Patent Application
20140245630
The following is a tabulation of some prior art that presently appears relevant:
Inventions applying microwave energy to drying of solids have been around nearly since the residential microwave oven entered the marketplace in the mid 1960's. For example, in U.S. Pat. No. 3,410,116 (1968) to Levinson a device is described that could be used as a microwave oven or converted to other uses, including drying clothes. The source of drying energy was a microwave generator, though not described in detail. However, drying of household laundry using microwave energy has some inherent problems that were not addressed in this early patent and, perhaps, not completely addressed since. One of the most difficult issues is that of arcing that can occur between metal contained in clothing (zippers, rivets, buttons, coins, etc.) and a grounded surface of the dryer. Microwaves are nearly totally reflected by most metal surfaces, but some skin penetration does occur with energy transfer to the metal. That energy releases electrons in the metal. According to classical electromagnetic theory, a free charge can exist only on the surface of a conductor, as the electric field within the metal must remain zero. Accordingly, free electrons will distribute as a surface charge and in such a way as to provide the appearance of a uniform electric field emanating from the surface when viewed at a great distance (infinity). If that electric field is represented by lines perpendicular to the surface and directed outward, those lines for a flat surface will be equally spaced. At a curved surface, the charge will aggregate so that the field lines will be closely spaced at the surface but will diverge and appear equally spaced at a distance. The density of the surface charge will be inversely proportional to the radius of curvature. This means that sharp points on the metal surface can create a very high electric field when viewed close to that surface. It is possible that the electric field could exceed the breakdown voltage of air, causing the air molecules to ionize and become conductive. An arc results from the transfer of electrons from the metal surface to ground. The energy associated with that arc can create high temperatures that can damage associated objects. In a later attempt to control arcing, in U.S. Pat. No. 4,523,387 (1985) to Mahan, a non-conductive liner was added to the drum. In the present invention, the drum surface must remain conductive to facilitate heat transfer from the jacket. In U.S. Pat. No. 5,270,509 (1993) to Gerling, an electronic system to monitor the electric field in the drum was added which could interpret a sudden drop in that field strength as arcing, and then reduce the microwave power. In U.S. Pat. No. 5,396,715 (1995) to Smith a fire suppression system is included triggered by a flame sensor. Smith makes the statement that thermal damage to fabric is the result of rapid heating of metal objects by microwaves. Rather, the damage is the result of the arc that may occur between the metal object and some lower potential, likely the grounded drum. However, some materials do readily absorb microwaves (some plastics, some liquids, even some crystals like graphite and doped silicon) and should such materials be included with clothes burning of fabric could occur. Checking pockets will always be part of successful microwave drying. Still, arcing remains a problem which inhibits the application of microwave drying to residential laundry.
According to Paschen's Law, the breakdown voltage for air will initially decline as the pressure is lowered from normal atmospheric pressure, reaching a minimum as the pressure is continuously reduced. For a vacuum dryer operating in that region of the Paschen Curve with negative slope, arcing in the dryer may be an even greater problem. A dryer operating at reduced pressure must expect a greater incidence of arcing. If, as suggested, the potential difference that causes arcing results from a building of surface charge on metal objects, a mechanism that conveys that charge to ground could avoid arcing. Such a mechanism exists naturally with saturated fabrics. Liquid water, except when extremely pure, has the ability to conduct surface charge to ground. However, as the surface of the fabric dries, even though water still exists inside the fibers, that conductivity will be gone. So, at some point during the drying process, metal surface charge will again build. If microwave energy is replaced at that point with conventional heating, some overall efficiency is lost, but that loss can be minimized with good sorting of loads.
One reason to apply microwave energy to drying is to provide higher energy efficiency than the conventional method of hot air drying. Microwaves will be absorbed by water molecules without being significantly absorbed by the surrounding air. Air may still be used to convey the vaporized water from the dryer, but the energy associated with that air will not represent wasted energy. It is also possible to minimize the temperature rise of the solid material using microwaves. If there is no air movement, liquid water will only be removed by raising the water temperature to the point where its vapor pressure is equal to the atmospheric pressure; i.e., to its atmospheric boiling point. However, if the incoming air is relatively dry, the liquid water can be removed at a lower temperature by contacting the water with moving air. The driving force to transfer water molecules from the liquid surface to the air is proportional to the vapor pressure of the water, which in turn is determined by temperature. The higher the water temperature, the faster the water will be removed. The same is true for a higher air flow rate, and for better contact between solids and air. It occurred to some that reduced pressure drying of solids using microwaves has the potential to further reduce the drying temperature by increasing the driving force for a given temperature. That is, for the same temperature, the partial pressure of water vapor will be greater at reduced total pressure, improving the rate of transfer of water molecules to the air. For example, U.S. Pat. No. 4,250,628 (1981) to Smith and Uthe describes a method and apparatus for drying fabric under vacuum using microwave energy and indirect heating from hot water at the bottom of the drying chamber, which does not rotate. Air from a blower is added to the chamber at the end of the cycle. There is no discussion of the potential for arcing.
