BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the vaporization of liquids and the pressurization of vapors in capillary force vaporizers. More particularly, the invention relates to new developments in the assembly and configuration of capillary force vaporizers, as well as systems and methods that incorporate these new features. 
         [0003]    2. Description of the Pertinent Art 
         [0004]    Many applications utilize gases that have been generated from liquid sources. Vaporization devices have been designed to vaporize liquids and release the resulting vapor under pressure. In applications in which a pressurized vapor stream is desired, prior art devices generally require that liquid be supplied to the device under pressure, or that the vapor is otherwise pressurized by external means. For example, in pressurized boiler systems, liquids are generally required to be supplied under at least as much pressure as that of the produced vapor. Pressurized liquid sources are usually inconvenient to use, heavy to transport, potentially explosive, and prone to leakage. It is desirable, for many applications, to produce pressurized vapor streams directly from liquids that are either at or near atmospheric pressure. 
         [0005]    Several devices that achieve the foregoing goal are known in the art as capillary pumps, capillary vaporization modules or capillary force vaporizers. These devices all generate pressurized vapor directly from unpressurized liquid by applying heat to cause liquid to boil within a capillary member, and by at least partially constraining the evolved vapor to allow pressure to increase. Vapor exits the device through one or more orifices as a high velocity jet. Other features, which these devices have in common, are that they all are thermally powered, compact, and generally have no moving parts, thereby offering certain advantages over other techniques used for liquid vaporization and vapor pressurization. Several capillary pumps, capillary vaporization modules, capillary force vaporizers as well as devices and systems in which they may be incorporated are variously described in: U.S. Pat. Nos. 6,162,046, 6,347,936 and 6,585,509 to Young, et al.; U.S. Pat. No. 6,634,864 to Young, et al.; U.S. Ser. No. 10/6981,067 to Young, et al.; U.S. Ser. No. 11/355,461 to Rabin, et al.; and PCT/US2006/018696 to Rabin, et al. 
         [0006]    While a number of the devices mentioned above offer advantages over alternative liquid vaporization technologies, many of the devices are insufficiently robust to enable operation either for extended periods of time, or over a variety of operating conditions. For instance, certain of the devices, when operated for minutes or hours, exhibited fine performance characteristics. However, during longer periods of operation, namely on the order of days or weeks, several anomalies in materials performance were noted. Furthermore, none of these devices are intended or adapted for use in pressurized environments. That is, none of the devices described or presented above are configured for use either with pressurized vapor streams or with pressurized liquid feed sources. The foregoing devices are primarily intended for use in converting non-pressurized liquid into vapor, where the vapor that is generated by the capillary device is ejected at pressures that are near atmospheric pressure. The ability to generate vapor at pressures higher than atmospheric pressure is desirable for a number of reasons. The present invention, therefore, provides advanced capillary force vaporizers for use in various types of environments under a variety of operating parameters. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention seeks to overcome certain limitations of, and provide advanced features over prior art capillary force vaporizers (CFVs) for the vaporization of liquids and the generation of pressurized vapor. The CFVs described herein are suitable for use under a variety of operating parameters. These operating parameters include, but are not necessarily limited to: reliability over time in cases where a CFV device is used intermittently; increases in time intervals during which a CFV is in active operation; variations in power density to the CFV; using different liquid feed or feed combinations; changes in environmental operating conditions; and so on. The capillary force vaporizers of the present invention also feature novel operating parameters that provide better reliability and improved responses to changes in input heat and power, in addition to offering other advantages over prior art devices as will be discussed in greater detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a graph of bubble point versus flow rate for water through a variety of monolithic materials evaluated according to the present invention at a feed pressure of about 0.14 kg-f/cm 2  (2 psi); 
           [0009]      FIG. 2  is a bar graph showing maximum power test results for various monolithic porous materials evaluated according to the present invention; 
           [0010]      FIG. 3  is a plot of heater temperature versus time for a non-preferred CFV operating at 100 Watts; 
           [0011]      FIG. 4  is a schematic cross sectional view of a capillary force vaporizer according to one embodiment of the present invention; 
           [0012]      FIG. 5  is a schematic cross sectional view of a capillary force vaporizer according to a second embodiment of the present invention; and 
           [0013]      FIG. 6  is a schematic cross sectional view of a capillary force vaporizer according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In early applications that involved the use of capillary devices, a liquid was fed into or positioned within the device at or near atmospheric pressure. The liquid feed was generally a fuel or combustible material, and the purpose for the capillary pump or capillary vaporization module was the generation of flames for cooking or for providing light. Accordingly, the devices were typically operated at temperatures that exhibited flame temperatures up to about 1090° C. (2000° F.), with the surface of the device reaching temperatures in excess of about 350° C. (660° F.). However, these prior art devices were prone to failure modes due to combustion of the very materials they attempted to vaporize and burn. Often, the devices would become clogged with the liquid fuel feed being used. Worse yet, many devices were prone to cracking due to constraints placed upon device components by the very nature of the peripheral glaze used in attempts to seal and pressurize the device. 
         [0015]    During the course of the investigations and inventions described herein, capillary force vaporizers (referred to herein as CFVs) have surprisingly been shown to be amenable for use in a larger variety of application areas. Many of these involve operation at temperatures much lower than capillary devices of the prior art, namely at temperatures of 250° C., preferably below 200° C. and often below about 150° C. For instance, CFV devices have been shown to be useful with non-fuel feedstocks such as: triethylene glycol; insecticides such as allethrin and transflutherin; inhalation wellness compounds such as eucalyptus oil, menthol and camphor oil; water; perfumes, fragrances, fragrance mixtures and scenting compositions; solvents such as isopropyl alcohol, toluene and acetone; mixtures for fuel cell feedstocks such as methanol-water mixtures; and saline solutions. Other non-fuel feedstocks which may also be used with CFVs of the present invention include, but are not necessarily limited to: nicotine formulations, for example, those used for smoking cessation as well as for tobacco alternative applications; formulations containing morphine, tetrahydrocannabinol or THC, and other pain management compounds; formulations containing substances traditionally inhaled or ingested through smoking; various glycol ethers formulations such as those used, for example, in eye wetting and eye lubrication applications; as well as various antihistamine formulations, for example, those containing loratadine, which may be useful in treating allergic rhinitis, conjunctivitis and pink eye; as well as combinations of any of the foregoing. Feedstocks in addition to those enumerated above may also be used with CFVs of the present invention. Accordingly, the preceding list is intended to provide representative examples of CFV feedstocks and should not be regarded as exhaustive. 
         [0016]    According to one embodiment of the present invention, a CFV comprises a porous member and a heat source such as a heater. The porous member further comprises an insulator and an optional vaporizer. The heater further comprises one or more orifices for the release of vapor, and a grooved surface in heat-exchanging contact with the porous member for the collection of vapor and the concomitant increase in vapor pressure. Optionally, a CFV may also comprise a mechanical force generator or retainer useful for maintaining the porous member and heater in heat-exchanging contact. 
