Vacuum Timber Dryer
Vacuum Drying is carried out in a closed vessel designed for 150mbar (negative) absolute pressure use. The chamber can be squarish but needs to be structurally reinforced. A cylindrical vessel is best for vacuum applications. Heating inside an 80% vacuum atmosphere is a challenge. The 20% of available air mass makes heat transfer difficult. Heat transfer depends on mass flow rate There is only 20% air mass left in the vessel under vacuum pressure of 200 mbar absolute. The airflow must be increased substantially to increase the mass flow for heat transfer to be effective.
Microwave heating and high frequency heating can heat up the wood very in a vaccum atmosphere very quickly without air-circulation fans. The principle of drying under vacuum is the same. Tritherm had been involved in the research and development of such heating system for vacuum drying of timber.
A Microwave Vacuum Dryer developed by Tritherm that can dry 60mm Green Heart timber in 45 hours from 49% to 10% moisture content. The drying results were excellent, but the drying capacity makes it economically not viable.The vessel having a gross volume of 20 m3 could only load in about 4m3 of timber. The stacking of the timber in a 2.4GHz micowave dryer has to be very well spaced to allow the 2.4 Ghz micowaves with short wave-length to bounce of the cylindrical walls of the vessel and penetrade every piece of wood with minimum blockage.
Image on the left: The very first High Frequency Heated Vacuum Dryer developed jointly by Jiyaun and Tritherm in 2008.
The HF heated vacuum dryer dries well provided the system has a 50-Ω matchboxes that can automatically tune the frequency with a crystal oscillator to amplify the required power in several stages (Jones 1996). In the first stage, solid-state devices bring the power up to a few kilowatts and then thermionic valve circuits can boost it further. The use of matchboxes assures balanced impedance. for higher energy conversion efficiency.
The main purpose of having reduced pressure is to lower the boiling point of water. If the wood core temperature is heated 60°C at a vacuum pressure of 200mbar absolute, the water in the wood core will start to boil and will become vapor. Vapor movement to the wood surface is faster than water migration by diffusion. Evaporation takes place at the wood face and causes a cooling effect on the surface. Unlike conventional drying, where the wood is hotter on the outside and cooler on the inside and causes the wood surface to dry quickly. This causes much stress that results in the wood twisting, bending, cracking and even case hardening.
For vacuum drying, the temperature gradient is reversed. The drying stresses are low. The wood undergoes very gentle and fast drying with virtually no drying defects. Thick timber can be dried quickly without defects. Drying time can be 3 to 5 times faster than conventional drying.
Hot water is circulated in the finned tube heat excahnger placed inside the vessel. Water at 80°C is suitable for vacuum drying as the wood needs to be heated up to slightly above 60°C.
When the circulating air becomes saturated the vessel losses vacuum, the vacuum will be operated to scuk out the wet vapor. However, the vapor needs to be cooled so that the steam can be condensed. This is normally done outside the vessel in a cooling heat exchanger when the air is being sucked out by the vacuum pump. The mixture of liquid and air are then separated inside the separator tank. The cool dry air is then sucked out by the vacuum pump and the water at the bottom of the tank is automatically discharged periodically. Air needs to be sucked out to maintain the vacuum in the vessel because water turning into vapour will increase the pressure. Below is a picture of a Vacuum Pump Station where the warm wood moiture vapor is being cooled and separated so that the vaccum pump sucks only dry cooled vapor while the water at the bottom is drained outsire periodically.