Abstract:
An apparatus and method for generating water vapor or humidity, useful in a variety of applications, including a wood drying kiln. The apparatus may contain a hydrophilic disk, a water supply structure that supplies water so that the water is deposited on the hydrophilic disk, and a motor for rotating the disk at a speed sufficiently high so that water which is deposited on the disk is caused to leave the disk in the form of a mist of particles or droplets which are sufficiently small to evaporate rapidly to generate the water vapor. The disk may be made hydrophilic by covering with a fabric.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a system and a method used for generating water vapor so as to provide a controlled humidity environment. More particularly, it relates to an apparatus and method which may have, among other applications, the control of humidity in a wood drying kiln.  
           [0003]    2. Prior Art  
           [0004]    Lumber drying kilns put 20,000 to 100,000 board feet (or more) of various species of lumber, with various dimensions in the kiln, and then heat the closed kiln to between 1000 and 1800 Fahrenheit (approximately 380 to 820 Celsius) for many days to extract moisture from wood. Thicker pieces of harder wood take longer to dry, sometimes up to 45 days (or longer). A major difficulty in drying wood is that the surface of the wood dries out more quickly than the center of a given piece. The result is checking, cracking, crazing or warping of the wood surface. Since surface clarity is the selling feature for cost of the dried wood, the value of the wood is greatly diminished or reduced to zero due to such checking, cracking, crazing or warping.  
           [0005]    In general, the normal approach used in a wood drying kiln is to gradually increase the temperature, while controlling the moisture levels in the kiln using the differential between wet bulb and dry bulb temperature sensors.  
           [0006]    The benefits of controlling humidity in the kiln include maintaining a given drying schedule and obtaining surfaces with no checking, cracking, crazing or warping, or shortening the processing or drying time. The higher temperatures allow shorter drying periods for reaching the desired 6 to 8% moisture levels of dried wood by maintaining the humidity at a higher level when higher temperatures are used.  
           [0007]    A conventional approach for maintaining higher humidity is to use hot steam in the kiln. However, the use of steam often raises the dry bulb temperature of the kiln at too high a rate. Further many plants do not have sufficient steam capacity to provide the required amount of steam. Typically a plant has a given boiler, and over the course of many years has added several kilns. There is enough steam available to dry the wood, but not enough to condition it by controlling the humidity level.  
           [0008]    Most boilers used in the kiln drying industry are rated to provide steam at 15 lbs. per square inch. A general rule is that five BTU of steam energy are needed per board foot per hour for conditioning. Typically, in a seven day drying cycle the first two days dry off the surface water (or ice). After that, for the next four to five days, moisture is added to the kiln to prevent the checking. As an example, in a 30,000 board foot kiln, the amount of steam required at 5 BTU/board foot times 30,000 board feet equals 150,000 BTU per hour, or approximately five horsepower continuously.  
           [0009]    Another approach is to use high-pressure steam to inject atomized water into the kiln. This method is expensive and requires energy for the boiler to produce or atomize the moisture, while keeping the dry bulb temperature from dropping.  
           [0010]    A variety of approaches have been suggested for vaporizing liquids. For example, U.S. Pat. No. 4,604,109 by Koslow discloses an apparatus for separating contaminants from a fluid using a vacuum chamber and at least one rotating plate disposed within the chamber. The contaminated fluid is flung of the rotating disk in the form of a fine mist of droplets from which the contaminants evaporate. The purified droplets are then collected.  
           [0011]    U.S. Pat. No. 4,833,895 to Johnson discloses a spin disk evaporator including a pan suspended from a cage enclosing a motor rotating a spin disk and a fan for moving an air stream across the pan and outward for the evaporator. The disk includes a cone-shaped tip immersed in water held in a sump portion of the pan. Rotation of the disk causes a film of water to be picked up by the tip and moved across the water transfer surface in a circumferential wall of the disk. Water collected on the inner surface of the wall separates and moves through grooves from the inner surface of the wall across the end of the wall to the outer surface. Water exits from the grooves and a circular head and is dispersed into the air stream as extremely fine water particles.  
