Patent Application
20050141343
The invention pertains to the field of process equipment. More particularly, the invention pertains to the attachment of blades to a rotating drive shaft, such as for example in scraped surface heat exchangers.
Scraped surface heat exchangers are a type of mixing apparatus well known in industry. Scraped surface heat exchangers generally feature an outer cylindrical housing tube and a central rotatable drive shaft disposed in the center of the outer housing tube. An annular space is provided between the central drive shaft and the outer housing tube, and material is forced from one end of the scraped surface heat exchanger through the annular space between the shaft and tube.
In general, the material enters the space between the shaft and tube near one end of the scraped surface heat exchanger and travels longitudinally along the scraped surface heat exchanger and exits near the other end of the scraped surface heat exchanger. During this time, the material can be mixed by blades which are attached to, and extend outward from, the central shaft and are rotated by the central shaft. The material may also be subject to temperature gradients as it travels along the scraped surface heat exchanger so that the material is heated or cooled.
The blades are sometimes very generally flat blades which are mounted in a pivotable fashion proximate to the surface of the central drive shaft. The blades extend outward at a relatively narrow angle from the central drive shaft and generally have a tip feature at their distal end which is in sliding contact with the inner surface of the outer housing cylinder as the blades are rotated.
The blades are generally elongated, and typically several blades are provided along the length of the scraped surface heat exchanger to occupy the length of the inner volume of the scraped surface heat exchanger. The blades serve at least some of several functions. For example, the blades can enhance overall mixing of the material as it passes along the inner volume of the scraped surface heat exchanger. The blades also can contact the inner housing of the scraped surface exchanger to in effect scrape material off the inner surface so that it does not build up on the inner surface of the housing cylinder. Further, the blades can add to general flow patterns within the heat exchanger which facilitate temperature transfer from the outer housing of the heat exchanger to the material itself. The outer housing may have a fluid jacket or other heating or cooling source to impart a desired temperature to the outer housing, so that the material can be heated or cooled as it travels through the device.
It has been known to mount the blades to the central driving shaft using so-called “pins”. The pins are individual items that are attached, usually by welding, to the drive shaft surface and have some sort of receptacle area to accept a part of the blade so the blade is pivotally positioned. Two or more pins are usually used on the length of a single blade to pivotally support one blade at the locations of the pins. Previously, the blades have had a cutaway portion forming a hinge shaft or single “attachment beam” that is received in the receptacle slot of the pin to form a hinge configuration.
As noted above, the pins are typically spaced apart longitudinally along the length of the drive shaft, with two or more pins being used for each blade. For simplicity and manufacturing costs reasons, for a given length of heat exchanger it is typically desirable to reduce the number of blades and even further to reduce the number of pins. Thus, it is desirable typically to use relatively longer blades if possible, and it would be desirable to reduce the number of pins for each blade as well.
However, there can be some drawbacks to using longer blades and fewer pins when using the aforementioned prior art pin connection methods. In the prior art, the blades have tended to have relatively short attachment beams due to shear and buckling failure modes. Due to the possibility of flexing of the entire blade between the attachment points, there is a need to place the beams and pins at certain regularly defined intervals.
Another disadvantage with at least some conventional pin and beam attachment systems has been that during assembly of the device, the blade may fall out of its pin receptacle depending on the manufacturing angle of the shaft, pin, and blade.
A different disadvantage with at least some conventional pin and beam attachment systems has been that scraping of the top of the pin against the inside of the outer tube can occur during installation and removal of the drive shaft with the pins and the blades. By way of example, for installation of a drive shaft, it is common practice to first mount each blade into its pivotal connection with its respective pins. Then, the drive shaft itself with all the blades attached (and sometimes flopping loosely to some extent) is gradually manually slid until the tube into its full length runs along the full length of the housing cylinder and then the ends of the drive shaft can be secured to respective bearings and end caps of the housing cylinder. Removal of the drive shaft is through reverse process. Since the clearances between the outside of the drive tube and even more so the outside of the pins, with respect to the inside of the housing tube tend to be very small, it is a very difficult process to manually slide the lengthy drive shaft into the outer housing tube without having the pins contact and to some extent scrape along the inside of the drive tube. Such scraping can be highly undesirable because the surface finish of the inside of the housing tube should generally be kept at a prescribed smoothness if possible. Keeping the inner surface of the housing tube at a prescribed finish helps prevent material from getting caught in surface imperfections on the inside of the tube, which then makes it difficult to clean the inside of the tube during cleanings. Moreover, deep scratches on the inside of the outer housing can lead to premature wear of the blades which are constantly in a rotating sliding contact with the inside of the outer housing tube.
One conventional approach to overcoming this problem has been to insert some form of relatively soft plastic shim or skid between the drive shaft and the housing tube, most particularly at the bottom where gravity would tend to pull the drive shaft down, and then to gently slide the shaft into the tube. The shims can be inconvenient, however, because first of all they need to be present at each assembly and disassembly, and second they may be difficult to insert before a shaft is removed and/or be difficult to remove after a shaft has been installed. Further, in vertical mounted units, the skids can be very difficult or impossible to use.
Accordingly, it would be desirable to have an arrangement where the usually metallic mounting pins do not come in contact with the inner surface of the outer housing tube of the heat exchanger during insertion and removal of the drive shaft with the pins and blades attached.
Accordingly, there is a need in the art for a blade attachment apparatus and method that can overcome the above disadvantages in some instances, at least to some extent, for example by providing desirable support to the blade as well as by providing some degree of locking of the blade to prevent it from falling out during installation, while permitting some pivoting during assembly and operation.