A rotating drum is helpful in most drying applications to provide continuously good contact between air and solids which facilitates mass transfer of water vapor to air and removal from the dryer. However, a rotating drum presents sealing problems with both microwaves and air, and especially so if the drum operates under vacuum. For example, U.S. Pat. No. 4,765,066 (1988) to Yoon describes an elaborate non-contact sealing system to allow for drum rotation without leaking microwaves by controlling clearances at a small fraction of the energy wavelength. The unit did not operate under vacuum.
All similar inventions heretofore known fail to take full advantage of the minimum temperature and energy efficiency possible with microwave drying by combining a rotating drum, reduced pressure, and energy recovery (including waste heat from the microwave generator), while effectively sealing both air and microwaves and avoiding arcing.
In accordance with one embodiment a sealed drum, rotating on a horizontal axis, loaded with clothes or other fabrics, is supplied with outside air and irradiated with microwave energy from a generator that rotates with and is located inside the drum, under partial vacuum. Air exits the dryer through a filter and flows through a vacuum pump which returns this moisture-laden air to the outer jacket of the drum and finally the air is directed to a trapped and vented drain. If the load may contain metal objects, then, when a moisture sensor in the vacuum pump discharge detects a preset reduced moisture level, the microwave generator is turned off and an electric heater turned on to raise the temperature of incoming air and continue the drying process without allowing arcing from any metal objects in the fabric. Unlike microwave energy, this heated air will transfer energy to the fabric as well, but the temperature will at least be limited by the saturation temperature of water at reduced pressure. When a further reduced moisture level is detected by the moisture sensor, the drying process is complete and all power to the apparatus—heater, vacuum pump, and rotating mechanism—is shut down. Air is directed to and from the rotating drum via dual flow rotating unions and through a double pipe which also serves as the axle for the drum. Inside each rotary union, the outer pipe is sealed via single mechanical seal while the inner pipe is sealed via stuffing box (although a mechanical seal is possible here also). When the load contains no metal, as with sheets, towels, etc., and for other solids, a selectable switch will allow microwave energy to continue drying until the final moisture level is reached.
In this embodiment, the continuous rotation of the drum provides good contact between fabric and air, the low absolute pressure inside the drum minimizes the temperature required to vaporize the water, the microwave energy targets water molecules while the energy normally wasted from the microwave generator is utilized to enhance drying, and the energy represented by the moisture laden air that leaves the drum is partially recovered via the outside heat transfer jacket of the drum, along with some of the heat of compression supplied by the vacuum pump. Using low voltage rotary contacts, the microwave source has been mounted inside the rotating drum. Most of the energy associated with the inefficiency of converting house current to microwaves is taken up by the incoming air and then transferred by convection to the solids in the dryer. Additional waste energy from the microwave generating equipment is transferred by conduction through the wall of the enclosure to the solids on the outside of that wall. Microwaves are sealed inside the drum by virtue of the small diameter of the orifice(s) for air flow into the drum, the small diameter of orifices in the support for the outlet filter media, and the conductive elastomeric seal and the small dimension of the gap between the form matching, overlapping door and the drum. Some of the latent heat of vaporization of the water and the heat of compression from the vacuum pump are recovered by directing air from the vacuum pump to the dryer jacket. The space between the inner drum and outer shell—the jacket—includes a spiral-wound baffle intended to maintain air velocity to promote heat transfer and prohibit bypassing as it directs the air flow across the drum and to the outlet pipe. As moisture condenses, the air flow rate and direction are intended to keep the liquid flowing so no buildup occurs that would inhibit additional heat transfer and condensation. This two-phase flow exits the dryer and enters a typical trapped and vented sewer line. As clothes are dried, the liquid water in the fabric will initially minimize any buildup of charge on metal objects by its conductivity. At some point the reduced moisture level will inhibit its ability to conduct charge to the grounded drum, at which point the energy source will be switched from microwaves to hot air, for those loads that may contain metal, to complete the drying of fabrics, thus avoiding arcing.
This embodiment provides a combination of low temperature drying and energy conservation and recovery not currently available. However, it should be noted that this embodiment, as with current technology, will provide its maximum energy efficiency when used continuously; that is, with one batch closely following the last. Otherwise, the energy associated with the elevated operating temperature of all dryer parts will be wasted following each batch, lowering the efficiency per batch.
Initially, an operator would open the door and lift the door to a position which it will maintain. The operator would then load the drum with wet clothes or other fabrics from a washing machine, close the door and use the locking handle 34 to seal the door. (The hinges 32 are slotted so that the door can move to the drum uniformly when the drum begins to experience negative pressure.) The operator would choose the proper position of the selector switch 45, depending upon whether the load may contain metal (zippers, buttons, rivets, etc.). Then, the operator would push the (momentary contact) start switch 46 to begin the drying cycle. The microwave generator is not switched on until the minimum vacuum is achieved, measured at the vacuum switch 41, indicating that the drum door is adequately sealed.
