Freeze dryer equipment

Abstract: A freeze dryer shelf (10) adapted to support articles to be freeze dried within a freeze drying chamber and having, a pair of opposed, flat parallel first and second plates (16,18) spaced apart from one another, a plurality of ribs (20) defining flow channels (24) for circulating a heat exchange fluid between the first and second plates (16,18), connections between the first and second plates and the ribs, and a thermal mass associated with the freeze dryer shelf, the first and second plates having predetermined thicknesses and essentially no local deformations in the first and second plates at the connections, wherein: the first and second plates (16,18) have a reduction in their predetermined thicknesses; the ribs (20) have elongated, opposed flat surfaces (26,28); and the connections comprise the first and second plates and the ribs at their said flat surfaces (26,28) internally brazed to one another and stress relieved substantially to prevent local deformations that would otherwise exist at the connections due to the reduction in the thicknesses of the first and second plates, whereby the reduction in the predetermined thicknesses of the first and second plates effect a reduction in the thermal mass associated with the freeze dryer shelf (10).

Description: The present invention relates to freeze drying equipment and more particularly to freeze dryer shelves for supporting articles such as substances or vials or trays containing the substances within the freeze dryers. In particular, the invention relates to such freeze dryer shelves in which the shelf also functions in the freezing and sublimation phases of the freeze drying process to freeze and heat the articles through circulation of a heat exchange (or diathermic) fluid through the shelf. 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 commonly disposed in a vertical stack that may be collapsible in order to stopper the vials in a manner known generally in the art. The shelves also serve to transfer heat between a heat exchange 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 subatmospheric pressures that are low enough to enable the moisture to sublime into a vapour. To this end, heat exchange 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 heat exchange fluid and thereby cause the freezing of the moisture contained within the articles. During sublimation, the heat exchange fluid is slightly heated by an external means in order to provide energy for the sublimation. Since the freeze drying process occurs in a low pressure environment, heat transfer between the articles and the heat exchange fluid occurs principally by conduction. As may be appreciated, it is critical that the shelves be as flat as possible in order to maximise the contact between the shelves and the articles. This maximisation of contact in turn maximises the degree of the conductive heat transfer between the articles and the shelves and hence, the heat exchange fluid. Previously, freeze dryer shelves have often been formed by two opposed stainless steels plates framed at the edges by a solid steel frame in order to form a space between the plates. Solid ribs traverse the space between the plates in order to form flow channels for the heat exchange fluid. In one type of design, the ribs are longitudinally welded to the plates and are configured to interlock in order to form the flow channels when the plates and ribs are assembled. In another type of design, the ribs are simply welded to one of the plates. Holes are then drilled into the opposite of the plates and such plate is plug welded to the ribs. The resultant raised weld beads are ground flush and polished. For example, it is known from US-A-4 109 396 that welding techniques can be used for the manufacture of freeze dryer shelves comprising the features as defined in the precharacterising part of Claim 1 of this invention. A problem with both types of such freeze dryer shelf construction is that the welds will tend to thermally stress the plates in the vicinity of the welding. In order to reduce concomitant straining and thus, local deformation of the plates near the welding, very thick plates are used in fabricating the shelves and solid ribs are used in forming the flow channels for the heat exchange fluid. The end result of the solid rib and thick plate construction of prior art freeze dryer shelves is that each shelf possesses a sizeable thermal mass or inertia. The result of this thermal mass or inertia is that a large fraction of the energy requirement of the freeze dryer during the cooling phase of the freeze drying process is wasted in cooling the shelves. In addition to the foregoing, the energy required in effecting the cooling is also wasted through heat leakage occurring during the cooling of the heat exchange fluid. In the refrigeration