DeMARCO, Francis, W. (1224 Cayuga Drive, Niagara Falls, NY, 14304, US)
| What is claimed is: 1. A method for constructing a freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer, the method comprising the steps of: applying a brazing material to at least one of a top plate, a bottom plate and an elongate divider having an open cross sectional shape; assembling the elongate divider between the top plate and the bottom plate, the brazing material being positioned in contact area interfaces between the divider and the plates; applying a clamping force to the plates to compress the divider between the plates, whereby the open cross sectional shape of the divider facilitates untwisting of the divider to permit area contact between the plates and the divider; and heating the clamped divider and plates to form brazed joints. 2. The method of claim 1, wherein the open cross sectional shape of the divider is a square "C" shape. 3. The method of claim 1, wherein the open cross sectional shape of the divider is an "I" shape. 4. The method of claim 1, wherein the open cross sectional shape of the divider is a "Z" shape. 5. The method of claim 1, wherein the brazing material is applied as a tape containing nickel. 6. The method of claim 1 , wherein the heating step comprises heating in a vacuum. 7. A method for constructing a freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer, the method comprising the steps of: applying a brazing material to at least one of a top plate, a bottom plate and an elongate divider having a cross sectional shape including at least one external curve; assembling the elongate divider between the top plate and the bottom plate, the at least one external curve of the elongate divider forming line contact interfaces with the top and bottom plates, the brazing material being positioned in the line contact interfaces; applying a clamping force to the plates to compress the divider between the plates, whereby the line contact interfaces between the plates and the at least one external curve of the divider are substantially maintained regardless of twist in the divider; and heating the clamped divider and plates to form brazed joints. 8. The method of claim 7, wherein the elongate divider has a hollow circular cross section. 9. The method of claim 7, wherein the elongate divider has a hollow oval cross section. 10. The method of claim 7, wherein the brazing material is applied as a tape containing nickel. 11. The method of claim 7, wherein the heating step comprises heating in a vacuum. 12. A freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer, comprising: a bottom plate; a top plate substantially parallel to the bottom plate and spaced away from the bottom plate; a plurality of hollow elongate dividers for separating circulating fluid flow channels between the top and bottom plates, the dividers having area interfaces with the top and bottom plates, the dividers spacing the top and bottom plates apart, each divider having an open cross sectional shape for reducing resistance of the divider to untwisting; and brazed connections between the dividers and the plates at each of the area interfaces. 13. The freeze dryer shelf of claim 12, wherein the open cross sectional shape of the divider is a square "C" shape. 14. The freeze dryer shelf of claim 12, wherein the open cross sectional shape of the divider is an "I" shape. 15. The freeze dryer shelf of claim 12, wherein the open cross sectional shape of the divider is a "Z" shape. 16. The method of claim 1 , wherein the brazed connection comprises a brazing material containing nickel. 17. A freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer, comprising: a bottom plate; a top plate substantially parallel to the bottom plate and spaced away from the bottom plate; a plurality of elongate dividers for separating circulating fluid flow channels between the top and bottom plates, each of the dividers having a cross sectional shape including at least one external curve for forming line interfaces with the top and bottom plates, the divider spacing the top and bottom plates apart, the line interfaces being substantially unaffected by twist in the dividers; and brazed connections between the divider and the plates at each of the line interfaces. 18. The freeze dryer shelf of claim 17, wherein the elongate divider has a hollow circular cross section. 19. The freeze dryer shelf of claim 17, wherein the elongate divider has a hollow oval cross section. 20. The freeze dryer shelf of claim 17, wherein the brazed connection comprises a brazing material containing nickel. |
Field of the Invention
[0001] The present invention relates generally to freeze dryer shelves for supporting articles such as substances or vials or trays containing the substances within freeze dryers. More specifically, the invention relates to the problem of fabricating freeze dryer shelves from components having inherent twist and warp imperfections.
Background
[0002] Freeze drying is a process that removes water from a product in the form of ice. In the freeze drying process, the product is frozen and, under vacuum, the ice sublimes and the vapor flows towards a condenser. Ice subsequently condensed on the condenser is removed in a later stage. Freeze drying is particularly useful in the pharmaceutical industry, as the integrity of the product is preserved during the freeze drying process and product stability can be guaranteed over relatively long periods of time.
[0003] Freeze dryer shelves are located within a freeze drying chamber of a freeze dryer for supporting articles such as biological substances or, more commonly, vials containing the biological substances to be freeze dried. The shelves are disposed in a vertical stack that may be collapsible in order to stopper the vials.