           [0012]    The approaches outlined above do not provide an apparatus or method for producing large quantities of water vapor in an inexpensive and efficient manner. Further, these approaches do not provide any suggestion of the manner in which water vapor can be produced efficiently and inexpensively for use in a wood drying kiln.  
         SUMMARY OF THE INVENTION  
         [0013]    It is an object of the invention to provide an apparatus and a method to readily generate water vapor efficiently and inexpensively.  
           [0014]    It is another object of the invention to use the generated water vapor to control the humidity in an apparatus such as a wood drying kiln.  
           [0015]    It is a further object of the invention to prevent the occurrence of undesirable defects in wood such as checking, cracking, crazing or warping, by inexpensively and efficiently controlling the humidity in a wood drying kiln.  
           [0016]    The invention is directed to an apparatus for generating water vapor comprising at least one hydrophilic disk; a water supply structure for supplying water so that the water is deposited on the at least one disk; and means for rotating at least one disk at a speed sufficiently high so that water, which is deposited thereon is caused to leave at least one disk in the form of a mist or aerosol of very small water particles which evaporate to generate the water vapor (humidity).  
           [0017]    A disk in accordance with the invention may comprise a water absorbing fabric affixed to the disk. An adhesive layer, such as an epoxy, may be disposed between the disk and the fabric for bonding the fabric to the disk. The fabric may be one of a woven and a non-woven fabric, and may be comprised of a material selected from the group consisting of a polyester, a cotton and a cellulose. Alternatively, a layer of a phosphate may be disposed on the disk so as to render the disk hydrophilic.  
           [0018]    The disk or disks may be rotated by an electric motor, a hydraulic motor or a pneumatic motor. The disk or disks may be directly attached to the shaft of the motor. The apparatus may be combined with a wood drying kiln, the apparatus being placed within the kiln to provide a source of water vapor as the temperature of the kiln is controlled to dry wood placed in the kiln.  
           [0019]    The invention is also directed to a method for providing water vapor, comprising rotating at least one hydrophilic disk at speeds sufficiently high to fling a fine mist of water particles from the disk; and providing water to said at least one disk.  
           [0020]    Another aspect of the invention is a method of drying wood in a kiln, comprising heating air in the kiln; adding water vapor to the air by providing water to at least one hydrophilic disk, rotating the disk or disks at speeds sufficiently high to generate a mist of fine water particles, and circulating the air to which water vapor has been added to dry the wood. This method may be thought of as lumber conditioning; that is the process used to relieve stresses that build up in lumber as a result of drying of the lumber. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein:  
         [0022]    [0022]FIG. 1 is a cross-sectional view of a wood drying kiln in accordance with the invention.  
         [0023]    [0023]FIG. 2 is a side elevational view of the humidifying apparatus, in accordance with the invention, of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Referring to FIG. 1, there is shown a cross-sectional view of a wood drying kiln  10  incorporating features of the present invention. Kiln  10  is of the package loaded type, although the present invention may be applied to various other types of kilns such as, for example, a lineshaft, double track, compartment kiln, or to applications other than kilns. Although the present invention will be described with reference to the single embodiment shown in the drawing, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used.  
         [0025]    Kiln  10  has a generally rectangular housing defined by insulated walls  12  and  14 , and an insulated roof  16 , the insulation being necessary to keep heat contained therein. A doorway  18  having a movable door  20  through which wood packages  21  may be placed into and removed from kiln  10  is provided. The wood is supported on a floor  22 , generally of concrete. A fan deck  24 , having air circulation openings  26  and  28 , together with walls  12  and  14  define a plenum chamber through which hot air is circulated to dry the wood placed therein. Openings  26  and  28  have respective heat control device  30  and vent  32 , the positions of which are controlled by respective ropes  34  and  36 , routed over respective pulleys  38  and  40 .  