         [0017]    The type of retainer contemplated for use herein may comprise: tensioning devices such as spring clips, clamps and clamping devices; friction fittings; snap closures; bayonet attachments; threaded screw closures; twist-lock closures; as well as the various types of spring systems known to those skilled in the art, including conical washers, wavy washers, bent leaf springs and coil springs; welding; chemical, physical or mechanical bonding; sintering; chemical reaction; as well as combinations of any of the foregoing. Note that a threaded screw closure or a twist lock closure may be comprised of the heater and porous member components without more, thus obviating the need for additional parts or hardware, etc. As discussed in greater detail below, gravity in the form of the earth&#39;s gravitational forces may also comprise one form of retainer acceptable for use with the CFVs of the present invention. Note further that regardless of the nature of the method used to provide a mechanical force or retain the heater in close proximity to the porous member as contemplated in the present invention, an important distinction between the present invention and prior art capillary vaporizers is that prior art devices often featured a sealing member, coating or shroud peripheral to the device components. By contrast, the retainer of the present invention contemplates the application of compressive forces among or between the CFV components. 
         [0018]    In addition to the foregoing, a CFV as contemplated herein may also optionally comprise a housing. In many applications, a housing may be useful for situating the CFV within or as part of a larger instrument, device, engine or apparatus; for ease in positioning a CFV in close proximity to heater control components; for convenience in situating a CFV in a particular location within a room; and so on. Various types of devices that may incorporate a CFV according to the present invention, particularly with respect to inline humidification, are described in U.S. Ser. No. 00/000,000 to Weinstein, et al., filed 30 Nov. 2006 for Inline Vaporizer. 
         [0019]    In operation of a CFV, capillary forces transport a liquid towards a heat source. The heat source vaporizes the liquid, such that it is emitted from the CFV at or slightly above atmospheric pressure. Liquid must be delivered to the heat source such that vaporization can occur in a controlled fashion. At the same time, heat and excessive vapor must be prevented from migrating from the heat source to the liquid to prevent failure of the CFV. Early prior art capillary devices that were used with combustible liquid feeds for heating, lighting and cooking applications comprised a wicking member and a heat source. The wicking member delivered the fuel to the heat source, but these early prior art devices produced insufficient fuel vapor for the intended application(s). It was postulated that the wicking member was unable to prevent excessive heat and pressurized vapor from traveling from the heater towards the direction of incoming liquid feed, thus resulting in diminished amounts of vapor being generated for combustion purposes. 
         [0020]    In later prior art capillary devices, the wicking member was fashioned from material having smaller pores. It was postulated that the smaller pores would result in higher capillary pressures realized in the wicking member and thereby prevent excessive vapor from migrating from the heater towards the liquid feed. However, the pores of these wicking members were too small, resulting in unacceptably higher capillary forces. The result was that the capillary devices that incorporated such wicking members essentially transmitted fuel at insufficient rates. 
         [0021]    More recent prior art capillary devices incorporated a combination of the above learnings. That is, a wicking member with larger pores was used in addition to the wicking member that had smaller pores. The region with smaller pores is referred to herein as a vaporizer layer or vaporizer, while the region with larger pores is referred to herein as an insulation layer or insulator. Together, the vaporizer and insulator comprise the porous member. The insulator conducts liquid feed towards the heater or heat source via capillary forces for vaporization of the liquid without permitting too much heat from flowing from the heater towards the advancing liquid feed. As is discussed in greater detail below, the overall dimensions, capillary pore size and liquid feed are all factors to be considered in optimizing a given insulator for a particular application. 
         [0022]    Surprisingly, it has now been found that preventing the occurrence of too much pressurized vapor from traveling from the heater towards the direction of the incoming liquid feed may be achieved in certain CFVs without requiring the use of a separate vaporizer. In prior capillary devices, the vaporizer had been considered a necessary layer for preventing too much gas or pressurized vapor from flowing or migrating from the heater towards the liquid feed. Vaporizers had been regarded as necessary, for example, where a CFV was to be operated with very high liquid feed mass flows, for extended periods of time and/or at very high operating temperatures as in combustion applications. 
         [0023]    More recently, the desire to operate CFVs at lower temperatures and pressures and for longer periods of time necessitated the reevaluation of suitable materials for use as insulators as well as vaporizers. While the insulator remains an essential feature of a CFV porous member, vaporizers or vaporization layers have been found to be optional where CFVs are operated at lower temperatures and do not involve the generation of very high internal pressures, as would be experienced with the combustion of liquid feeds such as fuels, or where very high mass flow rates and higher temperatures are required. Moreover, it has been learned during course of the present invention that CFVs for intermittent use, that is, applications that require bursts of small amounts of vapor without the buildup of high internal pressures, may also be developed without the inclusion of a vaporizer. 
         [0024]    Accordingly, a series of studies were conducted to evaluate materials suitable for use as vaporizers and insulators. Surprisingly, it has now been found that through prudent selection of insulator and vaporizer materials, the relative amount of vaporizer to insulator required for a particular application can be significantly modified if not entirely eliminated. Surprisingly therefore, it was learned herein that in certain instances, as in the use of CFVs for the vaporization of water and fragrances, it was possible for a CFV to operate successfully and have liquid feed converted to vapor without the use of a separate vaporizer in the porous member. In such instances, and without being bound by theory, it is theorized that the region of the insulator that is contacted by the heater serves as the region in which vaporization of the liquid feed occurs within the capillary pores of the insulator. Naturally, the absence of a vaporizer layer is not the most efficient technique for blocking gas generated from the liquid feed from moving away from the heater and through the insulator towards the incoming liquid feed. Without a vaporizer layer, the vapor generated by a CFV exhibits less homogeneity in vapor form and droplet size. Sputtering and emission of nonvaporized liquid may also occur. However, for most low mass flow applications, and/or applications in which CFVs are operated intermittently and without high boiling point liquids, vaporizers have been shown to be nonessential. Thus, when CFVs are used with liquid water fed at a rate of several grams per minute, some loss of water may be observed if a vaporizer is not used. At lower water flow rates on the order of a gram per munute or less, numerous trials have shown that vaporizers are not necessary. According to one embodiment of the present invention, therefore, a device for the generation of pressurized vapor from unpressurized liquid comprises:
       a) a porous member further comprising an insulator and an optional vaporizer, including a surface for receiving the liquid and an area for the pressurization of vapor that is produced from the liquid;   b) a heater for conveying heat to the porous member for vaporizing the liquid, the heater further comprising an area for the collection of vapor and at least one orifice for release of the vapor at a velocity greater than zero;   c) a retainer for situating the heater in heat-exchanging contact with the porous member; and   d. optionally, a housing;       
 
