The present technology of vacuum drying has its origins in 1962. Recently, it has received an interest that has grown in both research and industry. A rise in the use of vacuum drying is predicted soon, specifically across Europe in Europe and Asia.
In vacuum drying, the lumber is put in a tightly sealed drying chamber, and then the vacuum system draws a vacuum across the wood. The wood is theoretically dried through the evaporation of moisture at low temperatures of boiling (usually around 40 C). In cyclic vacuum drying the lumber is heated in a series of intervals by convection of hot air. When the wood has reached the temperature required the vacuum is pulled across the lumber. The drying continues until the drying rate is minimal after which the process repeats.
Numerous studies have been done on vacuum drying. In the end, it was discovered that vacuum drying greatly reduces drying times, particularly for heavy lumber and the drying performance generally is excellent. Vacuum drying is typically employed for species with high value as well as for lumber with large dimensions. Yet, there isn’t much research that has been completed on the practical aspect of the vacuum drying process. The absence of information on technical aspects and operating procedures has slowed the application of this drying technique.
Hypothesis
The principle of vacuum drying wood is based on the reality that water’s boiling point gets significantly reduced when the atmospheric pressure above it decreases. As the atmospheric pressure decreases and thereby creating a pressure gradient within the wood begins to develop. As the temperature of the wood increases or surpasses that of the water’s boiling point, the vapor is rapidly produced, as well as hydrodynamic bulk flow is resulting due to this pressure gradient. In this situation, wood’s permeability could be the primary factor governing water movement.
Two hypotheses are:
- If permeability is a significant element in permeability, the anisotropy should show that the moisture transfer in the direction of the longitudinal is much greater than that transferred in the transverse direction.
- If the boiling front (transition area that separates boiling and non-boiling areas) is present in the process of vacuum drying, then it is between the front and the top, the pressure difference is the main driving force. The water that is transferred from the region of boiling is transferred into steam by WVBF and Darcy’s Law applies. From the front of the boiling to the middle, the drying process is similar to that used in traditional kiln drying.
Objectives
Objectives
The ultimate purpose is to understand the mechanisms of vacuum drying. An in-depth understanding of the mechanism of the flow of moisture through vacuum drying may result in a way to control and monitor the rate of drying in order to enhance the efficiency of the vacuum drying process. The goals of this research are to demonstrate that, using the measurement of temperatures and pressures in various places within the lumber, and the permeability assumptions.
- A total pressure differential is the main driver that determines the speed of drying by vacuum.
- Moisture is removed mostly in the direction of longitudinal movement and the moisture is removed into steam form.
- Boiling takes place within the wood. It is evident that there’s a boiling area and the boiling front is moved towards the center of the wood as the drying process proceeds.
Mechanism of Moisture Movement
The Energy State of Moisture in Wood
Potential and Kinetic energy are the two main types of energy. The kinetic energy is related to velocity multiplied by. Since the flow of water within the wood is very slow, the potential for kinetic energy is typically very low (Siau 1984). However, the energy potential, related to the location or internal conditions, is crucial in determining the condition and the movement of water within the wood. Water flows continuously towards a decreasing energy potential.
The decrease in potential energy as distance increases is the force moving that creates moisture flow.
Free Water Bulk flow (Movement of Liquid Water)
The wood is flooded with moisture as bound water in the cell wall capillary water in liquid form, and as water as gas within the voids in the wood. Capillary water bulk flow is a reference to the movement of liquid through the interconnected voids and across the top of a hard surface because of molecules of attraction that bind the liquid to the. In wood that is saturated, it has equilibrium with the capillary water at the same altitude.
The pressure actually is atmospheric, and suction is unaffected. Wood that is untreated is subject to suction or subatmospheric pressure which is the equivalent of the negative potential of pressure. The loss of water can occur up to a certain threshold when the largest pore starts to be empty. The critical section is known as the suction for air entry within soil sciences.
As suction increases and gets stronger, more water will be pulled from the wood. The large pores that are unable to hold the water against suction will be emptied out. The gradual increase in suction will cause the drainage of smaller pores.
Water Vapor Bulk Flow
In temperatures that are near or above that of the water’s boiling point, the rapid production of vapor can result in significant total pressure gradients, in addition to partial pressure gradients for vapors. Water vapor is transported between low-pressure and high-pressure zones with a total pressure differential.
