– Compare of Air drying Kiln Drying and machine drying
Wood drying is a critical process and it directly decides the wood values, nowadays as the latest technology applied has greatly improved the drying process, there are now mainly three kinds of drying, namely Air Drying, Kiln Drying and Machine Drying, What are them and which drying methods should i choose, when people make up their minds to do the drying work, this is the question comes first, Just i have already give some ideas about the definitions of these three different generations of wood drying before, today i will show you in another way, that is the compares of these three drying methods
1.Cost Of The Drying
Cost always play a very important role in the business, after all we are creating value by less cost and better output, so the first compare will be Cost of the drying, I will list all the possible cost of drying here and give a conclusion
Cost of equipment and Installation
Air Drying: the main Equipment you need is the devices help you to stack the wood well, the stack sticks to keep the wood spaces, and of course you need a place to put your wood according to the species and sizes. The cost will depend on the land. It takes you thousands USD.
Kiln Dying: Besides the cost listed in air drying, you need build a room or workshop to shield the wood, fans to accelerate air running, boilers to heat the air, and devices to control the wetness of air, because drying too fast may cast the large stress result in cracks. The cost include the need of build the house, the land and the devices. It takes tens of thousands USD.
Machine Drying: The cost to install a machine drying system is mainly the cost the machine itself, because the modular design and pure electricity drive, it take little time and space. The price of the machine is tens of thousands USD.
So in case of the initial cost, Air drying is the best
Cost of drying Per CBM wood
Air Drying:the energy needed to dry the wood will be from the nature, mainly solar energy and the wind, but this process is hard to control
Kiln Drying: According to the test if purely powered by electricity the cost of drying per CBM wood for the kiln drying is about 160-240KWH, the detailed consumption will also closely related to the drying circle, but since the main drying method in the kiln drying is hot steam, boiler could also be used there to make use if the leftovers，so the cost appear lower than what it is.
Machine Drying: Due to the control system and the sealed oven, the machine drying system could make the best use of the power, take SW-4.0 From Shuowei Company as an examples, although the drying time is short,usually 4-10days to dry from green wood to 8% MC, the electricity needed is only 80-120 KWH, anyway there is still a shortcoming that is the machine drying could only uses electricity and has a high request of the power system.
To conclude, taken time and power cost into consideration, Machine system is the most efficient in the production process
Cost of Manpower
Air Drying: What you need do is just stack them, and since it is really a long period, you are not requested to finish the work in a short time, it is suitable for the small capacity producers, one or two people is enough.
Kiln Drying: There is a strict control of the drying process by the kiln, Many different factors, like the sticks and the wood directions, may cause irreversible quality problems, Anyone in this field could clear realize this point, you need put at least a team there to make the best use of the kilns and make everything under control, so the cost of manpower is high, normally for a 100 CBM Kiln you need 5-8 people there.
Machine Drying：There could be a automatic feed timber cart to help you load the wood, and since the main heating principle is high frequency, so no stickers and directions request are needed, the whole process is operated with the control unit, so for this kind of drying, one or two people is enough .
In conclusion, Air drying and Machine drying system have little request of manpower, but skilled workers are a basic need for the kiln drying, it also a main cost of the kiln drying process.
Cost of maintenance
For Air Drying there is no such need, because rare equipment is there, but for both Kiln drying and machine drying, this is important, anyway, the design and materials are different for different manufacturers, the reliability varies too, it is better to buy from a direct manufacturer and check with the existing clients to get the real performance.
After all, cost is never an isolated issue and it should never be denied only the price of an equipment, other wise, there will be some problems you do not want to be faced with around the corner, What makes profits is cost, if you buy cheapest but can’t be used well, it is only waste. In full consideration and compare, machine drying system is better than kiln drying while air drying is the cheapest for beginners.
2.Technology And Principles
Drying principles
Air drying: The basic drying principles for drying wood is to take the moisture inside the wood by heat and wind, the Air drying methods used this way, Logs are left to dry in the open or with cover overhead for a period of time, which can last for 8-16 months depending on the species. As time pass by, the moisture content in the wood adjusts to the environment it is in, which is usually 12%-15% MC.
Kiln drying: the conventional kiln drying is to create a man-made environment in a house or room, with high temperature (steam), low moisture (dehumidification) and wind (fans). Anyway in the real operation you will find it is hard to dry uniformly if you dry the wood fast in this way, so people need add water in the rood sometimes to keep the drying speed of the surface and core the same.
Nowadays, benefited from the development of technology, people try to add solar energy, vacuum drying, computer control into the regular kilns for better efficiency, but it is hard to combine them together based on the traditional structures.
Machine drying: Such design is firstly found in 1980s, the basic technology is the vacuum drying by the pumps, and also as a heat recourse, there are various combinations in the machine designs to make the best use of it, like heat board drying, high (radio) frequency drying, Microwave drying. As a new drying method，the design is highly related to automation and informatization from the very beginning. A more successful example is Shuowei’s high frequency vacuum wood drying kiln, it combines these two kinds of latest wood drying technology and featured fast and high quality drying, especially expert at thick and hard-to-dry wood.
Control methods
Air drying: You could do little things if you choose to use this kind of drying method, and because the nature is changeable, there may be a higher moisture content in the surrounding if the it is rainy days.
Kiln drying: The best control is always coming with the most experienced workers, as talked above, you need keep an eye on the way the wood stacked, MC in the room, the boiler and fans, meanwhile strictly follow the regulations of kiln drying to make sure no defects there.