         [0029]    wherein the porous member draws the liquid towards the heater via capillary forces. 
       A. Selection of Materials 
       [0030]    While numerous materials have adequate permeability to provide the flow of liquid required by the porous member of a CFV, a combination of materials properties is appropriate so that the capillary forces are adequate, excessive pressure does not build up and thermal and mass transfer requirements are met. In other words, the liquid feed must be transported to the vaporization region of the CFV and the flow of liquid within the porous member during operation of the CFV should result in sufficient cooling without concomitant pressure buildup so that the CFV maintains a reasonable temperature and pressure, even at elevated power levels. Capillary forces due to small pores in a candidate material are essential to accomplishing these goals. These capillary forces can be quantified and compared using “bubble point” measurements. 
         [0031]    The bubble point of a material refers to a specific test in which air is forced through a porous material that has been saturated with a liquid. The pressure at which a bubble of the liquid starts to form at a surface of the porous material due to gas permeation of the saturated material is known as the bubble point. Common porous materials include simple paper filters, gas diffusers, construction materials such as bricks, etc. The foregoing have very large pores and it is very easy to force a liquid through them. This is because any surface tension due to wetting of the pores by a liquid passing through the pores is relatively easily overcome. In these instances, the pressure required to initiate gas flow through the porous material can be very low. 
         [0032]    The smaller the diameter of the pores of a material, the higher the pressure needed to displace a liquid passing therethrough. Technical porous materials, that is, those designed for applications in which pore size distribution is critical, are often described by their maximum through pore size, which in turn is easily calculated from the bubble point. For vaporization applications, the bubble point should be high enough to prevent expulsion of liquid from the pores, but low enough to enable sufficient quantities of liquid feed to flow through the material. An acceptable value depends on the nature of the fluid and the specific application. Materials with very low bubble points can accommodate very large flow rates, while materials with very high bubble points can only accommodate very low flow rates. For most CFV applications, very low flow rates and thus materials with very high bubble points are not desirable. 
         [0033]    A variety of technical porous materials were evaluated for use herein. The materials which were studied include those described in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Materials Evaluated for Use in CFVs 
               
