longer.
2.3.6 The final treatment and storage of the finished material is usually 8h, and the temperature is close to or slightly higher than the temperature. In order to reduce the deformation of the finished product, the material after the discharge is preferably placed in a warehouse or work shed without direct sunlight for 14 days in order to eliminate internal stress and stabilize the moisture content gradient inside and outside the material.
Accept Small order!!
We are professional manufacturer for electronic Stationery since 1993, Providing one-stop solution from design, R&D, Quality control, mass production, Export etc., our key partner are staples, Essetle, Acco and so on, Technology and production is best for you. License: ISO9001,ISO14001,Reach, Rohs, CE,UL,FCC,SEDEX.
Our products including Manicure Tool,Manicure Set,Manicure Pedicure Set.Used carbon steel and stainless steel for the material, Polishing technology and electroplating technology
Manicure And Pedicure Set,Electric Manicure Pedicure Set Kit,Manicure Tool,Manicure Set,Manicure Pedicure Set
Ringsun Holding Ltd , http://www.ele-office.com