[0004] The shelves also serve to transfer heat between a circulating fluid such as alcohol, glycol, mineral oil, etc., and the articles to be freeze dried. During the freeze drying process, moisture present within the articles is frozen. After freezing, the articles are subjected to sub-atmospheric pressures that are low enough to enable the moisture to sublime into a vapor. To this end, the fluid circulating within the freeze dryer shelves is first cooled by an external refrigeration circuit in order to cause heat to be transferred from the articles to the circulating fluid and thereby cause the freezing of the moisture contained within the articles. During sublimation, the circulating fluid is slightly heated by an external heater in order to provide energy for the sublimation.
[0005] Since the freeze drying process occurs in a low pressure environment, heat transfer between the articles and the circulating fluid occurs principally by conduction and by radiation. As may be appreciated, it is critical that the shelves be as flat as possible in order to maximize the contact between the shelves and the articles. This maximization of contact, in turn, maximizes the degree of conductive heat transfer between the articles and the shelves and, ultimately, between the articles and the circulating fluid.
[0006] In one design, shown in FIG. 1 , a freeze dryer shelf 100 is constructed from two opposed stainless steel plates 110, 120 framed at the edges by a frame of square stainless steel tubing (not shown), forming a space 1 15 between the plates. Additional square stainless steel tubes 130 traverse the space 1 15 between the plates in order to form flow channels both between and through the tubes. The circulating fluid flows through the flow channels, transferring heat to or from the stainless steel plates 1 10, 120 and the biological substances to be freeze dried. An example of such construction is disclosed in U.S. Patent No. 5,519,946 to Renzi.
[0007] The freeze dryer shelf 100 is constructed by clamping the two opposed stainless steel plates 1 10, 120 together over the stainless steel tubes 130, with a brazing substance such as nickel sandwiched between the tubes and plates. The assembly is then heated in a vacuum furnace to form brazed joints 135 between the components.
[0008] The brazing operation to fabricate the shelves requires tight closure and proper seating of parts such that gaps are not present in a manner that would prevent the brazing material from joining parts of the assembly. Any gaps in the joints result in internal coolant leakage and therefore inefficient heat transfer, and reduced strength. The divider tubes of the shelf assembly must therefore be straight and free of any twist that would prevent close contact with the adjacent plate material.
[0009] In practice, the rectangular hollow tubes 130 used as dividers are often found to be twisted. To achieve a proper brazed joint in a freeze dryer shelf as described above, it is frequently necessary to straighten or untwist the square tubing before assembly; that process is difficult and time consuming. If the twisting of the tubes is not removed, the brazing operation may not yield satisfactory results as gaps may be present in the brazed interfaces.
[0010] There therefore remains a need for a freeze dryer shelf having enhanced manufacturability while maintaining efficient heat transfer and minimum internal coolant leakage.
Summary of the Disclosure
[0011] The present disclosure addresses the needs described above by providing freeze dryer shelves and methods for constructing the same. One embodiment is a method for constructing a freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer. A brazing material is applied to at least one of a top plate, a bottom plate and a hollow elongate divider. The elongate divider has an elongate cross sectional shape. The elongate divider is assembled between the top plate and the bottom plate, the brazing material being positioned in contact area interfaces between the divider and the plates. A clamping force is applied to the plates to compress the divider between the plates, whereby the open cross sectional shape of the divider facilitates untwisting of the divider to permit area contact between the plates and the divider. The clamped divider and plates are then heated to form brazed joints.
[0012] The open cross sectional shape of the divider may, for example, be a square "C" shape, an "I" shape or a "Z" shape. The brazing material may be applied as a tape containing nickel. The heating step may include heating in a vacuum.
[0013] In another method for constructing a freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer, a brazing material is applied to at least one of a top plate, a bottom plate and an elongate divider having a cross sectional shape including at least one external curve. The elongate divider is assembled between the top plate and the bottom plate, with the at least one external curve of the elongate divider forming line contact interfaces with the top and bottom plates. The brazing material is positioned in the line contact interfaces. A clamping force is applied to the plates to compress the divider between the plates, whereby the line contact interfaces between the plates and the at least one external curve of the divider are substantially maintained regardless of twist in the divider. The clamped divider and plates are heated to form brazed joints.
[0014] The elongate divider may have a hollow circular cross section, or a hollow oval cross section. [0015] Another embodiment of the invention is a freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer. The shelf includes a bottom plate and a top plate substantially parallel to the bottom plate and spaced away from the bottom plate. The shelf further includes a plurality of hollow elongate dividers for separating circulating fluid flow channels between the top and bottom plates, the dividers having area interfaces with the top and bottom plates, the dividers spacing the top and bottom plates apart, each divider having an open cross sectional shape for reducing resistance of the divider to untwisting. Brazed connections are between the dividers and the plates at each of the area interfaces.