The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus is provided that in some embodiments provides a protective rest nub on a blade.
In accordance with one embodiment of the present invention, a blade for use with a scraped heat exchanger having a drive shaft with at least one outwardly directed blade mounting pin, and a housing tube having an inwardly facing inner surface, is provided comprising a body having a cross section spanning between a first tip end and a second mounting end, and at least one projecting nub projecting generally radially outwardly from the blade body when the blade is in a mounted state, the nub dimensioned to project radially outwardly past the pin when the blade is in the mounted state, and dimensioned to provide a resting surface for the blade, the pin and the drive shaft and to support the blade, the pin and the drive shaft when the blade, the pin and the drive shaft are otherwise unsupported in the housing tube.
In accordance with one embodiment of the present invention, a blade for use with a scraped heat exchanger having a drive shaft with at least one outwardly directed blade mounting pin, and a housing tube having an inwardly facing inner surface, is provided comprising a body having cross section spanning between a first tip end and a second mounting end, and at least one supporting means projecting generally radially outwardly from the blade body when the blade is in a mounted state, the supporting means dimensioned to project radially outwardly past the pin when the blade is in the mounted state, and dimensioned to provide a resting surface for the blade, the pin and the drive shaft and to support the blade, the pin and the drive shaft when the blade, the pin and the drive shaft are otherwise unsupported in the housing tube.
In accordance with one embodiment of the present invention, a method for use with a scraped heat exchanger having a drive shaft with at least one outwardly directed blade mounting pin for mounting at least one blade, and a housing having an inwardly facing inner surface, is provided comprising mounting at least one blade to the drive shaft via at least one pin, and supporting the blade, the pin and the drive shaft on at least one projecting nub projecting radially outwardly from the blade, the nub dimensioned to project radially outwardly past the pin when the blade is in the mounted state, so that the nub provides a resting surface for the blade, the pin and the drive shaft to support the blade, the pin and the drive shaft when the blade, the pin and drive shaft are otherwise unsupported.
There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.
Preferred embodiments of the invention provide pivotal attachment of a blade to a drive shaft of a device such as a scraped surface heat exchanger. The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like elements throughout.
Turning to
A space 20 is provided between the drive shaft 12 and the inner surface of housing tube 14 through which material passes longitudinally along the length of the scraped surface heat exchanger and travels along the longitudinal axis of the shaft 12 and the housing 14. As the material moves through the space 20, it is scraped and/or mixed by the blade 16 due to rotation of the shaft 12.
The operative direction of rotation of the shaft 12 in
Turning to
Looking particularly at
The use of fewer pins 18 for a given length of blade 16 provides significant advantages in some circumstances. For example, the cost of fixing each pin 18 to the central shaft 12 is reduced. Moreover, a larger free area 42 (the area under the blade not interfered with by the base 22 of each pin 16) is further provided than would be available if more pins 18 were needed. Providing this larger free area 42 facilitates flow of the material under the blade which can enhance mixing flow properties and also reduce the force needed to drive the central shaft drive.
Another benefit of providing dual attachment beams 40, as opposed to a single attachment beam having the same effective length as the two attachment beams, is that with the dual beams 40, each beam 40 is relatively shorter and less susceptible to bending and buckling failure. Further, the central locking element 38 provides further stiffening due to its thickness.
The method of installation of a blade 16 to a pin 18 will now be described in more detail with particular respect to
Initially, as in
The interaction of the foot 38 and the cam corner 50 with the surface 28 of the pin 16 serves to translate the attachment beam 40 upward from its position in
The position shown in
The blades, pins and shafts may of any suitable materials as desired. In some preferred heat exchangers, the drive shaft 12 is stainless steel and the pins 18 are stainless steel items welded on to the shaft. The blade 16 is preferably made from a nonmetallic material in order to facilitate forming of desired blade shapes, and preferred blade materials include molded plastics such as Victrex PEEK (polyaryletherketone) or Ticona Celcon (acetyl copolymer). Alternatively, the blade can be for example a pressed metal such as stainless steel. A benefit of some embodiments of the present invention, is that the elongated dual attachment beams 40 can be provided, and due to the provision of the central supporting element 38 it can have a desirably long effective supporting length without being subjected to such stress that a metal strengthening insert would necessarily be required. However, the locking feature of some embodiments of some invention does not require that there be dual attachment beams with the locking feature provided on a central element. For example, alternative embodiments can include a single attachment beam extending across a single cutout. A locking feature including a foot 52, and a cam corner 50 may be provided at any location along the blade, most preferably in such an embodiment adjacent to one side of the cutout to interact with a corresponding feature on the pin. In such an embodiment, a single finger would preferably be used with a single beam, and of the saddle 56 if provided would be located to the side of the respective finger, rather than centrally between two fingers as shown in the embodiment of
Returning to
Since the blades may be selected to be made of polymer materials in many preferred instances, the nubs therefore provide a sliding surface of polymer material on the inside of the housing tube 14, which depending on the material of the pins themselves, which is typically metal, provides significantly less if any damage to the inside surface finish of the housing tube 14.
In the illustrated preferred embodiments, the nubs are integrally formed, most preferably by being molded integrally with the remainder of the blade 16. However, particularly if the material of the blade 16 does not provide desirable protective sliding characteristics, in alternative embodiments the nubs may be discrete components made of a different material from the rest of the blade that are bonded glued or otherwise affixed to an outward facing surface of the blade, with sufficient projection to achieve the resting protective properties described above.
The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.