             
          
           
               
                 Sample 
                   
                   
                   
               
               
                 No. 
                 Name 
                 Description 
                 Source 
               
               
                   
               
             
          
           
               
                 1 
                 ZAL-45AA 
                 High porosity alumina consisting of grains and fibers, 
                 Zircar Ceramics, Inc. 
               
               
                   
                   
                 with a mean pore size of approximately 5 microns, 
                 Florida, NY 
               
               
                   
                   
                 used primarily for thermal insulation in high 
               
               
                   
                   
                 temperature furnaces 
               
               
                 2 
                 Mott Grade 5 
                 Porous sintered metal with a mean pore size of 
                 Mott Corp. 
               
               
                   
                   
                 approximately 5 microns, used for gas and liquid 
                 Farmington, CT 
               
               
                   
                   
                 filtration 
               
               
                 3 
                 AF6 
                 Moderately porous alumina with a mean pore size of 
                 Refractron 
               
               
                   
                   
                 approximately 5 microns, used for liquid filtration 
                 Technologies Corp. 
               
               
                   
                   
                   
                 Newark, NY 
               
               
                 4 
                 AF15 
                 Moderately porous alumina with a mean pore size of 
                 Refractron 
               
               
                   
                   
                 approximately 14 microns, used for liquid filtration 
                 Technologies Corp. 
               
               
                   
                   
                   
                 Newark, NY 
               
               
                 5 
                 AF30 
                 Moderately porous alumina with a mean pore size of 
                 Refractron 
               
               
                   
                   
                 approximately 30 microns, used for liquid filtration 
                 Technologies Corp. 
               
               
                   
                   
                   
                 Newark, NY 
               
               
                 6 
                 AF50 
                 Moderately porous alumina with a mean pore size of 
                 Refractron 
               
               
                   
                   
                 approximately 50 microns, used for liquid filtration 
                 Technologies Corp. 
               
               
                   
                   
                   
                 Newark, NY 
               
               
                 7 
                 MeAF1 
                 Composite porous alumina with 2 distinct layers, each 
                 Refractron 
               
               
                   
                   
                 having a different mean pore size (approximately 1 and 
                 Technologies Corp. 
               
               
                   
                   
                 14 microns), used for water filtration 
                 Newark, NY 
               
               
                 8 
                 MeAF3 
                 Composite porous alumina with 2 distinct layers, each 
                 Refractron 
               
               
                   
                   
                 having a different mean pore size (approximately 3 and 
                 Technologies Corp. 
               
               
                   
                   
                 14 microns), used for water filtration 
                 Newark, NY 
               
               
                 9 
                 Micromass 
                 High porosity alumina with a mean pore size of 
                 Selee Corporation, 
               
               
                   
                   
                 approximately 5 microns, used in high temperature 
                 Hendersonville, NC 
               
               
                   
                   
                 furnaces as kiln furniture 
               
               
                 10 
                 T-Cast 
                 High porosity alumina microns, used primarily for 
                 Refractory 
               
               
                   
                   
                 thermal insulation in high temperature furnaces 
                 Specialties, Inc., 
               
               
                   
                   
                   
                 Sebring, OH 
               
               
                 11 
                 P-6-C 
                 Moderately porous alumina with a mean pore size of 
                 CoorsTek, Golden, 
               
               
                   
                   
                 approximately 6 microns, used for liquid filtration 
                 CO 
               
               
                 12 
                 P-10-C 
                 Moderately porous alumina with a mean pore size of 
                 CoorsTek, Golden, 
               
               
                   
                   
                 approximately 10 microns, used for liquid filtration 
                 CO 
               
               
                 13 
                 P-16-C 
                 Moderately porous alumina with a mean pore size of 
                 CoorsTek, Golden, 
               
               
                   
                   
                 approximately 16 microns, used for liquid filtration 
                 CO 
               
               
                 14 
                 P-40-C 
                 Moderately porous alumina with a mean pore size of 
                 CoorsTek, Golden, 
               
               
                   
                   
                 approximately 40 microns, used for liquid filtration 
                 CO 
               
               
                 15 
                 P-55-C 
                 Moderately porous alumina with a mean pore size of 
                 CoorsTek, Golden, 
               
               
                   
                   
                 approximately 55 microns, used for liquid filtration 
                 CO 
               
               
                   
               
             
          
         
       
     
         [0034]    The results of bubble point studies for several of the foregoing materials are shown in  FIG. 1 . The sample numbers and corresponding data for the materials evaluated in  FIG. 1  are provided in Table 2 below. The evaluations were carried out at a pressure of about 0.14 kg-f/cm 2  (2 psi) with water as the liquid feed. The materials were evaluated in the form of monolithic disks shaped 2 cm in diameter with a height of 1 cm. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Water Flow Rates and Bubble Points for Various Material Samples 
               
             
          
           
               
                 Sample 
                 Bubble Point 
                 Flow Rate 
               
               
                 No. 
                 (in kg-f/cm 2 ) 
                 (in cm 3 /sec) 
               
               
                   
               
             
          
           
               
                 4 
                 .021 
                 1.4 
               
               
                 6 
                 .021 
                 3.4 
               
               
                 5 
                 .028 
                 3 
               
               
                 15 
                 .056 
                 2.13 
               
               
                 14 
                 .070 
                 0.97 
               
               
                 9 
                 .098 
                 1.56 
               
               
                 10 
                 .106 
                 1.2 
               
               
                 1 
                 .127 
                 1.7 
               
               
                 12 
                 .176 
                 0.06 
               
               
                 3 
                 .232 
                 0.23 
               
               
                 13 
                 .352 
                 0.06 
               
               
                   
               
             
          
         
       
     