Machine drying:There are machine system helping you feed the wood, monitor the drying process and control the drying temperature, all you need do is turn on the machine and release the water inside wood, it is the most friendly way fro beginners
Components and spares
This part is mainly for the kiln drying and machine drying, we need add different systems into the drying equipment, but as mentioned, the traditional structure of drying kilns are not proper for such designs,because they are mainly houses, that is why even now the kiln drying are totally different as before conventional ones, now the kilns are always made of steels, but adding a motor to the bike will never makes it a car, there are still a big gap between kiln dying and machine drying system, the design of the machine drying are more reliable and it is much more tough for different work conditions.
Conclusion
In recent years, there is no new technology in the wood drying system and people are paying more attention to the control method and different combination of drying technology, for these trends, machine drying has Innate advantages.
3.Drying Result (Quality)
Drying time and Volume
Before this compare, i want to clarify why i do not say output directly, yeah output equals volume divided by time, but there is a big difference when running a business, the longer time the bigger cash pressure.
Air Drying: there is no limit of the volume as long as you have enough space, but the drying time is also as long as 8-16 months, anyway, even after a long time the water content will be just the same as the environment, which is 12-16%
Kiln Drying: Usually the volume of a kiln will be 100 CBM around or smaller, and the drying circle is 6-12 weeks.
Machine drying: Due to the limit of transportation and power request, the machine drying is usually designed with a smaller capacity like 10-20 CBM or even smaller, anyway the drying circle of machine drying could be 4-8 day
Moisture content
Now in the wood market, there are mainly two kinds of lumber, one is air dried, the other is kiln dried ,and the price of kiln dried wood is much higher, the main difference is the moisture content
Air drying: the principle and technology decides that there are litter human force in the drying, so the final MC is just to 12%-16%. Besides,during air-drying and for some wood species, resins continue to be liquid and runny, this will cause a serious defect
Kiln drying: Normally the MC could be controlled as low as 8% because the air could be dry in the kiln. And the high temperature dries off the resins in softwoods
Machine drying: The MC could be any target one as needed and resins are removed together.
Molding & Insects
Air drying:Air-drying doesn’t kill any insects, eggs, bug larvae, mold, and fungi in the wood, leaving it vulnerable to a potential infestation. And usually require chemical treatments to increase its lifespan.
Kiln drying: As kiln-drying process uses heat, it kills some of the insects eggs and mold.So, kiln-dried logs does not require a harsh chemical treatments and can last relatively longer
Machine drying: Both the vacuum and high temperature helps kill the insects,eggs larvae, mold and fungi, the wood and finish products benefit a lot, there is no need for any chemical treatments as the drying process effectively sanitizes it.
Strength and stress
Air drying: Air drying is a natural process, and this process doesn’t introduce any internal tensions in the wood. so, you can easily work on it using hand tools after the drying works. And air-dried doesn’t swell because the cells collapse and compress as the timber air dries.
Kiln drying: Kiln-drying can cause the wood to shrink. That, in turn, makes the wood fragile, breaking it off when using hand tools directly, after coming out of a kiln, the dried lumber needs to be kept in a controlled climate. When exposed, it starts absorbing moisture and may start swelling.
Machine drying: to solve this problem cause by high speed drying, the machine drying equipment also introduce a hydraulic pressure system to help keep the wood straight and during the process the internal tensions are released because the drying is evenly and from core to surface, which allows you to work on the dried wood directly.
Cracking and Swelling
Air drying: the wood is died outside in the environment, the drying process is slow and there will be s serious defect because the drying is unevenly, the wood are more easily to crack and big chances are that there will be swelling and bending.
Kiln drying: wood are dried in a closed chamber and the drying speed could be controlled by experienced workers, so the kiln dried wood is much more better with little cracking or swelling, but the fast drying also tend to cause stress in the wood as motioned so the wood are more likely to bend
Machine drying: the drying is in a sealed oven, the environment is vacuum and wet, what is more there is a pressure system, so there are no cracking swelling and bending at all, it is a most high quality drying way.
Color
Wood color is a critical attribute in some applications, particularly in high value-added products. Scientists research shows that discoloration increased with higher temperatures and longer drying time. if measure surface color change during drying of pine, you could find that drying temperature is significantly correlated with color change, with color change accelerating at temperatures higher than 60 °C
Air drying: the wood have a bad defect and there are some molds even
Kiln drying: the temperature will well pass 60°C which will cause discolor
Machine drying: with the help of vacuum drying system, the wood are drying with in a vacuum status, the vacuum pressure is -0.093 Mpa, under vacuum conditions, wood moisture can be boiled at 50-60 degree and vaporize rapidly, so the color could be well kept.
Pollution and Risks
Air drying: No such concerns
Kiln Drying: Just for the kilns drying with boilers, let alone you need a worker specially doing this job, you also need pay attentions to the fires, there is strict control of fire in a place where there are a lot of woods. And also the pollution produced by the combustion exhaust is a big concern
Machine drying: such equipment is purely powered by electricity, it is environment friendly, so no such concerns.
Applications
Air drying: Air dried lumber is preferred for outdoor patio furniture, fencing, decking and the pallet industry.
Kiln drying: Kiln-dried wood could be used for furniture, cabinets, flooring, and many other products.
Machine drying: Machine dried wood are with even better quality than kiln dried ones, and as it is less likely to cup and warp. This allows manufacturers to make a more durable product and some arts
4.Final Summary
Through these compare, what i want to stress is that new technology will always bring better work methods, if you do not follow the trend, you will be behind the ages, the wood drying is always developing! Machine drying are better drying methods in many aspects, i do hope people could know it more and benefit from it. For more machine drying system benefits , welcome your messages.
