Patent Application
20210190423
The present invention relates to the drying of vegetable material in a rotary dryer and, in particular, to the separation and hot-air drying by means of a Coand{hacek over (a)}-effect vegetable material dryer.
The drying of diverse materials such as municipal waste sludge, wood chips, fertilizers and harvested grain, relies upon a heated flow of air blown onto the vegetable matter. The removal of water from a mass of vegetable material by evaporation is often necessary to stabilize the material, either to transport or use the dried material or to preserve it and prevent spoliation due to molds or bacterial deterioration. Most commonly, a rotary dryer is used to present the vegetable material to a stream of air—very like the drying of laundry in a clothes dryer.
Rotary dryers use a tumbling action in combination with a flow of drying air in order to efficiently remove moisture from the vegetable material. Most often, rotary dryers are of the direct configuration, meaning that the drying air is brought into direct contact with the vegetable material.
Rotary dryers comprise a rotating drum, into which the material is fed in combination with a flow of heated drying air. Rotary drums include flighting or vanes; these vanes are the primary material handling mechanism in the rotary drum, conveying the material from the bottom of the rotary drums carrying the vegetable material to near the top where gravity draws the material from inner surface of the drum pouring down in a cascading motion. Material to be dried enters the dryer, and as the dryer rotates, the material is lifted up by a series of internal fins known as “flights” or “vanes” lining the inner wall of the dryer. The cascading action that the flights impart when dropping it through the air stream maximizes heat transfer between the material and drying air (in the case of direct dryers) as well as carrying moisture from the vegetable material. While the dryer will generally dry the vegetable material, pockets of moisture often remain. Some materials clump and potentially form balls containing that moisture. To dry the interior, these clumps or balls of vegetable material need to be broken up to expose that interior vegetable material to the drying flow. The interior vegetable material must be dried which wastes heated air on the already dried vegetable material. Not all vegetable material will be consistently balled or clumped and might, instead, coalesce in long and stringy chunks which can easily get trapped within the drying mechanism.
A conventional dryer includes a fueled fire in a combustion chamber which either provides heated gasses or directs a heated airflow to heat the interior of the rotary drum. Combustion chambers can be integrated into either co-current (airflow in direction of material flow) or counter current dryers, with the goal being to keep the material from coming into direct contact with the burner flame.
The conventional rotary dryer will include discharge breeching is where two main functions occur: vegetable material exits the dryer, moving on to screening, cooling, storage, or shipping, and the exhaust gas system removes off-gases from the system. At discharge end some openings in the cylinder allow discharge of the vegetable material into the discharge breeching. The breeching encloses the cylinder and bottom part is hoppered and flanged. The breeching face encircling the cylinder is often equipped with a special friction type seal. The top of the breeching has a flanged opening for drying air inlet.
At the feed side, the rotary drum is provided with a feed inlet head equipped with screw conveyor feeder and flanged opening for air and vapor removal. A seal is often provided between the feed inlet face and the rotary drum or cylinder. Exhaust gas systems provide a place for spent gases and hot air (and small particulates) to exit the system. Where a combustion chamber and discharge breeching meet the drum, a seal or pair of seals is needed to connect the stationary component to the rotating drum. The purpose is to keep air and material from leaving the drum prematurely or to escape to the ambient.
Direct dryers are used more frequently than their indirect counterparts, because of the efficiency they offer. Direct dryers rely on direct contact between the material and drying air to efficiently dry materials. Sweep air carries the evaporated moisture along with dust particles from inside the dryer to the exhaust system at the discharge breeching. This process ensures that the material is being dried to the required moisture percentage.
All direct rotary dryers must be equipped with exhaust gas handling equipment. Vegetable material is notoriously flammable among other issues that exist relating to the exhaust gas. Consider, for example, the Washburn A Mill fire on May 2, 1878 in Minneapolis where dried flour dust exploded with such a violent combustion that bits of the mill were found over a quarter of a mile away from the explosion.. For that reason, exhaust gas handling equipment is required and additional scrubbing might be necessary where there are unique emissions requirements based upon the material being processed as those requirements are expressed in local, state, and federal regulations.
As discussed above, there is a problem with the conventional rotary dryers in that clumping occurs. Indeed, because of the flow of air on the exterior of a flow of vegetable material tends to “skin” the exterior of vegetable material, the rotary dryers often form these clumps in normal operation. Such clumping prevents uniform drying as dried air cannot reach the interiors of clumps from drying. Disparate materials can often clump together, or stick to the interior of the rotary dryer during the industrial drying process.