         [0035]    It has now been surprisingly found that bubble points on the order of about several tenths kg-f/cm 2  (several psi) are suitable for use with certain CFV applications, such as with aqueous systems. Thus, for certain applications, a relatively small area of  FIG. 1  can describe optimal operating properties for CFVs. For samples having areas of about 3 cm 2 , which were used for the vaporization of water, the region of interest in  FIG. 1  lies between flow rates of 0.01 to 10 cm 3 /sec, more preferably between 0.1 and 5 cm 3 /sec, and most preferably between 0.5 and 3 cm 3 /sec having bubble points of 0.001 to 10 kg-f/cm 2 , more preferably 0.01 to 0.5 kg-f/cm 2 , and most preferably between 0.025 to 0.2 kg-f/cm 2 . 
         [0036]    For other fluids and operating conditions, higher bubble points may be appropriate. Where a CFV is used to provide vapor into a pressurized gas environment, for example, a sufficient bubble point is required such that the feed liquid does not leak or is not otherwise expelled from the CFV before it can be vaporized. In such instances, higher bubble points with concomitant higher liquid flow rates are generally more desirable. An expulsion of liquid without vaporization would prevent a stable flow of liquid feed from reaching the vaporization area adjacent to the heat source. Vaporizing liquids in elevated pressure environments can also require a pressurized liquid flow to the CFV, as will be discussed in greater detail below. 
         [0037]    Depending on the requirements of the specific CFV vaporization application, different combinations of these two parameters, that is bubble point and flow rate, may be desirable. For example, certain humidification applications require a bubble point on the order of about 0.01 to 0.15 kg-f/cm 2  and a flow rate or permeability of 1 cm 3 /sec when a pressure of 1400 kg/m 2  is applied to a disk 2 cm in diameter and 1 cm thick. 
         [0038]    As the bubble point of a material is determined by its pore structure, it does not depend upon the thickness of the material, but rather on the maximum through pore size. The permeability generally diminishes as the thickness of the material is increased. In order to provide sufficient flow for a desired power level and therefore vapor output rate, the thickness of a material can be manipulated. This is one reason why different vaporizer thicknesses may be used where it is desirable to prevent the buildup of too much backpressure within a porous member. See the discussions above describing the use of CFVs with fuels and combustible liquids. 
         [0039]    In the case of an insulator, by contrast, regardless of the material chosen, the insulator cannot be too thin or the temperature of the CFV will be excessive. An appropriate length for an insulator is that which, when fluid passes through it during vaporization, it can provide sufficient cooling to allow the CFV to remain at a temperature well below the boiling point of the fluid. In this manner, vapor should be produced only near the heat source and the vapor generated by the CFV is ejected mostly through the orifice. The optimal thickness chosen for a particular insulator will necessarily depend upon such parameters as the nature of the fluid to be vaporized, the duration of operation for the CFV, the liquid flow rate and necessary power level required, and the thermal conductivity of the material. 
         [0040]    If the bubble point and the permeability for a particular CFV vaporization application cannot be met by a porous member comprising a monolithic material, a CFV can be fashioned with more than one material comprising the porous member, as in the combination of an insulator and optional vaporizer, discussed above for fuel and combustible liquids. It should be noted that the proper combination of materials for insulator and vaporizer can be achieved in various ways using one or more components which vary in pore size distribution and other properties. 
         [0041]    Somewhat surprisingly, it has now been found that combinations of materials for porous members comprising insulators and optionally, vaporizers, may be satisfied according to the following preferred configuration. Materials with finer pores, and therefore higher bubble points, are situated adjacent to the heat source while materials having larger pores, and thus higher permeability, are situated remotely from the heat source. The finer pore region can supply the necessary capillary force and provide sufficient permeability even when relatively thin in comparison to the insulator thickness at a given diameter. The larger pore region tends to be thicker, so as to provide adequate thermal insulation. Naturally, CFVs containing layers with more than two different pore sizes can also be contemplated. Porous members may therefore comprise materials with constant pore sizes as well as materials with varying pore sizes, such as graded materials, in which pore sizes vary when transversing from a first surface to a second surface across the material. 
         [0042]    Composite structures that contain combinations of fine poor and large poor regions can arise, for example, from a single material containing a distribution of pore sizes that may be introduced during the manufacturing process. The result is a “graded” material, as there is a gradient in the pore size distribution in moving through the material. Graded materials may also be created in a multi-step process in which fine pore material is integrally bonded to the larger pore material. Alternately, the same result may be achieved by using two distinct components that are in intimate contact with each other. 
         [0043]    Permeability and bubble point alone are not sufficient parameters for predicting suitability for components in all CFVs, but they can be used to evaluate materials as new materials are developed. Other parameters that may be important factors can include evaluation of energy densities that a material can accommodate for high performance CFV applications, for instance. Several monolithic materials that were evaluated for maximum sustainable power levels are shown in  FIG. 2 . The materials were evaluated in the form of 20 mm diameter discs with a height of 10 mm. Note that replicates of trials are included in  FIG. 2  for samples numbered four and eight. 
         [0044]    The study summarized in  FIG. 2  was carried out in order to evaluate CFV materials for use in applications that encompass aqueous feed applications with power level requirements on the order of about 100 Watts. Sample numbers are those provided above in Table 1. The fact that a number of samples exhibited good reliability up to about 120 Watts indicates that these are more robust materials. Note that materials that exhibited stability at higher power levels are generally more preferred. 
         [0045]    Other variables that are used to evaluate suitable of a given material for use in a CFV include such factors as: response time; reliability of the material over time; potential to contribute contaminants to the vapor stream; ease and convenience of manufacturability and associated costs for doing so; etc. Table 3 below illustrates one set of results obtained for a variety of porous materials evaluated for possible use as the porous member in CFVs. The following operating characteristics were evaluated: 1) maximum power, that is, the maximum sustainable operating power, with higher values preferred; 2) heater temperature, in other words, the temperature of the heater with a CFV operating at maximum power, with lower temperatures being preferred; 3) CFV temperature, that is, the temperature of the CFV device when the CFV is at maximum power, with lower temperatures preferred; and 4) friability, in other words, the propensity of the porous material to generate particulate matter or give up other byproducts during operation of the CFV, which may also be termed “contaminants.” 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Evaluation of Porous Materials According 
               