[0016] In yet another embodiment, a freeze dryer includes a bottom plate and a top plate substantially parallel to the bottom plate and spaced away from the bottom plate. A plurality of hollow elongate dividers separate circulating fluid flow channels between the top and bottom plates. Each of the dividers has curved surfaces for forming line interfaces with the top and bottom plates. The dividers space the top and bottom plates apart, the line interfaces being substantially unaffected by twist in the dividers. Brazed connections are between the divider and the plates at each of the line interfaces.
Brief Description of the Drawings
[0017] FIG. 1 is a partial perspective view of a prior art freeze dryer shelf.
[0018] FIG. 2A is a partial perspective view of a freeze dryer shelf in accordance with the disclosure.
[0019] FIG. 2B is a perspective view of a divider illustrating a twisted condition. [0020] FIG. 3 A is partial perspective view of freeze dryer shelf in accordance with another aspect of the disclosure.
[0021] FIG. 3B is detail view of the freeze dryer shelf of FIG. 3A.
[0022] FIG. 4 is partial perspective view of freeze dryer shelf in accordance with another aspect of the disclosure.
[0023] FIG. 5 is partial perspective view of freeze dryer shelf in accordance with another aspect of the disclosure.
[0024] FIG. 6 is partial perspective view of freeze dryer shelf in accordance with another aspect of the disclosure.
[0025] FIG. 7 is a flow chart showing a method in accordance with one aspect of the disclosure.
[0026] FIG. 8 is a flow chart showing a method in accordance with another aspect of the disclosure.
Description
[0027] The freeze dryer shelves and methods of the present disclosure overcome the problems inherent in the prior art by using elongate dividers having cross-sectional shapes in which twist is not problematic. In one embodiment, those cross-sectional shapes may be shapes having curves that present line contact interfaces with the top and bottom plates.
[0028] As used herein, the term "cross sectional shape," as applied to elongate dividers, means the shape of a transverse cross section; i.e., the shape of a section taken perpendicular to a long axis of the elongate divider. [0029] An example of such a freeze dryer shelf 200 is shown in FIG. 2 A. A top plate 210 and a bottom plate 220 are spaced apart by elongate dividers 230 with hollow cross sectional shape. The dividers 230, together with the top and bottom plates 210, 220, define a flow channel 240 for the circulating fluid. Additional flow channels such as channel 250 are defined by the hollow cross sectional shape of the dividers 230. In practice, the circulating fluid may be circulated though both channels 240 and 250.
[0030] A divider 230 to be assembled into shelves may be received by the shelf fabricator in a twisted condition as shown in FIG. 2B, where a line 275 originally parallel to a longitudinal axis 276 of the divider 230 is offset by an angle 280 over the length shown due to twisting of the divider. The twisting may be due to material imperfections or manufacturing imperfections such as uneven heating or cooling as the tube is formed.
[0031] The cross sectional shape of the dividers 230 is circular; i.e., the divider is cylindrical. That shape presents external curved surfaces such that a tangential line contact interface is defined between the divider and the plates. Using the disclosed design of the divider 230, twisting of the tube does not affect the contact between the divider and the plate. Instead, the tangential line contact interface effectively adjusts to accommodate twisting of the tube, resulting in continuous contact regardless of twist.
[0032] Because the divider 230 need not be untwisted before it is used in the shelf assembly 200, a torsionally rigid member such as a member with a closed cross section as shown may be used. The tube may be used as received or, at most, after performing a straightening operation.
[0033] One skilled in the art will recognize that other shapes that result in a line contact between the divider and at least one plate may be used to avoid the necessity of removing twist from the divider. For example, the shelf 300, shown in FIG. 3 A, includes a divider 330 having an oval cross section. The oval cross section presents curved surfaces to the plates 310, 320, resulting in tangential line contact interfaces 336 (FIG.
3B) between the oval divider and the plates. A brazed connection 335 is formed within and around the line contact interface 336.
[0034] Other cross sectional shapes presenting at least one external curve may be used. For example, as an "S" cross section or an "I" cross section having curved flange sections may be used.
[0035] Twisting has almost no effect on the contact between the divider 330 and the plates 310, 320. Because the divider orients itself between the plates with the short dimension 312 of the oval spacing the plates, the divider need be straightened only in the direction of the short dimension because bending in a direction parallel to the plates 310,
320 does not significantly affect contact. Additionally, the oval cross sectional shape of the divider tends to stabilize the divider and prevent rolling during assembly of shelf components.
[0036] Another approach to minimizing the problems associated with dividers received in a twisted condition is to reduce the forces required to untwist the dividers.
The term "untwist," as used herein, means to reduce or remove a twist about a longitudinal axis of a divider.
