Drying and gas or vapor contact with solids – Process – Gas or vapor pressure is subatmospheric
Reexamination Certificate
2002-06-28
2003-10-21
Esquivel, Denise L. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor pressure is subatmospheric
C034S092000, C034S519000, C034S497000
Reexamination Certificate
active
06634118
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wood drying techniques. More specifically, the present invention relates to a vacuum drying technique for wood where the wood is placed in a collapsible, flexible container.
BACKGROUND OF THE INVENTION
Drying freshly cut wood is an essential step in producing lumber usable for construction and furniture. In the conventional technique for drying wood, the wood is placed in a kiln, and dry, heated air is circulated around the wood, slowly evaporating the moisture. Care must be taken so that the wood is not dried too fast, as this causes checking (cracking) of the wood due to nonuniform shrinkage. Kiln drying of wood tends to be costly, wasteful of energy, and slow. A large amount of heat energy is wasted by loss to the atmosphere; drying time can be about 2 months for a 4×4 inch piece of wood, for example.
To address the problems of energy consumption and speed, vacuum drying and radiofrequency drying have been developed. In vacuum drying, the wood is disposed in a vacuum chamber. Vacuum increases the evaporation rate, and greatly reduces the time required for drying. However, heat must be applied to the wood while in vacuum, which is problematic because vacuum prevents convection or conduction of heat energy to the wood. To overcome the problem of heating wood in vacuum, cyclical vacuum drying can be employed, in which the wood is alternatively heated in air, and exposed to vacuum, which cools the wood by evaporation. Cyclical vacuum drying is problematic because it tends to be slow, although it is still faster than conventional kiln drying.
In another known method, wood is heated internally by applying radiofrequency (RF) energy to the wood while it is in vacuum. RF/vacuum drying is problematic because it requires expensive electronic equipment and tends to cause nonuniform drying. Also, RF/vacuum drying requires heating by electrical energy, which is a relatively expensive form of heat energy (e.g., compared to heat from burning coal, oil, or natural gas). Other drawbacks of conventional vacuum drying techniques include the need for a large, rigid vacuum chamber, which is very expensive. It would be an advance in the art of wood drying to provide a wood drying technique that is fast, is energy efficient, does not require expensive equipment (e.g., RF generator or vacuum chamber), and provides improved drying uniformity compared to conventional wood drying techniques.
SUMMARY
The present invention includes an apparatus for drying wood having a vacuum-tight container for holding a piece of wood. The container has a flexible wall. A bath is provided for immersing the container and wood. A vacuum pump is provided for evacuating the interior of the container. Vacuum causes the flexible wall to press against the wood, thereby facilitating heat conduction from the bath to the wood. The bath can be heated.
The container can comprise a bag made with flexible walls. The bath can comprise water, air or other liquid. Also, the present device can have a heater for heating the bath. The bath can have a temperature in the range of 50-200 degrees or 60-180 degrees Fahrenheit, and the vacuum can be in the range of 0-400 Torr or 1-100 Torr (absolute pressure), for example.
A method of the present invention includes disposing a piece of wood in a container having a flexible wall, evacuating the container so that the flexible wall presses against the wood, and immersing the container and wood in a heated bath. The heated bath heats the wood through the flexible wall. The bath temperature can be in the range of about 50-200 degrees Fahrenheit or 60-180 degrees Fahrenheit, for example. The vacuum applied to the wood can be in the range of about 0-400 Torr or 0-100 Torr, for example.
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patent: 4893415 (1990-01-01), Moldrup
patent: 5852880 (1998-12-01), Harrison
Chen Zhangjing
Lamb Fred M.
Esquivel Denise L.
O'Malley Kathryn S.
Virginia Tech Intellectual Properties Inc.
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