Inside the drum, air is contacted with fabric which is tumbling as the drum rotates, accentuated by the drum baffles 25. Although three are shown, this number may be greater. Microwaves directed from the wave guide 22 fill the drum and are absorbed by water in the fabric before and after microwaves are reflected by the grounded metal inside surfaces of the drum. The drum can be made of various metals, but aluminum is a good choice for a fabric dryer to provide high thermal conductivity, resistance to water/air corrosion, and to minimize the weight of the unit. The drum must be designed for the anticipated operating vacuum. Design for full vacuum will provide maximum flexibility in any future vacuum pump substitution.
As air exits the drum it flows across a filter material to remove lint, such material supported by a metal housing 26 drilled with many small diameter holes designed to provide minimal pressure drop at the design air flow rate, with diameter at a small fraction of the wavelength of the microwaves generated. From the filter housing 26 air flows through the inside pipe of the opposite side rotary union 13 and then to the vacuum pump 17 inlet. From the pump discharge, compressed (approximately atmospheric pressure) air flows past the moisture sensor 42 and to the outer pipe of the same side dual flow rotary union 13. Through the annular area of the double-pipe axle of the rotary drum, the air flows into and through the drum jacket 28, from the jacket to the annular area of the opposite side double-pipe axle, and then transitions from rotating axle to the stationary discharge pipe through the same rotary union that conveys inlet air and rotating low voltage supply wires. The jacket area must exclude that required to accommodate the door. This will require abrupt changes in direction of the spiral path that directs the flow from the inlet to the outlet side of the jacket.
When a new drying cycle is initiated, the vacuum pump must provide an acceptable minimum vacuum at the vacuum switch 41 (indicating that the door seal is acceptable) before microwave heating can begin. Because the initial air flowing through the vacuum pump may be relatively dry room air, the final low moisture switch 42 must be bypassed briefly so that the heating can begin and moisture laden air can make its way to the moisture switch. Microwave drying will continue until the moisture sensor detects sufficiently low moisture to indicate the potential buildup of charge on metal objects (depending upon the selector switch position). At this point, energy from microwaves is replaced by that from room air heated by an electric resistance element 39, controlled by an in-line temperature switch 40, while the air flow rate and vacuum remain essentially unchanged.
Hot air vacuum drying continues until the moisture sensor detects a low moisture level that indicates the load is acceptably dry. At this point all power would be interrupted except that the (now open) final low moisture switch will be bypassed during that part of the final revolution required to bring the drum door to the normal load/unload position. A mechanical proximity switch 47 is held closed by the drum except at a single position of the drum. This means that once per revolution the proximity switch will open briefly. As long as the moisture switch is made, the opening of the proximity switch has no effect.
When the drum stops, the operator would open the locking mechanism and raise the door to the sustaining open position and manually unload the dryer. The operator could stop the drying process at any time by pushing the (momentary contact) stop switch 48.
An additional embodiment would employ the same basic design but would not include the electric resistance heater 39 and the second moisture switch 42 set point. This embodiment would be used to dry other solids, such as pharmaceuticals or fine chemicals. In this case, the lack of metal objects would eliminate the arcing potential. To eliminate the possibility of corrosion and chemical contamination, 316L stainless steel would be a better choice than aluminum for drum construction. The door assembly of the first embodiment is replaced with a welded flange and bolted butterfly valve with conductive elastomeric seal. The flange diameter would accommodate installation and service of the microwave generator canister. With the butterfly valve full open, it would be possible to replace the filter media. Depending upon the nature of the solid chemical being dried, the media inside the filter may differ as required to keep fine solids out of the vacuum pump and piping. This industrial dryer designed for gravity discharge may be elevated to accommodate the height of the product container.
From the above description, a number of advantages of the dryer apparatus are evident:
This dryer apparatus is a relatively simple machine that can be constructed using readily available components and uncomplicated fabrications that lend themselves to mass production. There will be some challenges. For example, the double pipe drum axle center line must coincide with the drum center line within a very small tolerance to make the drum rotate without deflection. And, the overall dimension from the rotary contacts 11 on one end to the vacuum pump suction piping on the other must be such that the unit could be placed in a typical residential laundry room. This consideration does not apply to the industrial application.
If such a unit should become widely used, the energy savings could offset a higher cost, and could have ramifications at the grid level that will be helpful to all. More sophisticated appliances are now more likely to be widely accepted by energy conscious consumers. The relatively rapid low temperature drying will have both energy and time saving advantages that will be interesting to industrial users who need to dry high value temperature sensitive pharmaceuticals and fine chemicals.
Although the description above contains a number of specificities, these should not be construed as limiting the scope of the embodiments but as merely providing illustrations of these embodiments. For example, the door of the dryer designed for residential laundry could have a variety of shapes and closure mechanisms. The ends of the drum could be formed shape rather than flat. The butterfly valve suggested for loading and unloading of an industrial dryer could be another type of valve with large diameter opening—perhaps an iris valve, if one could be designed to seal under vacuum. And, the transition from the drum to valve flange could be a shape other than cylindrical to encourage gravity discharge of non-free flowing solids.
Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given.