               
                 to Operating Characteristics 
               
             
          
           
               
                   
                 Maximum 
                 Heater 
                 CFV 
                   
               
               
                 Sample No. 
                 Power 
                 Temperature 
                 Temperature 
                 Contaminants 
               
               
                   
               
               
                 1 
                 ++ 
                 + 
                 + 
                 − 
               
               
                 2 
                 − 
                 − 
                 − 
                 ++ 
               
               
                 3 
                 − 
                 − 
                 − 
                 ++ 
               
               
                 4 
                 − 
                 ∘ 
                 − 
                 ++ 
               
               
                 5 
                 ++ 
                 + 
                 + 
                 ++ 
               
               
                 6 
                 − 
                 ∘ 
                 ∘ 
                 ++ 
               
               
                 8 
                 ++ 
                 + 
                 + 
                 ++ 
               
               
                   
               
               
                 Legend to Table 3: 
               
               
                 “−” indicates poor performance; 
               
               
                 “∘” indicates adequate performance; 
               
               
                 “+” indicates good performance; and 
               
               
                 “++” indicates very good performance 
               
             
          
         
       
     
         [0046]    The materials that were evaluated for inclusion in Table 3 are: ZAL-45AA; Mott Grade 5; AF6; AF15; AF30; AF50; and MeAF3; additional information for which can be found in Table 1 above. The results provided above in Table 3 indicate that not all porous materials are equally suitable for the development of high performance CFVs. Those samples that demonstrated the most favorable characteristics in Table 3 above include samples 5 and 8, which correspond to AF30 and MeAF3, respectively. According to one embodiment of the present invention, therefore, a capillary device for the generation of pressurized vapor from unpressurized liquid comprises an insulator characterized by material selected from among: ZAL-45AA, Mott Grade 5, AF6, AF15, AF30, AF50, MeAF3, Micromass, T-Cast, P-10-C, P-16-C, P-40-C and P-55-C; more preferably selected from among: ZAL-45AA, Mott Grade 5, AF15, AF30, AF50, MeAF3, Micromass, T-Cast, P-40-C and P-55-C; and most preferably selected from among: AF30 and MeAF3. 
       B. CFV Configuration 
       [0047]    During operation of CFVs having certain configurations, it has been surprisingly found that there can be a tendency for gas bubbles to evolve from the sides and bottom of the CFV. Without being bound by theory, it is speculated that these bubbles originate in a variety of ways: by accumulation of dissolved gases present within the feed liquid; as a result of unsaturated porosity in the porous member, namely in the insulator; etc. The bubbles can compromise operation of a CFV, since the accumulation of bubbles can obstruct the flow of liquid feed to the insulator, particularly when the liquid feed is water as in instances where the CFV may be used for humidification purposes. 
         [0048]    Depending on the liquid feed, the power level at which the CFV is operated and the details of CFV construction, failure in this mode can occur in minutes or hours. In the past, the evolution of gas bubbles was observed to occur during use of many vaporizers that were operated on the order of hours minutes or even seconds. By contrast, it has now surprisingly been found that when properly vented, CFVs can operate stably and reliably for times on the order of days to several weeks, even at high power settings. 
         [0049]    In many vaporizers of the prior art, there was no opportunity for the venting of gas bubbles, as in cases where the insulator was tightly contained within a sheath, tube, or similar containment device, or in situations where a deliberate attempt was made to seal the perimeter of the insulator, as with a coating or other integral housing. The accumulation of gas inside the porous structure of the insulator in these vaporizers can and often did cause serious problems with operation of the device due to interference by the gas with the flow of liquid to the heater. As alluded to above, this can cause overheating of the device and result in heater failure, thus rendering the CFV inoperable. However, not all prior art vaporizers were prone to failure. Due in part to microscopic roughness, device housings which only loosely enclosed the vaporizer may have fortuitously provided a region which was either empty or contained the fluid to be vaporized, and thus provided a de facto escape route for gas bubble buildup. 
         [0050]      FIG. 3  shows what can happen in instances where a CFV is inadequately vented. The graph shown in  FIG. 3  is a plot of heater temperature versus time for a CFV operating at 100 Watts. The increase in CFV heater temperature was accompanied by the formation of bubbles over time. Note that the heater temperature climbed while a constant power level was maintained, until the bubbles had a chance to escape. The temperature then immediately dropped and new bubbles started to accumulate. This thermal cycling process is detrimental to good control of the vaporization process, and can produce unnecessary strain on the power supply and control circuit. It can cause undesirable stress on CFV components, as well as place the CFV at risk for overheating and ultimately, failure. 
         [0051]    Various modifications can be contemplated for the CFV either to prevent bubbles from accumulating or to provide with an easy escape route in the event of bubble formation. Those familiar with the problem of gas generation and bubble formation will understand that too close a fit between the CFV porous member and a device housing can constrain bubbles. Accordingly, it is advisable to design the local environment of the CFV properly. A region can be provided between the porous member and the housing that is either continuous or discontinuous, such that evolving bubbles can be directed or channeled out of the CFV as quickly as possible. This may be accomplished, for example, by creating channels or gaps in continuous or selected portions of the device containment perimeter. The gap can be either fluid filled or contain a gas. A wide variety of configurations can be used to remedy the situation of bubble formation. Additionally, there may be features on the CFV housing, adjacent to the lower portion of the porous member or insulator thereof, which direct bubbles forming here to the device perimeter, so that they can escape. Another alternative approach is to reduce the height of the CFV container or housing, so that it does not entirely cover the peripheral area of the CFV, so that gas bubbles are easily dispersed into the ambient environment, away from the CFV. 