This is like free bulk water flow. Gas permeability is the main consideration in the bulk flow of water vapor. Even though water vapor’s density may be minimal, however, its volume-flow rate can be high in certain pressure ranges. A substantial amount of moisture can be transferred through the bulk flow of water vapor. When drying in a vacuum, total pressure fluctuations within wood trigger bulk flow of water vapor. There is a constant water vapor flow in the boiling region as a result of the process of water evaporation.
Diffusion
Contrary to the free flow of water caused by Capillary Forces (matric potential) diffusion involves the process of transferring material through the basically random movement of individual molecules in response in response to gradients of concentration. Small molecules, like water, are able to move across cell walls even if the wood is inaccessible and there is no pressure gradient.
The main reason to dry timber at greater temperatures is the fact that diffusion rates increase as the temperature increases. Diffusion plays a significant role in the drying process of lumber, regardless of the moisture content in impermeable timbers and also in permeable timber where there is a moisture level insufficient to allow the flow of hydrodynamic liquid through the lumens.
Vacuum Drying
In a vacuum drying process, the lumber is put in a sealed drying chamber. The system that is used for vacuum drying draws an air vacuum over the lumber to ensure that the water inside the lumber is at a boil before it is pulled out of the wood. Vacuum drying actually relies on the idea that the water’s boiling point decreases significantly as the atmospheric pressure on the wood is decreased.
History of the Vacuum Drying
The development of vacuum-drying wood dates back to the 1920s. A patent for the vacuum drying method for wood was issued in 1922 in Sweden. In 1962 the first commercial vacuum dryer made using the cyclic technique was constructed.
The year 1964 saw the first smaller dryer constructed using electrical resistance plates operating under a continuous vacuum. The dryer was developed in 1975. equipped with heated hot air was invented for the very first time. First, radio-frequency vacuum dryers were developed at the beginning of the 1970s.
Types of Vacuum Drying Systems
Vacuum drying systems are classified according to the method of transfer of heat to the wood into four kinds. Conduction is achieved through direct contact with the hot plate or an electric heating blanket, convection with steam that is superheated at high temperatures convection with hot air in cyclic systems as well as radio frequency dielectric heating. The cyclic vacuum drying system is a discontinuous drying process. The other systems are continuous.
In cycles of drying, the latent heat from evaporation is vital to dry. Once the latent heat generated by the residual heat stored in the wood is exhausted the drying ceases. There is no method to transfer heat to the wood under vacuum. If there is no vacuum, convection can be nearly not present, and radiant heat surrounding the wood is only able to warm the outside inside the stack.
In the process of radio-frequency vacuum drying, it is possible to place the wood dielectric positioned between metal electrodes or plates. When an RF electric charge occurs, these molecules alter their direction in a cyclic manner, resulting in a rapid movement of molecules. In the process, heat is produced. Radio-frequency-generated heat generates a large temperature within the wood in a brief amount of period of time.
Vacuum Drying Quality
The quality of vacuum-dried lumber is generally high. In general, less degradation occurs when drying with a vacuum than in traditional kiln drying. When drying with vacuum lumber, the color after drying, and is similar to the color it was before drying.
In addition, wood that is dried in a vacuum is lighter in color than dried lumber. Vacuum drying creates an extreme gradient in moisture that occurs on the surface of the sample due to the fact that it isn’t able to move enough humidity from inside to outside. The steep gradient in moisture develops even at the beginning of drying.
Property Change in Vacuum Drying
It has been noted that shrinkage in red oak that is dried by RFV is around 30 percent less than when dried using traditional kiln drying. In RFV, red oak was dry from the color of green to 5.5 percent MC and had the average shrinkage being 4.1 percent radially, and 7.5 percent tangentially. In contrast, for traditional drying, red oak was dried between green and 7.8 percent MC and had shrinkage of 5.6 percent and 10.3 percent in the tangential and radial directions, respectively.
The content of equilibrium (EMC) is considerably lower when drying in a vacuum than in traditional drying (Lee and Harris 1984). Taniguchi as well as Nishio (1991) stated that the lumber dried using RFV was found to have an EMC of 0.7 to 1% less than that dried with traditional kiln drying.
Conclusion
If you want to get good results in drying your wood or lumber, the best way to get it is by using a high-frequency vacuum machine. All of the professionals do their best to achieve the best wood quality. If you have any questions about the vacuum drying machine you can always contact us and we will discuss to you further details about this vacuum dryer.