         [0026]    As well known in the art, a control room  42  is provided for housing control and process condition recording equipment. A so called wet bulb temperature sensor  44  is used to generate a signal from which is derived a control for the position of vent  32 , while a so called dry bulb temperature sensor  46  is used to generate a signal from which is derived a control for the position of heat control device  30 . A fan  48  having a large diameter blade assembly  50 , typically on the order of thirty six to seventy two inches (91.4 cm to 182.9 cm) in diameter, which is turned by a motor  52 , is used to circulate the air in kiln  10 . The air is heated by steam containing tubes or pipes  54 . Air chimneys  56  with automatic vents  58  are provided in roof  16 . The operation of the fan  50 , baffles  30  and  32 , and the amount of steam circulated through pipes  54 , and other aspects of the drying process in kiln  10  (including periodic reversal of the direction of air flow by reversing the direction or rotation of motor  52 ), are controlled by suitable control devices in control room  42 .  
         [0027]    Referring to FIG. 2, in accordance with the present invention, instead of spraying steam into the fan deck region of kiln  10 , a humidifying apparatus  60  in accordance with the invention is used. A disk  62 , mounted for rotation therewith on the shaft  63 , of a motor  64 , is caused to rotate at a high rate, such as 3600 rpm. Motor  64  may be electrically, hydraulically, mechanically or pneumatically driven (or may be driven by any other equivalent means). A hub  65 , configured to receive shaft  63  and affixed thereto by any of several well known techniques, including at least one set screw  67 , may be welded to disk  62  in a manner which does not distort disk  62 . A source of water, such as a feed pipe  66 , drops water directly onto the spinning disk, at its center, or as close to the center as possible. The water feed to apparatus  60  by pipe  66  may be controlled by a relative humidity sensor (not shown), or wet bulb temperature sensor, so that water is supplied only when the humidity falls below a predetermined or preprogrammed level, which may vary with time, as the wood is dried. The water is caused, by centrifugal force, to be directed toward the outer circumference of disk  62 , where it is desired that it be flung therefrom in the form of a fine mist of water particles, in the order of 5 to 20 microns in diameter. In fact, the relationship that defines the size of droplets is explained in the above mentioned U.S. Pat. No. 4,604,109 to Koslow.  
         [0028]    A problem that may occur is that the water may not wet the surface of disk  62 , which may be formed of a stainless steel to assure adequate strength and corrosion resistance. In this case, the water will bounce off at positions other than the rim, and will not be adequately vaporized. This will result in less than the proper humidity being maintained, which may cause the abovementioned defects of checking, cracking or crazing to occur.  
         [0029]    In accordance with the present invention, disk  62  is hydrophilic. Disk  62  may have a hydrophilic coating or layer placed thereon to assure that the water wets the disk  62 , or is absorbed thereon, so that it may be carried to the outer periphery of disk  62  in a controlled manner before being propelled therefrom.  
         [0030]    While a very thin layer of a phosphate may be deposited on disk  62  to enhance surface wetting, there is a tendency for this surface layer to wear off in a relatively short time with use. While this approach is of value for some applications, it requires frequent maintenance. For example, it may be necessary to have spare disks and to change the disks in order to keep a process running. Such interruption or lack of continuity may be undesirable in some processes, such as the wood drying process described herein, and adds to costs, in terms of lost time and the requirement for an additional inventory of parts.  
         [0031]    Also in accordance with the invention, these difficulties may be avoided by affixing a hydrophilic layer  68 , such as a hydrophilic, non-woven fabric, to the surface (generally the upper surface) of disk  62  on to which the water is caused to flow. Layer  68  may be affixed to the surface of disk  62  with a layer  70  of an adhesive, such as, for example, an epoxy resin. To assemble the fabric to disk  62 , first the surface thereof is suitably prepared, by for example scoring, so that the epoxy will readily adhere thereto. Then, a thin layer  70  of the epoxy is applied to the surface of disk  62 . A piece of fabric  68  larger than the size of disk  62  is then stretched over disk  62 . Epoxy layer  70  is then allowed to set. Excess fabric extending beyond the circumference of disk  62  is then trimmed off, by use of a suitable cutting implement. If necessary, disk  62  may be spin balanced, for example by placement of suitable weights (not shown) placed at locations about its circumference, on the side opposite that of fabric  68  (the underside), in a manner well known in the art, to prevent undesirable vibrations which may adversely effect the life of bearings that support motor shaft  63 .  
