In the before passages, i make it clear that machine drying system is a better choice for wood drying, It is easier to realize easy fast and high quality wood drying because the new technology adopted.
The machine drying system is always based on vacuum drying technology, because there are a lot of benefits for this technology and it is relatively easy to accomplish. The machine drying system is always with a closed chamber, with the help of vacuum pumps,a low pressure environment is created. It is easy but means a lot, the low boiling point of water allows the moisture in the wet wood vaporizes fast, a low temperature drying ensures that there will be slight defect in color and strength, what is more the pressure gap will suck the water in the wood not which contribute to the drying speed.
So vacuum drying is quite popular in this field now, but only vacuum drying is not enough, we still need a way to offer the heat, that is also important
Together with my team, i collect the existing machine drying systems, and make some tests. The main aspects are in the work principles, the control methods , the energy consumption , the drying speed, and the drying defects.
Now there are mainly three ways to heat the wood in the industry, one is steam or hot air, another is the heating plates and the last one is high frequency technology.
Work Principles
The work principles of these three kinds of methods are totally different
Hot air: this is a very traditional way, it is just very similar like what the kiln drying does, heating the air and faster the air circulation inside the chamber, then the heat will pass from the surface to the core, but unequal stresses are set up during the process, with the outer portion in compression and the inner tension, so it requires skilled labor to control the stack method and the stickers and due to the process of drying is quick, continuous attention needs to be given to check seasoning defects.
Heating plates: the drying process is very much like heating your breakfast bread, but wood is not bread, the same problem exist for heat transfer process, it is easy to see that it needs a lot of space to put the heating plates, it could not handle the thick wood, and for some hard wood there will be a serious defects.
High frequency:High frequency is also called radio frequency. It’s heating principle is same with microwave, but wavelength is different which allows better penetration. High frequency generator generates electromagnetic fields between two electrodes through three phase alternating current. In this field, water molecules are polarized – rotated – heated by friction. It dry wood from core to wood surface wholly and uniformly from the ever beginning, even thick wood can also be well heated, no stress is accumulated.
So in the aspect of working principles, the high frequency get one point.
Control Methods
The machine drying system always integrated the control system and sensors which could collect the situation like the temperature in the chamber, so what we compare is to the way to change the heating process, and how to realize it, good news is almost all the machine drying system is powered by electricity, so it is relatively easy.
Hot air: the way to control the process is to control the heater (like the boiler) and the fans, the reaction speed is gradually and slow, you could balance the temperature and drying speed by them, a little complex for a new comer.
Heating plates: Change the power in could control the temperature and the drying speed, but there will be a certain lag in the operation and changes, because it needs time to cool the oil inside the heating plates
High frequency: because the change of electricity will directly affects the heating process, so it is faster and more accurate.
To conclude, for the control methods, the high frequency is better than the other two methods.
Energy Consumption
As confirmed above, most machine drying system is powered by electricity, so the most direct results will be the electricity consumption of each kind drying machine to dry per CBM wood
Hot Air: According to the test, For the 2’’ oak slab, to dry it from 50% MC to 20% MC the consumption is 108 KW.h. which means to remove 1% MC in one CBM wood you need about 3.6 KW.h electricity, and further to 8% MC as a finish MC, the average consumption for each CBM will be about 3 times higher than before, the consumption is 136KW.h, which means the need to remove 1% MC will be 11.3 KW.h, So the total cost is 240KW.h for this operation
Heating Plates: Regarding this drying technology we are testing by the latest Cyclic drying process control method,which means heating for 2 hrs and release about 0.5-1hrs, in this way the dealing is more efficient and with less defects, this kind of drying method is with the fastest drying speed, and it has a more strict request of the power supply equipment because the high electricity need in a short time. The final test results are 100-120KW.h for per CBM wood to reach 8% MC.
High frequency:In this testing we uses the Shuowei high frequency vacuum wood dryer, it is a mature product in this field and have some good designs, this machine have a semi-auto design that , the power will on and off according to the oven temperature automatically, and it applied a insulation layer,The lab test shows that using SW machine to dry Oak from 50% to 8% MC , per CBM wood will cost 80-120 KW.h So after all, the hot air lost this compare because the high electricity consumption
Drying Speed
Drying fast is a obvious advantage for the machine drying system, so this is also a very important data for machine drying system
Hot air: the drying speed is directly related to the wood thickness, if with standard operation which is suggestd to ensure the drying quality, for 2 inches hardwood boards it will take about 14 days to dry from 50% to 8% MC, and as the machine introduction says one more week for each inch increase.
Heating Plates: this kind of drying is direct and fast but we need control the temperature to make sure there are not serious defects, even so, for 2 inches hardwood board, the time you need is within 3 days, and if needed it could dry wood within 3 hours, but it is not suggested to to so, and there will be a limit for the thickness and wood species
High frequency: It is easy to see, this kind of drying is suitable for almost all the wood species, and the combination of high frequency and vacuum wood drying doubled the drying speed, for 2 inches hardwood boards, it takes about 5 days to reach the target MC. So the heating plates is the fastest but with limits about the thickness, the drying speed test High frequency performance better comprehensively.
Drying Defects
There will be some changes between the dried wood and the green wood, and we mainly pay attention on the following aspects, that is the color, the deformation, the cracks.
Hot air: it is very similar to the conventional kiln drying, the drying quality could be well controlled with the help of skilled worker, but it is not stable due to the man forced operation
Heating Plates: this is the most disadvantages of this kind of drying, what need to clarify again is that, for the heat plates pressing drying process, it is not suggested in some wood species, this is better for some thin boards and soft wood, it could prevent bending but cause some deformation, especially in the thickness shrink. And as it is proved in many tests by team from different countries, high temperature will be the key reason of discolor, for this kind of seasoning, you will have to put the surface of wood together with a heating plates, there will be a visible change for the color
High frequency: the main causes of the defects are carefully dealt with like the temperature and stress, so rare defects there in the combination of these two technology.
So in terms of defects, we need pay more attentions on the heating plates, and if no skilled staff, high frequency will be a better choice.
To sum up, my team has referred lots of data and do many tests to reach such a conclusion, high frequency drying and vacuum wood drying is the most ideal combination for the wood drying technology, that is also why we have been devoted in this field for so many years, and finally developed Shuowei Series high frequency vacuum wood dryer, we will be more than happy if any messages or comments from you.