One conventional means of preventing clumping is referred to as a knocking system which strikes the exterior of the rotary drum to “knock” material off the interior of the drum as it rotates. Unfortunately, after knocking, further drying is necessary to reach the newly exposed surfaces of the vegetable material. What is missing in the common art of drying vegetable material that avoids this clumping and allows drying air to reach the interior of such clumps of vegetable material as might occur as it is dried.
A cylindrical drum having a plurality of vanes which extend radially into the drum and extend axially substantially the length of the drum rotates about its axis such that the plurality of vanes convey vegetable material from a lowest point in the interior of the drum to a highest point in the interior of the drum, there, to drop the vegetable matter downward within the interior of the drum. A plenum feeds air through a nozzle opening in a rectangular slot extending in its longest dimension substantially the length of the drum. The plenum and nozzle being housed to form a Coand{hacek over (a)}-effect nozzle body having a tear-shaped housing. Two Coand{hacek over (a)} surfaces are situated in opposed relation and terminating at the nozzle. The Coand{hacek over (a)} surfaces to guide air into a combined flow. A hopper plate cooperates with the housing to form a hopper directing vegetable matter to collide with the combined flow. Negative pressure within the drum assures no propagation of the dust especially as the dust is drawn to feed end of the drum, i.e. pulled from the dry end of the knife to the exhaust and capturing dust in the wetter solids.
A rotary dryer includes a drum defining an interior. The drum, in operation, rotates about its axis in sealed rotatable engagement between a feed-end structure and a discharge-end structure. The feed-end structure defines an infeed hopper further defining an infeed port for admitting vegetable matter into the drum. The feed-end structure further defines an exhaust port. In operation, an exhaust blower draws an exhaust volume of heated air and dust from the interior of the drum through the exhaust port. A plenum passes into the interior of the drum. An intake blower blows an intake volume of heated air into the plenum passing the intake volume into the drum for drying vegetable matter. The intake volume is selected not to exceed the exhaust volume. A discharge-end structure defines a discharge chute for removing vegetable matter from the interior of the drum. The exhaust blower is connected to blow the exhaust volume of heated air and dust into a cyclone separator. The cyclone separator allows the heated air to escape to the ambient and to drop dust into the infeed hopper to mix with the vegetable matter as the vegetable matter is admitted into the drum.
The Coandã effect is the tendency of stream of fluid to stay attached to a curved surface, rather than to follow a straight line down in its original direction. The Coandã effect is also known as “boundary layer attachment” and was named after the Romanian discoverer Henri Coandã, who was the first to understand the importance of this phenomenon for aircraft development. The Coand{hacek over (a)}-effect describes a laminar flow of gas (or liquid) that follows a surface as it passes over it. To form a lamina, however, the flow must be organized with a rectangular cross-section.
A free jet of air entrains molecules of air from its immediate surroundings causing an axisymmetrical “tube” or “sleeve” of low pressure around the jet. The resultant forces from this low pressure tube end up balancing any perpendicular flow instability, which stabilizes the jet in a straight line. However, if a solid surface is placed close, and approximately parallel to the jet, then the entrainment (and therefore removal) of air from between the solid surface and the jet causes a reduction in air pressure on that side of the jet that cannot be balanced as rapidly as the low pressure region on the “open” side of the jet. The pressure difference across the jet causes the jet to deviate towards the nearby surface, and then to adhere to it. The jet adheres even better to curved surfaces, because each (infinitesimally small) incremental change in direction of the surface brings about the effects described for the initial bending of the jet towards the surface. If the surface is not too sharply curved, the jet can, under the right circumstances, adhere to the surface even after flowing 180° round a cylindrically curved surface, and thus travel in a direction opposite to its initial direction. The forces that cause these changes in the direction of flow of the jet cause an equal and opposite force on the surface along which the jet flows. This tendency to follow such surfaces is known as the Coand{hacek over (a)}-effect.
This signature structure, the Coand{hacek over (a)}-effect nozzle body 10, is recognizable for its inverted “tear drop” shape as it includes a plenum 14 conjoined with a nozzle 20 to form a housing 12 whose exterior defines these two Coand{hacek over (a)} surfaces 18 in opposed relation cooperating to entrain air into this resulting air flow 26. The organized flow of entrained air 24 remains highly organized as a laminar flow. This laminar flow of entrained air 24 efficiently amplifying the flow of air as it issues out of the rectangular nozzle 20. The Coand{hacek over (a)} surface 18 is curved to draw surrounding air over its generally smooth surface shaped such that air is drawn in and organized as a laminar sheet in flow of air there-over.