         [0032]    By hydrophilic, it is meant that disk  62  takes in water so that the water does not bounce off the disk  62  without being vaporized. The water is then available to be conducted to the edge of disk  62 , and leaves the rotating disk in the form of a very fine mist. It will be recognized that structures other than those shown herein may achieve the same result. For example, disk  62  may be configured as a sandwich structure or assembly with a top and bottom plate, and the top plate having a central opening for receiving water. A hydrophilic material may be disposed between the two plates to receive the water and to conduct the water to the periphery of the assembly. Alternatively, disk  62  may be machined with a series of preferably deep, radially extending grooves to receive the water and to conduct the water to its periphery. This disk may be the lower one of the sandwich type structure discussed above.  
       EXAMPLE  
       [0033]    Disk  62  may have a diameter of approximately 10 inches (25.4 cm) and a thickness of approximately 0.2 inch (0.5 cm). A non-woven fabric made of a hydrophilic material or a woven cloth, such as one made from cotton may be adhered to disk  62  by a water resistant epoxy, of a type well known in the art. The disk  62  may be rotated at 3600 rpm by a totally enclosed, fan cooled, fractional horsepower motor of a type well known in the art, suitable for high temperature applications. Such motors are readily available from a number of manufacturers. At this speed of rotation, the mist formed will be distributed about a theoretical particle diameter of approximately 18 microns. This distribution will be maintained as long as the rate at which water is deposited on disk  62  does not significantly approach a maximum rate Q (max)  defined in the abovementioned U.S. Pat. No. 4,604,109 to Koslow. In this example Q (max)  is approximately 10 gallons (40 liters) per minute.  
         [0034]    Such small water particles (or small droplets which may be characterized as an aerosol) evaporate at an extremely rapid rate, especially in the heated environment of a wood drying kiln. While such evaporation may cause some cooling, and additional heat may be needed to maintain a drying temperature program, the situation is fail-safe, in that a slightly lower temperature will not damage the wood, because it will not contribute to the creation of the checking, cracking, crazing or warping mentioned above.  
         [0035]    In accordance with the size of the kiln, more water vapor may be required than can be supplied by a single rotating disk. Several assemblies as described herein may be utilized, as required. In addition or alternatively, with suitable selection of motor size, as described in the above mentioned patent to Koslow, multiple disks may be mounted on a single shaft, and each disk supplied with water to be vaporized.  
         [0036]    Depending upon the particular application of the apparatus in accordance with the invention, the diameter of the rotating disk may be varied. Disk sizes much smaller and much larger than the example set forth herein may be used, with constraints imposed by the fineness of the mist that is to be generated, as set forth in the above mentioned patent to Koslow.  
         [0037]    It may also be desirable to provide protection against motor failure or an interruption in the supply of electricity to an electric motor used to rotate the disk, which could lead to water being dropped on the fan deck if the motor were not spinning to vaporize the water. This can be avoided by placing an electrically operated valve (not shown) in the water supply system to automatically shut down the flow of water if the motor speed drops or if the motor stops. A centrifugal switch (not shown), such as those used to switch motor start windings (that close when the motor is operating below a predetermined rotational speed) can be used to provide an appropriate control signal, with modifications to open below such speed and remove the electrical supply to a normally closed electrically operated water supply valve.  
         [0038]    In most cases, it is recommended that an anion/cation water softener be used to remove hardness and heavy metals, such as iron, for the local water supply that is used to provide water to the apparatus in accordance with the invention.  
         [0039]    Thus, the present invention provides superior product control, as there is no staining due to the deposit of moisture droplets upon the wood that is being processed. Maintenance requirements are greatly reduced as there are no steam injection nozzles to clean. Further, the ability to accurately control moisture, or relative humidity, with no steam related temperature increase allows more precise and reliable control, resulting in shorter, more effective conditioning time periods. The rotating disk typically ejects water particles of less than 20 microns in size at a speed of 140 mph (226 kph). These particles immediately vaporize, thus providing humidity at ambient kiln temperature. This is done with low energy use.  
         [0040]    It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.