circuit used in cooling the heat exchange fluid, an external heat exchanger is provided to transfer heat from the heat exchange fluid to a recoverable refrigerant such as FREON. Inevitably, there are thermal losses in the heat exchanger and the piping involved in conducting the cooled heat exchange fluid back into the freeze drying chamber. As may be appreciated, such heat leakage must be compensated for by increasing the amount of refrigeration provided by the refrigeration circuit and thus, the energy required to provide the refrigeration. The present invention generally provides a shelf design that provides the requisite shelf flatness while having less thermal mass than prior art freezer shelf designs. In addition, it provides a shelf design which minimises heat loss during the cooling of the heat exchange fluid. In accordance with the invention, there is provided a freeze dryer shelf adapted to support articles to be freeze dried within a freeze drying chamber and having, a pair of opposed, flat parallel first and second plates spaced apart from one another, a plurality of ribs defining flow channels for circulating a heat exchange fluid between the first and second plates, connections between the first and second plates and the ribs, and a thermal mass associated with the freeze dryer shelf, the first and second plates having predetermined thicknesses and essentially no local deformations in the first and second plates at the connections, characterised in that: the ribs have elongated, opposed flat surfaces and are in the form of hollow rectangular pipes; and the connections comprise the first and second plates and the ribs at their said flat surfaces internally brazed to one another and stress relieved substantially to prevent local deformations that would otherwise exist at the connections due to the reduction in the thicknesses of the first and second plates, whereby the first and second plates are able to have a reduction in their predetermined thicknesses and thereby effect a reduction in the thermal mass associated with the freeze dryer shelf. The freeze dryer shelf has a thermal mass associated therewith. This thermal mass is reduced in the present invention by providing first and second plates having a reduction in their predetermined thicknesses. The first and second plates are connected to the ribs at opposed flat surfaces of the ribs. It is to be pointed out that the reduction of plate thickness provided for in the present invention would be difficult if not impossible to connect by welding. Even if such structure were welded together, local deformations would occur at the connections which would interrupt the requisite flat, heat conductive surface to be provided by the shelf. It has been found, however, that if the connections comprise the first and second plates being internally brazed to the ribs at their flat surfaces and stress relieved, local deformations that would otherwise exist at the connection to the reduction in the thicknesses of the first and second plates will be substantially prevented. Generally, a freeze dryer shelf must present as flat a surface as possible to the articles to be freeze dried in order to maximise heat transfer by conduction. The invention can therefore provide a freeze dryer shelf adapted to support articles to be freeze dried within a freeze drying chamber of a freeze dryer and having internal flow channels for circulating a heat exchange fluid within the shelf, wherein the heat exchange fluid is adapted to be cooled by a refrigerant such that moisture within the articles freezes while the articles are being supported by the shelf, and: the freeze dryer shelf comprises a top heat exchange fluid section adapted to support the articles and a bottom refrigerant section in good thermal contact with the top heat exchange fluid section; the top heat exchange section has the flow channels for the heat exchange fluid; the bottom refrigerant section has flow passages for circulating the refrigerant through the lower refrigerant section such that the heat exchange fluid is cooled while circulating through the freeze dryer shelf; a first set of inlet and outlet means is present for introducing and discharging the heat exchange fluid into and from the heat exchange fluid section of the freeze dryer shelf, respectively; and a second set of inlet and outlet means is present for introducing and discharging the refrigerant into and from the refrigerant section of the freeze dryer shelf, respectively. As such, a freeze dryer shelf comprises an upper heat exchange fluid section and a lower refrigerant section in good thermal contact with the upper heat exchange section. The upper heat exchange section has the flow channels for the heat exchange fluid and the lower refrigerant section has the flow passages for circulating the refrigerant through