         [0052]    An example of a non-vented CFV is shown schematically in  FIG. 4  at  400 , while  FIG. 5  shows a schematic of a vented CFV at  500  according to one embodiment of the present invention. An alternate example of a CFV according to another embodiment of the present invention is shown schematically in  FIG. 6  at  600 . Note that like reference numbers are used throughout the figures to represent similar parts or features of the drawings. 
         [0053]    Turning first to  FIG. 4 , the device at  400  illustrates one example of a capillary force vaporizer according to the present invention. A porous member for the delivery of liquid feed via capillary forces comprises insulator  404  and optional vaporizer  402 . The porous member is in intimate contact with a heater, which further comprises heating element  406  and heat exchanger  408 . Heating element  406  is in intimate contact with heat exchanger  408 , which includes orifice  410  for the emission of pressurized vapor from vapor that collects and becomes pressurized at vapor collection channels  412 , situated at the interface between the porous member and the heater (not labeled). Electrical leads  414  connect heating element  406  to a power supply (not shown). 
         [0054]    As shown in  FIG. 4 , device  400  rests on ledge  418  of housing  420  disposed towards the interior, and situated along the lower portion, of housing wall  422  of housing  420 . Outer housing  416 , which may comprise a variety of shapes and configurations, variously helps to provide an attachment, anchoring or containment point for electrical leads  414 . According to one embodiment of the invention, electrical leads  414  may comprise a spring clip. According to another embodiment of the invention, electrical leads  414  may further comprise a bead, tab, hook or like feature for engaging outer housing  416  (not illustrated). Note that housing  420  is in continuous and intimate contact with the heater and porous member components of device  400  along housing wall  422 . 
         [0055]    Turning now to device  500  illustrated in  FIG. 5 , it should be noted that the overall configuration of housing  420  differs slightly from that shown for device  400  in  FIG. 4 . In  FIG. 5 , housing walls  422  of housing  420  are disposed at a distance somewhat remote from the porous member and heater components of  500 , resulting in the inclusion of an opening or gap  524  therebetween. While the remaining features of device  500  are relatively similar to those described previously for device  400 , it should be noted that the inclusion of gap  524  has surprisingly afforded significant advantages in operability and longevity for device  500  over device  400  and similar capillary force vaporizers of the prior art, as described above. 
         [0056]    Also somewhat surprisingly, it has been learned that gap  524  need not be very large in order to impart significant operating enhancements to a CFV as compared devices that lacks a gap. Thus, a gap comprising a total spacing of between approximately 3 mm, preferably 2 mm, and most preferably 1 mm between a CFV and housing wall  422  has been found to be adequate for those applications in which housings are needed, used or desired. If the gap is too large, there tends to be insufficient surface tension of the bubbles that may arise from within the CFV. The result is that the bubbles are unable to bridge gap  524  between the CFV and housing wall  422 . This can result in a tendency for feed liquid to leak out of the device if placed in an inverted position, especially where water is the liquid feed. 
         [0057]    While gap  524  is shown schematically in  FIG. 5 , it should be recognized that no one particular shape or configuration of spacing between a CFV and housing wall is contemplated, and the shape, placement and dimensions of gap  524  in  FIG. 5  are for purposes of illustration only. In fact, a number of different configurations have been tried and evaluated. If viewed from above the CFV, that is, along an imaginary line in the plane of the paper from the top to the bottom of the illustration in.  FIG. 5 , the configuration can best be described as: concentric circles; a circular array of 3 or 4 channels similar to a 3- or 4-leafed clover; a circular array comprised of many channels; and so forth. According to a preferred embodiment of the present invention, housing wall  422  and gap  524  are disposed in concentric configuration about a central CFV device, respectively. 
         [0058]    An alternate embodiment for a CFV is illustrated at  600  in  FIG. 6 . This device differs from  500  in  FIG. 5  in that housing wall  622  is shorter and therefore does not conceal or shield as much of the CFV as compared to housing wall  422  of  FIG. 5 . In such a configuration, there is less of a gap and more of the porous member, comprising optional vaporizer  402  and insulator  404  are exposed to the atmosphere. The result is that there are less bubbles to collect, as they may freely emanate from the device. 
         [0059]    In the extreme case, housing wall  622  can have a null height. That is, housing  420  may comprise a disk with ledge  418  upon which a CFV is situated. In such a case, there is no gap, and the CFV has an essentially infinitely open configuration. The only requirement in this case is that there be some point of attachment or anchor means for providing the necessary mechanical force to hold the various CFV components together. In light of the foregoing discussions, therefore, and according to one embodiment of the present invention, a device for the generation of pressurized vapor from unpressurized liquid may comprise:
       a) a porous member further comprising an insulator and an optional vaporizer, including a surface for receiving the liquid and an area for the pressurization of vapor that is produced from the liquid;   b) a heater for conveying heat to the porous member for vaporizing the liquid, the heater further comprising an area for the collection of vapor and at least one orifice for release of the vapor at a velocity greater than zero;   c) a retainer for situating the heater in heat-exchanging contact with the porous member; and   d) optionally, a housing including a housing wall present at a spacing of 3 mm, preferably 2 mm and most preferably 1 mm from porous member, heater and retainer.       
 
