Patent Application
20080121497
There are various applications of screw conveyors which are heated or cooled depending on the intended purpose. In one such application carbon black pellets are conveyed for use in rubber compounding. A preferred form of conveyance is a screw conveyor because the conveyor also acts as a feeder capable of delivering precise amounts of carbon black to a weigh hopper. Carbon black pellets are extremely small and fragile. The rubber mixing process is generally a batch operation requiring the movement of large volumes of carbon black in an intermittent fashion. As the carbon black is shuttled through the process some portion of the fragile pellets break down and become micron size particles of dust known as fines. Carbon black fines have an affinity for anything they come in contact with including themselves. Over time a build up of carbon black fines develops on the screw of the conveyor and eventually takes up so much volume that the screw must be removed for cleaning. Depending on the volume of carbon black moved through the screw this could be done several times per year at great expense due to labor and lost production. History has shown that by heating the screw to a temperature at or above 100° C., such as disclosed in U.S. Pat. No. 4,834,647, the carbon black loses its affinity for the surface of the screw and therefore no buildup of fines will occur. This results in a much more efficient and consistent process.
The current state of the art for heating the screw is to insert an electric cartridge heater inside the hollow screw pipe. The cartridge heater should be longer than the screw itself to facilitate mechanical attachment. If the screw conveyor is over 12 ft. long two heaters are required to be inserted from each end significantly increasing the overall length of the conveyor. The cartridge heater is stationary while the screw rotates around it. Heat is transferred to the screw from the heater mostly by radiation with the minimal amount of convection. This method is only about 40% efficient, meaning that to achieve a screw temperature of 100° C. requires a heater temperature of 250° C. The thermal profile of the heated screw is also not uniform. The hottest section is at the center of the screw with the ends of the screw being cooler. The uneven thermal profile can result in fines building up at the in-feed and discharge ends of the screw. Additionally, because the cartridge heater does not rotate with the screw the heater is subject to wear and vibration that shortens its useful life. The result is an expensive system both in initial cost and in the cost of operation and maintenance, The cost of down time from carbon black fines outweighs these costs in high volume production but smaller volume rubber mixing operations may not see a fast enough return on their investment.
Similar problems exist where screw conveyors, such as an auger screw, are used in applications involving cooling below ambient temperature.
An object of this invention is to provide improvements for heating or cooling a screw conveyor, particularly a volumetric screw conveyor.
A further object of this invention is to provide such a heated/cooled screw conveyor which is particularly thermally efficient and which provides for a more uniform temperature throughout the heated/cooled zone.
A heated/cooled screw conveyor includes a thermosyphon in the hollow screw shaft of the conveyor. The thermosyphon is in the form of a hollow tube in contact with the inner surface of the shaft for transferring heat/cold by direct contact with the shaft.
In a preferred practice of the invention the thermosyphon tube is mounted against the inner surface of the shaft in such a manner that an integral unit results whereby the thermosyphon rotates with the rotation of the shaft. Preferably, the thermosyphon includes a wick mounted within the thermosyphon tube in such a manner that liquid may flow between the outer surface of the wick and the inner surface of the thermosyphon tube while vapor flows in the opposite direction through the tubular wick itself.
The present invention addresses problems relating to the inefficiency and high operating costs of conventional heated screw conveyors. A key feature of the invention is the use of a thermosyphon to heat the screw instead of using a cartridge heater. The thermosyphon in the preferred practice of the invention is an integral part of the screw shaft rotating with the screw shaft and transferring heat by conduction through direct contact rather than radiation. The thermosyphon works on the principle of latent heat of evaporation. Since the latent heat of evaporation is large the thermosyphon has the capacity to transfer large amounts of heat at high speeds in both heating and cooling operations. Therefore a small heated section of thermosyphon will transfer the heat/cold along its entire length rapidly and efficiently. Theoretically the thermosyphon will be 2.5 times more efficient than conventional cartridge heater designs. The thermosyphon design will also heat/cold the screw uniformly from end to end because the thermosyphon will be an integral part of the screw.
Heat can be supplied to the thermosyphon from, for example, electric band heaters attached directly to an exposed end of the screw shaft as later described. These band heaters are commonly used items that are readily available from commercial supply houses. The same band heaters will work on any thermosyphon equipped conveyor regardless of length. The extremely larger cartridge heaters currently used are custom made and require several weeks for manufacturer. Their length is also custom fit to each individual conveyor. Thus, if one has twelve heated screw conveyors of customized length, there is a need to stock 12 spare heaters. Heaters are not interchangeable.
Screw conveyor 10 would be used as a heated screw conveyor when treating, for example, rubber in a molding process and could be used as a cooling screw conveyor when treating, for example, plastic material in an injection process.
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During manufacture the thermosyphon 36 is inserted into one end of the hollow screw shaft 18 until the thermosyphon abuts against one of the stop members. After being positioned within the hollow shaft 18 the other stop member is inserted to fix the location of thermosyphon 36. In a preferred practice of this invention the thermosyphon tube 40 is made of a heat conductive material and is preferably a copper housing while the screw shaft 18 is made of carbon steel or stainless steel. The coefficient of expansion is significantly higher for copper than for steel or stainless steel. This causes the thermosyphon outside diameter of tube 40 to increase more than the inside diameter of the inner surface of shaft 18 when both are heated. In this case, when the thermosyphon 36 is inserted into the hollow shaft 18 the assembly is heated to a temperature in excess of 288° C. The assembly will be held at such elevated temperature long enough for the thermosyphon copper housing 40 to yield and thus not return to its original diameter after the heat is removed. The result of this is a positive mechanical contact between the screw shaft 18 and the thermosyphon 36.
Since the screw shaft 18 and thermosyphon 36 form an integral unit by virtue of the expansion of thermosyphon 36 they will rotate together when the screw is in operation. The band heaters 38 are also attached to the screw 18 and will rotate in unison. To supply power to the band heaters 38 requires the use of a rotary contact 48. Rotary contact 48 has a set of stationary contacts that will connect to the power supply and a set of rotating contacts that will rotate in unison with the screw 18 and supply power to the band heaters 38.
An advantageous feature of the invention is that the heat source or cooling source would be located entirely at one end of the screw.
The present invention creates a significantly efficient heated screw assembly far superior to the conventional state of the art technology whether heated by electricity, steam or some other medium which does not come close to the thermal efficiency of the present invention. Such conventional systems create a thermal gradient along the length of the screw that requires significantly more heat energy input into the system than the recirculating closed loop system provided by thermosyphon 36. The Wattage input required for the present invention is much smaller because the heat transfer system is approximately 2.5 times more efficient. The band heaters 38 that can be used are less expensive and can be stocked for a variety of screw conveyors. The heat input can also be supplied by a variety of other sources, not simply electrical, but also including but not limited to steam, hot water, hot air and hot oil.
The present invention also allows for extracting heat from the material as it is being conveyed with the same thermal efficiency as described for heating. This opens the possibility of utilizing the invention for applications for cooling a product while conveying it as well as for heating. Thus, for example, the present invention could be used for cooling a rotating auger screw internally by various cooling mediums including, but not limited to, chilled water, chilled gases and cryogenic liquids. Thus, in a broad sense the invention utilizes thermal applicating structure which could be either for heating or for cooling a rotating screw. Use of the invention would result in a thermal efficiency greater than 95% and in providing a more uniform temperature throughout the heated or cooling zone which would be ±1° C.