the lower refrigerant section such that the heat exchange fluid is cooled while circulating through the freeze dryer shelf. The refrigerant may comprise any known refrigerant material, including a FREON or ammonia. It may also comprise a liquefied gas such as liquid nitrogen. A freeze dryer shelf formed in accordance with the invention does not require the use of an external heat exchanger to transfer heat between the heat exchange fluid and the refrigerant, but instead integrates the heat exchanger into the shelf design. The advantage of this is that the integral heat exchanger of the invention is exposed to the low pressure environment of the freeze drying chamber while heat exchange is taking place and thus is in effect vacuum insulated to substantially reduce heat loss. Additionally, heat loss into the heat exchange fluid that occurs along the external piping into the freezing chamber is also eliminated. As can be appreciated, a freeze dryer shelf designed in accordance with the invention provides a very energy efficiency design. As such, the invention could be used in its own right in increasing the energy efficiency of a freeze dryer. However, it can be advantageously incorporated into a freeze dryer shelf design to further increase the energy efficiency of a freeze dryer. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawings, in which: Figure 1 is a perspective view of a freeze dryer shelf in accordance with the invention, with portions of the shelf broken away to illustrate its internal structure; Figure 2 is a top plan view of a freeze dryer of Figure 1 with a top plate broken away to illustrate the internal structure of a top heat exchange fluid section of the freeze dryer shelf; Figure 3 is a top plan view of a freeze dryer shelf of Figure 1 with the top heat exchange fluid section of the shelf broken away to illustrate the internal structure of a bottom refrigerant section of the freeze dryer shelf; and Figure 4 is a fragmentary exploded perspective view of the freeze dryer shelf of Figure 1. With reference to Figure 1, a freeze dryer shelf 10 in accordance with the invention is illustrated having a top heat exchange fluid section 12. Heat exchange fluid section 12 supports the articles to be freeze dried and is designed to receive and circulate a cooled heat exchange fluid so that heat is transferred from the articles being supported to the heat exchange fluid. A bottom refrigerant section 14 is situated beneath heat exchange fluid section 12 and is in good thermal contact therewith. Refrigerant section 14 is designed to receive and circulate a refrigerant to cool the heat exchange fluid circulating through heat exchange fluid section 12. With additional reference now to Figure 2, the heat exchange fluid section 12 is provided with a pair of first and second plates 16 and 18. Both plates are flat, parallel and spaced apart from one another. A plurality of ribs 20 are provided between the space formed between first and second plates 16 and 18. Ribs 20 are spaced apart to define flow channels 24 for the heat exchange fluid. In this regard, ribs 20 are staggered relative to one are another in order to produce a parallel serial flow path through heat exchange fluid section 16, and thereby minimise pressure drop. Ribs 20 are preferably hollow rectangular tubes. They can additionally be any form having elongated flat surfaces, such as designated herein by reference numerals 26 and 28, in contact with first and second plates 16 and 18, respectively. If the ribs are solid, however, as in the prior art, the thermal mass of the shelf will of course be greater than the illustrated embodiment having hollow ribs. Heat exchange fluid section 16 is peripherally sealed by a frame 30 formed of rods having a square transverse cross-section (designated by reference numerals 32, 34, 36 and 38) connected end to end and connecting first and second plates 16 and 18. Heat exchange fluid flows into and is discharged from heat exchange fluid section 16 (as indicated by the arrowheads) by a set of first inlets and outlets formed by inlet and outlet pipes 40 and 42 connected to inlet and outlet tab portions 44 and 46, provided with internal drillings 48 and 50. Heat exchange fluid enters enters into and is discharged from flow channels 24 through apertures 51 defined in rods 32 and 34 and in communication with each of the internal drillings 50 of end tab portions 44 and 46. Inlet and outlet pipes 42 and 44 serve as connection points at which well known convoluted, flexible stainless steel hoses are welded. Such hoses run to an external circuit for the heat exchange fluid which conventionally includes a pump to circulate the heat exchange fluid and an electrical heater to heat the heat exchange fluid during the sublimation phase of the freeze drying process. A freezer shelf in accordance with