       C. Gravity as Suitable Mechanical Force 
       [0064]    It should be noted that in its simplest form, a CFV may be regarded as comprising a porous member, a heater and a retainer or mechanical force generator to place the heater and porous member in intimate heat-exchanging contact with one another. The porous member further comprises an insulator and an optional vaporizer and the heater further comprises a vapor collection region and at least one orifice for the release of vapor. 
         [0065]    A variety of mechanical force generators or mechanical force generating means have been discussed previously. See, for example, co-pending and commonly assigned application for patent PCT/US2006/018,696 filed 15 May 2006, which discusses mechanical force generation means such as clamping, springs, clips, etc. In addition to the foregoing, during the course of the present investigation, it was also surprisingly learned that even gravity as in the gravitational force present on or in proximity to the planet Earth, may provide a suitable mechanical force under certain conditions for successful operation of a CFV. Accordingly, in one embodiment of the present invention, a capillary force vaporizer may comprise:
       a) a porous member, further comprising an insulator and an optional vaporizer;   b) a heater; and   c) a retainer for situating the heater in heat-exchanging contact with the porous member;       
 
         [0069]    wherein the retainer may be selected from among: the gravitational force of the earth; tensioning devices such as spring clips, clamps and clamping devices; friction fittings; snap closures; bayonet attachments; threaded screw closures; twist-lock closures; as well as the various types of spring systems known to those skilled in the art, including conical washers, wavy washers, bent leaf springs and coil springs; welding; chemical, physical or mechanical bonding; sintering; glazing; chemical reaction; as well as combinations of any of the foregoing. 
       D. Operation of CFVs at Higher Ambient Pressures 
       [0070]    During typical CFV operation, capillary forces transport a liquid feed towards a heat source, which vaporizes the liquid such that it is emitted from the CFV at or slightly above atmospheric pressure. Providing that the air or other ambient environment into which the CFV was placed and operated was at or near atmospheric pressure, the CFV device functioned normally. However, if the CFV was operated in elevated atmospheres, back pressure on the CFV device would ultimately drive the feed liquid back into the device and prevent the successful emission of vaporized liquid. 
         [0071]    Somewhat surprisingly, it has now been found that under certain conditions, it is possible to generate pressurized vapor using a CFV and to eject that vapor into a vapor stream, even one at elevated pressures, without flooding or failure of the CFV device. The key to achieving this goal is to ensure that there is no pressure drop across the CFV device. However, the accomplishment of this task turned out to be less than completely straightforward. 
         [0072]    Through attempts to operate a CFV in certain pressurized environments, it was learned that the encounter of sufficient backpressure would prevent the ejection of vapor and result in the occasional flooding of the CFV. Although the capillary forces acting on liquid feeds in the CFV were enough to deliver material for vaporization into atmospheric environments, there was too much of a pressure drop if the vapor was intended for use in pressurized environments. Attempts to overcome the backpressure problem by mere pressurization of the liquid feed, however, often resulted in too much liquid being driven through the CFV such that large, visible droplets of the liquid feed could be seen dripping at the orifice(s) of the CFV device. 
         [0073]    Interestingly, it was learned that by pressurizing the liquid feed to approximate that of the ambient environmental pressure as closely as possible, it was possible to operate the CFV in order to generate vapor that would not be overwhelmed by the carrier or environmental pressure. According to an embodiment of the present invention, therefore, an apparatus for the generation of a vapor jet from a liquid for use in an environment having pressure at a first pressure above atmospheric pressure comprises:
       a) a porous member including a surface for receiving the liquid and an area for the pressurization of vapor that is produced from the liquid; and   b) a heater for conveying heat to the porous member for vaporizing the liquid, the heater further comprising an area for the collection of vapor and at least one opening for release of the vapor at a velocity greater than zero;   wherein the liquid is present at a pressure of at least that of the first pressure.       
 
         [0077]    Similarly, a technique for the generation of pressurized vapor in environments having pressures at a first pressure above atmospheric pressure can be contemplated as comprising the steps of:
       a) pressurizing a source of liquid feed to a second pressure; and   b) providing the pressurized liquid feed to a capillary force vaporizer, the capillary force vaporizer comprising:
           i) a porous member further comprising an insulator including a surface for receiving the liquid feed and an optional vaporizer, including a vaporization area for the collection and pressurization of vapor that is produced from the liquid; and   ii) a heater component for conveying heat to the porous member for the vaporization of the liquid and at least one opening for release of the vapor at a velocity greater than zero;   
               
 
         [0082]    wherein the second pressure is at least equal to the first pressure. 
         [0000]    It should be noted that it is not necessary to pressurize liquid feed to a CFV that is operating at or near atmospheric pressure. Thus, small variations in atmospheric or environmental pressure can be handled by the CFV without compromising the operation of the CFV device. 
         [0083]    The present invention has been described above in detail with reference to specific embodiments, Figures, and examples. These embodiments, Figures and examples should not be construed as narrowing the scope of the invention, but rather serve as illustrative examples to facilitate an understanding of the invention and ways in which the invention may be practiced, and to further enable those of skill in the pertinent art to practice the invention. It is to be further understood that various modifications and substitutions may be made to the described capillary force vaporizer devices, modules and systems, as well as to materials, methods of manufacture and use, without departing from the broad scope of the invention contemplated herein. The invention is further illustrated and described in the claims that follow.
The present invention relates to the vaporization of liquids and the pressurization of vapors in capillary force vaporizers. More particularly, the invention provides new developments in the assembly and configuration of capillary force vaporizers, as well as systems and methods that incorporate these features, thus providing capillary force vaporizers that exhibit enhanced operability and reliability during operation.