Importantly, to achieve this effect, the jet 22 must be long enough extending in a direction perpendicularly to the sheet containing
Moving, then, to
The resulting hopper 19 feeds the stacked vegetable material 34a into a collision region 28 wherein a flow of stacked vegetable material 34a collides with the air knife 26. In that collision region 28, this air knife 26 performs three important functions: 1) to tear, because of the distinct velocities and directions of the flows of the stacked vegetable material 34a and the air knife 26 breaking up clumped stacked vegetable material 34a into its smallest particles; 2) bathing the now broken up clumped stacked vegetable material 34a with heated air drying the vegetable material more completely and quickly (Small or thin objects have a large surface area compared to the volume. This gives them a large ratio of surface to volume. Larger objects have small surface area compared to the volume so they have a small surface area to volume ratio. A large lump, for example, has a small ratio, and may be smashed into powder to give it a large surface to volume ratio.); and 3) accelerating stacked vegetable material 34a moving it rapidly out of the collision region 28 in an aerosol of vegetable material 36 making room for more stacked vegetable material 34a to collide with the air knife 26. Each of these three important functions enhances the drying efficiency of the rotary dryer.
Also visible in
Also, another innovation is present in the form of the air withdrawal port 42 upon which more will set forth relative to the discussion of dust control set forth below. The air withdrawal port 42 is located within the interior of the drum 38 in a space where a vortex that the air knife 26 creates throws heavier vegetable material dust in an aerosol of vegetable material 36 outward to be captured by the vanes 66 at the outer extremes of the interior of the drum 38 leaving the air at the air withdrawal port 36 largely free of dust. Such dust that is there is drawn through the air withdrawal port 42 is generally quite fine as it is less influenced by centrifugal force than the heavier vegetable material 38. Thus, air is withdrawn through the air withdrawal port 42 to balance the volumes of air necessarily introduced through the plenum 14 to form the air knife 26. This feature of the invention allows the control of dust as described below.
While in normal operation, the inventive dryer might obtain as much as a thirty five percent reduction in moisture content within the vegetable particulate 30, stacked vegetable material 34a, and interspace vegetable material 34b shown herein. One can readily imagine that in beginning rotation, especially if some of the interspace vegetable material 34b already resides between the vanes 66 and if moist, its weight would tax the motor 56. To protect the motor 56, a thermal cut-off relay 74 is provided. Thermal cut-off relays 74 protect the motor 56 by cutting power from the motor 56 if the motor 56 draws too much current for an extended period of time. To accomplish this, thermal cut-off relays 74 contain a normally closed (NC) relay. When excessive current flows through the motor circuit, the relay opens due to increased motor temperature, relay temperature, or sensed overload current, depending on the relay type. Thermal cut-off relays 74 are similar to circuit breakers in construction and use, but most circuit breakers differ in that they interrupt the circuit if overload occurs even for an instant. Thermal overload relays are conversely designed to measure a motor's heating profile; therefore, overload must occur for an extended period before the circuit is interrupted. Peak and sporadic overloads are generally not damaging to the motor 56.
To place the same elements described with reference to
As demonstrated in the skeletal view of the rotary drum dryer in
In viewing
Also visible in
Having described fully the path of vegetable material through the drum 38, a second innovation in the rotary dryer is that of retaining dust within the dryer. As stated in the background above, vegetable material dust is extremely flammable. Consider, for example, the Washburn A Mill explosion. In the spring of 1878, the original Washburn A Mill exploded in a fireball of flames, thrusting debris hundreds of feet into the air. In a matter of seconds, a series of thunderous explosions—heard 10 miles away in St. Paul—destroyed what had been the city's largest industrial building, along with several adjacent mills. At the inquest into the deaths of the 18 workers, John A. Christian, the A Mill's manager, explained that the disaster had been caused by rapidly burning flour dust.
Dust presents another hazard as well. Using flour again as an example, because of its apparently benign and common nature, few people realize that it is a hazardous material. Workers in baking-related jobs may inhale flour dust when it becomes airborne. The dust can irritate the respiratory tract and lead to occupational asthma, also known as baker's asthma. The health problems can develop over 30 years. Naturally, other vegetable material dust can present equally or, in fact, to a far greater extent, a real danger to the workers who might breathe such dust.
