Drying and gas or vapor contact with solids – Apparatus – Gravity flow type
Reexamination Certificate
2001-09-20
2002-08-27
Walberg, Teresa (Department: 3742)
Drying and gas or vapor contact with solids
Apparatus
Gravity flow type
Reexamination Certificate
active
06438864
ABSTRACT:
BACKGROUND
The invention relates generally to centrifugal pellet dryers utilized to dry plastic pellets which have been cut from strands of plastic by a pelletizer, and more particularly, to a centrifugal pellet dryer apparatus including improved rotor and lifters which can provide enhanced dewatering capabilities. The invention also relates to additional improvements in the centrifugal pellet dryer apparatus which can further enhance dewatering capabilities.
Generally, a water slurry of plastic pellets is introduced into a pellet dryer for separation of the pellets from the water. The dry pellets can then be conveyed to a shipping container or to a location for further processing. Centrifugal pellet dryers are well known in the art. In particular, U.S. Pat. No. 5,611,150, to Yore, Jr., issued Mar. 18, 1997, which is hereby incorporated herein by reference, discloses a “Centrifugal Pellet Dryer” wherein a slurry of pellets and water is introduced upwardly through the bottom of the dryer, into a hollow region in the rotor and out through ports in the rotor. In this manner, the slurry is directed radially outwards from generally the center of the rotor. The rotor has lifters, i.e. blades, which carry the pellets and water upwardly through the dryer as the water is forced outwardly by centrifugal force through a cylindrical screen that surrounds the rotor. The dewatered pellets are discharged through an exit port near the top of the cylindrical screen while the separated water is drained through the bottom of the dryer housing. Prior to the centrifugal pellet dryer described in the '150 patent, centrifugal pellet dryers introduced the slurry of pellets and water through an entry port in the side of the pellet dryer. The slurry was injected inwardly towards the rotor and the lifter affixed to the outside of the rotor drove the slurry outwards against the screen as the rotor was rotated. In that configuration, the rotor breaks the stream of the slurry where it is introduced through the side of the pellet dryer, causing a large portion of the pellets to be deposited on the portion of the screen nearest the slurry entry port. In contrast, the pellet dryer disclosed in the '150 patent describes introducing the slurry of pellets and water through the center of the rotor and radially outward into the space between the rotor and the mesh member. By introducing the pellets and water through the center of the rotor, the slurry of pellets and water is radially, and more evenly, distributed by the rotor lifters about the interior of the pellet dryer. Thus, pellet impacts are not concentrated at any single area of the screen, in contrast to the conventional pellet dryers which injected slurry through a side port where most of the initial dewatering is done in the first quadrant adjacent to the slurry inlet, which loads up the screen and limits its effectiveness in the initial dewatering stage.
Introducing the slurry through the rotor also minimizes radial bearing loads that are present in the prior-pellet dryers since the lifter blades in those pellet dryers must break the stream of water and pellets in a single location which is off-center from the axis of the rotor. The feed of the slurry through center of the rotor also enhances the centrifugal force of the rotor to throw the water radially through the screen to thereby enhance the initial dewatering stage of the dryer.
Recently issued U.S. Pat. No. 5,987,769, to Ackerman, et al., issued Nov. 23, 1999, discloses an alternative manner of introducing a slurry of pellets and water generally into the center of the pellet dryer. However, the slurry is not injected radially outwards through the rotor as in the '150 patent. Rather, the slurry is injected axially upwards onto a lower face of the rotor. Lifters, or blades, are provided on a lower face of the rotor which throw the slurry radially outwards against the cylindrical screen. A disadvantage of this pellet dryer can be that the lower end of the rotor is not supported. This is because the slurry is directed upwards against the lower face of the rotor. Thus, the rotor is supported only at the top of the dryer, which can be disadvantageous since it is desirable to support a rotating member like the rotor at both ends to maintain axial alignment. Supporting the rotor at only one end can create problems with balance and vibration when the rotor is rotated, especially when rotated under a load as when slurry is being pumped by the rotor through the pellet dryer. Accordingly, it can generally be desirable to support the rotor at both ends to improve efficiency and longevity of the dryer.
Conventionally, it was generally believed that dewatering was best accomplished by impacting the pellets against the dewatering screen to remove the water. However, it has been discovered that dewatering can best be achieved by instead limiting the number of impacts between the pellets and the dewatering screen and increasing the number of impacts between the pellets and the lifters. Residual water on the screen can actually be reacquired by pellets through repeated impacts with the screen. The slurry of pellets and water can typically be introduced into the pellet dryer while the pellets are still hot. The internal heat of the pellets actually assists in the drying process. An additional reason why pellet impacts against the dewatering screen can be disadvantageous is that the screen acts like a “grater,” shaving or breaking off pieces of the pellets during impacts. These pieces of pellets, commonly called “fines,” can cause other problems with the operation of the pellet dryer and with disposal of the water removed from the slurry. Consequently, lifters which reduce the number of impacts with the dewatering screens can improve the drying efficiency and reduce fines. Lifters can be designed to reduce the number of impacts with the dewatering screens by creating the lifters with an inwardly curved surface which tends to direct the pellets away from the dewatering screen and back in towards the rotor surface and other lifters. This additionally results in increased impacts between the lifters and the pellets.
In prior pellet dryers, generally flat lifter blades are attached to the surface of the rotor in a helix configuration at a 45 degree angle. The 45 degree angle is what “lifts” the pellets upwards through the pellet dryer. The lifters are generally flat in that there is no curvature apart from the helix curvature imparted as a result of attachment to the cylindrical rotor. The flat lifters direct the pellets out into the dewatering screen. The lifters are also conventionally attached to the surface of the rotor in an evenly distributed manner, in that there are the same amount of lifters in each row along the entire length of the rotor. Prior art pellet dryers typically use 5-6 lifters evenly spaced radially around the circumference of the rotor. The lifters are also conventionally aligned horizontally in rows and vertically in columns.
Accordingly, there is a need for a lifter, rotor and pellet dryer apparatus which provides improved dewatering capabilities through increased pellet impacts with the lifters and reduced pellet impacts with the dewatering screen.
SUMMARY
According to the invention, a centrifugal pellet dryer apparatus, rotor and lifter is provided wherein the lifters can have a front surface configured to deflect pellets inwardly toward the rotor surface and other lifters, and the rotor can have lifters attached in a configuration designed to increase pellet impacts with lifter blades as well as providing different regions of lifters wherein different numbers of lifters can be provided in the different regions of lifters along the length of the rotor. The specially configured surface of the lifters tend to control the pellet path, keeping the pellets in the lifter envelope and away from the dewatering screens. The configuration of the lifters on the rotor, generally a higher concentration of lifters on the lower region of the rotor can further create increased pellet impacts wit
Buchanan Ingersoll P.C.
Patel Vinod D.
The Conair Group, Inc.
Walberg Teresa
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