[0037] It is known that open shapes provide considerably less resistance to torsion than a closed shape such as the rectangular hollow tube shown in FIG. 1
(Blodgett, W. Omer, Design of Welded Structures, The James F. Lincoln Arc Welding
Foundation. (Cleveland, Ohio 1968), Section 2.10). Utilizing such an "open" shape allows the divider to more easily conform to the required flatness in the assembly of shelf parts prior to brazing, and permits the divider to be more easily straightened before assembly of the parts.
[0038] As used herein, an "open" shape is a shape having a discontinuous perimeter, wherein the interior of the shape is not completely enclosed by the perimeter. For example, while an "O" is a closed shape, a "C" is an open shape.
[0039] An exemplary freeze dryer shelf 400, shown in FIG. 4, includes elongate dividers 430 in the form of beams having an open cross section in the form of an "I". The cross sectional "I" shape presents two flange sections 435 extending from opposite sides of the beam at right angles to a central web 436. The outside surfaces of the flange sections 435 present contact area interfaces for brazing to the top and bottom plates, while the web 436 provides a structural connecting member and flow divider in the shelf. Because the open shape of the "I" presents a lower resistance to twisting, the divider 430 is more easily untwisted either before the shelf is assembled, or during the clamping of the dividers between the top and bottom plates. The contact area interfaces formed by the flanges 435 are therefore free to align properly with surfaces of the top plate 410 and bottom plate 420, forming continuous braze joints.
[0040] Another exemplary freeze dryer shelf 500, shown in FIG. 5, includes a divider member 530 having an open cross section in the form of a squared "C", having two flange sections 535 forming the contact area interfaces. Because the open shape of the squared "C" presents a lower resistance to twisting, the divider 530 is more easily untwisted, either before or during assembly. [0041] In another exemplary shelf 600, the dividers 630 have "Z" shaped cross sections with flanges 635, 636 forming the contact area interfaces. The open "Z" cross sectional shape presents a lower resistance to untwisting prior to or during assembly of the shelf.
[0042] The freeze dryer shelf constructions disclosed above greatly improve the manufacturability of the shelves. One method 700 for manufacturing a freeze dryer shelf with a top plate, a bottom plate and a hollow elongate divider for use in supporting articles and exchanging heat with articles in a freeze dryer is shown in FIG. 7. A brazing material is first applied at 710 to the top and bottom plates, to the hollow elongate divider, or to both. The brazing material may be applied in the form of a nickel-based tape or paste. The brazing material is applied at least in contact interface areas where the plates and the dividers will make contact when assembled.
[0043] The dividers are then assembled at 720 between the top plate and the bottom plate. The dividers may be placed in a labyrinth pattern between the plates to separate flow paths of a circulating fluid. Additional dividers or other components may be placed around the periphery of the plates.
[0044] The dividers have cross sectional shapes including at least one curved surface forming line contact interfaces with the top and bottom plates. For example, a cylindrical divider forms a contact line with the plate where the plate is tangent to a circular cross section of the cylindrical divider. The contact line extends substantially parallel with a longitudinal axis of the cylindrical divider. Similarly, a divider having an oval cross section will form a line contact with the plate where the plate is tangent to the curved surface. The brazing material is positioned in the line contact interfaces. [0045] A clamping force is then applied to the plates at 730 to compress the divider between the plates. Because of the curvature of the divider surface in the area of contact, the integrity of the line contact interfaces between the plates and curved surfaces of the divider is substantially maintained regardless of twist in the divider.
[0046] The clamped divider and plates are then heated at 740 to form brazed joints. In a preferred embodiment, the freeze dryer shelf is heated in an evacuated oven. Upon heating to the melting point of the brazing material, surface tension of the brazing material causes the material to flow into the interface and create a sealed joint. Because the curved surface of the divider permits a substantially continuous line contact between the divider and the plate over the length of the divider, the brazed joint is also
substantially continuous and, in use, prevents flow of circulating fluid past a divider/plate interface.
[0047] Another method 800 for constructing a freeze dryer shelf for use in supporting articles and exchanging heat with articles in a freeze dryer is shown in FIG. 8. To fabricate a freeze dryer shelf in accordance with that embodiment, a brazing material is first applied at 810 as above to flanges of elongate dividers with an open cross sectional shape.
[0048] The elongate dividers are then assembled at 820 between the top plate and a bottom plate, with the brazing material positioned between the flanges of the divider and the plates. Elongate components such as solid bar or square tube members (not shown), more structurally rigid than elongate dividers with an open cross sectional shape, may also be assembled between the plates in addition to the dividers to add structural integrity and to enclose outer edges of the plates. For example, those rigid elongate components may be assembled around the periphery of the plates.
[0049] A clamping force is applied at 830 to the plates to compress the divider between the plates. The open cross sectional shape of the divider facilitates untwisting of the divider to permit area contact between the plates and the divider. The clamped divider and plates are then heated at 840 to form brazed joints.
[0050] The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Description of the Invention, but rather from the Claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.
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