the invention could be constructed in line with heat exchange fluid section 12 as outlined above. In such case, an external heat exchanger, well known in the art, would be provided to transfer heat between a refrigerant flowing in a refrigerant circuit and the heat exchange fluid. It is to be noted here that a refrigerant is not used alone to circulate through a freeze dryer shelf because it is impractical to provide a near uniform temperature distribution across the shelf with a refrigerant alone. Preferably though, a freeze dryer shelf in accordance with the invention is designed to act as a heat exchanger to transfer heat from the heat exchange fluid to the refrigerant. In the illustrated embodiment this is accomplished by providing freeze dryer shelf 10 with refrigerant section 14. With additional reference to Figures 3 and 4, refrigerant section 14 is peripherally sealed by a frame 54 formed by rods of transverse square cross-section (designated by reference numerals 56, 58, 60 and 62), connected end to end and connecting second and third plates 18 and 52. Refrigerant enters and is discharged from refrigerant section 18 by way of a second set of inlet and outlets formed by an inlet tube 64 which is welded to an inlet tab portion 66 and in communication with drillings 68 and 70 provided within inlet tab portion 66. A transfer tube 72 provides fluid communication from drilling 70 to an inlet manifold 74 abutting rod 56 of frame 54. Refrigerant is discharged from refrigerant section 14 by way an outlet manifold 76 abutting rod 60 of frame 54, another transfer tube 78 which provides fluid communication to drillings 80 and 82 within an outlet tab portion 84. An outlet tube 86 is welded to outlet tab portion 84 and is aligned with drilling 82. Inlet and outlet pipes 42 and 44 are welded to both inlet and outlet tab portions 44 and 84; and 66 and 46, respectively. Furthermore adjacent inlet and outlet tab portions 44 and 84 are welded to one another as are inlet and outlet tab portions 66 and 46. Although not illustrated, refrigerant lines would be welded to inlet and outlet tubes 64 and 86 to connect refrigerant section 14 within a refrigerant circuit. As such, the refrigerant lines would be located within the heat exchange fluid lines carrying heat exchange fluid to and from heat exchange fluid section 12 of freeze dryer shelf 10. Where connection is required within the refrigerant circuit of the refrigerant lines, the heat exchange fluid lines would be provided with rigid pipe-like sections without convolutions. Such rigid pipe-like sections would be provided with openings for passage of the refrigerant lines out of the heat exchange fluid lines, preferably by 90° bends provided in the refrigerant lines, penetrating the openings, and welded to the rigid pipe-like sections of the heat exchange fluid lines. Inlet and outlet manifolds 74 and 76 are of identical design and both are formed by square pipes provided with six lower equally spaced, slot like openings such as a slot-like opening 88 shown for manifold 74. In use of the present invention in a very small freezer, flow passageways for the refrigerant could be of arbitrary design. However, in large scale applications, fins 90 are provided which connect second and third plates 18 and 52. Fins 90 provide flow passages 92 for the refrigerant circulating between inlet and outlet manifolds 74 and 76 as indicated by the arrowheads of Fig. 3. Fins 90 are required in such large scale applications to provide a large heat transfer surface to conduct heat from the heat exchange fluid to the refrigerant. Fins 90 are preferably formed of a prefabricated material comprising a stainless steel sheet longitudinally embossed with elongated embossments of essentially rectangular transverse cross-section to provide alternating upper and lower elongated surfaces 94 in contact with second and third plates 18 and 52. Such a material is also transversely pierced by, for instance, piercing 96 to increase fluid contact. As with heat exchange fluid section 12, second and third plates 18 and 52 are internally brazed to the material providing fins 90 at surfaces 94 so that the assemblage is stress relieved. Such material can be obtained from Robinson Fin Machines, 13670 Highway 68, South Kenton, Ohio 43326. In addition to providing a large surface contact area for the refrigerant to conduct heat from the heat exchange fluid fins 90 also provide a sufficient structural support to refrigerant section such that freeze dryer shelf 12 can bear down on stoppers of vials supported by a shelf of identical design located beneath shelf 12. In this regard, shelf 12 is provided with 4 shelf support blocks 100, 102, 104, and 106 having openings 108, 110, 112, and 114 to receive support rods well known in the art to connect freeze dryer shelf 10 to identically designed