To contain such dust, the inventive rotary drum dryer exploits a closed-loop system to control the flow of heated air through the dryer. Referring, then, to
Returning then to the embodiment illustrated in
Because of the air withdrawal port 42 is placed in an advantageous location, free from most of the vegetable particulate 30, the air draw from the interior of the drum 38 is laden, mostly, with the lightest or smallest vegetable particulate 30 as it is the most readily swirled about in the turbulence the air knife 26 generates as it stirs the interior air. As is seen in
A squirrel cage blower, also known as a centrifugal blower, is so named since its construction looks similar to that of a hamster wheel. Squirrel fans are known for their superior energy efficiency compared to other types of blowers. They are also durable, reliable, relatively quiet, and capable of operating in a broad range of environmental conditions. These types of blowers use kinetic energy to increase the velocity and capacity of the air stream; thus differentiating them from positive displacement blowers, such as conventional fans, which use mechanical energy to physically move the air from the inlet to the outlet.
At the heart of the squirrel-cage blower 76 is an impeller which is a circular or cylindrical mechanism with a series of curved vanes 66. As the impeller rotates, the air surrounding it also rotates at the same speed. This action imparts a centrifugal force to the air, causing it to move radially outwards to the walls of the squirrel-cage blower 76 or fan housing 12. The air follows a spiral trajectory—increasing in pressure and velocity—until it exits the discharge end of the squirrel-cage blower 76. In the presently preferred embodiment of the inventive dryer, the squirrel-cage blower 76 has proven to be the most advantageous means to draw air from the interior of the drum 38.
The squirrel-cage blower 76 is a constant-displacement or constant-volume device, meaning that, at a constant fan speed, a squirrel-cage blower 76 moves a relatively constant volume of air rather than a constant mass. As such, regulating the speed of the squirrel-cage blower 76 regulates the volume of air the squirrel-cage blower 76 moves. In most conventional squirrel-cage blowers, the impeller is belt-driven such that motor, belt and pulleys are selected to spin the impeller at a selected speed based upon design parameters. Because the volume of air driven into the plenum 14 is known, each of the squirrel-cage blowers 76 that feed air into the drum 38 and that drawing air out of the drum 38 can be selected such that, in operation, a slight air flow into the drum 38, is drawn into the discharge chute 80 thereby preventing the escape of dust from the drum 38 actually “rinsing” discharged vegetable matter with a current of inflow air as it exits the drum 38.
In an alternate embodiment shown in
A simple feedback loop is available, therefore. By selecting the optimal position, a rotation speed of the squirrel-cage blower 76 can be selected to place the pressure sensing vane 84 in the optimal position 84a. Pressure within the interior of the drum 38 can be regulated by the speed of the squirrel-cage blower 76 and, thus, a variable drive blower can be used to maintain an optimized pressure within the drum 38. Variable drive blowers may use hydraulic or magnetic couplings (between the impeller wheel shaft and the motor shaft) to vary speed in a controlled manner generally by allowing the impeller wheel shaft to slip relative to the motor shaft.
In some embodiments blower speed controls can be integrated into automated systems to maintain the desired impeller shaft rotational speed. An alternate method of varying the fan speed is to use an electronic variable-speed drive to control the rotational speed of the motor which, in turn, is mechanically connected driving the fan. A variable speed motor controller offers a better overall energy efficiency than mechanical couplings, especially at greatly-reduced speeds.
In this manner, for example, optimum air flow may be achieved by using such as a phase-locked loop feedback mechanism, the volume of vegetable matter fed into the dryer will always receive the optimum flow of drying air based upon the position of the pressure sensing vane 84 and its proximity to the optimum position 84a. A phase-locked loop or phase lock loop (“PLL”) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a variable frequency oscillator and a phase detector in a feedback loop. The oscillator generates a periodic signal, and the phase detector compares the phase of that signal with the phase of the input periodic signal, adjusting the oscillator to keep the phases matched. Keeping the input and output phase in lock step also implies keeping the input and output frequencies the same. Consequently, in addition to synchronizing signals, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. If an oscillator can generate a frequency based upon a position of the pressure sensing vane 84, the rotational speed of the impeller and the position of the pressure sensing vane 84 can be correlated in this PPL manner.
Referring now to
Importantly, the moisture of the infed vegetable material 34 acts to coalesce the dry dust particles especially exploiting the mixing action of the infeed auger 40 upon the moist vegetable matter. In fact, the dust is folded into the vegetable material 34 which actually tends to prevent clumping by amalgamating this very dry dust into the interior of the flow of vegetable matter makes the vegetable matter very much more susceptible to be broken up by the air knife 26 at the base of the hopper 19 (See
Referring now again to
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
