Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1987-10-06
1990-01-16
Bennet, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 60, 34 77, 34 92, F28B 504
Patent
active
048934151
DESCRIPTION:
BRIEF SUMMARY
The application relates to a method for the removal of moisture from solid wood and other wood-based products, where the wood is first stacked with suitable air space in a drying chamber which is arranged to be able to withstand a vacuum and temperatures of up to 100.degree. C, after which the drying chamber is closed and evacuated during the application of heat for the boiling out of the moisture bound in the wood.
When felled, wood contains large amounts of water, and in most applications of wood it is necessary for the main part of this water to be removed from the wood before the wood is taken into use.
During the drying of wood, a number of special conditions manifest themselves: adjust itself to a wood dampness (moisture balance) which depends on the temperature and the humidity of the surroundings. the amount of hygroscopic water which is removed from the wood. which exceed 40-90.degree. C. At higher temperatures, most kinds of wood will be subject to damage in the form of discoloration and a reduction in the strength of the wood. traditional drying, the consumption of energy is thus between 2,000,000 and 6,000,000 kJ per m.sup.3 of dried wood.
Traditionally, the drying of wood is carried out by placing the wood to be dried in the open air (air drying), or by drying in special drying chambers where hot air is circulated around the wood (artificial drying). However, both of these methods are very slow. When drying in the open air, a drying time from a few months up to several years must be expected, while the drying time with artificial drying is from a week up to a few months.
In order to avoid damage to the wood during the drying, it is important that the shrinkage during the drying is uniform, and this is achieved by controlling the moisture balance dictated by the temperature and the humidity, so that the moisture in the surface of the wood does not at any time during the drying deviate too much from the moisture within the wood.
When air drying, the possibilities of controlling the drying process are very small, whereas artificial drying provides the possibility of reasonable control of the drying process. This is effected by regulating the humidity and the temperature of the air which is circulated around the wood during the drying. The humidity is controlled by means of a hygrometer or a dry- and wet-thermometer in the application of steam or dry air to the air around the wood.
To increase the speed at which the water is removed from the wood, for many years it has been normal practice to dry certain kinds of wood at temperatures in excess of 100.degree. C (FR patent publication No. 1,023,606). At temperatures above 100.degree. C (high-temperature drying), the water in the wood will be boiled out of the wood.
However, most kinds of wood cannot tolerate temperatures of more than 40-90.degree. C, and in order to increase the speed at which these kinds of wood can be dried, it has been suggested from several quarters that the drying should be effected under vacuum. Under vacuum, the boiling point of the water is reduced, and thus the water will be able to be boiled out of the wood, even though the temperature does not exceed 40-90.degree. C.
Traditionally, one distinguishes between two forms of vacuum drying: Discontinuous and continuous vacuum drying.
With the discontinuous vacuum drying, alternate changes are made during the drying between a heating phase and an evacuation phase. During the heating phase, the wood is heated by circulating hot air in the drying chamber (DK Pat. publication No. 44,287, US Pat. publication No. 4,058,906, U.S. Pat. publication No. 4,176,466, U.S. Pat. publication No. 4,194,296 and U.S. Pat. publication No. 4,198,763) or by high -frequency (dielectric heating) (U.S. Pat. publication No. 3,685,959). The heat which is applied during the heating phase is utilized during the evacuation phase for evaporation of water from the wood by establishing a vacuum in the drying chamber. During the evacuation phase, no heat is applied to the wood, and the wood will gradually cool
REFERENCES:
patent: 1328657 (1919-11-01), Fish
patent: 4127946 (1978-12-01), Buchholz
patent: 4467532 (1984-08-01), Drake
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