Agitating – Mortar mixer type – Movable mixing chamber
Reexamination Certificate
2002-06-11
2004-04-27
Soohoo, Tony G. (Department: 1723)
Agitating
Mortar mixer type
Movable mixing chamber
C366S228000, C432S118000, C034S599000, C110S227000
Reexamination Certificate
active
06726351
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention generally relates to regulating the flow of material within rotary equipment, and more particularly, to a method and apparatus for controlling the amount of material picked up and veiled while drying aggregate materials, such as asphalt, within a dryer drum.
2. Background of the Invention
Asphalt is typically produced by heat drying virgin asphalt aggregate and by adding to it and mixing with it liquid asphalt cement, fillers and other additives, often including recycled asphalt pavement. Often times, asphalt is also made by drying virgin asphalt aggregate and moving it to a batch plant tower for batch mixing with the asphalt and other additives.
Asphalt manufacturing machines producing paving compositions are well-known. Generally, such machines include an area or chamber for heating and drying the aggregate and a area or chamber for mixing the heated and dried aggregate together with other materials, such as asphalt cement, liquid asphalt, fines, etc. There are generally two types of mechanisms for making aggregate dryers, that being parallel flow and counterflow. In a parallel flow dryer, cold, wet material is introduced into the cylindrical dryer near the combustion zone. The material to be dried is continuously fed into the upstream end of the dryer where the burner is located. By contrast, the burner of a counterflow dryer is located at the downstream or discharge end of the dryer. The cold, wet material enters the dryer on the upstream end and moisture is gradually driven off and the aggregate temperature is raised as the material progresses downstream in the dryer. A drying drum, dryer or cylinder is where particles are moved generally from the front to the back while being heated by a centrally located burner to dry. Within the drum, fixed flights are attached to the interior cylinder wall to stir the mixture. These flights are the internal “paddles” secured to the dryer's wall used to deflect and direct the mixture/aggregate as the aggregate moves and travels through the dryer. Flights also enhance the mixing of the mixture and prevent the mixture from sticking to the interior wall or around the wall. These flights pick up the mixture at the bottom of the rotating drum and drop the mixture as the drum rotates. The amount and specific locations of the dropped mixture will depend on the material being dried, the speed of the rotation and the profiles of the flights. Often times the pattern of the flowing and cascading mixture within the drum caused by the flights is referred to as the veil or the veil profile because the mixture takes on a certain profile throughout the drum. Generally, the veiling aggregate is more dense on the uphill side of the rotating drum than the downhill side of the drum.
As the aggregate is tumbled and dried in the dryer, dust is naturally created and carried by the hot gases of combustion. Emission regulations prohibit the discharge of such gases with dust to the atmosphere. In addition, depending on the speed of rotation and the temperature of the dryer, the dust created may represent a portion of the fine aggregate material needed in the particular mix. As a result, dust collection or recovery systems, such as baghouses and cyclone separators, are used for the removal of the dust before further processing of the gases and exhaustion to the atmosphere. The dust and gas conveyed to a baghouse or other similar air or gas filtration means are separated; the dust is separated collected for later use while the gases are vented to the atmosphere.
The interior of such a dryer can reach 300° F.-500° F. Typically, in a counterflow system, the air temperature in the drum is around 400° F. with the temperature in this region of the burner reaching up to 3,000 degrees Fahrenheit. To reach the maximum rated capacity of an asphalt plant, the exhaust temperature exiting the stack of the baghouse should be in the range of between 220° F. and 250° F. at about 70° F. ambient temperature. Over time, it has been observed that the stream of gasses will loose about 10° F. to 30° F. from the time it exits the dryer and passes through the duct work and baghouse. By controlling the temperature in the baghouse stack, one can control the efficiency of the dryer. In short, it has been found that one way to control efficiency of the aggregate dryer, or mixer drum, is to control the stack temperature of the baghouse.
Compounding the above observations, it has been noted that if the exhaust stream entering the baghouse exceeds 250° F., energy is being wasted. If the temperature exceeds 350° F., the nomax bags used for filtering the air can be damaged and loose their filtering ability. Replacing the bags is expensive.
Further, aggravating the above, unfortunately, mixer drums or aggregate dryers are not run at the same levels all of the time. They are operated in a wide range of production rates. If, for example, an operator reduces a production level to fifty percent (50%), the exhaust temperature generally will fall below 200° F. This creates a dangerous condition because if the exhaust temperature drops below 180° F., moisture can accumulate in the baghouse. Moisture combined with dust form mud which can blind the bag, effectively shutting down the baghouse.
One way to control the baghouse stack's temperature is to adjust the aggregate being picked up and veiled; in short, controlling the veil. This is accomplished by manipulating the flights to change the drum's profile. Thus, if one can control the flights as one adjusts the production level, one is theoretically capable of controlling the baghouse stack's temperature and the system's efficiency.
Over the years there have been many attempts to design variable flights. One of the most common approaches today is to have one or more people enter the dryer and physically remove some of the flights from the interior of the dryer when the production level is to be decreased. If the production level is to be increased, the flights removed are physically reattached and put back into place. This approach has many down sides. First, it is labor intensive and time consuming. Second, it is very imprecise.
In another approach, the internal flights are set or established at 75% rated capacity of the system. This approximation permits one to theoretically operate within a 50%-100% production level. However, this is not true. As the system approaches 100% capacity, the exhaust temperature will rise substantially and the efficiency of the dryer will be reduce accordingly. In the same vein, attempts have been made to remove some of the flights within the dryer to accommodate lower production levels. However, again, as production rates increase, the exhaust temperature rises substantially, diminishing system efficiency. At times, the exhaust temperatures to the baghouse become so great, that the baghouse controls shut off the burner fuel valve and hence the burner in the dryer. This is because baghouses are equipped with sensors to monitor the temperature of the exhaust gas stream and to shut off fuel to the burner when the temperature exceeds about 350° F. A way around this is to physically add flights as production increases, a laborious task.
Other teachings learned over time include the fact that combustion flights should not carry aggregate a substantial distance up the interior side wall of a drum as the drum rotates and then allow the aggregate to fall vertically down the interior face of the combustion flighting. This is because the deflection of dust and aggregate particles can enter the combustion zone, impinging on the combustion process. The flights must also be able to withstand intense heat (2,400° F. to 3,200° F.) for extended periods of time without warping, distorting, or simply burning up; thus preventing the aggregate from falling between the flights and contaminating the combustion zone.
Others have tried methods and means for modifying flights. Such attempts are shown in U.S. Pat. Nos. 5,083,382 to Brashears; 5,515,620 to Butler; 3,641,68
Dillman Equipment, Inc.
Soohoo Tony G.
Wallenstein Wagner & Rockey Ltd.
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