shelves located above and below freeze dryer shelf 10. Freeze dryer shelf 10 can be fabricated in a variety of sizes, for instance 600 mm x 450 mm or 600 mm x 900 mm or 900 mm x 1200 mm, or even 1500 mm x 1800 mm. The 600 x 900 mm and the 900 x 1200 mm shelves can incorporate ribs formed by about 9.525 mm square pipe. The 600 mm x 450 mm shelves can incorporate ribs formed by about 12.7 mm x 6.35 mm rectangular pipe, and the 1500 mm x 1800 mm shelves can incorporate ribs formed by about 19.05 mm square pipe. In all embodiments, the pre-fabricated fin material can be approximately 0.2 mm thick, and 6 mm to 8 mm in height and width. The spacing between ribs depends upon the pressure to which the shelf is subjected and the mechanical strength required. In smaller shelves, 70 mm centre to centre is sufficient, while for the larger shelves, for instance 1500 mm x 1800 mm, a 45 mm spacing can be used. All of the components used in a sterilised application for freeze dryer shelf 10 (as an example in manufacturing biological preparations) should be fabricated from stainless steel. In order to fabricate shelf 12, a well known type of nickel brazing substance which can comprise a nickel powder on a self-adhesive backing is sandwiched between first plate 16 and ribs 20 and 22; between ribs 20 and 22 and the second plate 18; between the underside of second plate 18, and the prefabricated fin material; and between prefabricated fin material 90 and third plate 52. The assemblage is then again sandwiched between graphite blocks or any heat conductive material and then placed within a vacuum induction furnace. The assemblage is then heated in the furnace in a temperature that ramps from room temperature to approximately 10° C of the melting of nickel, approximately 482° C. The temperature is then stabilised and then again steadily increased up to the melting point of nickel and the crystallisation temperature of the stainless steel. This temperature is stabilised for between 15 and 20 minutes in order to stress relieve the assemblage of components. Thereafter, the furnace is cooled down for about 12 hours until 204° C is reached, at which point, the entire assemblage is quenched with an inert gas which can be nitrogen. Thereafter, the assemblage is allowed to cool to room temperature. Frames 30 and 54 are then welded to the plates and preferably ground, smoothed, and polished. The end result of the construction method outlined above, is that freeze dryer shelf 12 is fabricated without welding and is thus made with less thermal mass than prior art shelf designs. In this regard, first, second, and third plates in any embodiment can be as low as about 1.0 mm thick. In the prior art, the steel plates making up the freeze drying shelves could be as much as about 4.0 mm thick.

Claims: A freeze dryer shelf (10) adapted to support articles to be freeze dried within a freeze drying chamber and having, a pair of opposed, flat parallel first and second plates (16,18) spaced apart from one another, a plurality of ribs (20) defining flow channels (24) for circulating a heat exchange fluid between the first and second plates (16,18), connections between the first and second plates and the ribs, and a thermal mass associated with the freeze dryer shelf, the first and second plates (16,18) having predetermined thicknesses and essentially no local deformations in the first and second plates (16,18) at the connections, characterised in that: the ribs (20) have elongated, opposed flat surfaces (26,28) and are in the form of hollow rectangular pipes; and the connections comprise the first and second plates (16,18) and the ribs (20) at their said flat surfaces (26,28) internally brazed to one another and stress relieved substantially to prevent local deformations that would otherwise exist at the connections due to the reduction in the thicknesses of the first and second plates (16,18), whereby the first and second plates (16,18) are able to have a reduction in their predetermined thicknesses and thereby effect a reduction in the thermal mass associated with the freeze dryer shelf (10). A freeze dryer shelf (10) according to Claim 1, further comprising: a rectangular frame (30) located between and welded to the first and second plates (16,18) to peripherally seal the freeze dryer shelf (10); and inlet and outlet means penetrating the rectangular frame for introducing and discharging the heat exchange fluid into and from the freeze dryer shelf (10), respectively. A freeze dryer shelf (10) according to Claim 1 or Claim 2 in which the rectangular pipes (20) are situated between the first and second plates (16,18) so as to form a series/parallel arrangement of flow channels.

Inventors: RENZI ERNESTO, RENZI, ERNESTO
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1993
Main classification: F26B5
IPC codes: F26B25, F26B5